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Inverter drives for single phase motors

Use of inverters with cap run induction motors

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Ed Dinning 108/04/2019 16:46:35
24 forum posts

Hi Gents, has anyone recommendations to make for the use of inverters on 230v. single phase, to run an induction motor(single phase, cap run, 230v); or have they successfully used them ?

Maker, supplier, likely price ?

A simple unit is all that is requires as the motor drives a fan and only 2 speeds are needed. One near max and the other about 1/2 speed.

Many thanks, Ed

Tim Stevens08/04/2019 17:25:58
1126 forum posts

I'm not sure what exactly you need to invert - the supply and the motor being both 230v single phase.


John Haine08/04/2019 18:04:14
2836 forum posts
141 photos

Ed wants to vary the speed, hence the frequency.

Ed, I'm afraid that you are not likely to easily find an inverter for this job, as capacitor run motors have only a limited range of frequency over which the capacitor can maintain approximately a 90 degree phase shift. There has been some discussion on here about running a single phase motor without its capacitor on 2 phases from a VFD but I don't know if anyone has done it successfully. 2-phase inverters don't seem to be made.

People do make fan speed controllers, whether they will work with your motor I don't know.


Wout Moerman08/04/2019 18:33:15
47 forum posts
2 photos

You want to use an inverter as a variable frequency drive? I don't know how much you can vary the frequency for a sine phase motor without it stopping. The capacitor is optimised for 50 or 60 Hz and I don't know if it generates a good phase shift at half the frequency.

An alternative can be to remove the capacitor and power the motor as a 3 phase. For some single phase motors this is possible. Not if it starts with a centifugal switch I think. But if it has 3 identical windings it is possible. Then power it with a 1 to 3 phase VFD.

Ed Dinning 108/04/2019 19:18:23
24 forum posts

Hi Gents, thanks for the inputs.

The application is a forced draught fan for a wood burning boiler.

Original equipment is a 250w cap run induction motor that is working well. It originally used an Italian made inverter as a 2 speed VFD.#This has died with no obvious faults and no way to get a schematic.

The PCB logo is (RD)² . It appears to have many more facilities for control than were needed, but no details of how to activate them so we can have full speed and about 1/2 speed,

I've since found Invertek drives and their Optidrive range has a single phase in, single phase out inverter for fan and pump use that can be configured as I need it, as well as many other features that I will never use.

Anyone familiar with this unit ?


Andrew Johnston08/04/2019 20:30:37
5120 forum posts
595 photos
Posted by Ed Dinning 1 on 08/04/2019 19:18:23:

Anyone familiar with this unit ?

No but you need to be certain what sort of motor you have. The Invertek single phase in and out are intended for permanent split capacitor or shaded pole motors. Shaded pole motors in particular are used for fan applications as they are cheap and robust and the poor starting torque is not a problem. In addition the locked rotor currents are only 20% or so above rated current.


norman royds 208/04/2019 22:00:43
43 forum posts

I have a brook Crompton parkinson range mater inverter v220/240 input output 370 w I bought board with dc controller and a single phase inverter and single to 3phase so do not know what it was for but it looks very complex for a fan regards norm hope this helps you

Ian S C09/04/2019 13:03:12
7468 forum posts
230 photos

Some two speed fans of around 40 Watt use a filter choke as found in older power supplies, when that is switched in the speed drops, 20/25 % at a guess. I would have thought that being able to switch the motor from 2 pole to 4 pole to adjust speed.

Ian S C

Russ B09/04/2019 13:42:22
560 forum posts
21 photos


How about using a 110v transformer and a rotary switch to run through the 110v or bypass it. This keeps your CAP running at the correct frequency for the phase shift

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