Stepper Motor back EMF protection for Un-Powered Circuit

[war_isbest2005]

Dear All,
I have two stepper motors connected on single shaft for driving a load. Only one motor is powered at a time.I have concerns over backemf generated in the on-powered motor when the other motor is actively driven. This back emf will go to my Un-powered driver and can cause damage. Is there any way to protect my un-powered driver. I figured i will use a dual pole dual through relay and will switch the motor coil to open terminals . Now i have 2 queries
1. What shall i put on the floating terminals of the relay. Shall i short them or connect a resistor across the terminals.
2. There is a scenario in the system when both motors will be off and the load may move because of vibrations/shock. Can i prevent the load from moving if i short the relay under this condition.

 

[dr pepper]

Just one way would be to put a vdr or transorb (both voltage dependant devices) across each of the coils on both motors, select a voltage well below the motor driving device's rating and above the power supply (note if using a full bridge the voltage across the motor could be double the supply), then the voltage spike will be clamped below that the driver can stand off.
There are of course other means which will follow...

 

[Colin]

Have you tried just putting a diode on each line?