Shunt-Wound Motor Final Report

8/3/2019 Shunt-Wound Motor Final Report

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Shunt-Wound Motor

I. Introduction to DC Motor

What is a DC Motor?

Direct Current (DC) motor is a fairly simple electric motor that uses

electricity and a magnetic field to produce torque, which turns the motor.

Electric motors operate through the interaction of magnetic fields and

current-carrying conductors to generate force. Electric motors are found in

applications as diverse as industrial fans, blowers and pumps, machine

tools, household appliances, power tools, and disk drives. They may be

powered by direct current, e.g., a battery powered portable device or

motor vehicle.

II. Shunt-Wound Motor

Shunt motor the field winding is connected in parallel or in shunt withthe armature winding. Shunt-motor speed varies only slightly with changes inload, and the starting torque is less than that of other types of dc motors.

III. Important Terms to Know

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A. Motor Principle of Operation

The speed-torque relationship for atypical shunt-wound motor is shown.A shunt-wound DC motor has adecreasing torque when speed

increases. The decreasing torque-vs-speed is caused by the armatureresistance voltage drop andarmature reaction. At a value ofspeed near 2.5 times the ratedspeed, armature reaction becomesexcessive, causing a rapid decrease

in field flux and a rapid decline in torque until a stall condition is reached.

The characteristics of a shunt-wound motor give it very good speedregulation, and it is classified as a constant speed motor, even thoughthe speed does slightly decrease as load is increased. Shunt-woundmotors are used in industrial and automotive applications where precise

control of speed and torque are required.

The resistance in the field winding is high. Since the field winding isconnected directly across the power supply, the current through the field isconstant. The field current does not vary with motor speed, therefore, thetorque of the shunt motor will vary only with the current through thearmature. Once you adjust the speed of a dc shunt motor, the speed remainsrelatively constant even under changing load conditions. One reason for thisis that the field flux remains constant. A constant voltage across the fieldmakes the field independent of variations in the armature circuit. If the loadon the motor is increased, the motor tends to slow down. When this happens,the counter emf (electromotive force) generated in the armature decreases.

This causes a corresponding decrease in the opposition to battery currentflow through the armature. Armature current increases, causing the motor tospeed up. The conditions that established the original speed arereestablished, and the original speed is maintained.

Conversely, if the motor load is decreased, the motor tends to increasespeed; counter emf increases, armature current decreases, and the speeddecreases. In each case, all of this happens so rapidly that any actual changein speed is slight. There is instantaneous tendency to change rather than alarge fluctuation in speed.

It is the type generally used in commercial practice and is usuallyrecommended where starting conditions are not usually severs.

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B. Winding Diagram

A wave winding on adrum-type armature. Noticethat the two ends of eachcoil are connected tocommutator segmentsseparated by the distancebetween poles. Thisconfiguration allowstheseries addition of thevoltages in all the windingsbetween brushes. This typeof winding only requiresone

pair of brushes.

The field winding is connected in parallel with the armature winding so thatterminal voltage of the generator is applied across it

The shunt field winding has many turns of fine wire having high resistance

Therefore, only a small part of armaturecurrent flows through shunt field windingand the rest flows through the load.

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A. Circuit Diagram

A shunt motoris a DC motor that hasthe field wiring connected in parallel with the

armature. A parallel circuit is often called ashunt. In a shunt motor, the field wiring is ashunt. DC shunt motors are used whereconstant or adjustable speed is required andstarting conditions are moderate. The fieldterminal wires extending from the shunt fieldof a DC shunt motor are marked F1 and F2.

The armature windings are marked A1 andA2.

B. Reversing the Rotation

The direction of rotation of a DCshunt motor can be reversed bychanging the polarity of either thearmature coil or the field coil. Theillustration shows the electricaldiagram of a DC shunt motorconnected to a forward and reversingmotor starter. You should notice thatthe F1 and F2 terminals of the shuntfield are connected directly to the

power supply, and the A1 and A2 terminals of the armature winding areconnected to the reversing starter.

When the FMS (forward motor starter) is energized, its contacts connect the A1

lead to the positive power supply terminal and the A2 lead to the negative powersupply terminal. The F1 motor lead is connected directly to the positive terminalof the power supply and the F2 lead is connected to the negative terminal. Whenthe motor is wired in this configuration, it will begin to run in the forwarddirection.

When the RMS (reverse motor starter) is energized, its contacts reverse thearmature wires so that the Al lead is connected to the negative power supplyterminal and the A2 lead is connected to the positive power supply terminal. The

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field leads are connected directly to the power supply, so their polarity is notchanged. Since the field's polarity has remained the same and the armature'spolarity has reversed, the motor will begin to rotate in the reverse direction. Thecontrol part of the diagram shows that when the FMS coil is energized, the RMScoil is locked out.

I. FieldThe outer part that doesnt move is

called the Field. There are pairs of staticmagnetic poles in the field. These polescan be permanent magnets orelectromagnets made from coils woundaround each pole. The motor case orframe (also called a yoke) closes thebackside of magnetic path betweenneighboring poles.

A field frame is the stationary part in a DC

motor or generator. The stationary part is calleda stator in an AC motor, but is called a field framein a DC motor. The field poles are metal piecesmounted to the field frame that are used as fieldwindings. The field poles are constructed of thinsheets of steel laminated together, similar to theconstruction of AC motors. The field windings aremagnets or stationary windings used to producethe magnetic field in an alternator or motor. Inmost cases, the field windings are made bycoiling wire around the field poles.

Interpolesare auxiliary poles placed between themain field poles of the motor. The interpoles areconnected in series with the armature windings, withone terminal of the interpole connected to thebrushes and one brought out to connect to the DC

power supply. The interpoles are made with larger size wire than the main fieldpoles, in order to carry armature current. They are smaller in overall size than themain field poles because they require fewer windings. Interpoles are also known ascommutating field poles.

No Field Condition

In order for a DC motor to turn, there must be the magnetic lines of force

from the armature and the magnetic lines of force from the field poles. As shuntmotors age and corrosion becomes a problem, a runaway condition may presentitself. When the shunt field is opened and current is available only to thearmature, the motor speed will increase dangerously.

It would seem that without the shunt field the motor would stop. However,the large metal pole shoes of the DC machine support a substantial residualmagnetic field. This residual magnetism is just enough to ensure that themagnetic principles that sustain the armature movement are present.

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The residual magnetic field is not, however, substantial enough to develop asuitable CEMF in the armature. Without the proper proportion of CEMF, currentflow to the armature increases. The more current to the armature, the greaterthe torque and the faster the damaged shunt motor rotates. A no field release isemployed by shunt motors to prevent such a casualty. When the shunt field isde-energized, the no field release disconnects the motor from the circuit.

Runaway

In a shunt-wound motor, decreasing the strength of the field decreases theinduced voltage, increasing the effective voltage applied to the armaturewindings. This increases armature current, resulting in greater torque andacceleration. Shunt-wound motors run away when the field fails because thespinning armature field induces enough current in the field coils to keep the field"live".

II. Armature

Drum-Wound Armature

The armature windings

are placed in slots cut ina drum-shaped ironcore. Each windingsurrounds the core sothat the entire length ofthe conductor cuts themain magnetic field.Thereason why shunt-wound motors usedrum-type armature

because, the Gramme-ring armature is seldom used in modem dc motors. Thewindings on the inside of the ring are shielded from magnetic flux, which causesGramme-ring type of armature to be inefficient. The direction of current flow ismarked in each conductor in the figure (view A) as though the armature wereturning in a magnetic field. The dots show that current is flowing toward you onthe left side, and the crosses show that the current is flowing away from you onthe right side.

ARMATURE REACTION

There are individual magnetic lines of force from the field poles and thearmature. Magnetic fields tend to combine. Additionally, the magnetic lines offorce are distorted (or concentrated) by an iron core. It shows the field flux (viewA) and the armature flux (view B) individually. View C shows the distortioncaused by the interaction of the two fields and the armature core movement.

This distortion is known as armature reaction.

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The armature current in a motor is forced to flow in the opposite direction to thatof the CEMF. In a motor, the main field flux is always distorted in the oppositedirection to armature rotation (view C); The resultant field in the motor (view C)is strengthened at the leading pole tips and weakened at the trailing pole tips.

This action causes the neutral plane to shift to A'B'.

The armature reaction is overcome in a motor; that is, by the use of laminatedpole tips with slotted ends, interposes, and compensating windings. To furtherensure successful commutation, small slots on the brush rigging permit a slightbrush position adjustment. By placing a tachometer on the motor shaft, anindication of motor efficiency may be obtained. Adjust the brush position for thefastest armature rotation in the absence of sparking.

III. BackEMF, Induced EMF, or Counter EMF(electromotive force)

Resistance of the armature widings has only a minor effect on armaturecurrent. Current is mostly determined by the voltage induced in the windings bytheir movement through the field. This induced voltage, also called "back-emf" isopposite in polarity to the applied voltage, and serves to decrease the effectivevalue of that voltage, and thereby decreases the current in the armature.

Counter e.m.f. opposes the current, which causes the armature to rotate. Thecurrent flowing through the armature, therefore, decreases as the counter e.m.f.increases. The faster the armature rotates, the greater the counter e.m.f. Forthis reason, a motor connected to a battery may draw a fairly high current onstarting, but as the armature speed increases, the current flowing through the

armature decreases. At rated speed,the counter e.m.f. may be only a fewvolts less than the battery voltage.

Then, if the load on the motor isincreased, the motor will slow down,less counter e.m.f. will be generated,

and the current drawn from theexternal source will increase.

In a shunt motor, the counter e.m.f.affects only the current in thearmature, since the field is connectedin parallel across the power source. Asthe motor slows down and the countere.m.f. decreases, more current flowsthrough the armature, but themagnetism in the field is unchanged.Compared when the series motor slowsdown, the counter e.m.f. decreases andmore current flows through the fieldand the armature, thereby

strengthening their magnetic fields. Because of these characteristics, it is moredifficult to stall a series motor than a shunt motor.

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IV. Terminal or Source Voltage

The armature circuit and the shunt field circuit are connected across a dc sourceof fixed voltage Vt.

V. Motor Speed

A motor whose speed can be controlled iscalled a variable-speed motor; dc motors arevariable speed motors. The speed of a dcmotor is changed by changing the current inthe field or by changing thecurrent in thearmature.

When the field current is decreased, the fieldflux is reduced, and the counter emf

decreases. This permits more armaturecurrent. Therefore, the motor speeds up. Whenthe field current is increased, the field flux is

increased. More counter emf is developed, which opposes the armature current.The armature current then decreases and the motor slows down.

When the voltage applied to the armature isdecreased, the armature current is decreased,and themotor again slows down. When thearmature voltage and current are bothincreased, the motor speeds up.

In a shunt motor, speed is usually controlled

by a rheostat connected in series with thefieldwindings, as shown in figure. When theresistance of the rheostat is increased, thecurrent through thefield winding is decreased.

The decreased flux momentarily decreases thecounter emf. The motor then speeds up, andthe increase in counter emf keeps thearmature current the same. In a similarmanner, a decrease in rheostat resistance

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increases the current flow through the field windings and causes the motor toslow down.

VI. Speed Regulation

This type of motor runs practically constant speed, regardless of the load. It isthe type generally used in commercial practice and is usually recommended

where starting conditions are not usually severs. Speed of the shunt-woundmotors may be regulated in two ways: first, by inserting resistance in series withthe armature, thus decreasing speed: and second, by inserting resistance in thefield circuit, the speed will vary with each change in load: in the latter, thespeeds is practicallyconstant for anysetting of thecontroller. This latteris the most generallyused for adjustable-speed service, as inthe case of machine

tools.

The shunt woundmotor is used whereconstant speed isrequired regardless ofload; for instance, withfans or pumps. Thestarting of a d.c. motorrequires a circuitarrangement to limit armature current. This is achieved by the use of a starter.A number of resistances are provided in the armature and progressively

removed as the motor speeds up and back e.m.f. is developed. An arm, as partof the armature circuit, moves over resistance contacts such that a number ofresistances are first put into the armature circuit and then progressivelyremoved. The arm must be moved slowly to enable the motor speed and thusthe back e.m.f. to build up. At the final contact no resistance is in the armaturecircuit. A 'hold on' or 'no volts' coil holds the starter arm in place while there iscurrent in the armature circuit.

If a loss of supply occurs the arm will be released and returned to the 'offposition by a spring. The motor must then be started again in the normal way.An overload trip is also provided which prevents excess current by shorting outthe 'hold on* coil and releasing the starter arm. The overload coil has a soft iron

core which, when magnetized sufficiently by an excess current, attracts the tripbar which shorts out the hold on coil. This type of starter is known as a 'faceplate'; other types make use of contacts without the starting handle butintroduce resistance into the armature circuit in much the same way.

VII. Power

Power flow diagram of a DC motor is shown in figure 40.1. A portion of the inputpower is consumed by the field circuit as field copper loss. The remaining poweris the power which goes to the armature; a portion of which is lost as field

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copper loss (Pfl), armature loss(Pal), core loss (Pcore) and mechanical loss (Pmech loss).Remaining power is the gross mechanical power developed of which a portionwill be lost as friction and remaining power will be the net mechanical powerdeveloped.

DC Shunt Motor Power Flow

A separately excited DC motor has two input power sources, to the armature circuitand the field circuit. Shunt and series motors only have one power source, thearmature circuit terminals. The power to the armature terminal is:

Considering the losses in the circuit, there are losses in the field and armaturewinding resistances. For shunt and separate excitation, the armature losses are:

All of the field power is converted to field copper losses,

The power converted to the mechanical system is calculated the same way for bothseparate, shunt and series excitation.

If there are no mechanical losses, then the output power will equal the powerconverted. If there are mechanical losses, they are deducted from Pconv to give thefinal output power.

No load rotational losses = Core loss - friction and windage No load rotational losses: Input power with machine operating at normal

speed and excited to produce the calculated internal voltage under full loadcondition (subtract voltage drops across brushes and series fields).

Friction and windage: Power input at normal speed with machine unexcited

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I. Losses

Losses occur when electrical energy is converted to mechanical energy (in themotor). For the machine to be efficient, these losses must be kept to a minimum.Some losses are electrical, others are mechanical.

A. Electrical

Electrical losses are classified as copper losses and iron losses:

Copper losses occur when electrons are forced through the copper windingsof the armature and the field. These losses are proportional to the square ofthe current. They are sometimes called I2R losses, since they are due to thepower dissipated in the form of heat in the resistance of the field andarmature windings. Heat is generated any time current flows in a conductor.Copper loss is an I2R loss, whichincreases as current increases. The amountof heat generated is also proportional to the resistance of theconductor. Theresistance of the conductor varies directly with its length and inversely with

its cross-sectional area. Copper loss is minimized in armature windings byusing large diameter wire.

While Iron losses are subdivided in hysteresis and eddy current losses. Asthe armature rotates in the magnetic field, the iron parts of the armature aswell as the conductors cut the magnetic flux. Since iron is a good conductorof electricity, the EMF s induced in the iron parts courses to flow throughthese parts.

B. Rotational

Mechanical losses occur in overcoming the friction of various parts of themachine.

Rotational losses consist of: bearing friction loss friction of the rushes riding on the commutator windage losses

Windage losses are those associated with overcoming air friction in setting upcirculation currents of air inside the machine for cooling purposes. Theselosses are usually very small. When there is no load on a shunt motor, theonly torque necessary is that which is required to overcome friction andwindage. (Windage is a mechanical loss due to the friction between themoving armature and the surrounding air.) The rotation of the armature coilsthrough the field pole flux develops a CEMF. The CEMF limits the armaturecurrent to the relatively small value required to maintain the necessarytorque to run the motor at no load.

A. Hysteresis

Hysteresis loss is a heat loss caused by the magnetic properties of thearmature. When an armature core is in a magnetic field, the magneticparticles of the core tend to line up with the magnetic field. When thearmature core is rotating, its magnetic field keeps changing direction. Thecontinuous movement ofthe magnetic particles, as they try to align

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themselves with the magnetic field, which produces molecular friction. This,in turn, produces heat. This heat is transmitted to the armature windings. Theheat causes armature resistances to increase.To compensate for hysteresislosses, heat-treated silicon steel laminations are used in most dc motorarmatures. After the steel has been formed to the proper shape, thelaminations are heated and allowed to cool. This annealing process reduces

the hysteresis loss to a low value.

B. Eddy Current

The core of a motor armature is made from soft iron, which is a conductingmaterial with desirable magnetic characteristics. Any conductor will havecurrents induced in it when it is rotated in amagnetic field. The currents thatare induced in the motor armature core are called EDDYCURRENTS. Thepower dissipated in the form of heat, as a result of the eddy currents, isconsidered a loss.

I. Efficiency

A. Common efficiency

To calculate a motor's efficiency, the mechanical output power is divided by the electrical input

power: , where is energy conversion efficiency,Pe is electrical input power, andPm is

mechanical output power.

In simplest case Pe = VI, and Pm = T, where V is input voltage,I is input current,T is output

torque, and is output angular velocity.

B. Electrical efficiency

Efficiency calculation of motor, first calculate the input power and then

subtract the losses to get the output mechanical power as shown below,

in

in

in

out

P

lossesP

P

P ==

Where:

( )voltageDCTerminal

lossesrotationalLosses

Losses

2

=

+=

++=

=

t

fatin

ftaa

inout

V

IIVP

IVRI

PP

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http://en.wikipedia.org/wiki/Energy_conversion_efficiencyhttp://en.wikipedia.org/wiki/Energy_conversion_efficiency

Shunt-Wound Motor Final Report

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