Single Phase Motor Speed Control

[discobobby]

Can anyone tell me how i can varie the speed of a 230v single phase ac motor. Preferably as easy and as cheap as possible.

Many thanks

Bob

[buildersmate]

You have to feed it from a variable frequency drive (VFD). The speed is otherwise 'locked' and fixed to that derived from the 50Hz sine wave from the 230V mains.

These things used to be prohibitively expensive (compared to the cost of the motor), perhaps not so now.

Stuff on VFD here.
http://en.wikipedia.org/wiki/Variable-frequency_drive

[Hymie]

Buildersmate’s solution is somewhat expensive – much cheaper to use a lamp-dimmer type circuit that are commonly used on electric drills.

This item on ebay looks just the job (and cheaper than building it yourself):-

http://www.ebay.co.uk/itm/High-torque-50V-220V-10A-AC-Motor-Speed-SCR-Controller-Power-2000W-/230897430868?pt=UK_BOI_Industrial_Automation_Control_ET&hash=item35c28f3154

The unit claims a rating to 220Vac – but I suspect that is just because of the 220V mains in China. You will need to enclose the unit in a suitable box (as is, the hazardous mains is accessible).

Alternatively you could purchase a standard light dimmer switch (suitably rated) and wire to a mains plug & socket.

[buildersmate]

Hymie
OK, so that's true of voltage control but only within a narrow band (perhaps up to 20%) of speed reduction. Go outside the narrow range and the torque produced will fall off very quickly.

[BRIGHT SPARK]

Is the motor driving a constant or a variable load.....an important factor to consider when looking at speed control on the cheap.

[Hymie]

Buildersmate is correct in that using a light dimmer switch type circuit (chopping the mains voltage) will result in poor speed regulation/control with a varying torque/load. At low speeds it is possible that the motor will stall.

But depending on the motor application, such a circuit may do just what you want.

I would add that since the light dimmer switch will be driving an inductive load, it should be rated at least 1.5 times the motor rating.