I am using a 90vdc motor and a custom built, variable speed controlled DC drive using a 15 amp IGBT. I do not know much about the DC drive as it is "potted" and all components are sealed. I only know there is a 15 amp IGBT. The motor lead is fused with a 5 amp fast blow fuse. The motor has to be able to be stalled and handle polarity reversal. I have found that the motor will run at full speed at times but never blow the motor fuse. Is this a problem with EMF and would a varistor solve the problem. Also does it matter where on the motor leads this component is used(I.E. before or after the fuse, close to the motor or could i put it further from the motor?)

Your help would be greatly appreciated

 

I have found that the motor will run at full speed at times but never blow the motor fuse. Is this a problem with EMF and would a varistor solve the problem. Also does it matter where on the motor leads this component is used(I.E. before or after the fuse, close to the motor or could i put it further from the motor?)

Can you elaborate?

When it runs at full speed, does it mean it does this without you giving the command to do so?
Why a varistor that protects against transient voltages should help to avoid the problem you have?

 

The motor runs at full speed when the command is half speed (from potentiometer). The potentiometer is set at 5 and the motor runs as if it were set at 10.

 

I was told to use a varistor because of polarity reversal,that diodes would not work because of the reversal.

 

I think something has gone wrong with the controller and that is why the motor runs at full speed when you only tell it to go half speed. A MOV (varistor) can help prevent voltage spikes from hurting the controller by absorbing the energy. It is best to place the varistor close to the motor.

However, this will not repair the controller. That is a second job to be done.

 

I suspect that a spike (transient) is the cause of the problem. There are no signs of a direct short. All the components are in tact. There are no burn marks or smell. The only component failure is the IGBT and possibly the rheostat. The motor's polarity is reversed quite often and the motor has a tendency to be stalled mid travel. I think that the stalling of the motor for a period of time followed by polarity reversal may be causing this transient. I have replaced the IGBT (I was wrong about its amp rating).

Thank you for your help with this matter i appreciate it

Jason