Condensed Brushless DC Motors

Axsys Technologies, Inc.Motion Control Products7603 Saint Andrews Avenue, Suite HSan Diego, CA 92154

Ph: (800) 777-3393Fx: (619) 671-9292

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© Copyright 2005 Axsys Corporation Rev. 07/2005Axsys believes the information in this publication is accurate as of its publication date. Such information is subject to change without notice.Axsys is not responsiblefor inadvertent errors. All brands and product names aretrademarks of their respective owners.

Axsys Technologies’ Motion Control Products Division is experi-enced and ready to build a high-quality system solution for yourspecific application.

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Motion Control PackagesFor decades,Axsys has providedsolutions for complex applications such asultrasonic imaging systems, semiconductorprocess, mechanisms, aircraft actuation andmissile guidance systems.The mechanismsection of the Axsys Technologies BrushlessDC Motor Manual gives a brief overview.

Design GuidesAt the end of this catalog, you will find aDC Motor Design Guide, which promptsyou to answer the necessary questionsabout your current motor requirements.The Design Guide is especially useful whenyour project calls for a custom solution.Werecommend you make copies of it to use asworksheets when deciding on your brush-less motor requirements.

Axsys Motion Control ProductsIntroduction

HeritageAxsys has been serving thecommercial,industrial and defense motioncontrol industries for 40 years, supplyinghigh performance components and systemsthat stand up to the most rigorous environ-ments.We supply high performance prod-ucts that withstand 700 g’s of shock, 15,000lbs of pressure, and temperature ranges from-55ºC to +200ºC, for applications fromaerospace to medical, textile to robotics.

From our inception,Axsys hascontinuously challenged technological limitations, developing state-of-the-artmotors, position feedback devices, andelectromechanical assemblies for the rapidlyevolving commercial, industrial, defense,

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Introduction

aerospace, and medical industries. Equippedfor innovation, with a world-class team ofengineers and a global support network,Axsys is uniquely able to apply this highperformance technology to your systemneeds, at a competitive cost.

Design and ManufacturingYour Axsys design begins with adetailed review of your specifications by ourengineers. If you have not established formalspecifications, our engineers will help devel-op them with you.We use computer aideddesign programs, design, and process controlspecifications to assure the product will meetyour specifications.

Axsys manufacturing cycle brings with theestablishment of a Materials RequirementsPlan (MRP).With the aid of an integratedcomputer planning/scheduling system,detailed production schedules are generatedto ensure on-time material delivery, optimaloutput, and inventory levels.

Customer ServiceAt Axsys, service does not endwith the delivery of your products. Serviceand support are our most important respon-sibilities, and we meet them with our net-work of technical support staff that stretchesaround the world. Our engineering, manu-facturing and quality experts - in fact, allemployees in our entire organization - areready to serve you from concept, throughdevelopment, to order delivery, and beyond.

Quality AssuranceWe have established the internal qualitysystems required for high-reliability com-mercial and defense programs.Axsys’ QualitySystem is certified to ISO 9001:2000.







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Manufacturing OperationAxsys MCP has a modern 60,000square foot facility in San Diego, staffed bydedicated employees.We can easily accom-modate contracts from the developmentphase through full-scale production.

We encourage factory tours. Our operationsinclude an extensive CNC machine shop,automated armature winding station,organized work centers, and environmentaland performance testing equipment.

Brushless DC Motors

The mechanical switching of currentassociated with brush motors is replacedwith electronic switching in brushlessmotors. Brushless DC motors are not simplyAC motors powered by an inverter, insteadthese devices use rotor feedback devices sothat the input wave forms are kept in propertiming with the rotor position. Some formof electronic commutation switching is nec-essary for all motors, except in the limitedangle devices.

Brushless DC motors with suitable controlelectronics can be directly substituted forsimilarly-sized brush DC motors. BrushlessDC motors provide several advantages:

1. Brushless units may be operated atmuch higher speeds and at full torqueat these speeds, resulting in a motorwith considerable power output for itssize. High speed operation is especiallydifficult for conventional DC motorsbecause the high energy that must beswitched by the brushes is destructiveand shortens motor life. In brushless

motors this energy can be handled bythe drive circuits.2. The stator, which is wound member,may be mounted in a substantial heatsink to minimize temperature rise andprolong bearing life.

3. Where long life is a requirement, theabsence of brushes normally increasesthe motor’s life expectancy to that ofthe bearings.

4. In high-cleanliness applications,unacceptable brush wear particles areeliminated.

5. The EMI (ElectromagneticInterference)normally associated with thearcing of the brush commutator interfaceis eliminated in the brushless motor.Brushless DC controllers are generallyfree of major EMI contributors.

6. For explosive environments, a brushlessmotor can be used without specialhousing elements necessary toexplosion-proof a conventional DCmotor.

7. Although brushes have been usedextensively in space environments,their preparation is expensive and timeconsuming.The brushless motorrequires much less preparation.

Typical ApplicationsBrushless DC motors have the sameelectrical performance operating (transferfunction) characteristics as brush-commu-tated DC motors, and can be used in thesame applications.They provide high start-ing torque, variable bi-directional speed







operation, and precision position and veloc-ity servo loop capabilities.They can alsoreplace AC motors in spindle and rotarytable drives where the higher torque DCmotor can drive the spindle directly, elimi-nating the need for a belt and pulley.

Basic ComponentsA brushless motor system consists of fourbasic subassemblies:1. A stator wound with electromagneticcoils which are connected in single andpoly-phase configurations.2. A rotor consisting of a soft iron coreand permanent magnet poles.3. A rotor position sensor providing rotor

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Applications Equipment Brushless DC Advantage

Velocity Servos Disk Memory Spindle DriveLong Life,Low-EMI

Video Tape RecorderHigh Speed,

No Brush Debris

Silicon Water SpinnerHigh Speed,Long Life

CAT ScannerLow-EMI,

Low Audible Noise

Infrared ImagerHigh Speed,

Low-EMI

Artificial HeartsLong Life,

No Brush Debris

Cryogenic Compressor No Brush Debris

Fuel PumpNo Arching

No Brush Debris

Air Bearing SpindleNo Brush Debris

High Speed

Position Servos Space VehicleVacuum Operation

Low-EMI

Optical System No Brush Debris

Stable Platform Low-EMI

High Power Density Robot Low Thermal Resistance

Airborne ActuatorLow Thermal Resistance

Vacuum Operation

The gearless DC motor drive is ideally suited tohigh acceleration applications requiring improvedresponse for rapid start/stop actions.







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position to the required resolution.4. Commutation logic and switching elec-tronics to covert rotor position infor-mation to the proper stator phase exci-tation.

StatorThe stator for a brushless DC motor is alaminated steel core with coils of magnetwire embedded and connected in two ormore phases so that by sequentially excitingthese phase winding, a rotating electromag-netic field can be generated.

Since both iron and copper losses in thebrushless DC motor take place in the stator,generated heat is easily transferred to thesurroundings.A motor with an inner rotor isbetter in this regard due to the larger statorarea in contact with the mounting surfaces.

RotorsIn all continuous rotation brushless DCmotors, the permanent magnetic field is onthe rotor.The winding supplying the rotat-ing electromagnetic field are in the stator.The rotor can be designed inside of the sta-tor, as in a conventional motor, or outside ofthe stator. See Figure 1A and 1B.The innerrotor design is generally used in incrementalmotion application where low inertia andfast response are required.The outer rotorconfiguration provides a more rigid struc-ture for the permanent magnets and hashigher inertia. It is used in high speed appli-cations where a stiff structure is required tocounter the centrifugal forces acting on thepermanent magnets, in velocity loop appli-cations where the additional rotor inertia isdesired for velocity stabilization, or whenthe additional inertia is insignificant relativeto the total inertia as in a memory diskdrive.

Position SensorsThere are several ways to sense brushlessrotor position. Rotor position sensing isnecessary so the stator winding excitationcan be controlled to keep the electromag-netic field in leading quadrature withrespect to the rotor field.These methods fallinto three categories: photo-electronic,electromagnetic, and magnetic.

PhotoElectronicA set of photo-transistors and LEDs arecoupled across a shutter which is keyed tothe rotor and has windows in the properpattern to control the phase excitation. If ashaft angle encoder is required in the sys-tem for normal shaft position sensing, aseparate pattern track can be included onthe disk for motor commutation.The out-put of an absolute encoder can also be usedin a sine cosine ROM or as input to a digi-

Stator Leadwires

PermanentMagnetsRotor Core

Windings

Stator

A) Inner Rotor Brushless DC Motor

Stator Leadwires

Rotor Core

Stator WindingsPermanentMagnets

B) Outer Rotor Brushless DC Motor

Fig. 1 Brushless Motor Configurations







tal comparator to develop the commutationwaveform.

ElectromagneticElectromagnetic sensors use a soft iron tar-get and a set of wound coils. Changes inthe coil inductance are sensed and decodedto verify rotor position. Eddy currentdevices using metallic targets can also beused in the same manner. Brushless resolverare commonly used in sine cosine systemsas the source of the phase quadrature wave-form.

MagneticHall effect sensors, magneto-resistors, ormagneto-diodes are used in magnetic sen-sors.These devices work directly off of therotor poles so that the alignment of thesensors can be accomplished during themanufacture of the stator.They become apart of the stator assembly so that the userneed only install the rotor to have analigned system.

ElectronicsThe electronics module, which can beinternal to the motor housing or placed onan external printed circuit board, receivesthe signals form the position sensors anduses digital logic to develop the wave-formsthat are used to control the switches.Theseswitches are usually power MOSFET,IGBT, or bipolar transistor devices.Theselection of the type of power switchdepends largely on the application andincludes factors such as the motor voltage,peak motor current, PWM frequency, andthe operating characteristics of the motor.

In general, power MOSFET’S, because ofthe very low “on” resistance exhibited bythis type of device, are the switches of

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choice while IGBT’S are most common inhigh voltage brushless motors driver wherethe motor voltages are on the order of sev-eral hundred volts.

Also, for high power applications, mostmodern three phase brushless drivers aredesigned to operate directly off of the 230VAC or 440 VAC power line, eliminatingthe need for separate power supply lines, andthe need for a separate power supply to gen-erate the switch bus voltages.These drivesystems almost always use IGBT powerdevices because of the high voltagesencountered and the high efficiencyachieved with direct line powered operation.

In a sine-cosine system, the electronics mod-ule is a two phase bipolar amplifier.The sen-sor output are first decoded or demodulate(if required), and then amplified by a fourquadrant power amplifier.To reduce powerdissipation in the amplifier, pulsewidth mod-ulation is often used.This type of system ismore costly than the digitally switched sys-tem. For this reason, is used only when theapplications needs the smooth, low rippletorque of a sinusoidally-commutated brush-less motor.

Brushless DC motor controllers offer manypower levels and commutation options.Standard controller products are availableproviding closed loop control of speed ortorque, accepting Hall sensor, opticalencoder or resolver position sensor signals.Often in low power applications, the controlelectronic are an integral part of the motorand range from single commutation andswitching circuits to complete speed controlsystems. Many IC’s combining commuta-tion, current sensing, fault detection, and







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power switch drive functions provide costeffective Brushless DC motor control.

Commutating Brushless DCMotors

Torque and Winding CharacteristicsThe basic torque waveform of a brushlessDC motor has a sinusoidal or trapezoidalshape. It is the result of the interactionbetween the rotor and stator magnetic fields,and is defined as the output torque generat-ed relative to rotor position when a constantDC current is applied between two motorleads.This torque waveform is qualitativelyequivalent to the voltage generated wave-form at the two motor leads when themotor is driven at a constant speed byanother motor.The frequency is equal to thenumber of pole pairs in the motor times thespeed in revolutions per second.

The brushless DC motor will exhibit torquespeed characteristics similar to a convention-al DC motor when the stator excitation is inproper alignment with the rotor’s magneticfield.The stator excitation may be squarewave or sinusoidal. Ideally, the stator excita-tion may be square wave or sinusoidal.Ideally, the stator excitation should beapplied in a sequence to provide a constantoutput torque due to the finite commuta-tion angle.The commutation angle is theangle the rotor rotates through before thewinding are switched. Ripple torque is typi-cally expressed as a percentage of average topeak torque ratio. It is present whenever thewinding are switched by a step functioneither electrically via solid state switches ormechanically via brushes.

Rotor PositionElectrical Degrees

90 180 270 360

0BTorque

iKt

0

Torque withCommutatedExcitation

Commutated ±Excitation

1 1 1 13 2 4 14 1 3 2

0ATorque

Figure-2. Switch mode commutation of a twophase brushless DC motor.

Figure-3. Switch mode commutation of a threephase delta wound brushless DC motor.







In brushless DC motors designed for squarewave excitation, the ripple torque can bereduced by reducing the commutationangle by using a higher number of phases,which also improves motor efficiency.Figure 2 shows the commutation pointsand output torque for a two phase brushlessmotor.The commutation angle is 90 elec-trical degrees which yields the largest rippletorque of about 17% average to peak.Athree phase delta system is shown in Figure3.The commutation angle is 60 degrees andthe ripple toque is approximately 7% aver-age to peak. Since two-thirds of the avail-able winding are used at any one time,compared to one-half for the two phasesystem is more efficient.

The torque waveform indicated in Figure 2and 3 have a sinusoidal shape.A trapezoidaltorque waveform can be obtained by usinga salient pole structure in conjunction withthe necessary lamination/ winding configu-ration. In practice, the trapezoidal torquewaveform does not have a perfectly flat top.Manufacturing and other cost considera-tions result in an imperfect trapezoidalwaveform.An example is Figure 4 wherethe generated voltage waveform across twoterminals of a brushless motor designed fortrapezoidal torque generation is Shown.Figure 5 is an example of a 12 phase brush-less motor with a trapezoidal torque wave-form.With the center terminal of the phas-es connected to the supply voltage, thephases are switched to ground during theindicated commutation angle of 30 electri-cal degrees. Only three phases are “on” atany one time.This motor was designed tosatisfy the requirements of high efficiencyand minimum ripple torque for a precisionpointing and tracking space gimbal applica-tion. In applications requiring smooth

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Figure-4. BEMF Trapezoidal waveform.

Rotor Position-Electrical Degrees

60 120 180 240 360

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Figure-5.Torque waveform for 12 phase brushlessDC motor.







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operation at low speed, or where the motoris operated in a position loop, a sinusoidaldrive system should be considered. Figure 6is an example of a two phase motordesigned for sine wave drive.The torque output of each phase is:

TA = IA KT Cos uTB = IB KT Cos uWhere

IA = current in phase AIB = current in phase BKT =torque sensitivity of motoru = rotor position in electrical degrees

If the motor currents are supplied in thefollowing relationships:

IA = I CosuIB = I Sinu

The torque output of the motor is:

T = TA + TB

T = I KT(Sin2u + Cos2u )T = I KT

This analysis indicates that the sinusoidallydriven brushless motor has linear character-istics similar to a conventional DC motorand has minimum ripple torque.Threephrase winding can be connected in eitherwye or delta configuration. Excitation canbe switch mode or sinusoidal drive.Theswitch mode drive is the most commonlyused system because it results in the mostefficient use of the electronics.Two switchesper phase terminal are required for theswitch mode drive system.Therefore, onlysix switches are required for either the wyeor delta configuration.

The delta winding form a continuous loop,so current flows through all three windingregardless of which pair of terminals isswitched to the power supply. Since theinternal resistance of each phase is equal,the current divides unequally, with two-third of the total current from one windingto another as the winding are commutated.

For the wye connection, current flowsthrough the two winding between theswitched terminals.The third winding isisolated and carries no current.As thewinding are commutated, the full load cur-rent must be switched from terminal to ter-minal. Due to the electrical time constantof the winding, it takes a finite amount of

0A

IA

0B

IB

Rotor Position Electrical Degrees

0A Torque withIA Const.

0B Torque withIB Const.

IA = I COSu

NormalizedTorque

IB = I SINu

90 180 270 360

90 180 270 360

1.0

0.5

T = IK SIN uB T

2

T = IK COS uA T

2

Figure-6.Two phase sinusoidal excitation.







time for the current to reach full value. Athigh motor speeds, the electrical time con-stant may limit the switched current fromreaching full load value during the commu-tation interval, and thus limits the generatedtorque.This is one of the reasons the delta-configuration is preferred for applicationsrequiring high operating speed. Other con-siderations are manufacturing factors whichpermits the delta configuration to be fabri-cated with lower BEMF constant, resist-ance, and inductance.A lower BEMF con-stant allows the use of more common lowvoltage power supplies, and the solid stateswitches will not be required to switchhigh voltage. For other than high speedapplications, the wye connection is pre-ferred because it provides grater motor efficiency when used in conjunction withbrushless motors designed to generate atrapezoidal torque waveform.

Sensor Timing and AlignmentA brushless DC motor duplicates the per-formance characteristics of a DC motoronly when its winding are properly com-mutated. Proper commutation involves thetiming and sequence of stator windingexcitation.Winding excitation must betimed to coincide with the rotor positionthat will produce optimum torque.Theexcitation sequence controls the polarity ofgenerated torque, and therefore the direc-tion of rotation.

Rotor position sensors provide the infor-mation necessary for proper commutation.Sensor output is decoded by the commuta-tion logic electronics.The logic signals arefed to the power drive circuit which acti-vates the solid state switches to commutatethe winding.

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The commutation points and output torquefor a two phase brushless motor were shownin Figure 2.The commutation angle is 90electrical degrees, and the winding areswitched “on: 45 electrical degrees beforethe peak torque position.The current polari-ty must be reversed for negative torquepeaks.The commutation waveform for thismotor are shown in Figure 7. Sensor outputand alignment is shown in figure 7A. TheS1 output leads the phase B torque positionby 45 electrical degrees.

There are several methods for aligning theS1,2 sensors with respect to the stator wind-ing.As shown in Figure 2, the peak torqueposition of phase B coincides with the zerotorque position of phase A and vice versa.Sensor S1 can be aligned to the phase Bwinding by applying a constant current tophase A.The rotor will rotate to phase A’szero torque position. S1 should be posi-tioned so that its output just switches from alow to high logic state at 45 electricaldegrees counter-clockwise from phase A’szero torque position.

Another method is to align the position sen-sors to the BEMF waveform. Since theBEMF waveform is qualitatively equivalentto the torque waveform, the motor can bedriven at a constant speed by another motor,and the position sensors aligned to the gen-erated BEMF waveform.The sensor transi-tion points relative to the BEMF waveformshould be as indicated in Figures 2 and 7.For critical applications which require thecommutation points to be optimized, themotor should be operated at its rated loadpoint, and then the position sensors shouldbe adjusted until the average winding cur-rent is at its minimum value.To facilitatesensor alignment,Axsys can supply stators







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with reference marks to which the positionsensors can be aligned.

The commutation points and output torquefor a three phase brushless motor was shownin Figure 3.The commutation angle is 60electrical degrees, and the winding areswitched “on” at 30 electrical degrees beforethe peak torque position, and switched “off” at 30 electrical degrees after the peaktorque position.The current polarity for

each phase during each commutation seg-ment is shown in Figure 3B.The commu-tated output torque versus motorposition(in electrical degrees) is shown inFigure 3C.

0A

+VSQ4

0 60 120 180 240 300 360

CW Rotor Position - Electrical Degrees

S1 01

S2 01

Q1 offon

(S1 l S2)

Q2 offon

(S1 l S2)

Q3 offon (B)

Q4 offon

(S1 l S2)

(D)

(A)

(S1 l S2)

-VSQ2

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Figure-7. Sensor output, commutation logic, andand winding excitation waveform for atwo phase brushless DC motor.

Figure-8. Sensor output, commutation logic andwinding excitation waveform for a three phasebrushless DC motor.







To identify each of the 6 commutation ter-minals, a minimum of three logic signals arerequired.The commutation waveform areshown in Figure 8.The three sensors arespaced 60 electrical degrees apart, and havea 50% duty cycle.

As indicated in Figure 3 and 8, sensor S1can be aligned to the [A-[B zero torqueposition.

This can be accomplished by applying aconstant current to the [A-[B terminals.The rotor will rotate to the [A-[B zerotorque position.Then S1 should be posi-tioned so that its output just switches froma low to high logic state S2 and S3 shouldbe positioned 60 and 120 electrical degreesrespectively from S1. Other alignment con-siderations are as previously discussed forthe two phase motor.

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Sine Wave DriveThe sine wave drive control scheme is thebest choice when the brushless motor isrequired to duplicate the performance char-acteristics of a conventional DC torquemotor. Brushless DC torque motorsdesigned to provide sinusoidal torque wave-form have the additional advantage of mini-mum ripple torque and high reliability when

compared to the brush-commutated DCtorque motor.

The basic equations for controlling motorcurrent and the resulting output torque for atwo phase Brushless DC torque motor wereshown in Figure 7.The control methodconsists of developing drive currents that area sinusoidal function of the rotor position.AS mentioned previously, position informa-tion can be obtained from a variety of

H1 H2

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V( )

10B

0A

ControlInputvoltage

VC

K2 VV( )

I0A

Power Amplifiers K 3 AMPV( )


Figure-9. Functional control circuit for a two phase brushless DC torque motor with Hall Effect rotorpositioning sensing.







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devices. Hall effect elements, sine cosineresolvers, optical encoder, electromagneticand electrostatic pickups, magneto-resistors,and magneto-diodes are currently beingused as rotor position sensors.

The specific requirements of each applica-tion usually dictate the position sensing sys-tem selections.The linear Hall effect element is the sensing systems most frequentlyused because of its small size, low cost, andsimplified processing electronics. Figure 9 isa functional control circuit for a two phasebrushless DC torque motor with linear Halleffect rotor positions sensing. To obtain Halldevice output voltages proportional to thesine and cosine of the rotor’s position, theHall devices are mechanically displaced 90electrical degrees from each other, and are

then align with the stator’s BEMF ortorque waveform.The magnitude anddirection of the motor drive current is reg-ulated by varying the magnitude and polar-ity of the Hall device control current.

The control equations are as follows:(Refer to Fig. 9)

IC =KIVCVH =KHICBCos u

WhereIc = Hall device control current(Amp)Vc = Input control Voltage (Volt)K1 = Current amplifier gain (Amp/Volt)VH = Hall device output voltage (Volt)KH = Hall constant (Volt/Amp-KGauss)B = Rotor permanent magnet field vector

H1 H2 H3_

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VK1

I C

_

+

K2

VH

( )

VV( )

_

+

K2

VH

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+

K2

VH


_

+

_

+

_

+

K 3

K 3

K 3

VV( )

VV( )

0A0C

0B

Power Amplifiers

AMPV( )

10B

0A

10C

ControlInputvoltage

VC

Figure-10. Functional control circuit for a three phase brushless DC torque motor with Hall Effect positioningsensor. positioning sensing.







u = Angle between the plane of the Hallelement and the rotor permanentmagnet field vector(degrees)

The current in [A winding is:I[A =VHK2K3

= K2K3 KHICB Cos u

= K2K3 KHK1BVC Cos u

Similarly, the current in [B winding is:

I[B= K2K3KHICBVC Sin uThe torque output of each phase is:

T[A = I[AKT Cos u

T[B = I[BKT Sin u

Substituting for I[A and I[B

T[A =VCK1K2K3 KHKTB Cos2 u

T[B =VCK1K2K3 KHKTB Sin2 u

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Let KC = K1K2K3 KHKTBThen the motor’s output torque is:

T = T[A + T[B =VCKC (Cos2 u + Sin2 u)T =VCKC

Therefore, the output torque is directly pro-

Direction Control Input

PowerMosfetDriveCKT

Q1

+V

Q3-V

OB

Q2

+V

Q4-V

OA

+VS

+VS

S1

S2

Figure-11. Functional drive circuit for bidirectionalcontrol of a two phase brushless DC motor.

Figure-12. Functional drive circuit for bidirectional control of a three phase brushless DC motor.







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portional to the input control voltage.Thesame control scheme can be applied to athree phase brushless DC torque motor asshown in Figure 10. The Hall devices aremechanically displaced 120 electrical degreesfrom each other and are aligned with theBEMF waveform of each phase.The controlequations are as previously derived for thetwo phase system, except the output torqueis:

T =VC KC (Sin2 u + Sin2 u (0 + 120) +Sin2 (u - 240))T = 1.5 VC KC

and, as before, the output torque is directlyproportional to the input control voltage.

Square Wave DriveA square wave drive system provides themost efficient utilization of the control elec-

tronics, and yields the lowest system cost.Power dissipation in the output stage isminimized by operating the power switchdevices in the complete OFF or fully ONstage.The output stage can be interfaceddirectly with the commutation logic cir-cuit, and the whole system can be digital.This allows the use of lower cost digitalintegrated circuits to decode the positionsensor output and to sequence the drivecircuit power semiconductors.

Figure 11 shows a simplified drive circuitfor bi-directional control of a two phasebrushless DC motor.The timing and exci-tation waveform for this motor were shownin Figure 7.The rotor position sensors areHall effect switches and their outputs areprocessed by Exclusive OR gates. This fea-ture allows the direction of rotation to bereversed.A logic “1” on the direction con-

+VS

+VS

+VS

0A

PWM INPUT FROMVELOCITY CONTROL CIRCUIT

S1

S2

S3

+VS

PROMorCustomI.C.

Q1

CURRENTSENSEOUTPUT

+VS

Q2 Q3

Q4 Q5 Q6

Direction ControlInput

0C

0B

Figure-13. Functional drive circuit for bi-directional control of a three phase brushless DC motor.







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+VS

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PWM INPUT FROMVELOCITY CONTROL CIRCUIT

S1

S2

S3

+VS

BCD TODECIMALDECODER

MotorWindings

Q7

Q8

Q9

Q10

Q11

Q12

Q1

Q4

CURRENTSENSEOUTPUT

Q2

Q5

Q3

Q6

+VSS









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trol input will invert the sense of the posi-tion sensor output. Referring back to Figure7, we note that inverting the sensor polaritywill reverse the polarity of excitation oneach motor terminal thus making it producetorque in the opposite direction.

The logic statements shown in Figure 7b areimplemented with AND and INVERTgates. Power MOSFETS are used to switchthe motor winding. Speed control isobtained by pulse width modulating thepower MOSFET during its ON commuta-tion period.The MOSFET drive circuitconsists of the gate drive and appropriatelevel shifting for the “P” and “N” channeldevices.

A functional drive circuit for bi-directionalcontrol of a three phase brushless DC motoris shown in Figure 12.The timing and exci-tation waveform for this motor was shownin Figure 8. The main features of this con-trol scheme are:

1. Rotor position feedback is obtainedfrom three LED-photo-transistors.

2. Schmit-trigger inventers are used toshape the photo transistors outputwaveform.

3. AND and INVERT gates are used toimplement the logic statements ofFigure 8B.

4. PNP and NPN power Darlingtontransistors are used to switch the motorwinding.

5. Flyback diodes across each transistorsprovide a transient path for thecommutated winding inductiveenergy.

6. Speed control is achieved by “servoing”the supply voltage to the transistorbridge.

A single PROM or custom integrated cir-cuit can be used to replace the EXCLU-SIVE OR,AND, and INVERT logic ICs.This is shown in Figure 13.

The required logic can also be implement-ed with a BCD-to-decimal decoder asshown in Figure 14. Since the decoder out-put has only one unique “high” outputstate for any combination of logic inputs,both the PNP and NPN power transistorsmust be switched OFF and ON simultane-ously for each commutation point.Thiscontrol function is performed by drivertransistors Q7 through Q12.

A simple low-cost drive used to control thebrushless DC spindle motor in a disk driveis shown in Figure 15. Only three powerMOSFETS are required to commutate thewye connected winding with the center tapconnected to the supply voltage.The wind-ing are switched over 120 electricaldegrees. Due to the relatively high inertialload, the increased ripple torque has negli-gible impact on system performance.

vE = KVT

R

B

L

I

Figure-16. Equivalent electrical circuit of theBrushless DC motor.







Clamping zener diodes are used to protectthe power MOSFETS from over-voltagetransients produced when the inductivewinding are switched OFF.

Brushless Servo SystemsThe brushless DC motor, when properlycommutated, will exhibit the same per-formance characteristics as a brushcommu-tated DC motor, and for servo analysis thebrushless motor can be represented by thesame motor parameters. It can be modeledby the equivalent circuit of Figure 16.Thismodel can be used to develop the electricaland speed-torque characteristics equationsfor brushless DC motors.

The electrical equation is:

VT = IR + Ldl/dt + KB(v) (1)

Where

VT = the terminal voltage across the activecommutated phase

I = the sum of the phase currents intothe motor

R = the equivalent input resistance of theactive commutated phase

L = the equivalent input inductance ofthe active commutate phase

KB = the back EMF constant of the active

v = the angular velocity of the rotor

If the electrical time constant of the brush-less DC motor is substantially less than theperiod of commutation, the steady stateequation describing the voltage across themotor is:

VT = IR+KBv (2)

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The torque developed by the brushless DCmotor is proportional to the input current.

T = I KT

Where KT = the torque sensitivity(oz-in/amp)

If we solve for I and substitute intoEquation (2) we obtain:

VT = T/KTR + KB(v) (3)

The first term represents the voltagerequired to produce the desired torque, andthe second term represents the voltagerequired to overcome the back EMF of thewinding at the desired speed. If we solve (3)for rotor speed, we obtain:

v =VT/KB - TR/KBKT (4)

which is the speed-torque equation for apermanent magnet DC motor.

A family of speed-torque curves representedby Equation (4) is shown in Figure 17.

The no-load speed can be obtained by sub-stituting T=0 into (4).

v NL

Torque StallT

VTVN

V2V1

( No Loadspeed)

Spee

d

Figure-17 Speed torque characteristic curves.







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v NL =VT/KB

Stall torque can be determined by substitut-

ing v=0 in (4)

TSTALL = KT VT/R = IKT

The slopes of the parallel straight line speedtorque curves of Figure 17 can be expressedby:

R/KBKT = v NL/ TSTALL

Since the speed-torque curves are linear,their construction is not required; the servodesigner can calculate all needed informa-tion from the basic motor parameters.

Application ExampleA typical constant velocity application suchas the spindle in a memory disk drive mayhave the following requirements:

Operating speed v = 3600 RPM = 377 rad/sec

Load Torque T L = 12 oz-inAvailable supply Vs = 24V ± 10%Max Voltage drop VCE(MAX) = 1.5 Vacross switching transistors

The requirement is to select an appropriatemotor and verify that it will meet operatingspeed torque requirements under worst caseparameter conditions.

The motor selected has the followingparameters:

KT = 5.4 ± 10% oz-in/ampKB = 0.038 ± 10% Volts rad/secR = 0.90 ± 12.5% ohms

Assume the motor has an internal losstorque.TLOSS = 0.5 oz-in, and the windingwill see a maximum temperature rise(ambient plus internal heating) of 25° C.

The minimum voltage available to themotor is:

VT(MIN) =Vs(MIN) - 2VCE(MAX) = 21.6 - 3.0= 18.6 VThe maximum resistance of the windingincluding tolerance and temperature rise is:RMAX = R (1 + 12.5%){1 + (0.00393)

(DT)}RMAX = 0.9(1.125){1 + (0.00393)( 25)}

= 1.11 ohm

The maximum voltage across the motorwith worst case parameters is:

VT(MAX) = ( RMAX)(TL + TLOSS)/(KT(MAX))+ KB(MAX)v

VT(MAX) = ( 1.11)(12.5 + 0.5)/(5.94) +(0.0418)(377)VT(MAX) = 18.1 V

Therefore, the motor selected will meet therequirements under worst case conditions.The block diagram and transfer functionfor a brushless DC motor when coupled toa load can be constructed from Equation (1) and the following dynamic torque

VT+

e IR + SL

IK T

K B

-e I

(J + J ) S+FL

T+ -TF + TL

v (S)

M

Figure-18 Block diagram of brushless DC motormodel.







equation:

T = (JM + JL))dv/dt + Fv + TF + TL (5)

WhereJM = the motor moment of inertiaJL = the load moment of inertiaF = the damping factor representing allmotor and load viscous frictionTF = the motor friction torqueTL = the load friction torque

Taking the Laplace transform of (1) and(5) yields:

VT = IR + SL + KB v(S) (6)

T = (JM + JL))S + Fv(S) + TF + TL (7)

Figure 18 is a block diagram representingEquations (6) and (7) and is identical to themodel used for the brush commutated DCmotor.

Some Common PitfallsMotor windings are inductive.This givesrise to two considerations that should notbe overlooked.The first is motor speed.Since the winding electrical time constantsare in the area of one millisecond, commu-tating frequencies above several hundredhertz need special treatment. Commutatingfrequency is equal to the number of polepairs in the machine times the speed inrevolutions per second. For units that mustoperate at higher speeds some provision forshifting the commutation points must bemade.This can be done either mechanicallyor electronically.Also, the motor inductancecauses high voltage spikes to appear acrossthe power transistors as they are switchedoff.These must be allowed for in the designeither by use of high voltage transistors orprotective zener diodes or other transient

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suppressors.Almost all brushless motor sys-tems require current limiting to avoid inad-vertent demagnetization of the permanentmagnet rotor during starting and fast rever-sals.The logic system must be examined forpossibility of improper outputs duringpower application. For instance, if the logicstates are such that both the transistors atone corner of the three phase delta areturned on, the resulting short circuit will bedisastrous.An easy and fruitful test duringbread boarding is to step the logic throughits sequence and note that the motor stepsthrough its rotation with no reversals or longsteps.This assures that the logic is correctand that the motor is connected properly. Ifthis is not done, it is possible that the motorwill run, but that during one segment of itsrotation its torque will be reversed or non-existent. High current will result, but may beoverlooked inadvertently.

A Simple Test MethodMany of our customers have found that anincoming test of the motor torque is expen-sive to instrument and perform. In thesecases we have suggested that the incomingelectrical inspection be made on the backEMF constant of the motor (Kb).Thisparameter is directly proportional to thetorque sensitivity and is more easily meas-ured.The technique involves driving themotor at a constant speed and measuring thegenerated voltage at the motor terminals.Acceptance limits can be set based on theback EMF constant and assurance of a com-pliant motor is given by this test.

Thermal ConsiderationsOn each of the following data pages, themaximum allowable winding temperature isspecified.The maximum operating tempera-







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ture of the winding depends on the loadduty cycle and the thermal paths to systemheat sinks.The value given for motor resist-ance and resistance dependent parameterson the data sheets are defined at 25°Cwinding temperature. If the winding isoperated at a different temperature, the tem-perature coefficient of copper (0.4% per °C)must be taken into consideration.

Handling, Storage, InstallationThe motors in this Source Book are alldesigned with rare earth magnets and requirecareful handling.These magnets have verystrong attractions to magnetic materials.Without cautious handling, injury could occur.

When motors are stored in their originalshipping container, they are well protectedfor normal storage conditions.Those motorsthat are housed have permanently lubricatedbearings and do not require changing thebearings unless storage packaging has beendamaged.The device should be closelyinspected to see if damage or contaminationhas occurred.

Installation of brushless motors has only ahandling problem.As the size of the motorincreases, not only does the weight affecthandling, but the magnetic attraction is aproblem. If the motor is shipped with themagnet assembly inside the armature, itshould be left this way to keep from damag-ing the magnets.There is a nonmagneticshim placed in the air gap to keep the mag-net assembly close to the center of thearmature.This helps to align the motor tothe mating mounting diameters.Themounting diameters of the housing and hubor shaft should have chamfers to help duringinstallation. Once the motor is in place, theshim is removed and the motor should be

free to rotate. It is always recommended toinspect the area of the air gap looking forany chips from magnets or magnetic chipsthat may have adhered to magnets andremoved before operating the motor.

Glossary

Commutation – Sequenced switching ofelectrical power so that it is properly distrib-uted in place and time to a motor winding.This function is performed by carbonbrushes and a copper commutator in a con-ventional DC motor.Electrical Degree – An angular measuredimensioned so that one pole pair contains360 electrical degrees. In a two polemachine an electrical degree equals amechanical degree.The number of polepairs in a machine equals the number ofelectrical degrees in a mechanical degree.Exclusive OR gate – An electronic logicelement in which the output is on wheneither of the two inputs is on but not whenboth are on or off. It can be seen that if oneinput is held off the state of the outputis the same as the state of the second inputbut if the first input is held on the state ofthe second input is reversed at the output.

Hall Effect Element – Semiconductorwhich produces an output voltage propor-tional to the magnetic flux density perpen-dicular to the surface of the semiconductorand an input control current. It is common-ly packaged with signal conditioning elec-tronics to provide a linear or digital output.LED (Light-Emitting Diode) – A two termi-nal electronic component that emits lightfrom its semiconductor junction when elec-trical current flows through it.







Mounting – The information given oneach data page are user requirements forcorrect axial and concentric orientation ofthe motor components. It is required topreserve the specified performancecharacteristics.

Phase – A motor winding which will setup magnetic poles in a specific position inthe motor when a current is flowing in thewinding.There is usually more than one

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phase in a brushless motor.The polepositions for each phase are angularly dis-placed from one another so that a movingmagnetic field may be set up in a stationarycomponent.

Photo Transistor – A semiconductor devicein which the electrical current flowingthrough it is controlled by the amount oflight falling in on the junction.When there

Axsys’ DC torque motors are designed using premium materials that offer unique space and weight savingswhile generating maximum power output. Limited angle torque motors do not require commutation elec-tronics and have near zero cogging.







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is no light the current is very small.A greater amount of light increases the cur-rent flow.

PROM – A programmable read only memo-ry is an integrated circuit where data isentered by field programming techniques inwhich fusible links are blown, or in whichsome other permanent modificationis made to the device structure.They can beemployed to eliminate conventional combi-national logic circuits.Rotor – Rotating element that drives theload.

Solid State Switch – A semiconductordevice which is used to switch the power tothe motor windings. Commonly useddevices are transistors and MOSFETS.

Stator – Stationary element.







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ConversionTable

To Convert:

From To Multiply by

Weight

ounces (force)ounces (force)pounds (force)pounds (force)

grams (force)Newtons

grams (force)Newtons

28.350.278453.64.448

Distanceinchesinchesfeet

centimetersmetersmeters

2.542.54 x 10-2

Torqueounce-inchesounce-inchespound-feet

gram-centimetersNewton-metersNewton-meters

72.017.061 x 10-3

1.356

Angular VelocityRPM

degrees/secondrev/second

radians/secondradians/secondradians/second

0.10471.745x10-2

6.283

Inertiaounce-inch-sec2

pound-feet-sec2

gram-centimeterKgram-centimeter

7.06 x 104

0.367

Power Rate ounce-inch/sec2 kilowatts/second 7.061 x 10-3

Conversion Table







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Performance/Winding Data:

Peak Torque:ooz-in o N-m

Motor Constant:ooz-in/ W oN-m/ W

Torque Sensitivity:ooz-in/Amp oN-m/Amp

Back EMF Volt/rad/sPower WattCurrent AmpVoltage VoltResistance OhmsInductance mH

Max Winding Temperature: 155°C is stan-dard for Brush type, 220°C is standard forBrushless type.Other Max.Winding Temperature ifrequired °C

Environmental Requirements:

Temperature of Operation:Minimum °C Maximum °CShockVibrationAltitudeOther

Requested by:

NameTitleCompanyAddressCityStateZipCountryPhoneFaxEmail

DC Motor Design Guide

Application_____________________________________________________________________________________________________________________________________________________________________

Physical Requirements:

o Brushlesso Brusho Inner Rotatingo Outer Rotatingo Limited Angleo Framelesso Housedo Maximum ODo Maximum Lengtho Minimum ID

For Housed Motors Only:

ODLengthShaft ODShaft Length

For Brushless Motors Only:

Commutation:o Hall Sensors o Resolvero Encoder o None

Drive Output Waveform:o 6 Point Trapezoidal o Sinusoidal

Winding:o Single Phase o 2-Phase o 3-Phaseo Delta o Wye o Open Delta

!W!W







© Copyright 2005 Axsys Corporation Rev. 07/2005Axsys believes the information in this publication is accurate as of its publication date. Such information is subject to change without notice.Axsys is not responsiblefor inadvertent errors. All brands and product names aretrademarks of their respective owners.

Axsys Technologies’ Motion Control Products Division is experi-enced and ready to build a high-quality system solution for yourspecific application.





Axsys Technologies, Inc.Motion Control Products7603 Saint Andrews Avenue, Suite HSan Diego, CA 92154

Ph: (800) 777-3393Fx: (619) 671-9292

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