Speed control of 1phase motor

rQx

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Oct 2010
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Hi,
I'm very familiar with speed control of 3 phase motors with VFD but how do you achieve this on a 1 phase motor if you need remote control of speed and start/stop etc
/Tim
 
I have seen a few 1phase in 1 phase out inverters on the market but never had a need for them. See Eaton DC1 for example. I suspect there may be some 1 phase motors that it won't work with, so read the manual first.
 
I connect small single phase AC motors to AB Powerflex 4/4m drives all the time. In fact I've got a display this year at Automation fair (two actually) that are doing just that. You don't get any direction control but you do get speed control. If you don't need speed control then all you need, depending on the size of the motor is a motor start capacitor. They need to be sized to the motor and that's beyond my knowledge but it is pretty easy info to find out.

https://youtu.be/azWUlDClTjs
 
What motor are you using in the video? My guess is that it’s a shaded pole motor.

Shaded pole motor will work with a single phase variable frequency drives but they are very low power motors.
A Variac can only change the output voltage the input and output frequency will remain the same. Lowering the voltage supplied to an AC motor will reduce the motor output power / torque with a given load the motor will slow down at the same time the motor will heat up eventually the motor will overheat.
One type of motor that is used all the time as variable speed single phase motor is what is called a universal motor used in power tools and appliances. These motors are nothing but a series wound DC motor. No VFD is necessary just vary the supply voltage and you vary the motor speed for a given load if the load increases then you need to increase the voltage to the motor to maintain the speed. While these motor are designed as DC motors and have brushes they work equally well at low frequency ac (50 – 60 Hz)
We could spend several pages on the different types of single phase motors and why you can’t change the frequency to change speed of each of them.
 
It will depend on the type of single phase motor you have. The vast majority of them use a capacitor to start turning in the correct direction, then that capacitor is switched out of the circuit by a centrifugal switch once it gets to speed. These types of single phase motors cannot be speed controlled at all, because when you lower the speed, the centrifugal switch recloses and puts the capacitor back in, which will make it overheat and possibly damage the drive transistors.

That leaves only two types of single phase motors that can be speed controlled, Shaded Pole (SP) motors, and Permanent Split Capacitor (PSC) type that can possibly be speed controlled. SP motors are very low torque to begin with and you can effectively control the speed by simply lowering the voltage because that reduces the torque even further. So the load slows down, but the motor impedance changes so it doesn’t overload. You CAN use a VFD on them, but it’s essentially overkill. PSC motors have two capacitors in series with the two windings (start and run) permanently. Because the caps are in series with an inductor (the windings) they are less affected by the harmonics from a VFD and this type can be run from one.

There are VFDs designed to operate PSC (and SP) single phase motors. They are extra expensive compared to similar 3 phase motors. I have heard of people using 3 phase VFDs and running single phase (probably PSC) motors, but the VFD cannot have output phase imbalance protection because obviously, you can’t connect 3 output phases to only two wires. Apparently the PowerFlex 4 is one of those.
 
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