WILLIAM A. FLEISHER
Manager of Advanced Technologies
Kollmorgen Motion Technologies Group
Radford, Va.

Edited by John R. Gyorki

When sizing and selecting small dc motors, various approaches can be used to determine electrical ratings and motor constants. Picking the wrong approach or using the wrongmotor constant, however, can lead to a mediocre motion system. The data also can make comparisons between different manufacturers' motors grossly inaccurate. One way to ensure a robust motor-control system on the first pass would be to study all manufacturers' dc motor parameters, ratings, and tolerancing methods, and analyze their electrical and thermal effects. Also, analyze the tests that different manufacturers use to develop their motor parameters, then normalize the constants and evaluate the motors under one uniform set of conditions. The following details just such a procedure.

One programmable drive can handle a wide variety of motors in a closedloop motion system, provided it contains accurate values for electrical and mechanical time constants, peak power, peak torque, and motor constants. This Servostar drive, for example, controls brushless motors, direct-drive motors, and servomotors using parameters derived from tests routinely performed to set up drives for robust motion systems.

PERTINENT PARAMETERS

Five major parameters determine most motor constants needed to size a motor: torque sensitivity, K_t; resistance, R_m; peak current, I_p; inductance, L_m; and inertia, J_m. From these calculate peak torque, motor constant, response time, acceleration, time constant, and other performance factors using the Motor parameter derivations table. The same principles apply to linear-force motors, but rather than using rotational inertia and torque, calculate inertial mass and force.

Other critical factors, such as temperature rise per watt, cogging, viscous loss, ripple, friction, and motor weight let motion designers determine maximum duty cycle and output profiles to verify requirements. Regardless of the application, use parameter tolerances when calculating worst-case conditions.

Parameters for brushless dc motors powered by sine-wave drives differ from those controlled by conventional six-step dc pulse drives. And because a major trend is toward more brushless motors with resolver feedback, manufacturers are beginning to provide servomotor data sheets that list the torque sensitivity, K_t, in rms units to make the math easier when converting output torques and other parameters. Although other methods may be used to transform dc values into rms, it's not as simple as multiplying or dividing by a single factor. The Sine-wave drive conversions table translates six-step to sinedrive parameters without first deriving multiplying factors.

Numerous software packages are available to help take the drudgery out of defining motion profiles and selecting the optimum servomotor, amplifier, and power supply. Motioneering from Kollmorgen is such a tool on CD-ROM that runs in Windows. It calculates shunt regeneration for multiple axes, and simulates foldback circuits to ensure that the motor and amplifier combination considers the amplifier's time constant.

Another approach that might prove misleading is a comparison among two or more motors, skewed by the test conditions under which the motors are rated. It's often difficult when comparing similar motors to understand why one appears to have greater peak torque, faster response, or higher acceleration. The first thought that comes to mind is that one motor is better than the other. Although each motor may be honestly rated and run at the nominal point of each specific parametric rating, the test conditions may differ enough to provide widely different values in a control system, thus creating an unfair or unbalanced data comparison. The only way out of this is to examine the different ratings and the various methods commonly used for tolerancing, and apply conversion factors or transformations to put them all on an equal footing.

For example, consider two motors of equal size using comparable design techniques and materials. Both motors dissipate the same power and have the same motor constant K_m, but one produces twice the peak torque. Checking out the test method might show that the higher torque came from twice the motor's rated peak current. Further examination may reveal that the higher rated unit can survive only when current peaks for less than one second, while the more conservatively rated motor may operate at its peak rating for several seconds.

FIRST-RATE MOTORS

One major rating method deals with motor input current, usually specified as a peak value. Occasionally, manufacturers provide continuous limits for constant-duty cycle or steady-state torque. Some push the peak current levels relatively hard. They are limited only by wire-current density or brush-current density in brushtype motors. A higher current generates more torque, although system efficiency doesn't improve because input power increases. This is why designers should compare motor constant K_m, a parameter independent of input power, to find a performance index for motor volume.