Study Unit DC Motor and Generator TheoryDC motors and generators are widely used in industrial...

DC Principles

Study Unit

DC Motor andGenerator Theory

By

Robert Cecci

DC motors and generators are widely used in industrial applications. Both motors and generatorsare devices that produce energy. A motor converts electrical energy from a power source into me-chanical energy. A generator converts mechanical energy into electricity.

DC motors are commonly used in industrial equipment. One reason is that DC motors are gener-ally easier to control than AC motors. DC motors provide the mechanical energy needed to drivecranes, hoists, elevators, material handling equipment, conveyors, precision machinery, andmeasuring equipment.

While storage batteries can be used to provide current for lighting and motors, they need contin-ual recharging in order to remain effective. In contrast, a DC generator can provide a more eco-nomical source of electric power to run industrial devices.

In this text, you’ll learn how DC motors work and how they’re controlled. Then, you’ll learnabout generator design and construction.

When you complete this study unit, you’ll be able to

� Describe the function of a commutator and brush assembly in a DC motor

� Explain how permanent magnet DC motors and stepper motors operate

� Identify series-wound, shunt-wound, and compound-wound motors and discuss theirapplications

� List the steps used to reverse a DC motor’s direction

� Describe how the speed of a DC motor is controlled

� Explain the basic principle used to generate direct current

� List the factors that affect the strength of an induced voltage

� Explain how the field connections of series-wound, shunt-wound, and compound-wound generators differ

� Explain why it’s necessary to shift brushes in a DC generator

� Discuss how interpoles and compensating windings can produce better generatoroperation

� List the various types of machine losses and calculate machine efficiency

Preview

iii

DC MOTORS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

DC Motor Construction

How a DC Motor Works

Motor Types

DC Motor Connections

Interpoles

Reversing a DC Motor

Controlling a DC Motor

GENERATOR BASICS . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Generating a Voltage by Electromagnetism

Direction of Current

Generator Construction

Generator Operation

Variations in Induced Voltage

Commutator and Brush Action

Field Connections

Field Excitation

Armature Reaction

Counteracting Armature Reaction

Eddy Currents

Hysteresis Loss

DC GENERATORS IN INDUSTRY . . . . . . . . . . . . . . . . . . . . . . 43

Types of Machine Losses

Machine Efficiency

Voltage Regulation

Operating Generators in Parallel

POWER CHECK ANSWERS . . . . . . . . . . . . . . . . . . . . . . . . . 49

EXAMINATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

Contents

v

DC MOTORS

DC Motor Construction

When you walk through an average industrial plant, you’ll verylikely see many different motors used in a variety of applications.

DC motors are used on cranes, elevators, conveyors, and on variousprocess and batch control manufacturing equipment. A workingknowledge of different types of DC motors, and how DC motors arecontrolled, is very important to an industrial electrician or electronictechnician.

While the construction of the different types of DC motors variessomewhat, all DC motors contain the same principal moving parts.Understanding what these parts are and how they work together willenable you to troubleshoot or repair a motor quickly and easily.

Generally speaking, an electric motor converts electrical energy intoturning motion. A motor uses a DC power supply and electromag-netic forces to cause an armature to turn, producing turning force, ortorque. The turning force is then used to drive another machine orpiece of equipment, such as a crane, hoist, pump, elevator, or even agenerator.

The principal parts of a motor are the following:

� A housing

� Field poles and field windings

� A rotating armature

� A commutator and brush assembly

� An output shaft

Figure 1 shows an external and a cutaway view of a typical, small DCmotor. This type of motor provides the turning motion to run a smallmachine. In Figure 1A you can see the outer case or housing that con-tains the rotating armature and the field poles. The front and rear cov-ers of the housing are called the end bells. The end bells contain thebearings that hold and support the shaft on which the armature ismounted.

DC Motor and Generator Theory

1

The manufacturer’s nameplate is typically located on the side of themotor. The nameplate lists the manufacturer’s specifications, such asthe armature voltage, current, and revolutions per minute (RPM).This information is useful when repairs are needed.

The output shaft is the turning part of the armature that’s connectedto another machine. The base is attached to a work surface to anchorthe motor. The brushes and brush holders are located in the rear endbell. The brush holders located on each side of the motor housinghold the brushes in a stationary position inside the housing. A view ofthe rear end bell and the brush holders is shown in Figure 1B.

Now, examine the simplified cutaway drawing of the motor inFigure 2A. Note the position of the armature, the field poles andwindings, the commutator, and the brushes. The field windings areconductive coils wound around metal cores called the field poles. Thefield poles are securely mounted to the inside of the motor housing.The field windings are insulated from the housing and are connectedto terminals that extend out through the housing.

When current is applied to the field windings, a stationary magneticfield is generated between the field poles. The field windings arewound around each pole in the same direction to create a stationarymagnetic field.

2 DC Motor and Generator Theory

FIGURE 1—Figure 1A shows an external view of a DC motor. Figure 1B shows the rear end bell, the brushes,

and brush holders.

Figure 2B is a cross-sectional view of a motor showing the field poles,field coils, armature, and armature windings. Note how the magneticfield (represented by the dashed lines) flows through the armatureand into the motor housing.

The armature (sometimes called the rotor) is a rotating device that’smounted on a shaft and positioned in the magnetic field between thefield poles. The armature contains current-carrying conductors calledwindings wound around a laminated steel core. The armature wind-ings fit into the slots in the surface of the armature core.

Note that the windings are represented by circled dots and circledplus signs in Figure 2B. The windings represented by the circled dotsare carrying current out of the page toward you. The windings repre-sented by the circled plus signs are carrying current into the pageaway from you.

The commutator is a round device made up of copper segments sepa-rated from each other by an insulating material (such as mica). Theends of each armature winding are connected to the copper segmentsof the commutator.

The brushes are floating electrical contacts that slide over the surfaceof the commutator as the armature rotates. Most DC motors havebetween two and six brushes. Brushes are often made from naturalgraphite, a soft type of carbon.

DC Motor and Generator Theory 3

FIGURE 2—Figure 2A is a cutaway view of a DC motor. Figure 2B is a cross-sectional view of the motor.

The brushes are held in place around the commutator by the brushholders. Springs inside the brush holders hold the brushes against thecommutator with the correct pressure.

The armature and commutator are illustrated in Figure 3. Figure 3Ashows a side view of a typical armature. In this view, you can see therear shaft that fits into the bearing in the rear end bell. Note the posi-tion of the commutator near the rear shaft. On the opposite end of thearmature, you can see the output shaft. The output shaft passesthrough the bearing in the front end bell, through a hole in the endbell, and then out of the motor. This shaft can then be connected to amachine or load. Figure 3B is a rear view of the armature showing thecommutator segments, the laminated steel core, and the armatureslots.

Figure 4A is a close-up view of the commutator and the armaturewindings. Figure 4B is a cross-sectional view of the armature, arma-ture windings, and commutator segments. Note how the end of eacharmature winding is connected to a commutator segment.


FIGURE 3—This figure shows

a side view and an end

view of the armature from

a typical DC motor.

How a DC Motor Works

A DC motor operates on the principles of electromagnetism. The fieldpoles are positioned inside the motor housing with their oppositemagnetic poles across from each other. A stationary magnetic fieldexists between the field poles (Figure 5).

When current flows into an armature winding, the winding becomesenergized, and a magnetic field develops around it (Figure 6). Themagnetic field is positioned perpendicular to the winding. The wind-ing then develops a north and a south pole.


FIGURE 4—Figure 4A shows a close-up view of the commutator and armature windings. Figure 4B is a

cross-sectional view of the armature showing the armature slots, armature windings, and the connections of

the windings to the commutator.

FIGURE 5—A stationary

magnetic field exists

between the field poles in a

motor.

Now, let’s combine all the forces together. Figure 7 shows an illustra-tion of the field poles and field windings, the commutator andbrushes, and one armature winding. The combination of electricaland magnetic forces shown here is known as the motor action of elec-tromagnetic induction.

To understand the motor action of electromagnetic induction, remem-ber that opposite magnetic forces attract each other and like forces re-pel each other. In Figure 7, the north pole of the armature winding isattracted (pulled) to the south pole of the field magnet. At the sametime, the south pole of the armature winding is attracted (pulled) to


FIGURE 6—When an

armature coil is energized, it

develops a magnetic field

all around it. The winding

also develops a north pole

and a south pole, as

indicated in the figure.

FIGURE 7—This simplified

illustration shows a motor

with one armature winding.

Note the positions of the

brushes, commutator, field

poles, and armature

winding. The interaction of

the magnetic fields

generated by the field

winding and the current in

the armature winding

causes the armature to

rotate.

the north pole of the field magnet. These pulling forces cause the ar-mature to rotate one half-turn in the direction shown by the arrows(clockwise).

When the armature has rotated one half-turn, the north pole of the ar-mature winding faces the south pole of the field magnet. Opposingmagnetic forces are holding the armature winding in place. Thus, un-less the magnetic poles are somehow reversed at this point, the arma-ture will remain locked in this position. And, unless the armatureturns continuously, the motor can’t provide the turning force neededto drive a machine. So, what keeps the armature turning? The answeris the commutator and brush assembly.

The commutator’s function is to continuously reverse the direction ofcurrent flow into the armature winding. When the current is reversed,the magnetic poles of the armature winding reverse (north becomessouth and south becomes north). Since like magnetic forces repel eachother, the field magnet poles will repel the winding, forcing the arma-ture to turn. Thus, while the field pole’s magnetic field stays constant,the armature’s magnetic field constantly switches polarity due to theaction of the commutator.

Look at the commutator assembly in Figure 7 again. Each end of thearmature winding is connected to one set of commutator segments.These segments are located 180 degrees away from each other. Notethat the brushes are also located 180 degrees away from each other.The purpose of the brushes is to conduct current from the externalpower source into the armature winding only when the armaturewinding is located next to the field poles.

As the armature winding rotates away from the field poles, its com-mutator segment disconnects from the brushes. At the same time, thenext commutator segment and its attached winding becomes con-nected to the brushes. Therefore, the brushes and commutator onlyallow current to flow in one direction in the armature winding that’sadjacent to the field poles. The commutator action keeps a constantforce on the armature windings, and the armature winding rotatescontinuously.

In an actual motor, the connection of the armature windings is morecomplicated than the connections shown in Figure 7. Remember thata real DC motor contains many armature windings and could containmore than two field poles. The interaction of all the magnetic fieldsaround these components creates the powerful turning force neededto run a motor.

Fleming’s left-hand motor rule can be used to illustrate the forces in aDC motor. Position your left hand as shown in Figure 8. In this posi-tion, the thumb points in the direction of the motion of the armaturewinding. The first finger points in the direction of the magnetic field,and the middle finger points in the direction of the current flow.


Motor Types

Many different types of DC motors are used in industrial applica-tions, from very small motors rated in ounce/inches of torque tolarge, powerful 1,000 horsepower (HP) motors. In this text, we’llgroup DC motors into two basic categories: precision and nonpreci-sion motors. Nonprecision DC motors provide the turning forceneeded to operate cranes, elevators, and heavy conveyors. All of thesemachines need simple turning force to run, but they require littleclose control of speed, starts, and stops of the motor. In contrast, thespeed and positioning of a precision motor can be accurately con-trolled through the use of an electronic control system. Precision mo-tors are used on machine tools, inspection equipment, and on someprocess control equipment.

Now, let’s look at some special types of DC motors.

Permanent Magnet Motors

All DC motors require a magnetic field to operate. Most larger DCmotors count on field windings to provide a stationary magneticfield. However, some motors contain strong permanent magnetsbonded to the inside of the motor housing instead of field windings.These permanent magnet motors are usually found in small cordlesstools and appliances. The power source in such a cordless appliance isa battery pack containing rechargeable nickel cadmium cells (NiCadcells). The output voltage of one NiCad cell is 1.2 V. Therefore, the ar-mature voltage in these motors is either 2.4 VDC, 3.6 VDC, 4.8 VDC,6.0 VDC, or 12.0 VDC (all multiples of 1.2 V). A small permanentmagnet DC motor is shown in Figure 9.


FIGURE 8—Fleming’s

left-hand motor rule can be

applied to the forces in a

DC motor.

One advantage of a permanent magnet motor is that there are no fieldwindings to energize with electricity. Current is delivered directly tothe armature through the commutator and brushes. Thus, the motordraws less current from the battery. This allows for longer periods ofoperation between chargings.

Permanent magnet DC motors aren’t limited to cordless tools and ap-pliances. Some newer-model permanent magnet motors are used asautomotive starters. High-technology permanent magnet DC motorsare also used in DC servo systems that control the positions of ma-chine tools and other process equipment. In such a servo system, a ta-chometer and position feedback devices are connected to the motor toprecisely control the motor’s position and speed.

Stepper Motors

For many years, stepper motors were used to provide the precise mo-tion needed to position machine tools and process equipment. A step-per motor is very similar to an electrical ratchet. Each pulse from themotor driver causes the motor’s shaft to turn a set amount of degrees.Usually, the amount of movement is either 1.8 degrees (full step) or0.9 degrees (half step). In recent years, stepper motors have generallybeen replaced by DC and AC servo systems.

DC Motor Connections

In different DC motors, the number of windings and brushes maydiffer. In addition, the electrical connections between the armaturewindings and the field windings may vary. These windings may beconnected in series, in parallel (shunt), or in a combination of both(compound). The different connections provide different types of mo-tor function. Let’s take a closer look at each of these connection types.

Series-Wound DC Motors

In a series-wound or series-connected DC motor, the field windings areconnected in series with the brushes connected to the armature windings.


FIGURE 9—This

permanent magnet DC

motor contains four

permanent magnets

bonded to the motor’s

case instead of field

windings.

A series-wound motor has a very high torque at start-up, and thespeed of the motor is easy to control. Series-wound motors are widelyused in cranes, elevators, and other heavy lifting equipment wherelots of power is needed at the start-up to get the load moving. How-ever, series-wound motors tend to slow down as the load increases.Also, if a load is suddenly removed, the motor turns at a higher-than-normal rate of speed.

A schematic diagram of a series-wound DC motor is shown in Figure 10.The field windings in this type of motor consist of a few turns of veryheavy gage wire around each pole shoe. Heavy gage wire is neededfor the field windings and the armature windings because a largeamount of current must pass through the windings and through theseries-connected armature.

Shunt-Wound (Parallel) DC Motors

Shunt-wound or parallel-wound DC motors operate much differentlythan series-wound motors. In a shunt-wound motor, the field wind-ings are connected directly across the armature windings (Figure 11).

The field windings in a shunt-wound motor are made up of manyturns of thin gage wire. The armature windings are also smaller gagewire. The parallel connection of the windings causes the current to besplit between the field winding and the armature windings.

Shunt-wound motors have a lower starting torque than series-woundmotors, but shunt-wound motors have better speed control. So, whenthe load on a shunt-wound motor changes, the motor speed remains


– +

FIGURE 10—This

illustration shows the

connection of the field

winding and the

armature in a

series-wound motor.

FIGURE 11—A shunt-wound

DC motor has a shunt field

connected in parallel with

the armature.

almost constant. Shunt-wound motors are used in applications thatrequire constant speed, such as drilling machine spindles, lathes, andconveyor drives.

One variation of the shunt-wound motor is the stabilized shunt mo-tor. This motor contains an additional field winding connected in se-ries with the parallel field winding and armature winding. Thisadditional winding helps to further regulate motor speed againstvarying load conditions.

Compound-Wound DC Motors

A compound-wound DC motor contains both a series field windingand a parallel field winding connected to the armature. (This configu-ration is very similar to the stabilized shunt motor discussed in theprevious section.) Both the series winding and the parallel windingare wound on the same field pole (not on separate field poles) so thatthe magnetic fields from each winding add together. The series wind-ing is heavier gage wire than the parallel winding since the largestamount of current flows through the series winding.

Compound-wound motors provide the advantages of fairly high startingtorque and good speed regulation for varying loads. Figure 12A showsa schematic diagram of a compound-wound motor while Figure 12Bshows a pictorial view.


FIGURE 12—This figure

contains both a schematic

and a pictorial diagram of

a compound-wound motor.

Interpoles

One other connection variation you should be aware of is the additionof interpoles to a DC motor. In a motor, as one commutator segmentmoves out from under a brush and another makes contact with abrush, sparking can occur at the commutator. To reduce this spark-ing, interpoles can be placed in the motor. Interpoles are additionalfield windings placed on poles located midway between the fieldpoles (Figure 13).

The interpole coils are connected in series with the armature; there-fore, they carry current that’s proportional to the armature current.The interpole coils are wound so as to produce a magnetic field with apolarity opposite to the polarity of the magnetic field created by thearmature current.

Now, take a few moments to review what you’ve learned by complet-ing Power Check 1.


FIGURE 13—Shown here is the location of the interpoles in a larger DC motor.


Power Check 1

At the end of each section of your DC Motor and Generator Theory text, you’ll be askedto check your understanding of what you’ve just read by completing a “Power Check.”Writing the answers to these questions will help you review what you’ve learned sofar. Please complete Power Check 1 now.

Write a short answer for each of the following questions.

1. What type of pole can be added to the field windings to prevent brush sparking in aDC motor?

__________________________________________________________________________

2. The commutator of a DC motor is made up of many segments of copper. What typeof insulating material is used between these segments?

__________________________________________________________________________

3. What is another name for the armature?

__________________________________________________________________________

4. Why are the field windings of a series-wound motor made up of a few turns ofheavy gage wire?

__________________________________________________________________________

__________________________________________________________________________

5. What is the step angle of a DC-powered stepper motor if it’s being driven at half step?

__________________________________________________________________________

Check your answers with those on page 49.

Reversing a DC Motor

During the operation of a motor in an industrial setting, it’s fre-quently necessary to reverse the direction of the motor. When a motoris reversed, the armature spins in the opposite direction. This reversalof the armature is periodically necessary to change the direction inwhich a piece of equipment is moving. Thus, for example, a motorruns in one direction to lift a crane; the armature direction must thenbe reversed to lower the crane.

In DC motors that contain field windings, the armature direction caneasily be reversed by reversing the polarity of either the field wind-ings or the armature windings. This action is shown in Figure 14.

Figure 14A shows standard electrical connections in a DC motor. Thepositive terminal of the power supply is electrically connected to fieldwinding 1 and brush 2. The negative terminal of the power supply iselectrically connected to field winding 3 and brush 4. The armatureturns in a clockwise direction.

In Figure 14B, the polarity of field windings 1 and 3 has been re-versed. Field winding 3 is now connected electrically to brush 2, andfield winding 1 is connected to brush 4. The armature turns in a coun-terclockwise direction.


FIGURE 14—Shown here

are two different

methods of reversing a

DC motor.

In contrast, in Figure 14C, the polarity of the armature windings hasbeen reversed instead of the field windings. This action also causesthe motor to reverse and turn in the counterclockwise direction.

In practice, it’s much easier to reverse the polarity of the armaturewindings than to reverse the connections to the field windings in atypical compound DC motor.

On older industrial equipment, motor reversing is done with specialswitches called drum switches, or with reversing relays called contac-tors. In modern DC-powered equipment, motors are reversed by elec-tronic control systems that reverse the polarity of the voltage that’sfed to the armature windings.

In a permanent magnet DC motor, the polarity of the permanentmagnets can’t be changed. Thus, the motor can only be reversed byreversing the polarity of the voltage to the armature.

When a permanent magnet DC motor is placed in a cordless appli-ance, a double-pole, double-throw switch is used to reverse the motor(Figure 15). By changing the switch position from the right to the left,you change the polarity of the voltage to the armature. An on-offswitch is used to energize or deenergize the motor.

Controlling a DC Motor

In precision industrial applications, motor speed control is very im-portant. There are many different methods of controlling the speed ofa DC motor. The most common method is to control (vary) the cur-rent to the armature windings and to maintain a fixed current in thefield windings.


FIGURE 15—By using a

reversing switch, it’s easy

to make a permanent

magnet DC motor reverse

direction.

Rheostat Control Systems

On older industrial equipment, armature voltage was controlled by ahigh-power variable resistor known as a rheostat. The rheostat wasconnected in series with the armature (Figure 16).

In this type of system, alternating current (AC) is supplied to a recti-fier assembly. The rectifier converts the AC voltage to a smooth DCvoltage. Part of this DC voltage is tapped to provide power for thefield windings. The rest runs through a rheostat to the armaturewindings. The rheostat can be adjusted to allow varying amounts ofcurrent to enter the armature.

When the rheostat is adjusted to a low resistance setting, a largeamount of DC current passes into the armature windings and the mo-tor turns at high speed. When the rheostat is adjusted to a high resis-tance setting, only a small amount of current passes into the armaturewinding, and the motor turns much slower.

In some older industrial systems, many rheostats were placed in thearmature circuit. Relay contacts or switch contacts were connected inseries with the rheostats to select which rheostat would be in the ar-mature circuit. In this type of arrangement, the control system was asimple multispeed DC motor drive system.

There are many disadvantages to a rheostat control system. First,some of the electric energy is dissipated (lost) as heat. Second, therheostat system doesn’t allow the motor speed to be adjusted in incre-ments—it’s either slow or fast. No smooth speed changes are possible.Finally, rheostat systems require too much maintenance to the relayor switch contacts and to the rheostat’s wiper and windings.

Silicon-Controlled Rectifier (SCR) Systems

Later, another type of motor control system was developed calledan infinite or stepless speed control system. The first type of steplesssystem used vacuum tubes to control the armature or field winding


FIGURE 16—One method of

controlling the speed of a DC

motor is to place a rheostat

in series with the armature as

shown here.

currents. These systems have now largely been replaced with mod-ern, solid-state stepless motor drive systems.

Modern DC motor controllers use solid-state switching devicesknown as silicon-controlled rectifiers (SCRs), triacs, or transistors tocontrol either the armature or field current. A block diagram of anSCR speed controller is shown in Figure 17.

In an SCR speed control system, the field windings are powered by aconstant voltage from a DC power supply. The armature voltage,however, is controlled by a triggering device and an SCR that’s con-nected in series with the armature.

If the SCR is triggered early or often in the AC cycle, a large amountof voltage reaches the armature. As a result, the armature turns fast. Ifthe SCR is triggered late in the AC cycle, or at irregular intervals, asmall amount of voltage reaches the armature, and it turns muchslower.

H-Bridge Control Systems

A more elaborate speed controller called an H-bridge motor controlleris used in permanent magnet servo motors. These motors must be veryclosely controlled so that they can offer exact positioning of machinetools, inspection, and process control equipment. An H-bridge systemis shown in Figure 18.


FIGURE 17—A simplified diagram of an SCR speed controller is shown here.

An H-bridge motor controller is a type of switching transistor driver.Figure 18 shows the power driver stage of the motor controller. Thisstage contains four power transistors or high-current switching tran-sistors. The base leads of the power transistors are located at points A,B, C, and D. These inputs are low-voltage inputs that turn the transis-tor on to apply power to the motor. When transistors A and D areturned on, the left side of the armature becomes positive and the rightside becomes negative, causing the armature to turn in one direction.If transistors A and D are turned off and transistors B and C are acti-vated, the left side of the armature becomes negative and the rightside becomes positive, causing the armature to turn in the oppositedirection.

An H-bridge motor driver uses pulse-width modulation (PWM) tocontrol speed. In PWM, the period of time that a group of two transis-tors is activated is varied to change the speed. This allows the arma-ture speed to be closely controlled while maintaining full voltage tothe armature windings. PWM signals for slow and fast speeds areshown in Figure 19.

PWM signals are created at a special circuit within a servo motor con-troller system. If a motor needs to run at high speed, the motor con-troller first issues a PWM signal like the one in Figure 19A, then thecontroller increases the “on” time of the signal until it looks like thesignal in Figure 19B.


FIGURE 18—Here’s a

simplified drawing of an

H-bridge DC motor

controller. Each transistor

acts as a high-speed

switch.

PWM signals are applied to the base terminals of the power transis-tors. Special circuits within the motor driver allow only two transis-tors to be turned on at one time (such as A and D or B and C). Thisprevents a short circuit across the power supply.

Field Weakening

A final type of motor speed control is called field weakening. Thistype of control is generally offered for motors of two horsepower ormore. In a field weakening system, the field winding voltage is variedinstead of armature voltage to provide speed control. A steady DCvoltage is fed to the armature at all times.

This system works opposite from the way you think it should. Whenfull voltage is applied to the armature and the field, the motor turnsslowly. As the field voltage is reduced, the motor gains speed. This“weakening” action continues until the field voltage reaches a cutoffvoltage at which no further speed increase can occur. Field weaken-ing is often used to control spindle systems in machine tools such asboring machines and lathes.

Now, take a few moments to check your learning by completingPower Check 2.


FIGURE 19—When comparing the two PWM signals, notice that the voltage remains constant while the “on”

time of the voltage changes.


Power Check 2

Fill in the blanks in each of the following statements.

1. An H-bridge motor driver uses _______ to control speed.

2. In a _______ speed control system, the field winding current is varied instead of thearmature voltage.

3. A/An _______ motor controller is sometimes used in permanent magnet servo mo-tors.

4. A _______ motor control system doesn’t allow the motor speed to be controlled inincrements.

5. In DC motors that contain field windings, the armature direction can be reversed byreversing the polarity of either the _______ or the _______.

6. A permanent magnet DC motor can only be reversed by reversing the polarity of thecurrent to the _______.


GENERATOR BASICS

Generating a Voltage by Electromagnetism

Generators and motors are very similar machines. The basic parts of amotor and a generator are the same. In fact, if the armature of a DCmotor is turned mechanically, the motor will act as a generator andproduce a voltage. Or, if a voltage is applied to the armature of a gen-erator, it will begin to rotate and the machine will act as a motor.Therefore, the basic difference between a motor and a generator isthat a generator converts mechanical energy into electrical energy,while a motor converts electrical energy into mechanical energy.

In simple terms, electric current is produced inside a generator by therelative movement of a conductor through a magnetic field. When aconductor is moved through a magnetic field, a voltage is induced inthe conductor. By “relative movement” we mean that either the con-ductor moves or the magnetic field moves in the generator. (In a realgenerator, the necessary relative movement of the magnetic field andthe conductor is produced by some outside source of mechanicalpower, such as a gasoline or diesel engine.) The relative movement ofthe conductor and magnetic field induces a voltage on the conductor.This process is called the generator action of electromagnetic induction.

The voltage produced in a conductor by electromagnetic inductioncan be used to perform useful work. If the energized conductor isconnected to a complete electrical circuit, the voltage becomes movingcurrent. The current can then be used to run other machines or equip-ment. The current produced by a generator in this manner is calledthe load current.

Figure 20 shows a simplified diagram of a generator mechanism. Aconductor is being moved upward through the magnetic field pro-duced by the two field magnets. The galvanometer that’s connectedto the conductor indicates that a voltage is being induced on theconductor.

Note that when the conductor is moved upward through the mag-netic field, the current produced moves in a counterclockwise direc-tion. If the conductor is moved downward through the magnetic fieldinstead of upward, the current will flow in the opposite direc-tion—clockwise.

Note that the current produced by the rotating coil-and-magnet de-vice in a generator is always alternating current (AC). If direct current(DC) is needed for a particular application, the current output fromthe generator must be rectified to convert it from AC to DC. Most in-dustrial applications require DC power.


Direction of Current

In a generator, the direction of the motion of a coil, the direction of themagnetic field, and the direction of conventional current flow (posi-tive to negative instead of negative to positive), in the conductor areall related in a fixed way. If any two of these directions are known,the third direction can be determined by applying Fleming’s right-hand rule of electromagnetic induction (the generator rule). The ruleis illustrated in Figure 21 and is stated as follows:


FIGURE 20—The upward

motion of the conductor in

the magnetic field causes a

current to be induced on

the conductor.

Rule: Extend the thumb, the forefinger, and the middle finger of your right

hand so that the middle finger is at a right angle to the thumb and forefin-

ger. Point the thumb in the direction of the coil motion and the forefinger in

the direction of the magnetic field. The middle finger now points in the di-

rection of the induced voltage. The induced current flows in the same direc-

tion as the induced voltage.

Note that according to this rule, if you reverse either the directionof the magnetic field or the direction of the motion of the coil, the di-rection of the induced voltage (and the resultant current) will also bereversed.

You can understand Fleming’s right-hand rule better if you apply it toFigure 20. In the figure, the coil is moving upward, as indicatedby the thumb, and the magnetic field is directed from right to left, asindicated by the forefinger. The current flows toward you, as indi-cated by the middle finger.

Generator Construction

DC generators and DC motors are very similar in construction. Bothtypes of machine contain the same basic parts (housing, armature,commutator, brushes, and so on). A simplified drawing of the parts ofa DC generator is shown in Figure 22. The figure shows the positionof the field poles and field windings, the commutator, brushes, andarmature coil.

Note that the field poles shown in this drawing are electromag-nets—conductive coils wound on iron cores. While permanent mag-nets can be used to generate the magnetic field in small generatorsand motors, they have disadvantages that limit their use in larger ma-chines. For example, the magnetic strength of a permanent magnetdecreases over time. Also, the strength of a permanent magnet can’tbe varied or controlled. For these reasons, electromagnets are used tosupply the magnetic field in most DC generators.

The coil of wire around the north field pole (N) and the south fieldpole (S) forms a field winding. When this winding is connected to aDC power source, current will flow into the winding and produce amagnetic field between the field poles. (Note that while this figurecontains only two field poles, a real generator contains many fieldpoles and windings equally spaced around the frame that holds thearmature.)


FIGURE 21—Fleming’s

right-hand rule shows the

relationship between the

direction of coil motion,

the direction of the

magnetic field, and the

direction of conventional

current flow in a generator.

The magnetic field that the electromagnets produce is called the mainfield. The current required to energize the electromagnets is called thefield excitation. This current is provided by a device called an exciter.In most machines, the armature of the generator itself is the exciter.However, a generator’s exciter may also be another generator, a bat-tery, or a supply of rectified AC voltage. In Figure 22, the exciter con-nected to the field winding is a battery.

The strength of the magnetic field produced between the field polesdepends on two factors: the number of turns in the field windingsand the amount of current applied to the windings. Also, a field polewith an iron core produces more magnetic flux than a field pole with-out an iron core.


FIGURE 22—The field winding

in this generator produces the

magnetic field.

FIGURE 23—The armature

core of a DC generator is

made of laminated steel

punchings that are 164 in.

(0.4 mm) thick. (Courtesy of Allis-

Chambers Corporation)

The rotating part of the generator, the armature, is shown in Figure 23.The armature core is made of laminations or punchings of specialsteel or iron. The core contains slots in which the armature conductors(the windings) are placed. The armature windings are spaced aroundthe armature core in fixed positions. The ends of the windings areconnected to the commutator.

The armature is mounted on a shaft and positioned inside the mag-netic field generated by the field poles. The armature shaft is held ateach end by bearings in the end bells of the generator housing. Thesebearings must be strong enough to support the armature and alsowithstand the force exerted by the gear or belt used to drive the arma-ture. The bearings may be ball or roller bearings (Figure 24) or sleevebearings. Roller bearings are used for heavy-duty applications wherethe stress is too great for a ball bearing. A sleeve bearing is a cast-ironor steel shell with a Babbitt lining. Babbitt is a soft metal often usedfor bearing linings because it prevents cutting of the shaft. Bearingcare is a major part of any generator maintenance program.

The armature is turned by an outside mechanical force. In a verysmall generator, the turning force could be provided by a hand crank.However, the armatures of larger industrial generators are turned bysteam or water turbines, gasoline or diesel engines, or even electricmotors.

A generator’s commutator is a band divided into segments. The endsof the armature windings are connected to the commutator segments.The segments are insulated from each other and from the shaft by aninsulating material, such as mica. Brushes are positioned around the


FIGURE 24—Although ball

and roller bearings are

similar in construction, the

roller bearing is used for

heavier duty applications.(Courtesy of SKF Industries)

commutator to make a sliding electrical contact with the commutatorsurface. Adjustable springs are used to maintain proper brushpressure against the commutator. The brushes provide a conductivepath for the current generated between the rotating and stationaryparts of the machine. The brushes connect the commutator electricallyto the external circuit.

Low-voltage, high-current generators use brushes made of pulverizedcopper mixed with carbon. The brushes must be uniform in qualityand must not groove the commutator segments. The brushes must bestrong enough to prevent them from chipping or breaking due to ma-chine vibrations. As the armature turns, the brushes should polish thecommutator to a chocolate brown color, indicating that good contactis being maintained between the brushes and the commutator.

Brush holders support and hold the brushes alongside the commuta-tor. One brush holder may contain a set of several brushes if the cur-rent generated is too high for just one brush to handle. The entireassembly of brushes and brush holders is often called the brush rig-ging. Since the proper operation of DC machines depends greatlyupon the brush rigging, it’s vital that the person responsible for gen-erator maintenance have a good understanding of brushes and brushholders.


FIGURE 25—Two types of brush holders are shown here. Figure 25A is the box type and Figure 25B is the

reaction type. The proper operation of DC generators depends greatly on the brushes and brush holders.(Courtesy of D. B. Flower Manufacturing Company)

There are two basic types of brush holders: the box type and the reac-tion type. The box-type brush holder (Figure 25A) consists of a box,open at both ends, in which the brush is free to slide. If the box is toobig, the brush will be loose in the box and vibrate; if the box is toosmall, the brush may stick. A reaction-type brush holder is shown inFigure 25B. The brush used in this type of holder must be set at anangle against the commutator; it’s held against the commutator in ei-ther direction of rotation. The top of the brush (not shown) isbeveled at a sharp angle, so that the spring which presses it againstthe commutator also holds it against one face of the holder. Brushpressure can be changed by adjusting the spring. Because the brushisn’t as free to move up and down, reaction-type holders are seldomfound on machines that vibrate considerably.

Generator Operation

The basic generator mechanism shown in Figure 26 contains one con-ductor loop called a single-turn coil. The single-turn coil is a basicarmature that rotates within the magnetic field. The curved arrowat the top of the illustration indicates the clockwise rotation of the ar-mature. The magnetic lines of force (imaginary lines that make up a


FIGURE 26—As this

single-turn armature is

rotated in the magnetic

field, an AC current is

generated and measured

by the galvanometer.

magnetic field) are indicated by the horizontal arrows pointing fromthe north field pole to the south field pole.

The long straight sections of the coil are called coil sides. As the looprotates, the coil sides cut across the magnetic lines of force, causing acurrent to flow in the coil. Essentially, each coil side operates as aseparate straight conductor moving through a magnetic field. Whenthe coil rotates in a magnetic field, a voltage is generated on each sideof the coil.

The ends of the coil are connected to slip rings that rotate along withthe coil. A slip ring is different from a commutator in that the slip ringis one continuous band of conductive metal (such as copper) that’s in-sulated from the armature shaft. The stationary brushes slide over theslip rings and conduct the generated current to an external circuit.The voltage that appears at the slip rings of this generator is an alter-nating (AC) voltage.

When the coil in Figure 26 is rotated, each of the two coil sides cutsacross the magnetic lines of force at the same speed. Thus, thestrength of the voltage induced in one side of the coil is always thesame as the strength of the voltage induced in the other side of thecoil.

If you think carefully about this action, however, you can see thateach of the coil sides cuts the lines of force in a different direction. Forexample, as the loop rotates in a clockwise direction, the upper coilside cuts down through the magnetic lines of force. The lower coilside cuts up through the magnetic lines of force. The voltage inducedin one coil side, therefore, is opposite to the voltage induced in theother coil side. However, since the two coil sides are connected in aclosed loop, the voltages add together. The result is that the total volt-age generated by one full turn of the coil is equal to two times thevoltage generated in each coil side. The total voltage is collected bythe brushes and is applied to an external circuit.

Now, suppose that one end of the coil in Figure 26 is bent around tomake another full turn before connecting to the slip ring. This newcoil contains two conductor loops and is called a double-turn coil. Thedouble-turn coil has four coil sides. The total voltage generated by thedouble-turn coil is therefore equal to two times the total voltage gen-erated by a single-turn coil.

Similarly, if the number of conductor loops in a coil is increased tothree, the coil would have six coil sides and would generate threetimes as much voltage as a single-turn coil. Thus, to generate the highvoltages needed to run machinery, the armatures in real generatorscontain many coil turns.

The strength of the voltage induced in a generator coil depends onthree factors: the strength of the magnetic field, the length of the coil,


and the speed of rotation. First, the greater the number of magneticlines of force in the field through which the coil moves, the more linesthe coil will cut in a given time period. Second, the longer the coilsides, the greater the number of magnetic lines of force cut. Third,the faster the conductor moves, the more magnetic lines it will cut ina given period of time. So, by increasing any of these factors—thestrength of the magnetic field, the length of the coil sides, or the speedof armature rotation—you can increase the amount of voltage inducedin the generator.




Power Check 3


1. In an electric generator, a relative movement between magnetic lines of force and a_______ cutting across the lines generates a voltage.

2. In an armature having a single-turn coil, the voltages produced in each coil side_______ because the coil sides are connected in a closed loop.

3. The process of producing a voltage in a generator is called the _______.

4. The _______supply the magnetic field or flux in DC generators.

5. The _______ connect the commutator of a DC generator to the external circuit.


Variations in Induced Voltage

When a coil rotates in a magnetic field, the voltage generated doesn’thave a steady fixed intensity or value. Nor does the induced voltagealways flow through the coil in the same direction. The value of in-duced voltage changes as the coil rotates. For some coil side positions,a great many magnetic lines are cut; for other positions, few or nomagnetic lines are cut. The direction of voltage changes because atone instant a coil side travels downward through lines of force, whilea half-turn later, it travels upward through the same lines.

Figure 27 shows an end view of the sides of an armature coil. One ofthe coil sides has been highlighted so we can track its movementthrough the magnetic field. In Figure 27A, the armature coil is in avertical (up-and-down) position between the field poles. Thehighlighted coil side is positioned parallel to the magnetic lines of


FIGURE 27—The maximum number of magnetic lines of force are cut when the conductor moves at right

angles to the magnetic field. Since the direction of the conductor motion reverses with respect to the

magnetic field at views (A) and (C), the direction of the induced voltage reverses.

force. Therefore, the coil side isn’t cutting through any magnetic lines,and no voltage is induced in the coil side at this point. In Figure 27B,the armature coil is in a horizontal (side-to-side) position between thefield poles. The highlighted coil side now cuts the magnetic lines atright angles. At this point, the maximum number of magnetic lines isbeing cut, so the maximum possible voltage is being induced in thecoil side.

In Figure 27C, the armature coil continues to rotate toward the verti-cal position, and the coil side cuts fewer lines of force. Therefore, theamount of voltage generated decreases toward zero. In Figure 27D,the armature coil rotates again toward the horizontal position and thegenerated voltage increases back to maximum. However, note thatthe coil side now cuts the lines in the opposite direction from that inwhich it cut them during the first half of the rotation. Finally, the ar-mature coil returns to the original vertical position shown in Figure 27A,and the generated voltage decreases again to zero.

The voltage generated by one rotation of the armature coil is shownin the voltage waveform graph in Figure 28. The voltage waveform orcurve, also called a cycle, represents the voltage wave during onerevolution of the coil. The curve moves up from the zero voltagepoint to indicate the increase in voltage in one direction, falls back tothe zero point, then slopes down below the zero point to indicate theincrease in voltage in the opposite direction.

The positions of the armature coil shown in Figure 27 can be relatedto this graph. The position shown in Figure 27A (zero voltage) is thestarting point (0) on the graph. As the armature coil begins to turn,the voltage waveform begins to rise upward from the zero point.When the armature coil is horizontally aligned with the field poles, asin Figure 27B, the maximum voltage is induced in it. This is the highpoint of the curve in the graph in Figure 28.

The armature coil continues to turn until it reaches the positionshown in Figure 27C, and the voltage waveform slopes downwarduntil it again reaches the zero position. As the coil continues to turn,


FIGURE 28—The current or

voltage produced in a

generator coil during a

complete revolution is

actually an alternating

current or voltage.

the waveform moves below the zero point. When the coil reaches theposition shown in Figure 27D, the waveform reaches its lowest point.Finally, as the coil rotates back to its original position, the waveformslopes back upward to the zero point on the graph.

You can also relate the waveform to the generator shown in Figure 26.Since the generated voltage changes its direction every half cycle, thecurrent also changes direction every half cycle. The current flowsthrough the external circuit, from one slip ring to the other, then re-verses and flows in the opposite direction.

Commutator and Brush Action

Remember that the voltage that’s generated at the slip rings of thegenerator in Figure 26 is an AC voltage; that is, the voltage is movingin two directions. In order to be used in DC applications, this AC volt-age must be converted to DC voltage (a voltage moving in only onedirection). The commutator performs this conversion in a DC genera-tor. Without a commutator, all generators would be AC generators(that is, alternators).

Figure 29 shows a basic DC generator with commutator and brushes.The ends of the armature coil are connected to the commutator


FIGURE 29—In the basic DC

generator shown, the

commutator consists of two

segments of a slip ring. Each

end of the single-turn coil

connects to a commutator

segment. The segments are

insulated from each other

and from the shaft by an

insulating material, such as

mica.

segments. The brushes are stationary and are positioned at either sideof the commutator. The brushes collect the generated current from thecommutator segments and provide a connection to the external circuitand the load.

Just as the direction of the voltage generated in the armature coil re-verses direction, each commutator segment moves out from underone brush and moves in under the other brush. It can be seen fromFigure 29 that the brush marked with a plus sign (+) only collects cur-rent from the armature coil side that moves upward along the southpole of the permanent magnet. Similarly, the brush marked with aminus sign (–) only collects current from the armature coil side thatmoves downward along the north pole of the permanent magnet.Therefore, the polarity of each brush stays the same, and the currentsupplied to the external circuit and load is in one direction. Note thatinside the armature coil, the direction of current flow is from minus (–) toplus (+).

In summary, the commutator and brush assembly interchanges theconnections to the ends of the coil at the instant the polarity (the volt-age direction) reverses. The action of reversing the connections to thecoil to obtain a DC rather than AC voltage is called commutation. Thecurrent and voltage at the brushes are actually voltage pulses (pulsesof voltage), as shown in Figure 30. The resulting output voltage is apulsating DC voltage.

Field Connections

Generators, like motors, may have their components connected in se-ries, in parallel (shunt), or in a combination (compound). Figure 31Ashows the connection diagram of a shunt-wound generator. The fieldwinding, called the shunt field, is connected directly in parallel to thearmature. The line voltage across the line terminals is equal to thevoltage across the armature and the voltage across the shunt field, asin any parallel-connected circuit.

The connections of a series-wound generator are shown in Figure 31B.The field winding is connected in series with the armature. Since allthe elements in a series circuit receive the same amount of current, the


FIGURE 30—The action of

reversing the

connections to the coil to

obtain a direct rather

than an alternating

voltage is called

commutation. The

resulting output voltage

of a DC generator is a

pulsating DC voltage.

current through the armature is the same as the current through theseries field, and is also equal to the line current.

Generators containing a combination of series and parallel connec-tions are called compound-wound generators. The compound-woundgenerator in Figure 31C contains a shunt field winding connected inparallel with the armature and in series with a series field winding.The resulting connection is called a short-shunt compound-woundconnection. The generator in Figure 31D contains a shunt field wind-ing connected in parallel with the armature and a series field wind-ing. This connection is called a long-shunt compound-woundconnection.

Field Excitation

DC generators are often classified according to the way in which theirfield windings are excited. If the current used to excite the field wind-ings is drawn from the armature of the machine, the field is termedself-excited. The schematic drawings in Figure 31 show various typesof self-excited field connections.

When the field windings are excited by current produced by an out-side source (an exciter), the field is said to be separately excited. Theschematic drawings in Figure 32 show two types of separatelyexcited field connections. In both connections, the shunt field is ener-gized by an exciter. However, in Figure 32B, the series field is connectedin series with the armature and receives the same line current as thearmature.


FIGURE 31—The field windings in generators may be connected in parallel or in series with the armature. In

compound-wound machines, both series and parallel connections are used.

Armature Reaction

Whenever current flows through a conductor, a magnetic field developsaround that conductor. In a DC machine, the magnetic field neededto operate the machine is produced by a direct current in the fieldwindings.

A simplified drawing of a generator with two field poles is shown inFigure 33. The field windings and armature coils are shown as smallcircles in the figure. The circles with dots are carrying direct currenttoward you. The circles with X’s are carrying direct current awayfrom you.

Figure 33A shows the magnetic flux produced by the field poleswhen the field windings are energized. The energized field poles actas north and south poles (N and S). The direction of the magneticlines of force between the poles is indicated by the arrows. Note thatthe armature coils aren’t energized in Figure 33A. The brushes arelocated exactly midway between the poles, or in the neutral position(indicated by the vertical broken line).

In Figure 33B, the field poles aren’t energized. However, the armaturewindings are supplied with direct current, and a magnetic fielddevelops around the armature windings. The direction of thismagnetic field is indicated by the circular broken lines and arrows,from the bottom to the top. The armature develops a magnetic fieldwith the north pole at the top (N) and the south pole at the bottom (S).

Figure 33C shows the magnetic flux that results when current is ap-plied to both the field poles and the armature windings. This is themagnetic field produced during normal operation of a DC generator.


FIGURE 32—Separately

excited machines are either

shunt-wound or

compound-wound, but they

can’t be simply

series-wound.

The magnetic fields of the armature windings and the field combineand interact, and the resulting magnetic lines of force flow in the di-rection indicated in the figure. The effect that the armature’s magneticfield produces on the main field is called the armature reaction.

This armature reaction opposes the main magnetic field and reducesits strength. In addition, the armature reaction causes a shift of theneutral position of the main field. Since the neutral position must belocated at right angles to the magnetic lines of force, the armaturereaction causes the original neutral position (the dashed line inFigure 33A) to be shifted in the direction of armature rotation. Thenew neutral position is shown in Figure 33C.

When the neutral position of the main field shifts, the position of thebrushes must shift also. If the brushes remain in the original position,dangerous sparking will occur at the commutator.


FIGURE 33—The uniform magnetic field produced by the field windings is affected by the varying magnetic

field of the armature. The armature field causes a reaction which shifts the neutral position.

How does sparking occur? Well, ideally, the coil that’s short-circuitedby the brush (that is, the coil being commutated) shouldn’t cut acrossany magnetic lines at the moment that the segment to which it’s con-nected is under the brush. However, if the neutral position is shifteddue to armature reaction, the coils will cut across some magnetic linesduring commutation. As a result, the cutting of the lines induces avoltage in the coil being commutated and causes heavy currents toflow through the coil and into the brush. These currents often causesparking at the commutator. Sparking can damage the surface of thecommutator and the brushes.

Counteracting Armature Reaction

There are several methods used to counteract armature reaction andprevent sparking at the commutator. One method is to shift thebrushes to a new neutral position, as shown in Figure 33C. In thisnew brush position, no voltage is induced in the coils being commu-tated, and consequently no sparking. On a generator, the brushes areshifted in the direction of armature rotation, which is shown to beclockwise in Figure 33C. Depending upon the application, the shiftmay be up to 20 degrees.

Brush shifting has several disadvantages, however. The magneticfield strength needed varies with the armature current; therefore, forbest results, the brushes should be shifted for each new armature cur-rent value. This shifting is seldom practical. Moreover, the direction ofshift changes with the direction of rotation. Thus, the position of thebrushes should be changed with every change in rotation direction.

Another method used to overcome the effects of armature reaction isusing commutating poles or interpoles. The interpoles are additionalfield windings placed on poles located midway between the fieldpoles, and directly over the armature coils being commutated.

The interpole coils are connected in series with the armature; there-fore, they carry current that’s proportional to the armature current.The interpole coils are wound so as to produce a magnetic field with apolarity opposite to the polarity of the magnetic field created by thearmature current. For example, Figure 33B shows the location of thetwo brushes, one over the north pole of the armature and the otherover the south pole. When interpole coils are used, one is placed sothat it forms a north pole at the top of the armature; the other forms asouth pole at the bottom. Therefore, each interpole is midway be-tween the field poles. The interpoles prevent armature reaction be-cause there’s no magnetic field perpendicular to the magnetic fieldcreated by the field windings and the brushes.

A third method of counteracting armature reaction is to use compen-sating windings, which are coils embedded in the surface of the pole


next to the armature. These windings are connected in series with thearmature so that their strength is proportional to armature current.

The effects of compensating windings are shown in Figure 34. Themagnetic field in the armature conductor opposes the magnetic fieldin the compensating winding; therefore, the effect of armature reac-tion is eliminated.


FIGURE 34—The result of the two opposing magnetic fields is such that, at every point under the poles, the

two fields balance, and the effects of armature reaction are eliminated.

Eddy Currents

The steel of the armature surface that’s near current-carrying conduc-tors and (to a lesser extent) the steel of the face of the field pole aresubject to magnetic field variations. Remember that a voltage is in-duced in a coil when the coil cuts through magnetic lines of force or issubject to a magnetic field change. Therefore, when a voltage is in-duced in an armature winding, a voltage is also induced in the steel ofthe armature slot that holds the winding. These voltages cause cur-rents called eddy currents to flow in the steel, resulting in an energyloss.

To reduce these eddy currents, the steel is alloyed with silicon to in-crease its resistance. It’s also laminated (composed of thin insulatedsheets) to decrease the total length of the paths that the currents maytravel (Figure 35).

Hysteresis Loss

As the armature rotates, all points on it are exposed to alternatingmagnetic field directions. The power used or lost in reversing themagnetic field direction in the armature core is called hysteresis loss.This loss varies with the number of reversals, the density of themagnetic field, and also the material of the armature (the loss ishigher for some steels than for others). Therefore, a special steel witha low hysteresis loss is used for armature cores.

Take a few moments now to check your learning by completingPower Check 4.


FIGURE 35—To reduce the

flow of eddy current, an

armature core is made

up of steel laminations

about 164 in. (0.4 mm) thick.

The armature shown has

four laminations, and the

power is one-fourth that

of a solid steel core.


Power Check 4


1. In a generator, the _______ converts a generated AC voltage to a DC voltage.

2. The current through the armature coil and the current through the field coils com-bine to produce a distorted field. This distorted field effect is called _______.

3. Your foreman asks you to see that the compensating winding in a DC generator isconnected properly. You do this by making sure that it’s connected in _______with the armature.

4. _______ are embedded in the pole face of the field coil.

5. _______ reduce the flow of eddy current in an armature.

6. _______ are placed midway between the main field coils to reduce commutatorarcing.

Use the illustrations provided to answer the following questions.

7. The connection diagram shown here for a self-excited generator represents whichtype of connection?

_________________________________________________________________________

(Continued)


Power Check 4

8. In which of the following figures is the field winding connected directly in parallelwith the armature?

A C

B. D.

9. Which of the diagrams shown in Question 8 represents a generator with a separatelyexcited field?

___________________________________________________________________________

10. Look at the position of the brushes in the drawing shown below. In what direction isthe generator’s armature rotating?

___________________________________________________________________________


DC GENERATORS IN INDUSTRY

Types of Machine Losses

The cost of operating machinery and equipment is a major concern inall types of industry. The energy lost during the production of currentis one major source of increased operation costs. This is true whetherthe voltage is produced in a plant or by a utility company.

The energy used by a machine is always greater than the energy itproduces. For instance, the mechanical energy required to turn a gen-erator is always greater than the electrical energy produced. The dif-ference between the energy used by a machine (input power) and theenergy produced (output power) is called the total machine losses.

There are two types of total machine losses. One type is mechanicalloss, a loss that occurs whether or not the machine is connected to aload. (For instance, a generator could be driven while not electricallyconnected to an external circuit.) Mechanical losses include bearingfriction, windage, and brush friction losses.

The other type of loss is an electrical loss. An electrical loss is depend-ent on (or proportional to) the load applied to a machine. Electricallosses include brush contact losses, copper losses or I2R losses (thesquare of the current times the resistance), field and armature wind-ing losses, iron losses, and stray load losses.

Let’s take a closer look at some of these machine losses to see howthey affect overall machine efficiency.

Bearing Friction and Windage Losses

Bearing friction losses and windage losses (losses due to wind or airturbulence caused by the rotating armature) are mechanical lossesthat consume the power required to rotate the armature at normalspeed, with no armature current flowing and no field flux. Factorsthat affect bearing friction include the type of lubrication used and thearea of the internal surfaces contacting the bearings. Windage lossesare affected by the shape of the moving surface. The more “wind”created by the moving surfaces, the greater the windage losses.

Brush Friction Losses

The brushes sliding on a commutator surface act like brakes on the ar-mature, due to the friction between the commutator surface and thebrushes. The magnitude of these brush friction losses depends on thespeed with which the commutator is moving, the total brush area,and the material the brushes are made of.


Brush Contact Loss

Brush contact losses occur as a result of electrical resistance. This lossis equal to the voltage drop that occurs between the brush and thecommutator surface multiplied by the current. (With an open circuit,therefore, brush contact losses are zero.) The loss increases with theload, but it remains constant once the load is applied. Thus, the loss issaid to be a load dependent loss. In practice, the voltage value de-pends upon the type of brush material, and whether shunts (alsocalled pigtails) are used with the brushes.

Copper Losses

Copper losses or I2R losses in the armature and field windings de-pend on the amount of current flow and the resistance of the wind-ings. In practice, the resistances used are corrected to 75° C (167° F)since this is the average expected temperature of such machine wind-ings. Although some machines have different winding temperatures(depending on the load and on whether the armature is speciallycooled), the important point to remember is that I2R losses vary withthe square of the current. For example, if the current triples, the I2Rlosses increase by 32, or nine times.

Iron Losses

Iron losses in an armature core are also called core losses, since theyrepresent power used up in the core. These losses consist of eddy cur-rent losses and hysteresis losses. Eddy current losses occur as a resultof the armature material acting as a conductor. For this reason, the ar-mature is laminated to reduce the effective conductor length. Hystere-sis loss is the power required to reverse the direction of the magneticflux in iron or steel.

Stray Load Losses

When generators are tested, machine losses can be determined bymeasuring the difference between the power input and the poweroutput. The difference can also be calculated without actually meas-uring these quantities. However, the actual measured loss will alwaysbe greater than the calculated loss. This “extra” loss is called a strayload loss. A stray load loss can result from eddy current losses in thearmature conductors, or losses caused by magnetic field distortion re-sulting from armature reaction.

Machine Efficiency

In all types of equipment, efficiency is very important. The efficiencyof a generator can be calculated easily. First, you must determine the


total power loss of the machine by adding together all the variouslosses. The total power input is equal to the power output plus thetotal loss, as illustrated in the equation below:

power input � power output � power loss

The calculated efficiency of a machine is equal to the ratio betweenthe power output and the power input, and is usually expressed as apercent. You can use the following formula to calculate the efficiencyof a machine as a percent:

efficiencypower output

power input� � 100

To use this formula, the power output and the power input must beexpressed in the same units; that is, both must be expressed in eitherwatts, kilowatts, or horsepower.

So, let’s calculate the efficiency of a generator with a power output of90 kW and power losses of 10 kW.

power input � power output � power losspower input � 90 kW � 10 kW

power input � 100 kW

First, determine the total power input. Addthe power output and the power loss. Thepower input is 100 kW.

efficiency �

power output

power input

� 100 Write the efficiency formula.

efficiency �

90 100

100

kW

kW

� Substitute 90 kW for power output and 100kW for power input.

efficiency =90 100

100

� Multiply 90 � 100.

90 100

100

9 000

10090

�

� �,

% Divide. Remember that the answer is a per-cent. Answer: The efficiency of this machineis 90%.

Voltage Regulation

The amount of voltage produced by a generator depends on the ar-mature speed and on the strength of the generator’s magnetic field.Since the magnetic field is produced by the current that flows throughthe field winding, we can also say that the amount of voltage pro-duced depends on the armature speed and on the value of the fieldcurrent. If the armature speed or field current or both increase, thevoltage increases. Likewise, if one (or both) of these factors decrease,the voltage decreases.


As a rule, the voltage produced by a generator isn’t controlled by in-creasing or decreasing the armature speed: it’s best to run the genera-tor at the manufacturer’s rated speed. Therefore, in practice, thevoltage is controlled by increasing or decreasing the field current (theexcitation) as needed. Various types of voltage controllers are used.Many are completely automatic, and use electrical and electronic con-trol circuits to adjust or regulate the voltage to the desired values.

Operating Generators in Parallel

In order to supply greater current, DC generators in power stationsare connected in parallel through common bus lines. For shunt-wound generators, the load is kept in balance by rheostats or by elec-tronic controllers that are connected in series with each shunt field.The voltage output is increased (while current is kept equal) by in-creasing the field excitation of both generators. For generators of une-qual capacities, the loads are generally divided in proportion to theirratings.




Power Check 5


1. If you increase the load on a certain DC generator in your plant, the brush-contactloss will _______.

2. If the current in the armature increases from 10 A (amperes) to 20 A, the copperlossesin the armature would be _______ times as much as they were before the increase.

3. Your foreman asks you to calculate the output power of a DC generator. He thenasks you to divide that value by the input power and multiply the result by 100.You’ve just calculated the _______ of the generator.

4. The output voltage of a DC generator is usually controlled by adjusting the_______.

5. _______ losses include bearing-friction losses, brush-friction losses, and windagelosses.

6. _______ losses are equal to the difference between measured input power andmeasured output power.

7. _______ losses are made up of brush-contact losses, copper losses, iron losses, andstray-load losses.


NOTES


1

1. An interpole

2. Mica

3. The rotor

4. Because a large current will flowthrough these windings

5. 0.9 degrees

2

1. pulse-width modulation (PWM)

2. field weakening

3. H-bridge

4. rheostat

5. field windings, armature windings

6. armature

3

1. conductor

2. add together

3. generator action of magnetic induction

4. field poles and windings

5. brushes

4

1. commutator

2. armature reaction

3. series

4. Compensating windings

5. Laminated cores

6. Interpoles

7. A series connection

8. A

9. D

10. The armature rotation is clockwise.

5

1. increase

2. four

3. efficiency

4. field excitation

5. Mechanical

6. Total machine

7. Electrical

Power Check Answers

49

NOTES

50 Power Check Answers

DC Motor and Generator Theory

When you feel confident that you have mastered the material in this study unit, complete thefollowing examination. Then submit only your answers to the school for grading, using one ofthe examination answer options described in your “Test Materials” envelope. Send your answersfor this examination as soon as you complete it. Do not wait until another examination is ready.

Questions 1–25: Select the one best answer to each question.

1. Which of the following DC motors provides the highest starting torque?

A. Series-wound C. Compound-wound

B. Shunt-wound D. Generator-wound

2. What is the efficiency of a DC generator that requires 100 watts of energy to supply 80 watts of outputpower?

A. 8% C. 80%

B. 12.5% D. 125%

3. Which of the following DC motors acts as a form of electrical ratchet?

A. Servo C. Stepper

B. Permanent-magnet D. Syncro

Examination 51

EXAMINATION NUMBER:

08600602Whichever method you use in submitting your exam

answers to the school, you must use the number above.

For the quickest test results, go tohttp://www.takeexamsonline.com

4. Why is steel alloyed with silicon used as the core material for the armature of a DC generator?

A. To reduce eddy-current flow in the armature coreB. To increase the hysteresis loss in the coreC. To decrease the resistance of the armature conductorsD. To reduce the resistance to current flow in the core

5. Which of the following losses can happen even when a DC generator isn’t connected to a load?

A. I2R loss C. Copper loss

B. Brush friction loss D. Iron loss

6. If a DC generator with an efficiency of 94% has an output of 240 volts at 100 amperes, the machinepower losses for this generator would be

A. 1175 watts. C. 1532 watts.

B. 1487 watts. D. 1655 watts.

7. What happens when the speed of a generator’s armature increases?

A. The load decreases C. The windage loss decreases

B. The field current reverses D. The induced voltage increases

8. To get better commutation in a DC generator, you should move the brushes

A. in the direction of rotation.B. in the direction of field polarity.C. against the direction of rotation.D. against the direction of field strength.

9. Which of the following DC motor drive systems controls motor speed by controlling the period of thewaveform to the drive’s output transistor?

A. Frequency-effect transition (FET)B. Frequency-drift commutation (FDC)C. Pulse-controlled modulation (PCM)D. Pulse-width modulation (PWM)

10. In a generator, the voltage induced in a rotating armature coil is at maximum when the coil

A. moves parallel with the magnetic lines of force.B. cuts the fewest magnetic lines of force.C. is at right angles to the magnetic lines of force.D. is in a vertical position between the field poles.

11. Which of the following methods can be used to cut down on brush sparking in a DC generator?

A. Install interpoles in the armature core.B. Install compensating coils in the field pole faces.C. Move the brushes against the armature rotation.D. Connect commutating coils in parallel with the armature coils.

52 Examination

12. Technician A is told to inspect and clean the brush holders and replace the brushes in a DC generator.Which of the following areas would the technician be working on in this case?

A. The base C. The rigging

B. The commutator D. The shaft

13. Which of the following devices did older DC motor speed controllers often use to control armaturevoltage?

A. A battery C. A potentiometer

B. A rheostat D. A diode

14. What is the efficiency of a DC generator that requires 200 watts of energy to supply 160 watts ofoutput power?

A. 40% C. 90%

B. 80 % D. 125%

15. In a field weakening DC motor speed controller system, what happens when the voltage to the field isdecreased?

A. The motor speed will increase.B. The motor will slow down.C. Sparking will occur at the commutator.D. Excess heat will be generated.

16. What type of generator connection is shown in the diagram below?

A. Separately-excited series-wound C. Separately-excited compound-wound

B. Separately-excited shunt-wound D. Self-excited series-wound

17. What part of a motor’s armature is in direct contact with the brushes?

A. The interpole C. The commutator

B. The end bell D. The case

18. The turning force produced by a motor is called

A. excitation. C. SCR.

B. horsepower. D. torque.

Examination 53

19. A DC generator’s load loss from hysteresis is one type of

A. bearing friction loss. C. copper loss.

B. brush-contact loss. D. iron loss.

20. When using an SCR motor speed controller, the speed of the motor depends upon the

A. voltage on the field windings.B. resistance in the armature circuit.C. triggering timing of the SCR.D. AC input voltage to the rectifier.

21. What type of generator connection is shown in the diagram below?

A. Self-excited compound-wound C. Separately-excited series-wound

B. Self-excited series-wound D. Separately-excited compound-wound

22. In which of the following figures is the maximum voltage being induced in the coil?

A. C.

B. D.

54 Examination

23. Which of the following types of motors is usually used in machine tools and precision positioningequipment?

A. A series-wound DC motorB. A DC permanent magnet servo motorC. A compound-wound DC motorD. A shunt-wound DC motor

24. If the current in a generator’s armature increases from two amps to eight amps, what is the increasein copper losses?

A. Four times C. Six times

B. Sixteen times D. Thirty-six times

25. What is the efficiency of a generator that produces 80 kilowatts of electric energy and has electric andmechanical losses of 20 kilowatts?

A. 58.3% C. 60%

B. 80% D. 100%

Examination 55

Study Unit DC Motor and Generator TheoryDC motors and generators are widely used in industrial...

Documents

Transcript of Study Unit DC Motor and Generator TheoryDC motors and generators are widely used in industrial...