Can Motor be used with VFD?

Has any one any information on a single phase 230v wound rotor slip ring motor? Can't seem to find any info on web, mostly 3 phase with slip rings. (would like to try VFD with it if possible )

james

A VFD will not work with a single phase motor, slip ring or squirrel cage. Do you mean single phase rotor 3 phase stator, or single phase both? If I remember my machines lectures correctly a resistor is connected in the rotor and varied to control the starting and running torque characteristics. Do you have such a motor?

I must admit the only time I have ever seen a slip ring motor was in the machines lab at university and they seemed to be a curiosity even then in 1968. Wikipedia says they have largely been superseded by squirrel cage motors with VFDs!

A slip ring motor would need to be controlled as a synchronous machine, rather like a permanent magnet motor, although there would need to be a field control circuit in the VFD too and it would want to be 3-phase. Many VFDs will control synchronous machines (you need a rotor position sensor / encoder / resolver to do so of course) but I'm not aware of any with field current control.

Are you certain it's a "slip ring motor"? It's much more likely to be a brushed motor if it's got extra connections for the rotor and only 2 for the stator. I'm trying to imagine what a single phase synchronous machine would look like.....

Gentlemen,

It is a slip ring wound rotor. no windings on the stator, all start and run windings are on the rotor along with the

centrifugal switch. You are correct John, it is a curiosity, at 1/4HP it's been on the lathe this last 35 years or more.

Plenty of ads for single phase to single phase stuff on the web, I am just testing the water, oh well, many thanks.

I will carry on with my clutch mod.

james

Intriguing. No windings on the stator you say and single phase? Sounds like an inside-out induction motor.

Most of the (few) references seem to be for 3-phase induction motors that have wound stators and the rotor windings brought out (via the slip rings) so you can vary the rotor resistance and thus achieve some form of speed / torque control.

Could you post some photos? Would love to see what one of these looks like up close.

Murray

Commutators have been replaced by slip rings in the past but I don't think it ever really caught on. A commutator needn't be cylindrical. The same thing can be done with disks. I'm sure I saw some discussion on the subject a long time ago. All a web search could bring up is something similar like this - the breaks in the ring can be seen

It possibly could have some advantages. I'd guess it interested the automotive people at some point which would be why I came across it.

John

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Edited By John W1 on 07/10/2015 21:10:37

Slip ring and commutators are two different animals and one cannot just replace the other.

Commutators are switching devices and are composed of segments, 2 minimun up to as many as you like (no max) whereas slip ring are just a means of making a continuous connection to something rotating. Either device can be cylindrical, flat or any other shape (although I've never come across a conical one)

Ian P

Murray,

Will see what I can do, as counter shaft is now in bits. Can give the motor a bit of TLC, years since I last had it apart.

james

regardless of most of the above, if this is an alternating current motor, single phase, slip ring or not, it is synchronous with the frequency of the supply, and a VFD, being a variable frequency drive, will vary the speed! What sort of lathe is it?

Phil

Just to add to my last post, Large slip ring motors used to be used for starting machines with high starting loads, or for driving machinery which needed to be accelerated up to the working speed more slowly than was possible with a direct on line type starter. The starter for a slip ring motor fed the rotor windings through a group of resistors which were switched out of circuit one by one as the motor and machine accelerated. These were VERY large motors, and they also had wound stators, this is a very small motor, with what I presume is a permag stator.. I am intrigued, any pics available?

Phil

As surprising as it might sound I am well aware that slip rings are normally continuous.

John

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Slip ring motors were a common fitment on travelling overhead cranes in our press shop and toolroom, the rotor resistors were switched by a large drum switch to give smooth operation to the crane movments.The dies were stored outside in a die park with a travelling gantry crane, one night we came in and the dayshift had wired the long travel motors direct on as the resistor bank was unserviceable and spares unavailable. Standing on the gantry walkway 25ft up in the air when that started off felt like being in a cartoon when their feet run off and the body catches up later. We paid a few visits to the crane that night as the fuses did not like direct on starting.

Mike

Gentlemen

Photos of rotor wound slip ring motor .

More photos in Album, my next job is to put it all back together. ( Murray, one may never ever see one of these again ). The second photo shows the centrifugal switch at the back of the slip rings. By the way I live in Cardiff..

james

Very interesting, ordinary motor turned inside out! the squirrel cage is in the stator and the induction coils in the rotor, used to create a high torque motor in a small frame size perhaps? any makers name on the motor?

Phil

Is it a Murad motor, it is certainly unusual?

Phil

Well, that IS an unusual motor design. I cannot imagine there are many other examples of the same motor still surviving so its definitely worth restoring. Thanks for posting the pictures too. To answer the original question, I would say that you could not control its speed running it off a VFD. I might operate happily over a limited range of frequency (say 40 to 60Hz) if you had a suitable single phase supply.

I wonder what advantages the original manufacturer saw in that method of construction, it must have been more expensive to produce and I am at a loss to see any functional benefits, in fact I can only see disadvantages.

Anyone got any thoughts on the following points?

Purpose of the ring of small holes in the outermost contact ring (cooling maybe?)

If the device between the windings and the contacts rings is the centrifugal switch, its not going to help the rotor balance and its adds to the motor shaft overall length.

I presume there are two slip ring connections (carrying L and N) so the start winding is connected via the centrifugal switch.

Nowadays lots of motors have the windings on the inner part of the motor but the difference is that its the inner that is stationary (so called 'outrunner' motors). Papst, amongst others make mains voltage, single and multiphase motors with an outer rotor (once very popular as tape recorder capstan drives) but the gain in performance given by the flywheel effect of a heavy outer rotor is no benefit as the motors get larger or more powerful as its harder to keep the enclosed windings cool.

Ian P

Phil.

Sorry I did not post photo of nameplate, it's a B.T.H. Rugby machine. Type BA147. Form DB1 , 1/4 HP,

2.3 amps at 1425 RPM . Continuous rating, usual 230/250v. It has been on the lathe for at least the last 35 yrs.

When I acquired the lathe, its an old Myford ML1/2 (not sure if motor was with it). Had to make counter shaft and motor base for it, at the time I was employed in the steelworks, have not that luxury now, pity, could have made my clutch.

james

Ian P.

The purpose of the ring of small holes is to fit balancing weights, as there are a couple on both the slip ring and on the cooling fan, when running it's as sweet as a nut.. This is the first time I have taken it apart since I

fitted it 35 odd years ago, thank you all for the interest you have shown in such a curiosity..

james

So, nowhere to connect a capacitor as there are only two connections to the winding, and no obvious sign of a shaded pole. So how does the darned thing start? Maybe the centrifugal switch shorts out a turn to create a shaded pole to get it started, then releases it when running. Or is there a capacitor hidden in the rotor?