The reason that you here the whining when they lock up is because you are not delivering enough amps to the motor for the speed you are trying to achieve. You must remember that you need to supply enough amps for all three motors simultaneously. so if you have 3 motors at 1.9 amps per phase, you will need to have at least a 12 amp power supply. If you are using a "chopper based driver" then you can lower that by 1/3. but no lower.
Hope that helps |