Electric Motors Whitepaper

7/27/2019 Electric Motors Whitepaper

http://slidepdf.com/reader/full/electric-motors-whitepaper 1/8

Sponsored by

Presented by

The relationship between motor loadand temperature is easy to remember:the higher the load, the higher the tem-perature at which the motor operates. Amotor’s maximum operating temperaturedetermines its maximum load, typically specified as the amount of power the mo-tor can deliver on a continuous basis.

Motor power is usually expressed in

terms of horsepower (hp) or wattage (W).One horsepower represents approximately 746 Watts. So a 7.5-kW electric motordoes the same amount of work as a 10-hpelectric motor.

Of course, the real goal identifies how much work the motor can perform overtime. Assuming there are no otherlosses in the system, a 1-hpmotor can lift a 550-lbweight one foot in onesecond, or 550 ft-lb/sec.

However, mechanical lossesdue to friction and heatlower that number.

In the InternationalSystem of measurement,force is measured in new-tons (N) and distance inmeters (m). So a 1-kW elec-tric motor delivers 1,000N-m/sec.

hp=550 ft-lb/sec W=N-m/sec Thus, the first and most important step

in electric motor selection is determiningload characteristics -- torque and speed versus time. Additional factors in motorselection are the mission goals, poweravailable, and cost.

The first step in sizing anelectric motor beginswith determiningthe startingand

General Information for All Motors

Electric motors, whether designed to operate on ac, dc, or both types of current,come in numerous shapes and sizes. All electric motors have one goal in common:

to convert electrical energy into mechanical motion. Whether the application uses

a general-purpose standardized electric motor or a motor intended for a specific

task, the selection process must satisfy the dynamic requirement of the machine

on which it’s applied without exceeding the motor’s rated temperature.

ELECTRIC

MOTORS

Schneider Electric Motion USA

1www.imshome.com April 2012



running torque parameters. Starting torque is theamount of torque needed to start motor rotationand build speed. Running torque is the torqueneeded to maintain a constant rpm.

Starting torque can vary from a small percent-age of full load to a value several times full-loadtorque. These variations are due to changes inload conditions or the mechanical nature of the

machine in which the electric motor is installed.Cause of the latter condition may be due to lubri-cant, wear of moving parts, or other reasons.

For an electric motor to gain acceleration, thetorque supplied by the motor must be more thanthat required from start to full speed. The greaterthe electric motor’s reserve torque, the morerapid the acceleration.

Many motor drive systems use gear reduc-ers to slow the rate of rotation of the equipmentshafts. As rpm drops, the torque available on the

driven shaft increases.T

1N

1= T

2N

2or T

1 /T

2= N

2 /N

1

where T is the shaft torque and N is the shaftspeed in rpm.

Calculating the acceleration torque needed forthese electric motors means reducing the rotat-ing component values to a common base. Partinertias are typically converted to their equiva-lent value at the drive shaft. Equivalent inertiaW

2K

22 of the load only is found from:

W 2K

22 = (W

1K

12 )(N

1 /N

2 )2

where W1K

12 is the load inertia in lb-ft2, N

1is

the load speed in rpm, and N2

is the electric mo-tor speed in rpm.

There are times when the electric motor loadis driven in a straight-line rather than a rotary motion. In these situations, a cam, cable, orrack-and-pinion mechanism translates the rotary motion of the motor into a linear distance. Forthese electric motor parts, the equivalent WK2 is

found from:

WK 2 = W(S/2πN)2

where W equals the load weight in lb, S is thetranslation speed in fpm, π is the constant pi, andN is the rotational speed in rpm.

Acceleration time

The acceleration time for an electric motor is

directly proportional to total inertia and inverselyproportional to the electric motor torque. For eletric motors with constant acceleration torque, ac-celeration time is given as:

t=WK 2(N 2-N

1 ) / 308Tx

where WK2 is the rotational inertia in lb-ft2, (N

- N1) the speed difference, and Tx the acceleratiotorque in lb-ft.

Use an approximation method to find theelectric motor’s acceleration time if accelerationtorque is not linear during the speed ramp-up. Omethod breaks up the speed versus torque curvesthe electric motor and driven machine into distinsegments. Acceleration time is then calculated foeach segment and the times added together.

Power rating

The most common equation for electric mo

power based on torque and rotational speed is

hp = (TN) / 5,250

where hp is the horsepower of the motor, T ithe torque in ft-lb, and N is the motor rpm.

At times an electric motor’s load is not con-stant but follows a definite cycle. In these situ-ations a horsepower versus time curve for thedriven machine is usually helpful. This curve dtermines both peak and root-mean-square (rmhorsepower. Rms horsepower indetifies the cotinuously needed electric motor rating.

While the peak load identifies the maximumload placed on the motor, it does not necessarigovern electric motor size. For example, a motwith a peak demand of 14 hp may only need 8hp for continuous operation. As long as the pedemand is not maintained for long periods of time, a 10 hp motor should handle the work loquite well. The determining factor for motor siis peak load versus duty cycle.

Duty cycle

Choosing the proper size electric motor de-pends on whether the load is steady, varies, fol-lows a repetitive cycle of variation, or has pulsating torque or shocks.

For example, electric motors that run continu-ously in fans and blowers for hours or days may be selected on the basis of continuous load. Butelectric motors located in devices like automaticacontrolled compressors and pumps start a numb




of times per hour. And electric motors in some ma-chine tools start and stop many times per minute.

Duty cycle is the ratio of motor on-time to theoverall on-off period. Logic dictates that motorsthat operate only a fraction of the time can besmaller than if they were called upon to work 100%.

dc=T on

/(T on

+T off

)x 100%

where dc is the duty cycle, T on

is the on time,and T

off is the off time.

However, this isn’t quite true. Starting a motortypically draws greater power, making the motorreach higher temperatures faster. A motor thatstarts and stops constantly may actually need arating above continuous load to let it handle theextra heat generated by so many start/stop cycles.

For most electric motors (except squirrel-cageelectric motors during acceleration and plug-

ging) current is almost directly proportional todeveloped torque. At constant speed, torque isproportional to horsepower. For acceleratingloads and overloads on electric motors that haveconsiderable droop, equivalent horsepower isused as the load factor. The next step in sizingthe electric motor is to examine the electric mo-tor’s performance curves to see if the electricmotor has enough starting torque to overcomemachine static friction, to accelerate the loadto full running speed, and to handle maximumoverload.

Electric motors possess a service factor.NEMA Standard MGI-143 defines the servicefactor of an ac motor as “...a multiplier which,when applied to the rated horsepower, indicatesa permissible horsepower loading which may becarried under the conditions specified for theservice factor...” In other words, multiplying theelectric motor’s nameplate horsepower by theservice factor tells how much the electric motorcan be overloaded without overheating. In gen-eral, service factors let electric motors: handlean occasional known overload; provide a safety

factor where the environment or service condi-tion is not well defined, especially for general-purpose electric motors; and obtain cooler-than-normal electric motor operation at rated load,thus lengthening insulation life.

Determining the amount of overload permis-sible is handled by the temperature rise figureon the nameplate data. That figure is always ex-pressed for the maximum amount of load. Forexample, two 100-hp motors with service factors

of 1.15 and 1.25 respectively each have servicetemperature rises of 90°C above ambient. Themotor with the 1.25 service factor is typically larger than the other motor to handle the extraheat generated by the higher rating service factor.

Efficiency

The efficiency of a motor determines how well

the motor converts the input electrical energy intomechanical motion. It is the ratio of power outputdivided by power input:

eff = P out

/ P in

If the motors shaft output power is given interms of horsepower, multiply the horsepower rat-ing by 746 to convert power output to Watts.

Motor efficiency has become a major specifica-tion in today’s energy conscious world. Small uni- versal electric motors typically have an efficiency

of about 30%, On the other hand, three-phase ma-chines can reach efficiencies of 95% or higher.

Electric motors lose efficiency in several areas,the most common being copper losses, iron losses,stray losses, and mechanical losses. The efficiency level of an electric motor depends on actual electricmotor load versus rated load. The highest efficiency occurs near the rated load andfalls off rapidly for under andoverload conditions. This is why proper motor sizing is neededfor greatest efficiency.

Because similar motors fromdifferent manufacturers may have quite different efficien-cies, the National ElectricalManufacturers Association(NEMA) established the NEMAPremium energy efficiency rat-ing. Motors classified as meet-ing NEMA Premium standardsmean the motors meet the mini-mum efficiency rating as set by

NEMA for that type and size of motor.AC Motors

The construction of ac motors is comprised of two main parts: the stator and the rotor. The sta-tor, or stationary part of the motor, creates a rotat-ing magnetic field around the rotor. The rotor, orrotating portion of the motor, tries to turn withthe rotating magnetic field created by the stator.

The easiest method of generating a rotating




April 20

magnetic field is with 3ϕ electrical power. Eachof the three phases, labeled as A, B, and C, is120° out of phase with the other two. The direc-tion of motor rotation is given as the sequenceof the phases: ABC or ACB. When applied tothree magnetic coils spaced 120° apart, the threephases generate the rotating magnetic field. Thespeed at which the magnetic field rotates in the

motor is called its synchronous speed.Ac motors may also generate rotating mag-

netic fields using two phases, and even a singlephase. Motors that generate the rotating mag-netic fields using two or more phases are alsocalled multiphase or polyphase motors.

How well the rotor manages to stay in syncwith the rotating magnetic field identifies thefirst major classification of ac motors: synchro-nous and asynchronous operation.

The synchronous speed of an ac motor is cal-

culated from two factors: the frequency of theapplied ac and the number of poles per phase inthe motor. Synchronous speed (N

SYNC) is calcu-

lated by:

N SYNC

= (120 × f) / P PH

where NSYNC

is the synchronous speed in rpm, f is the frequency in Hertz, and P

PHis the number

of poles per phase in the motor. So a 3ϕ ac motorwith four poles per phase operated on the 60-Hzac power would have a synchronous speed of

1,800 rpm.An ac motor is said to be synchronous whenits rotor rpm matches the synchronous speed of the motor. The rpm of asynchronous motors dif-fers from the synchronous speed. This differenceis called the motor slip and is usually expressedas a percentage of synchronous speed:

Slip % = (N SYNC

– N TRUE

) / N SYNC

× 100%

In this equation, NSYNC

is the synchronousspeed of the motor, while N

TRUEis the actual

rpm that the motor spins. If NSYNC matchesN

TRUE, then it is a synchronous motor with 0%

slip. However, most ac motors operate at speedsslightly below synchronous speed. These areknown as asynchronous motors.

For example, the previous four-pole motor ac-tually has a true rpm of 1,725. If its synchronousspeed is 1,800 rpm, what is its slip?

(1,800 – 1,725) / 1,800 × 100% = 4.2% slip

Induction AC Motors

The ac induction motor is the most commonform of asynchronous motor as well as the simplest and most rugged. Rotors in induction motors typically consist of a laminated, cylindricairon core with slots for receiving conductors. Tmost common type of rotor has cast-aluminumconductors and short-circuiting end rings. If th

conductors were viewed without the surrounding rotor core they would look very much likea squirrel cage. Consequently, these motors areknown as squirrel-cage induction motors. As trotating magnetic field of the stator cuts througthe stationary rotor, a voltage is generated withthe conductors of the rotor. The voltage produa current flow in the rotor, creating a magneticfield that interacts with the rotating field of thestator, and the rotor begins to turn.

As rotor speed increases, the speed at which t

magnetic field cuts through the rotor decreases,reducing the induced rotor currents. If the rotorshould match synchronous speed, the magneticfield induces no current flow in the rotor, and throtor would slow until equilibrium is reached.

Standard AC Motors

The construction of an ac motor influences torque, current, and full-load speed. The evolution and standardization of ac motors have re-sulted in four fundamental types of ac motors:

Designs A and B: These are general-purposac motors with normal starting torques andcurrents, and low slip. Fractional-horsepowerpolyphase ac motors are generally design B.Because of the drooping characteristics of desiB, a polyphase ac motor that produces the sambreakdown (maximum) torque as a single-phaac motor cannot reach the same speed-torquepoint for full-load speed as single-phase ac motors. Therefore, breakdown torque must behigher (a minimum of 140% of the breakdowntorque of single-phase, general-purpose ac motors) so that full-load speeds are comparable.

Design C: These motors are noted for theirhigh starting torque with normal starting curreand low slip. Design C motors are normally uswhere breakaway loads are high at starting, butwhich normally run at rated full load and are nsubject to high overload demands after runninspeed has been reached.

Design D: These motors have a high slip, bupossess a high starting torque with low start-ing current. However, they have a low full-load




www.imshome.com

Squirrel cage induction motor rotor

Aluminumend ring

Aluminumbars

Steellaminations

Shaft

speed. Because of the high slip, speed varieswidely under fluctuating loads. This ac motordesign is subdivided into several groups that vary according to slip or the shape of the speed-torquecurve.

Design F: Possesses a low starting torque, low starting current, and low slip. These ac motorsare built to obtain low locked-rotor current, let-

ting the motor come to a full stop under load.They are sometimes referred to as torque mo-tors in that they can provide a torquing actionwithout rotation. Both locked-rotor and break-down torque are low. Normally these ac motorsare used where starting torque is low and whereno high overloads exist after running speed isreached.

Wound-rotor AC Motors: Squirrel-cage acmotors are relatively inflexible with regard tospeed and torque characteristics, but a special

wound-rotor ac motor has controllable speedand torque. In this motor, the squirrel-cage is re-placed with an actual motor winding with accessto the winding made through slip rings on therotor assembly. With the rotor winding available,the characteristic of this motor may be changedby inserting resistance into the rotor winding.

Wound-rotor ac motors are generally startedwith secondary resistance in the rotor circuit. Asmotor rpm rises, the resistance is sequentially reduced to permit the motor to come up to fullspeed and torque. Thus, wound-rotor ac motors

can develop substantial torque while limitinglocked-rotor current. This ac motor resistancecan be designed for continuous service to dis-sipate heat produced by continuous operation atreduced speed, frequent acceleration, or accel-eration with a large inertia load. External resis-tance gives ac motors a characteristic that resultsin a large drop in rpm for a fairly small change inload. Reduced ac motor speed is provided downto about 50% rated speed, but efficiency is low.

Multispeed AC Motors

By physically reconnecting leads from the sta-tor windings, it’s possible to change the numberof poles in an ac motor, effecting a change inspeed. Typical synchronous dual speeds for 60-Hz ac motors are: 3,600/1,800 rpm (2/4 pole),1,800/900 rpm (4/8 pole), and 1,200/600 rpm(6/12 pole).

Multispeed ac motors provide their outputhorsepower in accordance with one of the fol-lowing load characteristics:

Variable torque: Variable-torque ac motors

have a speed torque characteristic that var-ies as the square of the speed. For example, an1,800/900-rpm electrical motor that develops10 hp at 1,800 rpm produces 2.5 hp at 900 rpm.Two times the speed produces four times theoutput power. Loads such as centrifugal pumps,fans, and blowers, have a torque requirement that varies as the square or cube of the speed. This acmotor characteristic is usually adequate.

Constant torque: Constant-torque ac mo-tors develop the same torque at each speed, thustheir power output varies directly with speed.

For example, an ac motor rated at 10 hp at 1,800rpm produces 5 hp at 900 rpm. These ac motorsare used in applications with constant torquerequirements such as mixers, conveyors, andcompressors.

Constant horsepower: These ac motors develop the same horsepower at each speed withtorque inversely proportional to speed. For ex-ample, a motor rated for 10 hp at 1,800 rpm stillproduces 10 hp at 900 rpm, but it’s torque valuedoubles. Typical applications for constant-horse-power ac motors include machine tools such as

drills, lathes, and milling machines.

Single-phase AC Motors

Single-phase ac motors have a unique problemin that there is no rotating magnetic field as seenin polyphase motors. Fortunately, through a com-bination of induction and rotation, a single-phasemotor will continue to turn once started. Theproblem becomes how to start the motor turning.

In addition to a main winding, single-phase

5 April 20



April 20

ac motors usually incorporate a start winding.Its purpose is to create a rotating magnetic fieldto start the rotor turning. Once the rotor reachessufficient rpm, the start winding is either discon-nected or the current flow through the windingis altered from its initial starting value.

Most single-phase induction ac electric mo-tors are commonly fractional-horsepower types,

although single-phase integral-horsepower areavailable in the lower horsepower range. Thetype of single-phase ac motor is typically identi-fied by its start winding connection, the mostcommon types being split-phase, capacitive start,the permanent-split capacitor or capacitive start/capacitive run, and the shaded pole motor.

Synchronous AC Motors

Synchronous ac motors are inherently constant-speed electric motors that operate in absolute syn-chronism with line frequency. As with squirrel-cage

induction ac motors, speed is determined by thenumber of pairs of poles and is always a ratio of theline frequency.

In synchronous ac motors,the squirrel-cage or woundrotor winding is replaced witha powered rotor winding ex-cited with a dc current. Thedc current creates a magneticpolarity in the rotor that locksinto the polarity of the statorfield and turns in lockstepwith it.

Synchronous ac motors aremade in sizes ranging fromsubfractional self-excitedunits to large-horsepower, di-

rect-current-excited ac motors for industrial drives.In the fractional-horsepower range, synchronous acmotors are used primarily where precise constantspeed is needed. A typical application of synchro-nous motors is in escalators. The constant speedof the synchronous motor keeps the escalator step

moving at a constant velocity as people step on andoff the escalator.In large horsepower sizes applied to industrial

loads, synchronous ac motors serve two importantfunctions. First, ac motors provide highly efficientmeans of converting ac energy to mechanical power.Second, by overexciting the rotor winding, synchro-nous ac motors can operate at leading or unity powerfactor, thereby providing power-factor correction.

A caveat in using synchronous ac motors:

Synchronous motors must start as induction mo-tors, and the motor much reach almost synchronospeed before excitation is supplied and the rotorlocked in synchronous step. This means synchro-nous motors must have some type of starting mechnism, and cannot be started across the power line acan most of the other ac motors. In a typical startusequence, rotor excitation is removed, and the win

ing shorted turning the motor into a wound-rotordesign. The motor is started in this condition untilapproaches its synchronous speed, when the shortthe rotor winding is removed and excitation applie

Motor starters & drives

Motors were once started by directly applyingpower to the motor through a set of switches or cotactors, and small ac motors are still done so todayHowever, these across-the-line starters were notorous for placing high demands on ac power systemproducing excessive torque on the output shaft of t

motor known to shear both shaft and motor cou-plings, and generate excessive interference througharcing at the contactor. As motor size increased, thimpact of these problems grew exponentially.

One of the earliest motor starting systems applicurrent limiting resistors to the motor ac power linto limit in-rush current during starting. The resis-tors were shorted out of the system once the motorwas rotating with sufficient speed. The resistorsconsumed power, and had to be vented to preventoverheating, especially if the motor was subject torepetitive stops and starts.

The advent of electronic motor controls offeredanother answer. The control could be programmeto apply power gradually as the motor came up tospeed as part of its normal operation. Motor contrlers performing that operation became known assoft-start controllers.

Though ac motor controls were used for constaspeed ac motor applications, dc motors held sway in the variable speed markets for many years. Thatstarted to change as the ability of solid-state devicegrew in handling higher power voltages and cur-

rents. The rotational speed of ac motors is more afunction of the frequency of the applied power thaits voltage. Ac motors operated at reduced voltagesbut the same frequency, overheated as motor slip,the difference between true rpm and synchronousrpm, grew larger. High-power solid-state devices,such as the power MOSfet, let designers develop variable-frequency motor drives that let motors ruat reduced powerline frequencies as well as lower voltages.

6www.imshome.com



www.imshome.com

Today’s motor drives use several banks of powerMOSfets to form 3ϕ ac power sources for ac induc-tion motors. Thus the 60-Hz ac power line can betransformed into any frequency at any voltage to ob-tain the desired rpm.

Motors for servomechanisms

Servomechanism motors, or servomotors for

short, are motors that power devices that employ feedback or error-correction signals to control itsmechanical speed, position, or other parameter.The feedback or error signal is typically suppliedby some type of encoder, a device that changesthe parameter to be controlled into a measureablesignal. In a simple servomechanism, the measuredsignal is compared to a desired value that representsthe intended operation of the servomechanism. If the measured and desired signals do not match, anerror signal is sent to the servomotor to move themechanism to the desired position.

Until recently, servomotors were strictly theprovince of dc motors. Today, with lower cost mo-tion control electronics and motor drives, two othertypes of electric motors are rapidly gaining on theirdc counterparts. They are the ac servomotor andthe stepper motor. While stepper motors are a formof servomotor, the term servomotor is generally re-served for the nonstepping-types of dc and ac mo-tors. The stepper motor is called just that, a steppermotor or stepmotor.

Ac servomotors are essentially synchronous mo-tors. However, the coil of wire typically found inthe rotor of most synchronous motors has been re-placed by permanent magnets. The magnet forcesthe rotor to rotate in lockstep, or synchronously,with the rotation of the magnetic field generatedby the motor stator. Because the magnetic field of the rotor cannot be turned off, the motor control-ler must know the rotational position and polarity of the rotor’s magnet and adjust the magnetic fieldpolarity of the stator accordingly. Hall effect devicesplaced at strategic locations around the stator sensethe rotor magnetic field.

Early ac servomotors used ferrite rotor magnets,but newer systems now use magnets fashionedfrom rare earths, such as neodymium and samar-ium-cobalt. The rare-earth magnets offer magneticfields several orders of magnitude stronger than thesame size ferrite magnet, permitting constructionof smaller and more powerful motors.

Stepper motors also use permanent magnets intheir rotor assembly. But the magnets terminatein a large number of individual north/south poles.

Pole counts of 50, 100, or more poles are possible.The poles are typically divided between two differ-ent stator coil windings. In operation, the magneticpoles of the rotor align with the magnetic field of the stator, locking the rotor in position. The mo-tor jumps from one position to the next usingdiscrete steps as the stator windings are energizedand de-energized. The position of the motor can be

determined by starting from a known position andcounting the number of steps, eliminating the needfor an encoder.

The most common stepper motor needs 200 stepsto complete one revolution (1.8°/step), though 50(7.2°/step), 100 (3.6°/step), and 400 (0.9°/step) stepsare also seen. Stepper motors may also work in themicrostepping region, where each full step is brokendown into subsequent microsteps. Upwards to 250microsteps/full step can be derived, resulting in up to51,200 steps required to complete one revolution.

The construction of stepper motors producesan inherently lower cost motor than conventionalservo motors. Step motors do not require tuning,allow for a greater inertia mismatch, and have very high torque density. This torque is 100% availableimmediately upon startup, which can be very ad- vantageous when doing short quick moves or whencoupled to high inertia loads. Because step motorsare synchronous motors with a high pole count,they are able to run smoothly at extremely slow speeds with no cogging.

Stepper motor technology does have some

disadvantages. The most critical drawback is theloss of synchronization and torque if a large loadexceeds the motor’s capacity. A high-inertia loadwill cause the rotor to slip, or not advance when thestep pulse is given. Step motors also tend to run hotbecause phase current is independent of the load.In some applications, if the motor needs to be over-driven by the load, it may be undesirable to feel thepoles of the step motor as the rotor is being pulledby the load.

Step motors also lose the ability to restart if theload applied exceeds the motor’s ability to pro-

duce the required torque. The area of the torquecurve where the motor can start rotating properly is known as the pull-in region. In this area, themotor will self-start if the load causes a loss of synchronization.

Various techniques have been developed to over-come this loss of synchronization. As stated, thedegree of rotation needed by a stepper motor canbe calculated by the number of steps required. Forexample, a 400 step/revolution motor attached to a

7 April 20



April 20

Schneider Electric Motion USA

ball lead screw needs 50 turns to reach the desiredposition. A quick calculation shows the controlneeds to execute 20,000 steps to complete the move.However, high loads may reduce the actual num-ber of steps taken by the motor due to rotor slip.By monitoring how many steps the motor actually performs compared to the number of steps com-manded, the motor controller can inject additional

steps to bring the rotor to the proper position.Such a system needs an encoder to monitor rotor

position. If the true location of the rotor equals orexceeds its desired position within preset bounds,the controller intervenes to prevent loss of synchro-nization. This is achieved by “slowing” or “acceler-ating” the stator’s magnetic field rotation to a speedthat equals the rotor, such that the rotor and statorlead or lag stays within bounds. In most instances,this means less than 2 steps leading or lagging. Thischange in stator speed continues until a change in

either commanded motor speed or load require-ments lets the motor produce sufficient torque atthe commanded speed.

Additional performance and efficiency can oc-cur through the introduction of variable currentcontrol that lets the controller supply only theamount of phase current necessary to perform amove. Using variable current control minimizesmotor heating while boosting system efficiency.

Integrated systems

A growing trend in motor control is the integratedsystem that combines motor, position encoder, andcontroller in one self-contained package under thetitle of intelligent motor control. Early designs keptall three devices separate. This could create someproblems if the capabilities of the devices were mis-matched. By integrating the controller with the mo-tor, the possibility of mismatch is eliminated.

Integrated systems basically need power and acommunication signal. The type of communicationsignal can take many forms, including serial RS-422/485, CANopen, Modbus, EtherNet/IP, EthernetPowerlink, EtherCAT, Modbus-TCP, Profinet,

DeviceNet, SERCOS III, and others as the list contin-ues to grow.Motor operation and synchronization are carried

out through digital data signals transmitted between

the motors and a master control system such asa programmable logic controller (plc) or processautomation controller (pac.) The plc or pac sends command to execute a particular function, and it’sup to the controller on the motor to carry out thatcommand. This distributed control system providfaster response and greater accuracies than that obtained by a single central control point running all

operations.One of the fastest growing communication pro

cols today is the Ethernet protocol. Many integratecontrol systems use the standard 8C8P connector(also known as RJ-45) and Cat-5/6 cabling to provcompatibility with other integrated motor controlsystems and standard computing systems.

Ethernet is a nondeterministic protocol. Thismeans that different pieces of information sent ovethe network between two devices can take differenamounts of time to arrive. However, the Ethernet

protocol is so fast that the delays encountered aretypically minimal, thus it’s been shown to providenear real-time synchronization of multiple motorinstallations.

Summary

DC motors used to be the only viable alternativwhen control of speed, position and torque were required. Today these functions can be accomplisheusing several alternate technologies. Ac motors,once confined to those applications that requiredconstant-speed, can now replace dc motors in almevery situation. The development of high-powersolid-state devices has led to the creation of vari-able frequency controllers that let engineers tailorthe characteristics of ac motors to their needs. Inaddition, advances in Stepper Motor technology, alow for very robust, dynamic motion control in anextremely cost effective package. Integrated MotioControl, combining motors, drives, and controlsinto a single package, let designers reduce machinsize, cost, and time-to-market. They also help redumachine complexity by eliminating compatibility issues between devices and lowering the number o

components in a system. This will undoubtedly leto smaller, lower cost, and more flexible machinedesigns.

Sponsored byPresented by

8www.imshome.com

Electric Motors Whitepaper

Documents

Transcript of Electric Motors Whitepaper