jmkasunich,

The same is true for an open-loop step motor drive. Desynchronizing (stalling) a step motor at high speeds results in breath-taking BEMF voltages; hundreds of volts even from a 24VDC powered drive. The power section bridge design must handle the resulting insult by shunting the returned energy back to the supply's bulk capacitor or dissipate it in an overvoltage protection shunt across the supply bus. A full-bridge power section design greatly simplifies the solution because the intrinsic drain to source diodes form a full bridge rectifier from the motor winding to the DC supply bus.

The problem is caused by the mechanical energy stored in the motor and load moment of inertia; just by itself, a NEMA-34 motor spinning at 6,000 RPM stores about 35J of energy. Experiments have shown well over 20J of that gets converted back into electrical energy, the balance being dissipated mechanically.

xXACEXx,

It's pretty fast accel/decel all right; The NEMA-23 test motor will literally jump off of the lab bench if it's not secured. Keep in mind this is on an unloaded motor and the purpose is to see how closely the results approach the theoretical max accel rate of the motor.

This is a funny screw-up: I used a really large square NEMA-34 motor as a weight atop the NEMA-23 and even that barely held it. Yesterday the NEMA-23 bucked it off and the NEMA-34 fell onto the prototype drive. That brought all further testing to a halt until the board was repaired. I have the motor screwed down to the bench now.:-)

A BLDC servo is definitely in the schedule now, based on the recent FOC circuit results and the fact both require remarkably similar control circuits. The main difference is a BLDC also requires a Clarke transform and its inverse. The Clarke transform converts a 3-phase rotating reference frame to a 2-phase or quadrature reference frame. Its inverse does the vice-versa thingy.

Mariss