VFD for Single Phase Motor


Controlling the motor speed offers many advantages. First, the VFD provides greater efficiency in both power use and transfer rate in a pump or motor. The VFD senses the load on the system and provides power to compensate. It also manages such problems as system malfunctions and overloads. This automatic, intelligent control can extend motor life, prevent system failure, and increase operational output.

Starting a single phase motors is different from a three phase motor but it is possible to have a VFD specially designed to meet the requirements of a single phase motor. But surely the motor needs to be an inverter grade, basically what it means is the insulation class of the motor to be improved. Can be cumbersome job to re-wind the single phase motors to meet the inverter grade (Class F / H) , but still i am wondering if there would there is a way to control the voltage spike at the inverter output like adding a first order or a second order filter to improve the voltage THD and remove the high frequency high voltage spike, then we can use a normal class B insulated motor with inverter.

Single phase input to VFD
So if you can't use a VFD with this design of single phase motor, what is the solution?  The answer is to input single phase to the VFD.  The VFD can act as a phase converter and output three phase to a three phase motor.
There are some considerations, particularly with sizing.  Some VFDs are designed and rated to input both single and three phase.  Check with the VFD manufacturer but you'll see something like this in the manual which denotes both phases.


Example
An application has a 230 VAC single phase input power source and needs to connect it to a conveyor that has a Variable Frequency Drive (VFD) connected to a 10 Horsepower 230 VAC 3 phase induction motor. Let us assume it has been determined that this application will operate well with a simple Volts per Hertz (V/Hz) VFD. The issue is, since there are no VFD manufacturers that offer a 10 Horsepower (HP) single phase input Variable Frequency Drive (VFD), we will need to de-rate a VFD with a three phase input for single phase input. Most manufacturers of VFDs only offer products up to 3 Horsepower (HP) for single phase input.  The 10 Horsepower (HP) AC motor nameplate reveals that the motor is rated for approximately 27 amps at 230 VAC. We must use the equation above:
VFD Input Current > Motor Current Rating * 1.73
VFD Input Current > 27 Amps * 1.73
VFD Input Current > 46.71
This application will need a 230 VAC 3 phase Volts per Hertz (V/Hz) Variable Frequency Drive (VFD) with an input current rated at or above 47.0 amps.
The operations of using VFD for single phase motor speed control are not complex. In this video, ATO will show you details about running a single phase motor on a VFD. It's important to note that there are two wiring methods for single phase motor connecting to ATO single phase VFD: wiring of removing motor capacitor & wiring of keeping motor capacitor. At first, you should choose the wiring method you needed, and then set basic parameters step by step.

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