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Single Phase motor speed control

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Stub Mandrel27/04/2010 21:52:02
4311 forum posts
291 photos
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I was given a 1/3 hp Hoover 1,420rpm capacitor start/run motor two days ago. It's probably 1974 vintage, beautifully made with tiny oilers at each end!
It's a nice size to replace the DC motor on a mini-lathe, but is there a cost-effective source of speed controllers for SINGLE-PHASE AC motors. A variable volatge and frequency inverter would be needed.
Any suggestions?
Ian S C28/04/2010 12:03:25
7468 forum posts
230 photos
Hi Stub Mandrel, best speed controller for that motor would be stepped pullys! Ian S C
Clive Foster28/04/2010 13:07:35
2882 forum posts
104 photos
A running single phase motor could be speed controlled by a VFD in a similar manner to a three phase motor albeit with a bit more attention to drive current control.  Both require rotating electric fields after all. The problem comes in starting the beast.  Theoretically you could arrange a suitable drive to the starting winding from a second output to get things going but this output would have to be permanently connected to the starter winding and only switched on briefly at start up.  As most single phase motors have a centrifugal switch to control start winding power installation is complicated by having to bypass this switch.  Which usually involves getting inside the motor.  Its all rather impractical from a product marketing viewpoint.  Just to make matters worse most single phase motor designs inherently have much more torque drop off than three phase motors once you move significantly off the design speed.
Your capacitor start and run motor is inherently a single (nominal) speed device.  When running it behaves as a two phase machine with the winding currents and phase separations defined by a careful balance of capacitance, winding inductance and reactive interactions.  Which only come out exactly right at one speed.  Move too far off and things get hot'n bothered quite rapidly.  Theoretically the capacitors could be removed and a VFD unit programmed to drive the windings directly and correctly for the desired speed but you'd virtually have to design a drive specific to the motor.  The washing machine and white goods people do this sort of thing to shave a few farthings from the price but they are making millions of the things.  Out in the real world a standard three phase motor and VFD set-up is easier and cheaper.
John Haine28/04/2010 13:19:57
4272 forum posts
251 photos
An interesting question.  In theory a capacitor start and run (CSR) motor is a 2-phase machine, where the quadrature drive for one phase is derived through the capacitor.  You can derive a 2-phase supply from a 3-phase one using an appropriate pair of transformers, so it ought to be possible to run a 2-phase motor from a 3-phase VFD.  Two problems in practice - the VFD output won't be very sinusoidal so the transformers might not work very well, and the CSR motor won't be very well balanced as the winding the capacitor supplies only has to generate enough flux to get the motor started (I think). 
John Haine28/04/2010 13:29:59
4272 forum posts
251 photos
Update - see Wikipedia
This says that actually the windings are identical so the motor should be balanced.
The transformer is called a "Scott Connected Transformer" =
...and it might be possible to buy them as a standard item?
Stub Mandrel29/04/2010 19:38:15
4311 forum posts
291 photos
1 articles
Thanks Folks,
I followed the Drives Direct advert over here -------->
and they do a 1/3hp motor and speed controller for £145
I think I'll fit a few pulleys and keep the Hoover for my Pott's spindle and a few things like that, and save up for one of the above as a guaranteed solution.
John Haine29/04/2010 20:18:57
4272 forum posts
251 photos
That sounds a good idea.  I did find some Scott transformers on the internet - made in India and look like they weighed a couple of tons!
Clive Foster30/04/2010 00:13:40
2882 forum posts
104 photos
Electric motor terminology can be a bit imprecise.   Capacitor start and run normally refers to motors with a capacitor permanently connected to the secondary winding with a second, larger capacitor connected in parallel, via the usual centrifugal switch, to give a good starting boost.  In general the windings on such motors are different and, although its in circuit all the time the "starting" winding usually doesn't contribute anything like half the output power.  These motors are normally designed to improve the torque curve  in comparison to standard single phase motors so they are much harder to stall and better able to run up to speed under load.
The permanent split capacitor (PSC) motor described in the wikipedia article as having identical windings is a very different beast with some rather bad behavioural characteristics making it unsuitable for general use.  Its characteristics are well matched to blower and fan jobs tho.  Starting torque is pretty low and torque rises pretty much in proportion to speed, just what a fan wants but not ideal for other jobs.  Its very efficient when running at its rated load and design speed but altering load, speed or both rapidly reduces efficiency leading to hot running.  Fractional and small horsepower versions will usually cook themselves to death in short order if run off load, sucking a lot of power while doing so.  I found this out the hard way by killing a 1/3 rd HP PSC motor on a drill press.  When running light the power draw would blow a 10 amp fuse.  The multi-speed, multi tap versions are usually small and not especially efficient.  Design is, I understand, something of an art.

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