1998 PT Design A317

Most rotary motionapplications todaydepend on motors.

Electric motors are domi-nant, though air and hy-draulic motors are pre-ferred in some applications,and engines or turbinesdrive others.

Several types of electricmotors turn shafts: dc, ac, servo, andstep motors. Also, a motor may be justa motor or it may be a gearmotor; thatis, it may have an integral gearedspeed reducer. Air motors are usuallyrotary vane or piston type; hydraulicmotors are rotary vane, piston, orgear type. Each has operating charac-teristics inherent to its basic design.Thus, motor selection is often a pro-cess of matching application require-ments to performance parameters ofthe various types, then selecting themost compatible.

Performance, however, is not theonly criterion in motor selection. Tooptimize selection, choose an enclo-sure that protects the inner workingsof the motor from the environment.Moreover, you must consider the bestmounting arrangement, such as base

ated from adjustable volt-age power supplies, Figure1. A stabilizing winding (asmall series field) helpsprevent speed increases asload increases at weak fieldsettings, Figure 2. Thiswinding has drawbacks inreversing applications,however, because winding

direction relative to the shunt wind-ing must be reversed when armaturevoltage is reversed. Here, reversingcontactors must be used. Where fastreversing is needed, the stabilizingwinding is omitted, and the motor isdesigned for stable operation.

Permanent-magnet motors —Operating characteristics of perma-nent-magnet motors are similar tothose of shunt-wound types. Fieldflux, however, is provided by perma-nent magnets instead of current in awinding. Motor torque is directly pro-portional to armature current overthe motor’s speed range. Comparedwith shunt-wound motors, perma-nent-magnet motors weigh less andare more economical to operate be-cause no power is needed to support afield.

Series-wound motors — As itsname implies, this motor has a fieldthat is in series with the armature,

or flange mount. Also, consider waysto protect the motor from overloadsand line fluctuations.

The following discussion deals withelectric motors for rotary motion. Forelectric motors for linear motion, seethe Linear Motion Devices ProductDepartment in this handbook.

DC MOTORSDirect current motors are used in

many industrial applications that re-quire adjustable speed.

In uses requiring quick stops, a dcmotor can minimize the size of a me-chanical brake or make it unneces-sary. This is done by dynamic braking(motor-generated energy fed to a re-sistor grid), or by regenerative brak-ing (motor-generated energy returned

to the ac supply).DC motor speed can be con-

trolled smoothly down to zero, fol-lowed immediately by accelerationin the opposite direction (withoutpower circuit switching). Also, dueto high torque-to-inertia ratio, dcmotors respond quickly to control-signal changes.

Motor typesDC motors in all but fractional

and low integral horsepower sizes(generally below 15 hp), havewound fields and are categorizedas shunt-wound, series-wound, orcompound-wound motors. In frac-tional and low integral horse-power sizes, permanent magnetsare used instead of wound fields inmany motor designs.

Shunt-wound motors — Stabi-lized shunt-wound motors are oper-

MOTORSDC MOTORS..........................................................A317 AC MOTORS..........................................................A318 SERVOMOTORS ....................................................A321 STEP MOTORS......................................................A322 MINIATURE MOTORS ...........................................A322GEARMOTORS ......................................................A323 MOTOR ENCLOSURES ..........................................A323 MOTOR PROTECTION ...........................................A325 MOTOR EFFICIENCIES...........................................A326INDUCED BEARING CURRENTS............................A327MOTORS ADVERTISING........................................A328

Figure 1 — Typical speed-torque curveand circuit for shunt-wound dc motor.

Figure 2 — Typical speed-torque curveand circuit for a shunt-wound dc motorwith various series stabilizer windings.

Figure 3. These motors are often usedto drive high starting torque loads,such as traction vehicles. Torquevaries approximately as the square ofcurrent. An increase in armature cur-rent is accompanied by a like increasein field current. At rest, the torque ishighest because no counter electromo-tive force (CEMF) is generated by thearmature. As the armature gainsspeed, the CEMF rises, reducing theeffective voltage, current, and torque.As the motor shaft load increases, thearmature slows to provide sufficientvoltage and current to match the loadtorque. If the load is removed, the mo-tor will race dangerously.

Compound-wound motor —Both shunt and series field windingsare used in compound motors. By ad-justing strength and direction of theseries winding relative to the shuntwinding, speed-torque characteristicscan be made to approximate those ofseries or shunt motors.

Controlling dc motorsDC motor speed is controlled by ar-

mature-voltage control, shunt-fieldcontrol, or a combination of the two.

Armature-voltage control —The type of control varies the voltageapplied to the armature, while fieldcurrent is maintained constant from aseparate source. Speed is proportionalto CEMF, which is equal to the ap-plied voltage minus the armature cir-cuit IR drop. At rated current, thetorque remains constant regardless ofthe speed (since the magnetic flux isconstant) and, therefore, the motorhas constant torque capability over

Reversing — This operation af-fects the power supply and control.When the motor cannot be stopped forswitching series fields before reverseoperation, compound and stabilizingwindings should not be used if fullload torque is needed in both direc-tions. Bi-directional operation mayalso affect brush adjustments.

Duty rating — DC motors carryone of three ratings:

• Continuous duty is applied tomotors that will continuously dissi-pate all the heat generated by inter-nal motor losses without exceedingrated temperature rise.

• Definite time, intermittent dutymotors will carry rated load for speci-fied time without exceeding ratedtemperature rise. These motors mustbe allowed to cool to ambient beforeload is repeated.

• Indefinite time, intermittentduty is usually associated with someRMS load of a duty-cycle operation.

Peak torque — The peak torquethat a dc motor delivers is limited bythat load at which damaging commu-tation begins. Brush and commutatordamage depends on sparking severityand duration. Therefore, peak torquedepends on the duration and fre-quency of occurrence of the overload.Peak torque is often limited by themaximum current that the powersupply can deliver.

Motors can commutate greaterloads at low speed without damage.NEMA standards specify that dc ma-chines must deliver at least 150%rated current for one minute at anyspeed within rated range, but mostmotors exceed this requirement.

Heating — The temperature of adc motor is a function of ventilationand losses in the machine. Somelosses — core, shunt-field, and brush-friction — are independent of load,and vary with speed and excitation.

Several methods can predict oper-ating temperature. The best methodis to use thermal capability curvesavailable from the manufacturer.

AC MOTORSAC motors can be divided into two

major categories: asynchronous andsynchronous.

The induction motor is the mostcommon form of asynchronous motorand is basically an ac transformerwith a rotating secondary. The pri-

its entire speed range.Horsepower varies directly with

speed. As the speed of a self-venti-lated motor is lowered, it loses venti-lation and cannot be loaded with quiteas much armature current withoutexceeding the rated temperature rise.Therefore, to obtain full load torque atlow speeds, motors are oversized orauxiliary blowers are added for suffi-cient cooling.

Shunt-field control — By weak-ening shunt-field current, motorspeed is altered, thus increasingspeed and reducing output torque fora given armature voltage. Field con-trol is good only for obtaining speedsgreater than base speed (the no-loadspeed with full field strength). Maxi-mum speed range by field control isabout 5:1, and this occurs for only lowbase speed motors.

Because the rating of a dc motor islimited by heating, the maximum per-missible armature current is nearlyconstant over the field-weakenedspeed range. This means that, atrated armature current, outputtorque varies inversely with speed,and the motor has constant-horse-power capability over this speedrange.

Selecting dc motorsChoosing a dc motor type and asso-

ciated equipment for a given applica-tion requires consideration of severalfactors.

Speed range — The minimumand maximum speeds for an applica-tion determine the motor base speed.

Allowable speed variation —Applications requiring constant speedat all torque values should use ashunt-wound motor. If speed changeswith load and speed variation must beminimized to less than 2%, a regula-tor employing tachometer feedbackmust be used.

Torque requirements — Thetorque requirements at various oper-ating speeds should be determined.Many applications are essentiallyconstant torque, such as conveyors.Others, such as centrifugal blowers,require torque to vary as the square ofthe speed. In contrast, machine toolsand center winders are constanthorsepower, with torque decreasingas speed increases. Thus, the speed-torque relationship determines themost economical motor.

A332 1997 Power Transmission Design

Figure 3 — Typical speed-torque-horsepower characteristics and circuit fora series wound dc motor.

1997 Power Transmission Design A333

mary winding (stator) is connected tothe power source, and the shorted sec-ondary (rotor) carries the induced sec-ondary current. Torque is producedby the action of the rotor (secondary)currents on the air gap flux.

The synchronous motor resemblesa dc motor turned inside out, with thepermanent magnets mounted on therotor. As an alternative, some are con-structed using a wound rotor excitedby a dc voltage through slip rings. Theflux created by the current-carryingconductors in the stator rotatesaround the inside of the stator in or-der to achieve motor action.

Induction motorsThese motors are probably the sim-

plest and most rugged of all electricmotors. They consist of two basic elec-trical assemblies: the wound statorand the rotor assembly.

The rotor consists of laminated, cy-lindrical iron cores with slots for re-ceiving the conductors. On early mo-tors, the conductors were copper barswith ends welded to copper ringsknown as end rings. Viewed from theend, the rotor assembly resembles asquirrel cage, hence the name “squir-rel-cage” motor is used to refer to in-duction motors. In modern inductionmotors, the most common type of ro-tor has cast-aluminum conductorsand short-circuiting end rings. The ro-tor turns when the moving magneticfield induces a current in the shortedconductors. The speed at which themagnetic field rotates is the syn-chronous speed of the motor and is de-termined by the number of poles inthe stator and the frequency of thepower supply.

acceleration from rest to the speedthat breakdown torque occurs.

Figure 4 illustrates typical speed-torque curves for NEMA Design A, B,C, and D motors.

• Design A motors have a higherbreakdown torque than Design B mo-tors and are usually designed for aspecific use. Slip is 5%, or less.

• Design B motors account formost of the induction motors sold. Of-ten referred to as general purpose mo-tors, slip is 5% or less.

• Design C motors have high start-ing torque with normal starting cur-rent and low slip. This design is nor-mally used where breakaway loadsare high at starting, but normally runat rated full load, and are not subjectto high overload demands after run-ning speed has been reached. Slip is5% or less.

• Design D motors exhibit high slip(5 to 13%), very high starting torque,low starting current, and low full loadspeed. Because of high slip, speed candrop when fluctuating loads are en-countered. This design is subdividedinto several groups that vary accord-ing to slip or the shape of the speed-torque curve. These motors are usu-ally available only on a special orderbasis.

Wound-rotor motors — Althoughthe squirrel-cage induction motor isrelatively inflexible with regard tospeed and torque characteristics, aspecial wound-rotor version has con-trollable speed and torque. Applica-tion of wound-rotor motors ismarkedly different from squirrel-cagemotors because of the accessibility ofthe rotor circuit. Various performancecharacteristics can be obtained by in-serting different values of resistancein the rotor circuit.

Wound rotor motors are generallystarted with secondary resistance inthe rotor circuit. This resistance is se-quentially reduced to permit the mo-tor to come up to speed. Thus the mo-tor can develop substantial torquewhile limiting locked rotor current.The secondary resistance can be de-signed for continuous service to dissi-pate heat produced by continuous op-eration at reduced speed, frequentacceleration, or acceleration with alarge inertia load. External resistancegives the motor a characteristic thatresults in a large drop in rpm for afairly small change in load. Reducedspeed is provided down to about 50%,

Where: Ns = synchronous speed f = frequency P = number of poles

Synchronous speed is the absoluteupper limit of motor speed. At syn-chronous speed, there is no differencebetween rotor speed and rotating fieldspeed, so no voltage is induced in therotor bars, hence no torque is devel-oped. Therefore, when running, therotor must rotate slower than themagnetic field. The rotor speed is justslow enough to cause the properamount of rotor current to flow, sothat the resulting torque is sufficientto overcome windage and frictionlosses, and drive the load. This speeddifference between the rotor and mag-netic field, called slip, is normally re-ferred to as a percentage of syn-chronous speed:

Where: s = slip Ns = synchronous speed Na = actual speed

Polyphase motors — NEMA clas-sifies polyphase induction motors ac-cording to locked rotor torque andcurrent, breakdown torque, pull uptorque, and percent slip.

• Locked rotor torque is the mini-mum torque that the motor developsat rest for all angular positions of therotor at rated voltage and frequency.

• Locked rotorcurrent is thesteady state cur-rent from the lineat rated voltageand frequencywith the rotorlocked.

• Breakdowntorque is the maxi-mum torque thatthe motor developsat rated voltageand frequency,without an abrupt

drop in speed.• Pull up torque is the minimum

torque developed during the period of

300

200

100

00 20 40 60 80 100

Design A

Design BTor

que

(% o

f rat

ed)

Speed (% of rated)

Design CDesign D

Figure 4 — Typical speed-torquecharacteristics for Design A, B, C, and Dmotors.

Nf

Ps = 120

sN N

Ns a

s

=−( )100

rated speed, but efficiency is low.Single-phase motors — These

motors are commonly fractional-horsepower types, though integralsizes are generally available to 10 hp.The most common single phase motortypes are shaded pole, split phase, capacitor-start, and permanent split capacitor.

• Shaded pole motors have a con-tinuous copper loop wound around asmall portion of each pole, Figure 5.The loop causes the magnetic fieldthrough the ringed portion to lag be-hind the field in the unringed portion.This produces a slightly rotating fieldin each pole face sufficient to turn therotor. As the rotor accelerates, itstorque increases and rated speed isreached. Shaded pole motors have lowstarting torque and are available onlyin fractional and subfractional horse-power sizes. Slip is about 10%, ormore at rated load.

• Split phase motors, Figure 6, useboth a starting and running winding.The starting winding is displaced 90

electrical degrees from the runningwinding. The running winding hasmany turns of large diameter wirewound in the bottom of the statorslots to get high reactance. Therefore,the current in the starting windingleads the current in the runningwinding, causing a rotating field.During startup, both windings areconnected to the line, Figure 7. As themotor comes up to speed (at about25% of full-load speed), a centrifugalswitch actuated by the rotor, or anelectronic switch, disconnects thestarting winding. Split phase motorsare considered low or moderate start-

power factor than other capacitordesigns. This makes them wellsuited to variable speed applica-tions.

Synchronous motorsWithout complex electronic con-

trol, synchronous motors are inher-ently constant-speed motors. Theyoperate in absolute synchronismwith line frequency. As with squir-rel-cage induction motors, speed isdetermined by the number of pairs

of poles and the line frequency.Synchronous motors are available

in subfractional self-excited sizes tohigh-horsepower direct-current-excited industrial sizes. In the fractional horsepower range, most synchronous motors are used whereprecise constant speed is required. Inhigh-horsepower industrial sizes, thesynchronous motor provides two im-portant functions. First, it is a highlyefficient means of converting ac en-ergy to work. Second, it can operate atleading or unity power factor andthereby provide power-factor correction.

There are two major types of syn-chronous motors: nonexcited and di-rect-current excited.

Nonexcited motors — Manufac-tured in reluctance and hysteresis de-signs, these motors employ a self-starting circuit and require noexternal excitation supply.

• Reluctance designs have ratingsthat range from subfractional toabout 30 hp. Subfractional horse-power motors have low torque, andare generally used for instrumenta-tion applications. Moderate torque,integral horsepower motors use squir-rel-cage construction with toothed ro-tors. When used with an adjustablefrequency power supply, all motors inthe drive system can be controlled atexactly the same speed. The powersupply frequency determines motoroperating speed.

• Hysteresis motors are manufac-tured in subfractional horsepowerratings, primarily as servomotors andtiming motors. More expensive thanthe reluctance type, hysteresis motorsare used where precise constantspeed is required.

DC-excited motors — Made insizes larger than 1 hp, these motorsrequire direct current suppliedthrough slip rings for excitation. The

ing torque motors and are limited toabout 1/3 hp.

• Capacitor-start motors are simi-lar to split phase motors. The maindifference is that a capacitor is placedin series with the auxiliary winding,Figure 8. This type of motor producesgreater locked rotor and acceleratingtorque per ampere than does the splitphase motor. Sizes range from frac-tional to 10 hp at 900 to 3600 rpm.

• Split-capacitor motors also havean auxiliary winding with a capacitor,but they remain continuously ener-gized and aid in producing a higher

A334 1997 Power Transmission Design

Figure 5 — Rings in shaded-pole motordistort alternating field sufficiently tocause rotation.

Figure 6 —Split-phase windings in a two-pole motor. Starting winding and runningwinding are 90 deg apart.

Figure 7 — Split-phase start inductionmotor.

Figure 8 — Capacitor-start motors aresimilar to split-phase motors.

1997 Power Transmission Design A335

direct current can be supplied from aseparate source or from a dc genera-tor directly connected to the motorshaft.

Synchronous motors, either singleor polyphase, cannot start without be-ing driven, or having their rotor con-nected in the form of a self-startingcircuit. Since the field is rotating atsynchronous speed, the motor mustbe accelerated before it can pull intosynchronism. Accelerating from zerorpm requires slip until synchronismis reached. Therefore, separate start-ing means must be employed.

In self-starting designs, fractionalhorsepower motors use methods com-mon to induction motors (split phase,capacitor-start, and shaded pole). Theelectrical characteristics of these mo-tors cause them to automaticallyswitch to synchronous operation.

Although dc-excited motors have asquirrel-cage for starting, the inher-ent low starting torque and the needfor a dc power source requires a start-ing system that provides full motorprotection while starting, applies dcfield excitation at the proper time, re-moves field excitation at rotor pull-out, and protects the squirrel-cagewindings against thermal damage un-der out-of-step conditions.

SERVOMOTORSThese motors are used in closed

loop control systems in which work isthe control variable, Figure 9. Thedigital controller directs operation ofthe motor by sending velocity com-mand signals to the amplifier, whichdrives the motor. An integral feed-back device (resolver) or devices (en-

DC servomotors

DC servomotors are high perfor-mance motors normally used as primemovers in numerically controlled ma-chinery or other applications wherestarts and stops must be madequickly and accurately. They havelightweight, low inertia armaturesthat respond quickly to excitationvoltage changes. In addition, very lowarmature inductance in these motorsresults in a low electrical time con-stant (typically 0.05 to 1.5 ms) thatfurther sharpens motor response tocommand signals.

DC servomotors are manufacturedin permanent magnet, printed circuit,and moving coil (or shell) types. Eachof these basic types has its own char-acteristics, such as physical shape,costs, shaft resonance, shaft configu-ration, speed, and weight. Althoughthese motors have similar torque rat-ings, their physical and electrical con-stants vary considerably. The perfor-mance of the servomotor is asdependent on the control scheme usedas much as the inherent characteris-tics of the motors themselves.

Brushless dc servomotors Brushless dc motors resemble a dc

shunt motor turned inside out. Per-manent magnets, located on the rotor,or a wound rotor excited by dc voltagethrough slip rings, requires that theflux created by the current carryingconductors in the stator rotate aroundthe inside of the stator in order toachieve motor action. The rotatingfield is obtained by placing three sta-tor windings around the interior ofthe stator punching. The windingsare then interconnected so that intro-ducing a three-phase excitation volt-age to the three stator windings(which are separated by 120 electricaldegrees) produces a rotating magneticfield. This construction speeds heatdissipation and reduces rotor inertia.

The permanent magnet poles onthe rotor are attracted to the rotatingpoles of the opposite magnetic polar-ity in the stator creating torque. As inthe dc shunt motor, torque is propor-tional to the strength of the perma-nent magnetic field and the field cre-ated by the current carryingconductors. The magnetic field in thestator rotates at a speed proportionalto the frequency of the applied voltage

coder and tachometer) are either in-corporated within the motor or are re-motely mounted, often on the load it-self. These provide position andvelocity feedback that the controllercompares to its programmed motionprofile and uses to alter its velocitysignal. The motion profile is a set ofinstructions programmed into thecontroller that defines the operationin terms of time, position, and veloc-ity. The ability of the servomotor toadjust to differences between the mo-tion profile and feedback signals de-pends greatly upon the type of con-trols and motors used. See theControls and Sensors Product Depart-ment.

Three basic types of servomotorsare used in modern servosystems: ac,based on induction motor designs; dc,based on dc motor designs; and acbrushless, based on synchronous mo-tor designs.

AC servomotorsThese are basically two-phase, re-

versible, induction motors modifiedfor servo operation. They are used inapplications requiring rapid and ac-curate response characteristics. Toachieve these characteristics, theseinduction motors have small diame-ter, high resistance rotors. The smalldiameter provides low inertia for faststarts, stops, and reversals. High re-sistance provides nearly linear speed-torque characteristics for accuratecontrol.

An induction motor designed forservo use is wound with two phasesphysically at right angles or in spacequadrature. A fixed or referencewinding is excited by a fixed voltagesource, while the control winding isexcited by an adjustable or variablecontrol voltage, usually from a ser-voamplifier. The windings are oftendesigned with the same voltage/turnsratio, so that power inputs at maxi-mum fixed phase excitation, and atmaximum control phase signal, are inbalance.

The inherent damping of servomo-tors decreases as ratings increase,and the motors are designed to have areasonable efficiency at the sacrificeof speed-torque linearity.

Induction type servomotors areavailable in fractional and integralhorsepower sizes.

Figure 9 — Typical dc servosystem witheither encoder or resolver feedback. Someolder servosystems use a tachometer andencoder for feedback.

and the number of poles.The rotor rotates in synchronism

with the rotating field, thus the name“synchronous motor” is often used todesignate motors of this design. Morerecently, this motor design has beencalled an electrically commutated mo-tor (ECM) due to its similarity to thedc shunt motor. In the dc shunt mo-tor, the flux generated by the currentcarrying winding (rotor) is mechani-cally commutated to stay in positionwith respect to the field flux. In thesynchronous motor, the flux of thecurrent carrying winding rotates withrespect to the stator; but, like the dcmotor, the current carrying flux staysin position with respect to the fieldflux that rotates with the rotor. Themajor difference is that the syn-chronous motor maintains position byelectrical commutation, rather thanmechanical commutation.

STEP MOTORSA step motor is a device that con-

verts electrical pulses into mechani-cal movements. Conventional motorsrotate continuously, but a step motor,when pulsed, rotates (steps) in fixedangular increments.

Step size, or step angle, is deter-mined by the construction of the mo-tor and the type of drive scheme usedto control it. Traditionally, step reso-lution has ranged from 90 degrees(four steps per rev) to a fraction of adegree, though 15 degrees (12 stepsper rev), to 1.8 degrees (200 steps perrev) has been most common. More re-cently, however, microstep motorshave been introduced that are capableof .0144 degree steps (25,000 steps perrev). Microstep motors are hybrid 200step per rev motors that are electri-cally controlled to produce 25,000steps per rev.

Step motors are usually used inopen loop control systems, Figure 10,though an encoder may be used toconfirm positioning accuracy.

stator design, two, three, or four phasewindings may be used.

Microstep motorsMicrostep motors are usually hy-

brid motors. The rotor consists of anarbor, bearings, and one, two, or threesets, or stacks, of toothed cylindricalmagnets. The toothed stator is woundso that alternate poles are driven bytwo separate phase currents. This re-sults in a 200 step per rev motor that,when the two windings are energizedproportionately, enable the motor tomake 125 intermediate steps betweeneach full step. Thus, using digitallogic control and bipolar pulse widthmodulation, the motor makes 25,000microsteps per rev.

MINIATURE MOTORSFilling the needs for a wider variety

of power devices, new miniature mo-tors are announced every day. Manyare less than 1/2-in. diam, even with agearhead, Figure 12. Typical applica-tions include medical systems, semi-conductor manufacturing, laser cutting systems, surface-mount as-sembly systems, winding machines,robotic handling equipment, microm-eter positioners, motorized poten-tiometers, screwdrivers, scales,

aircraft actuators, computer pe-ripherals, bar code scanners, andmany others.

To maximize the power-to-weight ratio, many miniature mo-tors are built with more costly buthigher strength magnets. Also, forreduced susceptibility to electricalnoise, some are designed with rela-tively low inductance.

Manufacturers often indicatemotor size with a Size designationof the diameter in 1/10s of an inch.Thus, a Size 30 motor has an out-side diameter of 3 in.

For feedback, manufacturers offera range of miniature optical and mag-

There are many types of step-motorconstruction. However, permanentmagnet (PM) and variable reluctance(VR) are the most common types.

PM step motorsThe permanent magnet step motor

is also referred to as a synchronous in-ductor motor. It moves in steps whenits windings are sequentially ener-gized, or it can operate as a low speedsynchronous motor when operatedfrom a two-phase ac power source.Figure 11 illustrates a permanentmagnet rotor surrounded by a two-phase stator. Two rotor sections (Nand S) are offset by one half toothpitch to each other. As energy isswitched from phase 2 to phase 1, aset of rotor magnets will align withphase 1, and the rotor will turn onestep. If both phases are energized si-multaneously, the rotor will establishits equilibrium midway betweensteps. Thus, the motor is said to behalf-stepping.

VR step motorsThe variable reluctance (also termed

“switched reluctance”) step motor isalso constructed with a toothed rotorand stator. There are not, however, anymagnets in the rotor. Depending on the

A336 1997 Power Transmission Design

Figure 10—Typical step motor system. Precise step systems have feedback loop (dotted line) using encoders or resolvers.

Figure 11 — Step motor with permanentmagnet rotor.

1997 Power Transmission Design A337

netic encoders, ac and dc tachome-ters, and resolvers. Some companiessupply line-driver encoders for elec-trically noisy environments and forinstallations with long distances be-tween the encoders and the motors.

Other miniature components of-fered for motion systems includeminiature brakes, controls, and servosystems with appropriate software.

Size vs. capabilitySome motors with diameters of less

than 1 in. offer over 100 W. Other mo-tors, such as a 48 Vdc motor, offerspeeds to 100,000 rpm.

Not all the miniature motors arerotary devices. Miniature linear stepmotors are used in applications thatrequire precise linear positioning. Forsome of these motors, resolutionranges from 0.008 to 0.000125 in. perfull step.

by the gear train, both must stop inthe same time. In severe cases, mo-mentary power failure may be all thatis necessary for a high-inertia load todestroy the gear train.

Overhung loads are applied to theoutput shaft of the gearmotor when-ever the gearmotor is connected to theapplication by belts, chains, or gear-ing. Applications requiring cams,hoisting drums, or switches at theoutput shaft can also cause very highoverhung loads on gearhead bearings.

It is inherent in gearmotors thatoverhung load capacity decreases asdelivered torque increases.

MOTOR ENCLOSURESNEMA standards MG1-1.25, 1-

1.26, and 1-1.27 define more than 20types of enclosures under the cate-gories of open machines, totally en-closed machines, and machines withencapsulated or sealed windings.

Open machines — Very few openmachines are made today. Most stan-dard motors are drip-proof andguarded, Figure 15, or at least semi-guarded. To improve the protectionoffered by drip-proof enclosures, somemanufacturers offer special processesor treatments to protect the windings

and bearings.Totally enclosed ma-

chines — Most totally en-closed machines are fan-cooled, Figure 16. However,some models in fhp ratingsare totally enclosed andnonventilated. Totally en-closed motors are more ex-pensive, but offer betterprotection. Outside venti-

lating air and contaminants are ex-cluded from interior parts of a totallyenclosed motor. The motor is not air-tight, but usual entrances to the inte-

Slightly different

Several miniature motor manu-facturers use motor designs thatdiffer from larger motors. For ex-ample, one design uses a hollow,ironless-core, rotor design, Figure13, for low inertia and fast re-sponse. The rotor fits over a sta-tionary magnet system. Only the

copper coil moves, unlike larger mo-tors where the whole coil and magnetsystem move. Such a design is 38 mmin diameter and is rated for 100 W.

GEARMOTORSA gearmotor, Figure 14, consists of

a motor and speed reducer in an inte-gral assembly. The motor portion ofthe assembly can be either dc or ac-powered. The speed reducer portioncan use spur, helical, or worm gears.Gearmotor sizes range from fractionalto above 200 hp.

The main advantage of a gearmotoris that the driving shaft can couple di-rectly to the driven shaft, thus in manysituations making belts, chains, or sep-arate speed reducers unnecessary.Also, because the motor and speed re-ducer are aligned during manufacture,field installation is simpler.

Starting and running torque mustbe considered separately becausestarting characteristics of the motorand gearing differ. Applications need-ing high breakaway torque requirecareful selection of the motor—split-phase polyphase, capacitor-start, andbrush types have high starting torque.

The gearmotor manufacturershould analyze any application withhigh inertia. This problem is espe-cially important with self-lockingright-angle gearmotors. Because therotor and load are rigidly connected

Figure 14 — Typical gearmotor in right-angle configuration.

Figure 12 — Typical of miniature motors,this dc motor is 0.511 in. in diam and 1.24in. long. It has an output power rating of2.5 W at 8,000 rpm. The constructionconsists of Neodymium magnets, preciousmetal brushes, and a moving-coil rotorthat uses a Rhombic-wound winding formaximum winding density. A matching0.511-in. planetary gearhead offers fiveratios from 4:1 to 1,118:1 and torquecapability to 0.35 Nm continuous and 0.53Nm intermittent.

Magnet Coreless rotor Housing

Shaft

Figure 13 — Some miniature motors use a coreless motor design, also referred to asironless core or moving coil. The concept replaces a rotating iron armature wound withcopper wire with a light weight copper coil. This coil rotates around a stationary magnetsystem.

rior, such as the conduit box, are gas-keted. Clearances like those aroundshafts are kept as small as possible.

Explosion-proof machines —Hazardous atmospheres require spe-cial totally enclosed motors. Motorsfor these atmospheres are designed tostandards established by Underwrit-ers Laboratories (UL). Only after amotor has been examined and ap-proved by UL can it be sold as an ex-plosion-proof motor.

Washdown — For those applica-tions that are subjected to washdownswith high-pressure liquid cleaners re-quire more features to protect the mo-tor from severe conditions. These aretypically encountered in such indus-tries as food processing plants anddairies. The motors are housed in asteel enclosure that is covered with anFDA approved epoxy paint or in astainless-steel enclosure. Seals at ev-ery opening are selected to prevent flu-ids from entering the motor as well asout of the bearings. Inside, all partsare covered with epoxy paint or othermaterial to retard corrosion, Figure 17.

Bakery service — For the bakingindustry, motors similar to the whitewashdown motors are rated for ser-vice according to Baking Industry

ponents — a rotor, stator and feed-back assembly, Figure 18, which areinstalled as an integral part of a ma-chine, Figure 19.

Frameless motors typically havecontinuous torque ratings from 100lb-in. to 10,000 lb-in. and speeds from300 to 20,000 rpm.

Advantages include:• Increased rigidity (stiffness).

Overall machine stiffness is depen-dent on the cumulative stiffness of allmechanical elements between the mo-tor rotor and load. These typically in-clude belts, pulleys, gear sets or gear-boxes, and couplings. In manyapplications, these can be eliminated

Sanitation Standards Committee(BISSC) requirements. Every bend isfilleted to prevent material build-upin a sharp bend, fold, or crack. Typi-cally the same internal and externalprotection is provided as is suppliedwith washdown motors, and the finalfinish is a glossy, smooth white.

Information on washdown andBISSC motors is excerpted from an ar-ticle by Baldor in the March 1994 is-sue of PTD.

Frameless motors — Frequentlydesigned into machine tools to powerhigh-speed spindles, frameless ac mo-tors are now finding acceptance inother machines that have space con-straints or unusual mounting requirements.

Recent technological advance-ments — new cooling techniques,more precise feedback devices,and expanding use of CAD —are extending the popularityof frameless motors intomore registration-orientedoperations such as paperconverting and printing.

Construction of framelessac motors, electrically, is nodifferent than conventionalpermanent-magnet and inductionmotors. The frameless designs can becontrolled by the same drives as thoseused to control frame-type motors.

Mechanically, however, framelessmotors are a horse of a different color.They are delivered as individual com-

A338 1997 Power Transmission Design

Figure 15 — Typical drip-proof andguarded enclosure.

Figure 16 — Typical totally enclosed, fan-cooled enclosure.

Figure 17 — Washdown duty motors include white dc motor (left), three-phase motor(right), single-phase motor (top) and the stainless steel motor in the foreground.

Figure 18 — In a frameless motor, thecasing around the stator windings providesmechanical stability for the windings and apath for transmitting winding heat to thecooling medium — typically a liquid thatcirculates through the finned structure.

1997 Power Transmission Design A339

by direct coupling.Moreover, shaft stiffness is a func-

tion of the cube of the shaft diameter.On frameless motors, the shaft diam-eter can be approximately three timesgreater than that of a conventionalmotor, thus giving 27 times morestiffness.

• Versatile motor cooling methods.Among them, fluid cooling enables acompact motor to deliver high power.

• Shafting options. Units can in-corporate a shaftless rotor with a borethrough it or with a hollow shaft. Thisdesign offers machine builders thefreedom to configure a variety ofshafting options. For example, mate-rial or process fluids can pass directlythrough a hollow shaft.

• Tailored bearing structure. Ma-chine builders can tailor bearingstructures to the precise needs of eachmachine. High-speed applications,such as spindles, can use a structurethat minimizes bearing heat. Low-speed applications, such as rotary ta-bles, can be designed to handle largeradial or axial loads.

• Compact design. Frameless mo-tors can be one-seventh the volume ofconventional framed motors with thesame power rating,

Are they for you? Although frame-

less motors offer many benefits, theyare not the right solution for every ap-plication. For example, if you arebuilding one or two machines, it willprobably be a waste of your time to de-sign a frameless motor into a ma-chine. Generally, doing so takes 50 to100 machines to pay for the initial de-sign investment.

Information on frameless motors isexcerpted from an article by Indramatin the November 1995 issue of PTD.

MOTOR PROTECTIONMotor longevity can often be im-

proved by protecting the motor fromoverheating and from fluctuations inthe incoming power distribution system.

OverheatingMotors can be protected from over-

heating by two basic methods — indi-rect and direct. Each offers advan-tages. In some cases, both methodsshould be used to assure maximumprotection.

Indirect method — the most com-mon method of three-phase motorprotection simulates the internal mo-tor conditions by sensing current to

the motor and the general condi-tions surrounding the motor.

• Thermal overload relays,Figure 20, are the most fre-quently used method of indirectprotection. In operation, motorcurrent passes through theheater in each overload relay(three relays are used for three-phase motors). An increase inmotor load increases currentboth in the motor and in the re-lay’s heater. If heater tempera-ture reaches a predeterminedpoint, the overload contacts open,turning off the motor.

• Magnetic overload relayscontain a fluid dashpot to retardthe trip time and approximatethe allowable motor heatingcurve. However, magnetic unitsare unaffected by cumulative mo-tor heating though thermal over-load relays are. Also, magneticunits are not ambient-tempera-

ture compensated.• Differential current systems usu-

ally serve motors of 500 hp or more,and use three current transformers.Each transformer senses the sum ofthe currents flowing through twowires of the same phase. This sum iszero during normal operation. If afault occurs—such as shorted turns orvoltage imbalance—the sum of thecurrents through one or more currenttransformers becomes significant.The control scheme senses this condi-tion and turns off the motor or pro-vides a warning signal.

Direct method — Used for pro-tecting small single-phase motors upto high-horsepower units, directmethods of motor protection sense thetemperature within the motor. Vari-ous methods offer different levels ofprotection. Almost all direct methodsdo a better job of protecting the statorthan the rotor. That is the reason forsometimes using more than onemethod, as discussed previously.

FeedbackSensor

RearBearings

CoolantOutlet

CoolantInlet

Shaft

MotorHousing

StatorWinding

Rotor WindingSupportandCoolingJacket

FrontBearings

Figure 19 — A complete frameless motor assembly includes a rotor, stator, andfeedback sensor supplied by the motor manufacturer. The machine manufacturerprovides the bearings, shaft, housing, and cooling medium.

Figure 20 — Typical control circuit withNC overload relay contact in series withmotor starter coil.

• Inherent is the simplest of the di-rect methods. Its device mounts in themotor end shield. Used in motorsfrom fractional through 10 hp, an in-ternal bimetal strip responds to sur-rounding temperature — the result ofambient temperature, internal motortemperature, internal motor heating,and the heat dissipated by a resistiveheater through which current passes.The bimetal strip operates one orthree contacts, depending on motorrequirements.

• Thermostats are also made withbimetal-operated, snap-action con-tacts. One or more thermostats in-stalled on motor-winding end turnssense true winding temperature.However, the unit is insulated fromthe winding and has significant massto heat. This combination induces athermal lag that makes the devicesuitable for sensing slowly changingtemperatures. Thermostats are oftencombined with thermal overload re-lays, which offer better protectionagainst locked rotor conditions.

Voltage surge protectionA voltage spike, or surge, can dam-

possibly causing insulation failure. Withcapacitors installed, the rate of rise is re-duced and the surge is more evenly dis-tributed across the entire winding.

Programmable unitsProgrammable motor protection

units are designed to protect largemotors, generally of 500 hp or more,and motors that are essential in a sys-tem or process.

Typically, the operator or program-mer inputs specific motor data, suchas full-load and locked-rotor time, al-lowable acceleration time, allowablenumber of starts in a specified time,and several other electrical parame-ters, plus allowable winding andbearing temperature.

The programmer can also establishwhat action the unit will take in theevent of a fault—indicate an alarm,shut down the motor, or initiate an or-derly process shutdown.

For efficient energy management,some programmable units give in-strument outputs to indicate lapsedrunning time, energy consumption,power factor, and power.

MOTOR EFFICIENCIESThe Energy Act of 1992 defines new

efficiency levels for several electricaldevices including motors. By 1997,however, motor manufacturers will nolonger be able to offer standard-effi-ciency motors. It is assumed that asmotors wear out, replacement motorswill be the high-efficiency offerings.

High-efficiency motors are built toreduce motor energy loss. Improve-ments in several areas, Figure 21, in-crease motor service life:

• Larger-diameter wire, increasingthe volume of copper by 34 to 40%.This change reduces copper losses thatresult naturally from current passingthrough the copper-wire windings.

• Larger wire slots to accommodatelarger wire. This reduces the amountof active steel in each steel lamination.

• Longer rotor and stator core tocompensate for the loss of steel and theresultant need to add more laminations.

• High-grade silicon steel lamina-tions approximately 0.018 in. thick,having an electrical loss of 1.5 W/lb.The chemical makeup and thinnergage of the laminations, plus a coatingof inorganic insulation on each piece,reduce eddy current losses. Special an-

age a motor almost instantly. Light-ning can cause severe motor damageeven if transformers and lightning ar-restors are installed on the incomingdistribution system. Power compa-nies frequently use power-factor cor-recting capacitors in distribution sys-tems. If these capacitors are switched,large voltage transients can be gener-ated, especially if proper snubbers arenot installed. Insulation failures inthe electrical system other than in themotor can produce voltage spikes fivetimes the normal line-to-ground crestvalue. With these possibilities forvoltage spikes, it is imperative thatproper transient suppression net-works be installed on equipment es-sential to an operation and on motorsoperating at 2300 V or more. For moreon a specific type of voltage spike, re-flected wave phenemenon, see thebox, “Reflected wave produced by A-Sdrives.” Two types of protection arecommonly used — lightning arrestorsand surge capacitors.

• Arrestors limit the maximumvoltage across the motor terminals.

• Capacitors limit the rate of voltagerise. If this rate is not limited, the firstfew motor windings must absorb a surge,

A340 1997 Power Transmission Design

Insulated gate bipolar transistors(IGBTs) let adjustable-speed drivesturn voltage on and off 18,000 to20,000 times a second. To do thismeans the voltage rise time is short,usually less than a microsecond.These short rise times combined withlong power lines between the driveand controller can produce voltagereflections, also called reflectedwaves that have high peak voltages.If the voltages are large enough, theywill generate potentially destructivestresses in the motor insulation. Thisphenomenon is not widespread, butusers should be aware of it.

Reflected wave phenomenon pri-marily affects 460-V and 575-V,IGBT-based drives. Generally, 230-Vapplications are unaffected becausethe reflected wave amplitudes arelow while the typical motor insula-tion is the same as a 460 V-motor.

The fast switching capabilities ofvarious drive switching devices, es-pecially IGBTs; long cable lengthsbetween motors and drives; and mis-matched surge impedance between

the cable and the motor contributeto the existance of this phenemenon.In addition, the insulation processfor motor windings may leave micro-scopic voids in the coating. Theseholes can be insulation failurepoints when voltage peaks are im-pressed on the stator winding by areflected wave, since as much as 60to 80% of the voltage can be dis-tributed across the first turn of themotor winding.

At this time, conclusive data is notavailable to determine the exactcause of insulation failure; motormanufacturers are split on what situ-ations result from the reflected wavephenomenon. But there are solutions.

• Provide a motor with insulationthat can withstand the typical ampli-tudes of reflected waves.

• Keep cable length short. • Install a filter at the drive output. • Install a terminating device that

will keep the amplitude of the reflectedwave below potentially destructive levels.

Reflected wave produced by A-S drives

1997 Power Transmission Design A341

nealing and plating of rotor and statorcomponents and use of high-puritycast aluminum rotor bars reduce hys-teresis losses.

• Higher-grade bearings reducesfriction loss.

• Smaller, more efficient designs re-duce windage losses in fan-cooled motors.

• Tighter tolerances and morestringent manufacturing-process con-trol reduce losses from unplannedconducting paths and stray load phenomena.

These design changes also result incooler running motors. Cooler opera-tion lengthens a motor’s service life intwo ways. For every 10 C reduction intemperature, motor insulation lifedoubles; high-efficiency motors tend tooperate 10 to 20 C cooler than stan-dard-efficiency motors.

The motors that must meet the newefficiency levels include: All motorsmade in or exported to the U.S., includ-ing all motors sold as part of a piece ofequipment; general purpose motors;motors rated from 1 - 250-hp; T-Frame; single speed; foot mounted;polyphase; NEMA designs A & B; con-tinuous rated; 230/460 volts; constant60-Hz frequency.

Exceptions include special purposeand definite purpose motors.

Efficiency ratings will be on the mo-

application, are considered the pri-mary causes of bearing currents, es-pecially in motors in the hundreds ofhorsepower. These imbalances aretypically caused by some form of non-uniform magnetic flux paths.

Harmonics in the voltage suppliedto the motor are becoming more com-mon because more facilities are gen-erating their own power, more motorsare powered by adjustable-speeddrives, and more drives now use IGBTrather than SCR and GTO technolo-gies in the drive inverter section. Asharmonic content in an adjustable-speed motor-drive system increases,motors that previously had no prob-lem with shaft currents may begin todevelop rapid bearing failures.

This shaft voltage seeks a completecircuit through its two bearings toground, or through its outboard bear-ing and the connected machinery. Un-less prevented from reaching highlevels, over 0.5 V, it can cause chemi-cal changes in the insulating grease,breaking it down and thereby makingthe grease act like an electrolytic in acapacitor.

When a motor operates on sinu-soidal power, a safe low level for volt-age along the length of the shaft isless than 0.1 V. If a motor operates offan adjustable-speed power, high-fre-quency transient-voltage spikes cancause this voltage to measure appre-ciably higher.

Determining the problem There are two methods to deter-

mine if bearing currents are the causeof unexpected motor bearing failure:measure the shaft voltage or examinethe bearings.

All motors have some level of shaftvoltage. Above a certain level, shaftvoltage is a failure indicator. Gener-ally end-to-end shaft voltage shouldbe less than 0.5 V. Normally, voltagelevels below this will not cause harm-ful bearing currents. Engineers canmeasure the shaft voltage with anyvoltmeter that has an impedance of10,000 ohms per volt or more.

When examining the bearings, lookfor specific types of damage. Bearingdamage results when current is bro-ken at the contact surfaces betweenrolling elements and raceways, Figure22. The size of the damage points de-pends on the magnitude of the inducedvoltage, the impedance of the current

tor nameplate. A recent efficiency la-beling standard requires that pre-mium-efficiency motors carry theNEMA nominal efficiency rating onthe label.

INDUCED BEARING CURRENTSProblems produced by electric cur-

rents passing through the bearings inan ac motor (expecially motors ratedin the hundreds of horsepower) hasbeen recognized since the 1920s. To-day, however, adjustable-speed (A-S)drives and plants generating theirown power are putting a modern twistto this problem that significantlyshortens bearing life.

Causes of induced shaft voltages

Magnetic imbalances and harmon-ics in the power line are the primarycauses of induced shaft voltages that,in turn, produce damaging bearingcurrents. Other causes include im-properly grounded electric arc weld-ing and static electricity from manu-facturing processes, such as pumpingor compressing applications.

Magnetic imbalances, which areproduced by the motor design or its

Figure 21 —Highefficiency motors

are built withlarger-diameter

wire, longer rotors,silicon steel

laminations andother features.

path, and the bearing type. The earlyobservable effects of this damage onthe bearings is pitting and fluting.

Figure 23 shows a series of electri-cal pits in a roller and in a raceway ofa spherical roller bearing.

More serious electrical damage oc-curs when current passes during pro-longed periods and the number of in-dividual pits accumulates. The result

Solutions

There are three ways of solving bear-ing current in any size motor: mechani-cal shaft grounding, harmonic suppres-sion, and insulating the bearings.

Some third party vendors have at-tached devices to the shaft with con-tacting elements that ground it. Thisis a workable solution, but not theonly one or necessarily the best, ex-cept for field retrofits.

To control the harmonic content inthe power systems, several manufac-turers offer line-to-line and line-to-ground sine-wave output and com-mon-mode filters for theiradjustable-speed drives These filtersreduce harmonics, audible noise, mo-tor temperature, and vibration, andthey increase the life of the motor andits windings. The cost of the filters aswell as their tendency to reduce mo-tor-drive efficiency because of extraheat generation determine whetherthis solution is workable.

Most standard motors in 440frames and smaller do not have insu-lated bearings. If bearing currentscause a problem, your motor manu-facturer can provide motors with in-sulated bearings. That is, place an in-sulating material between thebearing mounting ring and the frameof the motor, or between the shaft andthe bearing. Any object in contactwith the bearing ring — pipes, tubes,washers, and the bolts used to mountthe bearing ring to the frame — mustalso be insulated.

Insulated bearings are a commonoption on motors of 580 frame andlarger. And it is generally accepted, inthe U.S. at least, that adjustable-speed drives and motors above NEMA440 frames should have insulatedbearings to prevent the flow of shaftcurrents.

It is only necessary to insulate oneoutboard bearing to open the circuit.Special provisions are necessary incertain cases to maintain an open cir-cuit to the load or another motor, suchas insulted drive couplings in largetandem motor arrangements.

Insulating the bearing will noteliminate voltage on the shaft, but itwill prevent that voltage from usingthe bearings as a current path.

Excerpted from an article bySiemens in the May 1996 issue ofPTD. ■

is fluting, Figure 24. Fluting in anti-friction bearing races specifically in-dicates the problem is bearing cur-rents. Fluting can occur in ball orroller bearings and develop consider-able depth, producing noise and vi-bration and eventual fatigue from lo-cal overstressing. Once fluting isstarted, it is self-perpetuating untilthe bearing fails.

A342 1997 Power Transmission Design

Figure 24 — Fluting, an accumulation of pits, is a sure indicator of bearing currents.Once started, fluting is self-perpetuating until the bearing fails.

Figure 22 — When current is broken at the contact surface between the rolling elementsand the raceways, it produces arcing damage.

Figure 23 — Electrical pits are another indication of bearing currents. If left unchecked,this will eventually result in fluting.