Rated Voltage vs. CNC Power Supply Voltage

Perhaps one of the biggest points of confusion is the discrepancy between the coil voltage listed on a stepper motor’s datasheet and the usually much larger power supply used with it. If the datasheet lists a coil voltage of 3.4 volts, how is it that it can be used with a 48VDC cnc power supply? Or an 80 volt power supply for that matter?  

This is the deal. Ignore the voltage rating. Pay close attention to the current rating instead.

This can be done because most stepper motor drivers available are called chopper drives. These drives monitor the rise of the coil current and ignore the coil voltage. When the current gets to a predefined threshold (the maximum motor current), it shuts the power to the motor off. Or chops it. Don’t exceed the maximum voltage rating of your driver. That value is not a suggestion.


Take a look at the ratings given for this oyostepper.com stepper motor (part# ROB-09238):

Rated Voltage: 12V

Rated Current: 0.33A

Winding resistance: 32.6Ω

Winding inductance: 48 mH

The rated voltage of 12V is not a maximum value. What these ratings are actually saying is that if you apply 12 volts to a coil of this motor, it will set up a current of about 0.33A because the coil resistance is 32.6Ω.

If you are controlling a stepper with some H-bridges or a very simple type of driver, then you will have to limit the motor voltage to 12 volts to avoid exceeding the rated current, which is a maximum value.

In the case of a chopper drive, exceeding the rated voltage isn’t a problem. The higher voltage makes the motor reach magnetic saturation more quickly. The motor is strongest when fully saturated. The formula given below illustrates the point. It calculates the current through a coil after a given time frame.