AN1495

Speed limitations

Doc ID 8700 Rev 3

9/25

3

Speed limitations

Since the motor coil is primarily an inductance, the rate of current change in the coils is

limited by the L/R time constant of the motor. As the motor is operated at higher speeds, the

L/R time constant of the motor limits the rate of current change and the current can no

longer follow the desired sine wave. Figure 7 shows the motor current at a higher rotational

frequency. On this scope trace, we see two effects. First, the filter on the reference voltage is

starting to roll off the reference signal and second, the motor current is limited by the motor

time constant and it begins to look more like a triangle waveform than the desired sine wave.

Although moving the pole of the filter on the reference voltage will make the reference signal

appear more ideal, it will have little effect on the motor current at this point since the motor

current is primarily limited by the L/R characteristics of the motor. When approaching this

point, the motor will run smoothly in full step mode and the microprocessor could easily

change to full step drive.

If the step rate is increased further, the motor will stall when the current can no longer reach

a value large enough to produce the required torque. Figure 8 shows a typical current

waveform when the motor has stalled. The almost pure triangular current waveform is

similar to the triangular waveform that would result if the motor were being driven in the full

step mode at this step rate. At this operating point the current is entirely controlled by the

L/R time constant of the motor and no chopping is occurring.

Figure 7.

Microstepping waveforms: current limited by motor inductance

AM15148v1