permanentmagnetDC

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Permanent Magnet DC motorsare useful in a range of applica-tions, from battery powered de- vices like wheelchairs and powertools, to conveyors and door open-ers, welding equipment, X-ray andtomographic systems, and pump-ing equipment, to name a few.They are frequently the best solu-tion to motion control and powertransmission applications wherecompact size, wide operating

speed range, ability to adapt to arange of power sources or thesafety considerations of low volt-age are important. Their ability toproduce high torque at low speedmake them suitable substitutesfor gearmotors in many applica-tions. Because of their linearspeed-torque curve, they particu-larly suit adjustable speed andservo control applications wherethe motor will operate at less than5000 rpm

Inside these motors, perma-

nent magnets bonded to a flux-re-turn ring replace the stator field windings found in shunt motors. A wound armature and mechanicalbrush commutation system com-plete the motor.

The permanent magnets supply

the surrounding field flux, elimi-nating the need for external fieldcurrent. This design yields asmaller, lighter, and energy effi-cient motor.

Armature interaction

Unlike a shunt wound dc motor,a PM motor is free of interactionbetween the permanent magnetfield and the armature demagnetiz-

ing cross field. Shunt wound DCmotors experience significant in-teraction between the armatureand the stator. The stator’s low re-luctance (high permeability) ironcore ultimately weakens the fieldas the load increases. The result isa dramatic drop in the speed-torque characteristics at somepoint.

The PM motor’s field has a highreluctance (low permeability) thateliminates significant armature in-teraction. This high reluctance

yields a constant field, permittinglinear operation over the motor’sentire speed-torque range. In oper-ation with a constant armature voltage, as speed decreases, avail-able torque increases, Figure 1. Asthe applied armature voltage in-creases, the linear speed-torquecurves shift upwards. Thus, a seriesof parallel speed-torque curves, fordifferent armature voltages, repre-

Applying PMDC motors

When you need high starting/acceleration torque, predictable motor speed properties, compact size, and energy efficiency,

Permanent Magnet DC (PMDC) motors may be the solution. Proper application will help you get the most out of them.

RONALD BULLOCK, Bison Gear & Engineering Corp.

POWER TRANSMISSION DESIGN MAY 1995 33

PRODUCT FOCUS: MOTORS

Ronald Bullock is President of Bison

Gear & Engineering Corp., Downers

Grove, Ill.

S p e e d

Shunt

PM

Torque

Torque

S p e e d

Current

I 4 s t a l l I 3 s t a l l I 2 s t a l l

I 1 s t a l l

IFL V1 V2 V3 V4

V1 < V2 < V3 < V

Figure 1 — The high reluctance of PMDC motorsprevents the torque drop off common with

shunt-wound motors.

Figure 2 — As applied armature voltage increasesin a PMDC motor, the linear speed-torque curves

shift upwards.



sents the speed-torque properties of aPM motor, Figure 2. Speed is propor-tional to voltage and torque is propor-tional to current.

Speed control methods

Speed is controlled by varying the volt-age applied to the armature. Feedbackdevices sense motor speed and send thisinformation to the control to vary its out-put voltage up or down to keep speed at

or near the set value. Feedback tech-niques include voltage tachometers, opti-cal encoders, electromagnetic pulse gen-erators, and back emf monitoring.

Regulation is the ability of the motorand control system to hold speed con-stant over the torque range. It extendsfrom 0.1% for highly divided (e.g., 1000div/rev) optical encoders to 5% for a sim-ple back emf system. Most manufacturersprovide the servo constant data for pre-dicting system response.

Form factorIn practice, the voltage used to power

a PM motor is not a pure DC. It is derivedDC voltage formed by rectifying an AC voltage. Thus, the DC drive voltage has a wave or ripple component that is relatedto the frequency of the AC input.

Form factor, which is the ratio of Irms

to Idc, indicates how close the driving voltage is to pure DC. By definition, formfactor for a pure DC source, such as a bat-tery, is 1.0. For a power source, thehigher the form factor is above 1.0, themore it deviates from pure dc. Table A

shows typical form factors for commonly used voltage sources.

Most manufacturers of PMDC motorsrecommend that form factor not exceed1.4 for continuous operation. Half waverectification is also not recommendedbecause it increases the form factor.

Driving a motor with a higher form fac-tor control than intended can cause pre-mature brush failure and excessive inter-nal heating. If you use a control with a

high form factor, you may need specialbrushes and commutators, a high tem-perature insulation system, or a larger

motor. It may cost more, but a controlthat reduces form factor can reduce heat-ing effects in the motor.

Because PM motors lack armature in-teraction, they can generate high mo-mentary starting and accelerationtorques, typically 10 to 12 times full ratedtorque. Thus, they suit applications re-quiring high starting torques or momen-tary bursts of power. However, they arenot intended for continuous operation atthe high levels of torque they can pro-duce. This can cause overheating, whichcan result in non-reversible demagneti-

zation of the field magnets. A torque (current) limiting function in

drive controls limits stall conditions, plugreversing, and current draw, particularly during high torque demand periods, andprotects against detrimental overload.(Plug reversing is not recommended be-cause it subjects the armature to higher-than-rated-for voltages). Besides pre- venting the motor from overheating, acurrent limiter can help protect drivenmachinery from excessive motor torques.

Permanant magnets

A number of magnetic materials areavailable for permanent magnets. These

include ceramic oriented ferrites, rareearth permanent magnets, and Alnico, al-though Alnico’s use is waning. Table Bcompares these commonly used materials.

Ceramic oriented ferrites, typically made with barium or strontium, developinto products with lower energy than Al-nico. Therefore, they have become thematerial of choice in most PM motors, re-placing Alnico, because of their greaterresistance to demagnetization, ease of forming, and low cost.

Rare earth magnets may let engineerschoose a downsized PM motor, or boost

its power rating. They include samarium-cobalt and the more recently developedneodymium-iron-boron. Their character-istics, compared to the previously men-tioned materials, include high energy and low susceptibility to demagnetiza-tion. The cost of these materials, how-ever, remains high.

The choice of material depends on theapplication requirements. The motormanufacturer can provide selectionassistance.

POWER TRANSMISSION DESIGN MAY 199534


Table A—Comparison of driving voltage to pure DC.

Form factor Dc voltage source

1.0 Battery (pure dc)

1.05 Pulse width modulation (PWM)

1.4 Full wave rectification

1.9 Half wave rectification

Table B —A comparison of permanent magnet motor materials.

Type Cost Resistance Energyto demag product

Ceramicoriented ferrites Low Medium Low

Samarium cobalt High High High

Neodymiumiron boron High High High



Brushes

PMDC motors use a mechanical com-mutation scheme to switch current to thearmature winding. Commutator barsconnect to the armature windings. A pairof spring loaded brushes make mechani-cal contact with the commutator bars,carrying the current to the armature.Thus, the brushes link the power sourceto the armature field windings. The arma-ture commutator and the brushes act as arotary switch for energizing the windings.

Design and selection of brushes tendto be something of a black art. The idealbrush offers low voltage loss, negligibledust formation, no arcing, little commu-tator wear, and generates little noise. Inmany cases, these requirements are con-tradictory, forcing a compromise in brushselection.

At low applied voltage, the voltage

drop across the brushes isthe prime consideration inbrush selection. At higher voltages, the voltage drop inthe brush is less important.Other parameters, such asoperating speed, abrasive-ness, lubricity and cost, be-come dominant.

Commonly used brushmaterials include carbonand carbon graphite,

graphite, electro-graphitic,and metal-graphite. Table Ccompares these brushmaterials.

Metal graphite brushes consist of amixture of copper or silver with graphite.They have low voltage loss and high wearrate, limiting their usage to low speed,low-voltage motors.

Carbon and carbon-graphite brusheshave low current capacity, a relatively high voltage drop across the brush, andtend to be abrasive. These properties

limit their use to low speed, high volt-age, fractional horsepower motorapplications.

Electro-graphitic brushes use a form of graphite developed from carbon sub- jected to intense heat. They have highcurrent capacity, a relatively moderate voltage drop, and low abrasive proper-ties. They handle high-speed, high-volt-age and high-power motor applications.

Natural graphite brushes have aslightly lower current capacity than elec-tro-graphitic brushes. They tend to havegreater polishing action than electro-

graphitics, low friction characteristics,and an inherent softness. They fit appli-cations where high-speed operation andquietness are critical. However, theirsoftness, which accounts for their quietoperation, also gives them a limitedbrush life.

Other factors also affect brush life andperformance, including temperature, hu-midity, altitude, spring pressure, controlform factor, size and duty cycle.

POWER TRANSMISSION DESIGN MAY 1995 35

Table D— Electrical insulation systems

Class A 105 CClass B 130 CClass F 155 CClass H 180 C

Table C — A comparison of motor brush materials.

Type Voltage Current Usagedrop capacity limitations

Carbon, High Voltagecarbon-graphite High Low Low speed

Fractional hp only

Graphite (natural) Medium Medium Medium speed/highvoltage

Electro-graphitic Medium High Medium to highspeed/Mediumto high voltage

Copper graphite Low Low Low voltage/Lowspeeds

SIlver graphite Very low Very low Very lowvoltage/lLow speeds

Figure 3 — Too little spring pressure onbrushes can result in excessive electricalwear. Too much pressure can result inexcessive mechanical wear. Optimalspring pressure is between these regions.

Brush life

The bathtub curve

W e a r r a t e

Spring pressure

Electricalwear Mechanical

wear

Optimal range



If spring pressure is too low, excessiveelectrical wear may occur. If it is toohigh, excessive mechanical wear may oc-cur. As shown in Figure 3, brush life canbe represented as a bathtub shapedcurve. The optimal spring-pressure rangefor minimal wear is between the highelectrical and mechanical wear regions.

Low humidity, high temperature orhigh altitude environments may not haveenough moisture present to form the nec-essary lubricating film between brushand commutator bar. Special lubricant-impregnated brushes can correct the

problem.Under light load conditions, the low

current draw can cause poor lubricationof the commutator. Smutting of the com-mutator and uneven commutation oftenresult. To correct, either use lubricant-

impregnated brushes or downsize thebrush cross-sectioned area for propercommutation.

Overall, choice of the appropriatebrush material is application dependentand is best made with the advice of themotor manufacturer, who typically hasthousands of hours of brush test data.

Other considerations for PMDC motorselection include proper choice of enclo-sure and electrical insulation system. If safety factors dictate a totally enclosedmotor, it may be non-ventilated (TENV)or fan-cooled (TEFC). (For an in depth

discussion on safety, see PTD, “ExplosionProof Motors for Adjustable-speedDrives,” 9/1991, p. 35.) Electrical insula-tion systems, Table D, are tested for20,000 hours at a rated temperature without degradation (as recognized by

UL, CSA, BSI, and VDE). Subtract ambi-ent temperatures (usually 25 C or 40 C)to determine allowable rise. You cancompute ambient temperatures if youknow the operating load, motor effi-ciency and thermal rise coefficient. Lookfor a control with a ramping function tohelp avoid operation in extremely low temperature. s

POWER TRANSMISSION DESIGN MAY 199536


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