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If you briefly increase the voltage going to the stepper motor's coil when the power to that coil is switched on, or changes polarity, you can increase the maximum useful RPM, and or the motor's torque at the top of the stepper motor's speed range. This might be called "boosting" the stepper motor.

There are several ways to boost a stepper motor. In a stepper motor driver circuit that uses resistors to limit the motor torque you can put a capacitor across the resistor, which will pass current briefly at the start of each coil switch "on", or a coil's polarity change. A transistor can be arranged in parallel to the stepper motor's current limiting resistor to do the same thing, but with somewhat better results since the resistance vs. time can be adjusted with the components used.

The drive itself can also do the boosting, as in a chopper drive where the chopping is stopped for about one millisecond after the coil switch "on", or coil's polarity change, so that constant power is applied at the full supply voltage, there by having the current rise slightly above the holding current for a brief time.

If you over boost the motor you can cause it to burn out, or de- magnetize, because the running current will be greater than the normal holding current. Normally in stepper motors, the current decreases as the motor turns faster, causing the torque to decrease at higher speeds. Boosting seeks to keep the current almost even at all speeds. Boosting the current above the holding current will probably make the motor unstable because of a lack of breaking force, and will probably also burn the motor out from overheating.

In order for boosting to be effective the motor supply voltage should be 3 to 10 times the motor's holding voltage. In general the boosting should only last no more than about 1 to 4 milliseconds, so the stepper motor's coil current never rises to more than about 125% of the holding current at the peak.

SECTION: 5.40.70.1 R/C stepper booster circuit GRAPH1.