The VFD settings will have to be set for both motors so the small motor can easily fry before the VFD even notices the problem. The other danger is no overload protection on the smaller motor. If you are running your main motor at half speed, you will only get half speed of the second motor, and about half HP.
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Keep in mind that all motors will run at the same speed on the VFD output, you cannot individually vary one motor's speed. You are relying on the other motor to absorb the spike, so the small motor may not have much of an effect if the larger motor is still running Switching a motor while another is running can be done, but it would be best to check with your VFD manufacturer to make sure yours can handle it.
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There are load reactors for VFDs that may help reduce the spike from switching the motor, but anything you use must be rated for the high carrier frequency. Never put a capacitor on the output of a VFD, it can explode! The same holds true to any other device that is sensitive to high frequency signals. I share my VFD with a few machines, so finding a way to switch the output on this little motor while the main motor is running would make wiring simpler (perhaps?) Any ideas appreciated. I thought of using the single phase 220v and using a start cap and current sensing startup relay on the third leg to run it, cause it will be used very intermitantly. I could borrow my buddy's scope and experament, just wondered if any electronics junkies have run into this before? The application is I am trying to power a Hardinge lathe with a 5hp VFD and it has a small 1/10th hp 3 phase motor that runs a leadscrew that changes the pulley pitch, and thus the drive ratio. What about putting a bridge rectifier AC terminals across the contacts, and have the DC output go to an electrolytic cap? Not sure is this would work at all, just a brain storm. But can you run 1 motor and switch another on and off while in run mode? I know the switch contacts will still induce a spike, but mabe not as bad? Could a solid state device be wired across the switch contacts, simmilar condenser to reduce the spike? I was thinking along the lines of a MOV, but they do degrade with use. I know you are not supposed to interupt the output of a VFD because the inductive spike will be higher than the threashold of the IGBT's can handle.