Spwm

[anandan]

 

Hello all,

 

Can we control the speed of a single phase induction motor with SPWM?

If possible what minimum speed can we achieve?

 

Regards,

Anandan

[marke]

Hello anandan

 

Welcome to the forum

 

It is not generally practical to control the speed of a single phase motor using any form of PWM control. This is because the single phase motor uses a phase shift method to generate a second phase and this phase shift component is frequency dependent. These components will be subject to the PWM carrier frequency rather than the low frequency waveform.

Additionally, the single phase motor usually has a start switch that opens at close to full speed disconnecting the start winding. If the speed was reduced, the start winding would remain in circuit and probably fail.

 

If you modify the motor and bring out both windings without the phase shift component or the start switch, you could use a "two phase" inverter to provide an output for the run winding, and a lower voltage output for the start winding with a quadrature phase shift. By the time that you do this, it is generally cheaper to use a standard single phase input three phase output inverter with a three phase motor.

 

Best regards,

[jraef]

There are single phase PWM drives for single phase motors, but the manufacturers are careful to caution users that they only work on 2 kinds of 1 phase motor designs; PSC (Permanent Split Capacitor) and Shaded Pole. Both motors are characteristically low torque motors for things such as centrifugal pumps (in the case of PSC) and fans (in the case of Shaded Pole). Shaded pole motors can also be speed controlled by simple voltage control, i.e. a rheostat, so it is unlikely anyone would go to the expense of a PWM controller for them. Which leaves PSC motors as the only really viable application. Even so, the 1 phase VFDs are expensive, making it close to being a push to do as Marke suggested anyway; use a 3 phase motor and a VFD capable of 1 phase input, 3 phase output.