Electricity + Control May 2015

DRIVES, MOTORS + SWITCHGEAR

Test set-up.

A bench top motor dynamometer for drive testing

By G Craig, Techlyn

Because stepper motors are current driven, even at standstill, testing has required only a test oscillator, power supply and appropriate motor. During a soak test, reliability and heatsink temperature could easily be assessed.

T echlyn has manufactured stepper motor drives since the mid nineties. Our new positioning drive for brushless (and brushed) servomotors created a testing problem. With these motors the drive current is proportional to the position error, so some method of loading of a rotating motor was required. A ‘quick and dirty’ test could be performed by using a lever on the motor output shaft at standstill. However, this would result in asymmetrical currents in the motor and drive. Drive operating principle Figure 1 shows the most important function blocks. The motor pro- vides two feedback signals: • Three magnetic Hall effect sensors to indicate when the three sta- tor windings should change polarity in order to maintain rotation of the permanent magnet rotor • An incremental encoder to allow high resolution positioning

Figure 1: Most important function blocks.

Electricity+Control May ‘15

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