DC PM motor back EMF and controller

[mwardas]

The scenario is this:
I have a DC permanent magnet motor being that I want to drive using a Curtis controller at 24 volts and 50 amps, Using a suffiently sized contactor, will I be able to disconnect or connect the motor from the contactor while the motor is spinning at its rated speed? My application requires that at times, while running the motor at 24 volts and 50 amps at speed, that I have the ability to let the motor spin freely. Because of the motor back emf using PM motors, presumably with the controller connected across the motor, the motor will not spin freely-hence my desire to disconnect the motor from the controller so that there is no path for back emf through the armature. Other than contactor related issues, any thoughts as to problems I might encounter.

 

[Skogsgurra]

mwardas,

I am not sure which controller you are using, but most of the Curtis controllers (the 1225 for instance) have an Inhibit input. The Inhibit (active low) "kills" all action and should also switch off the bridge transistors. Switching them off will give exactly the same result as would a disconnect with a contactor. Much better, much cheaper, much cleaner.

 

[israelkk]

I am no electrical engineer but where the current energy will go? if you "disconnect" the motor's terminals whire its spinning you will receive a spark or a very high negative voltage which can hurt the winding isolation coating. You should route the current somewhere. I assume there is an electrical solution but you have to take it under consideration.

 

[baltic]

Another way to "virtually" disconnect the motor is to run controller with a zero current command. Then controller voltage generated due closed current loop will be exactly compensate for motor back EMF and will drop with motor speed. Motor current all will be PWM current ripple meaning zero on average as current command assumes.

Of course, this will lead to additional motor losses (power dissipation), mostly due to eddy current core loss.