Originally Posted by
Jared Sankovich
You can use a standard single phase induction motor with a single phase VFD if you're willing to do a few special things.
Let me discuss this a bit. The problem with a VFD on a single phase induction motor is the starting circuit, normally a capacitor and coil. I'm going to specifically address a capacitor start induction motor.
When a single phase capacitor start induction motor starts, the start capacitor is in series with the start coil. This generates a second phase which creates a torque on the rotor, causing the motor to begin rotating. When the motor reaches about 70% of full speed, the centrifugal switch opens and removes the starting circuit. The problem with using a VFD on a single phase motor has to do with this starting circuit and whether it's in the circuit or not.
You want the starting circuit to be in the system when you're first starting up the motor, but to then remove it in such a way that even if you reduce the speed below 70% of the full speed RPM it does not come back into the circuit.
Here's what you can do.
1. Get a momentary close single pole, single throw (SPST) push button switch.
2. Get access to your starting capacitor. It's usually on the outside of the motor so it's easy to get to.
3. Cut one wire going to the starting capacitor and, using some wire, hook in your SPST push button switch. Put the switch next to the VFD.
4. Hook the power wires of your motor to the VFD.
When you go to start your motor, set the VFD to close to the line hertz (usually 60 hertz). Hold your SPST switch down and turn the motor on through your VFD. This will start the motor normally, just as it it was connected to wall power.
Once the motor is up to speed, let go of the SPST switch. The starting circuit is now completely disconnected from the power. Go ahead and reduce the speed of your motor. When you reach 70% of the rated RPM the centrifugal switch will close but that will not affect anything because the starting circuit is disconnected by the SPST switch.
Us the motor and VFD normally, just as you would with a 3 phase VFD.
Induction motors on a VFD are generally constant torque systems when you're at an RPM less than the rated RPM. But since horsepower is a factor of torque and RPM, the motor will not be able to do as much work at lower RPMs because the HP will be lower.
Also, most general purpose induction motors are cooled by an internal fan. At lower speeds the fan does not move as much air so you have to be concerned about heat (true for both single phase and 3 phase motors) so monitor the heat and put an external fan pointing at the motor if it starts getting hot.
Mike
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