Hi all, I am building a Centrifuge for separating water and contaminants from oil and was thinking of using a washing machine motor directly coupled to the central shaft to avoid a wasteful gearbox or belt drive.

Can anyone provide me with a circuit for a simple VFD circuit to drive this motor to between 8,000 & 10,000 RPM or give me any pointers in designing my own?

PS, I have an electronics background but have no experience with AC Motor control.

Any help much appreciated.

Hello IanFiTheDwarf

Welcome to the forum.

First, there are a number of constraints to consider with your application.
The first is with the motor itself. If you are going to direct couple the motor, then you are going to have to overspeed it quite a lot and there may be mechanical issues with bearings etc. Single phase motors are difficult to speed control, so you really need to be using a small three phase motor.
When you alter the speed of an induction motor, you do so by varying the frequency of the voltage applied to it. Double the frequency gives double the speed, but in changing the frequency, you must also change the voltage to avoid over fluxing the iron. As you reduce the frequency, you must reduce the voltage to keep the V/Hz ratio constant.
When you increase the frequency, you should also increase the voltage, or you will reduce the flux as the speed increases and this reduces the amount of torque that you can get out of the motor.
Above rated frequency, it is difficult to increase the voltage due to the limitation of the supply voltage. You can only put out the same voltage as you put in unless you add a transformer.

The other major thing to consider, is the cost of the bits to build your own speed controller. It is far cheaper to buy a commercial unit that to attempt to build your own. Small units are pretty cheap and you are not faced with a massive amount of work to get the software etc working. If you really want to build your own, I suggest that you look at a development system as supplied by the chip manufacturers such as Atmel, Microchip etc, but that will still cost you more than a complete unit.

I would recommend a mechanical speed step up so that the motor does not operate much above rated speed, then use a three phase motor and inverter.
NB You can get many low cost inverter speed controllers that are single phase in and three phase out.

Best regards,

Cheers Marke,

I think single phase washing machine motors must be wound to run at much higher frequencies than 50 or 60hz as from what I have found searching the web some run at up to 20,000rpm, the following speck was listed for a 1500 spin belt driven washing machine motor

• Type : Single phase, UM/AC
• Power (Wash) (W) : 585
• Speed (Wash) (rpm) : 10750
• Start : Electronic speed control

I suspect the controller in a washing machine must be fairly simple to keep the cost down; I may try a bit of experimentation or reverse engineering

Hello IanFiTheDwarf

The fastest that an induction motor will operate when driven by the supply is 3600 RPM at 60 Hz and 3000 RPM at 50Hz. These are two pole machines.
Universal motors will operate at higher speeds, but these have brushes and need to be regularly checked.
In older machines, induction motors were used with gear boxes to change the speed.
In some modern machines, BDC motors are used and driven by special electronics and these can operate at much higher speeds.
You need to find out what type of motor you are looking at, and then find out the detailed information on that motor.

Best regards,

Cheers again Marke,

I can confirm that the motor is defiantly a brushed universal motor, it just never clicked to me that it was a universal motor, the tacho generator on the back should have given it away strait away.

I shouldn’t have to much trouble building a phase angle controller now that I know what I’m trying to control.

Many thanks for turning the lights on for me.

Regards, Ian.