Successfully implementing a brush-less (BLDC) servo motion controlsystem requires — at the very

least — understanding the operatingparameters of each element in the sys-tem: motor, amplifier, feedback devices,motion controller, and all power trans-mission devices, Figure 1. One of themost complex relationships centers onproperly matching servo motor and am-plifier. These two elements share signif-icant and subtle interactions that arekey to achieving optimum system per-formance.

To better understand the matchingprocess requires knowing how each op-

erates and some of the basic opera-tional equations.

Motor equivalent circuit

For a brush-type dc motor, the ampli-fier bus must deliver enough voltage andcurrent to satisfy Equation 1, which is de-rived from the brush dc servo motor’selectrical characteristics as shown inFigure 2.

This equation also applies to nonsinu-soidal commutation of BLDC motors.

This is not the case with sinusoidalcommutation, because all three windingsare excited simultaneously. The resultantvoltages and torque values are inter-twined with the amplifier switching fre-quency, characteristics of the outputpower stage, feedback device that pro-

Matching servomotorsand amplifiers

Properly matching brush-type and brushless dc (BLDC) motors and amplifiers is essential to obtaininga high-performance motion system. This article gives you tips on how to do it.

POWER TRANSMISSION DESIGN JULY 1997 41

JAY K. GREYSON and WILLIAM A. ASHWORTH, Cleveland Motion Controls

V R I L dI dt K NB M M M M E≥ + +( ) / (Hot

Jay K. Greyson is vice president and gen-eral manager and William A. Ashworth isprincipal application engineer of the TorqueSystems Div. of Cleveland Motion Controls,Billerica, Mass..

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Motor current (equivalent to

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Speed, krpm

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Hot motor resistance,

Torque

Peak torque

Brush drop (approx. 1- 2 volts)

Bus voltage, V

Applied motor voltage,V

Amplifier saturation voltage, V

),

ΩΩ

Digital motion controller

Servo amplifier

Encoder or resolver Servomotor

Power transmission

(gears, bearings, etc.)

Velocity feedback

Current feedback

Hall commutation (brushless

only)

Power

Position feedback

Load

Load inertia

Viscous friction

Figure 1 — Basic servo system with typical feedback circuits.

Nomenclature

mPRECISION MOTION CONTROL

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vides the commutation, and the motordesign. Thus, it is important that the si-nusoidal amplifier and BLDC motor betested as a system to avoid any perfor-mance surprises.

Amplifier switching frequency

In general, a high-switching frequency(20 kHz and more) by PWM amplifiers en-ables using motors with low inductance,high-energy magnets, and less wire. Suchmotors can operate at high speeds, withrelatively low voltage (including 24-Vdc),and yet offer fast response.

The minimum rated inductance is volt-age dependent. If it is rated at 2 mH at100V, it will be 6 mH at 300 V, because ofthe power loss in the amplifier that is afunction of the slew rate.

Torque ripple

A nonsinusoidal drive applied to a si-nusoidal motor may produce torque rip-ple (equation 2) approaching 15%. More-over, if 180-deg square-wave phasecurrents are applied to a mismatchedmotor, it can develop 40% torque ripple.

By contrast, torque ripple of a sinu-soidal drive theoretically can approachzero. In practice, torque is a function ofload inertia, motor design parameters —such as stator-tooth-tip design, winding

pattern distribution, rotor slewing, etc.— and current harmonics, because nowaveform is a pure sine wave. Therefore,the torque ripple may exceed zero, but itwill be much closer to zero than other al-ternatives.

Amplifier current limiting

Servo amplifiers contain two types of cur-rent limit. One protects against sustained(continuous) overcurrents to protect themotor from exceeding its maximum contin-uous torque rating. It also prevents the am-plifier from self destruction.

Set higher than the continuous value,the second type of current limit protectsagainst excessive short-term (peak) cur-rents. It enables the motor to deliver ahigh-output torque for short durations.Plus, it provides protection from motordemagnetization.

For brush-type motors, peak currentlimiting helps protect the brushes andcommutator from arcing and exceedingcurrent density limits.

Regardless of the type of motor anddrive, it is vital to ensure that the ampli-fier can supply sufficient current to gen-erate the continuous and peak torquesrequired by the application.

As an example of what can occur whensystem components are mismatched, Fig-ure 3 shows an amplifier that will allowthe motor to achieve its continuoustorque rating. However, the peak torqueof the motor will never be reached be-

cause the amplifier’s current-limit con-trol limits the motor’s output torque to101 lb-in. while the motor’s peak torquerating is actually 150 lb-in. In applica-tions that require peak torque to acceler-ate the load, this limitation may be a se-vere penalty to pay for ratherconservative protection against excessivepeak-torque overloads.

A worst-case scenario is indicated inFigure 4. In this example, the continuouscurrent rating of the amplifier is so lowthat it restricts the continuous outputtorque of the motor to 42% of its capability.Moreover, the peak torque capability ofthe motor is limited to 27% of its capacity.

Commutation limits

Brush-type motors are limited by thewatts that can be switched (commu-tated). Each such motor has a commuta-tion curve that indicates the speed andload that sparking between the brushesand commutator bars begins, Figure 5. Itis important to operate below this thresh-old to ensure rated motor life.

Motor winding construction

If an SCR amplifier is used, the motorwindings may need to be braced due to thehigh surge currents inherent in an SCR-

POWER TRANSMISSION DESIGN JULY 199742

Figure 2 — Brush-type dc and brushlessdc (BLDC) servo amplifier output stageand servo motor equivalent circuit.

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current limit

Peak

motor limit

Figure 3 — Typical BLDC motor operating curves with driveprotective (overcurrent) values.

Figure 4 — BLDC motor operating curves with conservative driveprotective (overcurrent) values. These settings are so low inrelation to the motor capabilities that they may prevent themotor from delivering sufficient accelerating torque.

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based drive. Adding inductors to reducethe rate of rise of this surge current is an-other possibility, but again this approachadds cost and lowers system efficiency.

Power supply and amplifier voltage

The amplifier must have sufficient busvoltage for the selected motor’s antici-pated peak speed and torque (Equation1). All voltage drops must be consideredincluding those in the motor and the con-necting cables.

To determine the power supply andamplifier voltage needed for the applica-tion of a brush system, the following gen-eral equation can be used.

A properly matched system must alsoconsider energy storage capacity of thebus filter capacitors. These capacitors

minimize the supply voltage ripple andensure that an adequate bus voltage isavailable upon demand. The capacitorsare also instrumental in absorbing ex-cess regenerated motor energy duringchanging load conditions, such as decel-eration. For certain applications, thesize of the bus capacitors and the addi-tion of a shunt regulator must be care-fully considered.

Form factor

Motor form factor is defined as the ra-tio of rms motor current to average motorcurrent, expressed as:

Excess ripple current in the motor cancause the motor to overheat, exhibit poorlow-speed performance, reduce systemresponse, and increase audible noise. De-pending on the amplifier being used, themotor may have to be derated substan-tially. For example, a derating of 30% to60% is typical when using a full-wave, sin-gle-phase, SCR type controller that typi-

cally has a form factor of 1.1 to 1.85 . How-ever, PWM controllers, can provide aform factor that approaches unity.

Impedance matching

The amplifier must be tuned to optimizeperformance by adjusting its control loops.This involves setting parameters such asthe velocity and current loop gains, notchfiltering, current limiting, etc.

Figure 6 compares the servo currentresponse vs. time for a matched and un-matched servo motor and amplifier.The initial response of a current loopchange is analogous to an accelerationmotion profile. The matched motor-am-plifier current response has a fasterrise time, less overshoot, and lower cur-rent ripple. The amplifier engineer cantune the motor current loop to maxi-

mize system powerover a wide frequencyrange by adjusting theinductance compensa-tion, which is usuallycalled “current loopgain.”

Ambient temperature

Many manufactur-ers rate motors andamplifiers at differentambient tempera-tures. Derate one ormore of the individualcomponents, i f theambient temperaturewil l be higher thanthe specified operat-ing ambient for thatcomponent.

Terminations andfeedback devices

To ease the installa-tion phase, assure theterminations betweenthe amplifier and mo-tor are compatible.Some motors are sup-plied with cables andplugs, and many ampli-

fiers are fitted with screw terminals.For feedback devices, two aspects

must be covered: terminations and avail-able power for encoders and other simi-lar devices. Assure there is a match forboth voltage and capacity.

To obtain information on the CMCMotionmaster software package formodeling and sizing motion systemcomponents, please circle 301 on thereader service card.

POWER TRANSMISSION DESIGN JULY 1997 43

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curr

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30.0

25.0

20.0

10.0

15.0

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Time, msec1816141210 86420-2

Current overshoot on unmatched set

Matched

Unmatched

Higher ripple on unmatched set

Figure 5 — Typical brush-type dc motor with rated,communication limits, and reak overcurrent values.

Figure 6 — Comparison of matched and unmatched servocurrent loops. These current values indicate output torque.

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