How does MID-7604 measure peak current?

What are the details about how the MID-7604 measures and regulates motor winding current?
We manufacture motion systems incorporating small step motors. One of our customers has ordered a motion system with the MID-7604. This is our first experience with the MID-7604.
When duplicating motor performance comparable to our other Step-motor Controllers, we must set the MID-7604 to 3X the motor's traditional rated peak current. For example, for a super-small MicroMo/Arsape Step motor, we traditionally limit the current to 250mA (RMS). However, with the MID-7604, we must raise the current limit to 700mA (peak) to achieve the same performance.
While we're not sure about the current regulation philosophy of our other Step-motor Controllers, the Step-motor Manufacturer's Specification Sheet limits the current to 250mA per phase with both phases energized to prevent overheating. Pumping in 700mA from the MID-7604 to achieve comparable performance just seems excessive. However, we're just playing with numbers here and not knowing what they mean. In the end, we're ignoring the numbers and instead, measuring motor torque and winding temperature in order to set the peak current switches on the MID-7604.
* How can we think of the current setting for the MID-7604?
* Is it not a PWM device that using width of voltage pulses to produce the current?
* What does "PEAK CURRENT" and "RMS CURRENT" mean?

Hey JThomas,
I'm sorry to hear that you smoked a stepper motor.There are a couple of reasons that your MID-7604 drive could appear to be operating strange.
There is a series of DIP switches on the front of the MID drive that allow you to set various things. One of those DIP switches is the current reduction switch. This switch is default in the ON position from the factory I believe so if it has never been turned off, chances are it's ON. When the Current Reduction switch is active, the stepper drive drops the current energizing the coils by 50% if there hasn't been a step in the last 500 ms. If you were gauging the strength of the drive based off of holding torque, then this could be the reason you were raising the current to a higher rating than you thought was neccesary. This settings is for customers who have an application with stepper motors that focus more on moving torque instead of holding torque.
Another possible reason for the increased current to match performance of your drives is if the bus voltage of your drives is some voltage other than 24 VDC. If your drives that you are used to using are say 36V drives, then at a lower current setting, they would provide greater torque at higher speeds because of the higher voltage being able to energize the coils faster.
Also, the KILL feature releases the axis from being actively driven. The HALT command simply locks the motor while leaving the coils energized to hold the position that it is currently in.
I hope some of this has helped you out. Let me know if I can elaborate on anything.
Thanks,
Evan D.
IMAQ/Vision R&D
National Instruments

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How does MID-7604 measure peak current?

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