Couple Stepper to AC motor

Question

Here's perhaps a strange question... I have a situation where I need both continuous rotation as well as incremental position control with hold capability. The approach that would work quite well would be to have a dual axis stepper motor between the AC motor and the load and have it disconnected (relays not just at the controller) while the AC motor is driving the load in a continuous fashion and then when I need to move it incrementally, turn off the AC motor, toggle the relays and use the stepper to do its thing... But I'm concerned that something would go badly after a while. The way I see it, if not powered, the stepper would just act as some kind of 4 phase generator, but considering that steppers usually run on low voltage & high currents, I am concerned that the voltages generated might be dangerously high or that the stepper is not designed for something like this.

Is it possible to drive a stepper motor like this for longer periods of time without damaging it? Anyone got any information about something like this? I was originally thinking of using a clutch, but the only way I could mount this is with the stepper between the AC motor and the load, not stepper behind AC motor. I would equally like to avoid gears if at all possible... The reason I cannot use the stepper in a continuous mode to drive the load is that I need a high HP motor for the continuous rotation and only a fairly low torque and low speed for the stepper part of the motion. I also don't think that I could use an encoder to then convert the AC motor to some kind of servo...

Anyone got some ideas on this?

Thanks in advance Joe

Answer 1

If one motor is completely disconnected when the other is operating, it is possible to drive the load with one motor turning the other. If the idle motor is disconnected, there should be no problem with the motor acting as a generator. If one motor is a permanent-magnet type, it will present some pulsed reluctance torque as a load to the other motor. That will result in some torsional vibration, but it would not necessarily be harmful to either motor or the load. Both motors must be mechanically capable of the maximum speed. If one motor is a permanent-magnet type, the voltage generated when it is idle must not exceed the motor's insulation rating. The motor that is in the middle must be mechanically capable of transmitting the torque of the motor on the end. The motor that is driving must be capable of driving the inertia of the idle motor. There may be alignment or overhung load issues if one motor shaft is much smaller than the other.