Understanding Kollmorgen Stepper Motor Current Units

The stepper motors' continuous current rating in the Kollmorgen documentation is in units of dc current.  This is based on the actual DC current that the motor can handle, and applies to older drives using full step or half step resolution.  Most stepper drives sold today take advantage of the microstepping technology.  Microstepping causes a 30 percent loss of current, because the wave form is closer to a sine wave than a square wave.  This means that the actual current is the rms of the rated current.  Therefore, the torque will be 30% less than the rating. 

This becomes an issue when setting the commanded current in the stepper drive.  Should it be set in rms or peak (DC)? 

Some stepper drives automatically compensate for the loss of current in microstepping mode by boosting the output current, so that the motor will produce the same torque as if it was not using microstepping.  Kollmorgen’s 6410 and P7000 drives automatically compensate for the current loss in microstepping mode by boosting the current to the peak value.  When in full step or half step modes, the current will be the DC current rating from the catalog.  When in microstepping mode, the output current will be the catalog current rating x 1.414 (peak value).  This allows the motor to produce the rated torque while using microstepping.

For drives that do not boost the current as compensation in microstepping mode, set the commanded current appropriately based on the mode being used.  For full step and half step, use the catalog current rating.  For microstepping, multiply the catalog rating by 1.414. 

Kollmorgen’s P5000 and P6000 only use microstepping.  There is no full step or half step mode in these drives.  Therefore, the documented current selections have been compensated such that the motor will produce the rated torque.  Select the current that closely matches the catalog current rating.