# How do I determine the power rating of a stepper motor?

I have in my possession two stepper motors that are clearly marked as 50 ohm, 22V motors. They have six leads each, which I believe makes them unipolar stepper motors.

It is my understanding (and experience) that unipolar stepper motors can be driven as bipolar stepper motors by ignoring the center lead, and that is what I am doing. When I measure the resistance between leads to determine which lead is the center of each winding, however, I find that the resistance between each "edge" lead and the center lead is 50 ohm, and the resistance between the two "edge" leads is actually 100 ohms. It is my guess that this is because when driven by a unipolar driver, current is never passed between the "edge" leads, and only ever from the center lead to one or the other "edge". This arrangement would make the 50 ohm rating make sense. Is this correct?

Because I am driving these motors as bipolar motors, however, I see double the resistance that a unipolar driver would see. I am also using a "chopper" driver, or constant-current driver, and I have the ability to select the maximum current for the motor. How do I calculate this? Is it a simple question of ohm's law, using the 100 ohm value I measured between the "edge" leads?

## 2 Answers

I would suggest driving the motor with 72% of rated current, or about 310mA. That should yield the same power dissipation as would be expected from unipolar drive at 440mA. When the motor isn't moving, the voltage would have to be about 31 volts. I wouldn't expect insulation problems at 31 volts, or even 50, but I don't know how far one could safely go. Perhaps measure the AC voltage when hand-spinning the motor quickly to see if there seems to be a limit.

If you are driving motor in non-standard mode, the only reliable way is to increase drive current while monitoring coil temperatures. When it reaches some 60-80C in continious operation - that's your power limit.

In any case, going much higher than 22V might cause insulation breakdown, so I would not go higher than 1.5x.

• I'm driving it (currently) at 12V, but the voltage isn't really the point. Chopper drivers (like mine) deliver (approximately) constant current, but not (at all) constant voltage. I would like to know what a safe current is. – Mark Mar 24 '11 at 15:47
• Well, That is was I saying. Increase current while monitoring temperature. Make sure that voltage is not over brakedown limit. That's it :-) – BarsMonster Mar 24 '11 at 16:08