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Phil G4SPZ 26th Nov 2021 2:34 pm

Vintage 1/4 horsepower induction motor
1 Attachment(s)
I can’t think of anyone else who might be able to help. Does anyone know anything about induction motors? A friend of mine has found an old model engineer’s lathe which he hopes to get going, and he has asked me to check out the motor. It has been decades since I studied electrical machines but I haven’t come across a motor quite like this before. It’s a 1/4 horsepower Metropolitan Vickers single-phase motor and I think it might be quite old.

Essentially, the motor has a wound rotor with centrifugal starting switch, and two slip rings which connect to spring-loaded carbon brushes held in the terminal block. But... the stator windings have no terminals and no apparent connections to the supply. It’s almost like a ‘squirrel cage’ or split-phase design, but inside-out. An extensive Google search has drawn a complete blank.

I have Megger tested it and it seems fine, and the rotor windings test okay, but am reluctant to apply power to the brushes without fully understanding the design.

The nameplate details are attached. Any advice you can offer will be greatly appreciated!


John M0GLN 26th Nov 2021 2:55 pm

Re: Vintage 1/4 horsepower induction motor
From your description I think it's a similar type but different make to one I have, if it is it's a 'Repulsion Start / Induction Run' type, infinitely more info' on Google than I can give you.


PS. from the web a basic description.

The repulsion-start induction-run motor, starts as a repulsion motor and runs as an Induction motor. The advantage of this starting scheme provided greater starting torque than a split phase motor could provide. The repulsion start motor rotor is wound similar to a direct current armature.

ex 2 Base 26th Nov 2021 3:32 pm

Re: Vintage 1/4 horsepower induction motor
hello Phil, if you PM your email address I will forward you three or four scans regarding Repulsion motors, as it is easier thank typing and avoids confusion. Your friends motor is very old, but has a lot of life yet. Ted

ex 2 Base 26th Nov 2021 3:57 pm

Re: Vintage 1/4 horsepower induction motor
Back again Phil, after reading yet again your request, I think I'm barking up the wrong tree. I can send you the repulsion details any way, I am familiar with Repulsion motor but they have a com rather than slip rings. A very useful book on single phase motor is "The repair of the small electric motor" by Karl Wilkinson, Karl also wrote another book Rewinding small motors. I have both. Ted

Lucien Nunes 26th Nov 2021 4:01 pm

Re: Vintage 1/4 horsepower induction motor
There mighe be some confusion here. Repusion and repulsion-start motors don't have sliprings, they have a commutator on which run a set of brushes that are shorted together but are not connected to the supply. The stator is energised from the supply. On a full-time repulsion motor the brushes remain in circuit; on an RSIR they are thrown off by centrifugal gear that also puts a shorting ring around all the commutator segments to simulate a cage rotor for running.

The OP's motor seems to be a conventional induction motor but 'inside out' as he puts it, with the energised (primary) winding on the rotor and a non-energised (secondary) winding on the stator. Motors with both rotor and stator wound, with the rotor brought out to sliprings, were common in larger sizes (5hp up) to enable rotor-resistance starting. This improved the starting torque by raising the power-factor of the rotor current at high slip. But invariably, the stator winding would be the primary and the rotor the secondary, not least because of the option to disengage the brushgear after shorting the rotor in a manner similar to the RSIR.

I cannot work out the advantage of the OP's motor's configuration nor its specific starting method. Does it have some kind of pole-shading? I can hardly think that would offer superior torque to a split-phase stator, nor can I see that the brushgear would have lower maintenance requirements than the tried-and tested centrifugal starting switch.

E2A I do have some Met-Vick and AEI catalogues of the era that will be worth a trawl. There was some cross-branding within AEI so it might technically be a BT-H product, which will be evident from its general construction. I might have a look for patents on starting methods once we know more about it technically.

Fascinating. Pics needed!

Phil G4SPZ 26th Nov 2021 4:42 pm

Re: Vintage 1/4 horsepower induction motor
Ted, John, Lucien,

Thank you all for your quick and helpful responses and your interest! Sadly it will be next Thursday before I can take more photos, but I’ll post some as soon as I can.

I agree it’s probably a very old motor and probably of a design that was quickly superseded. Any info from old catalogues would be extremely helpful!

I would very much like to get this old motor running, as it’s got ball bearings and seems very conservatively rated - as you said, Ted, plenty of life in it yet.

Thanks again all, pics will follow.

Phil G4SPZ 2nd Dec 2021 11:36 pm

Re: Vintage 1/4 horsepower induction motor
5 Attachment(s)
Today I attached a temporary mains lead to the carbon brush holders and cautiously powered the motor up from a Variac. Works a treat! The centrifugal starting switch operates correctly and the rotor spins up freely, albeit a tad noisily but it probably needs to be stripped down and serviced properly. No-load running current is around 2.25 amperes at 220 volts.

I attach some pictures which I hope will aid identification. The terminal block is damaged and the electrical wiring is purely for test purposes.


Lucien Nunes 3rd Dec 2021 2:23 am

Re: Vintage 1/4 horsepower induction motor

The centrifugal starting switch operates correctly
Ah, so it has a split-phase winding on the rotor? That implies there is no special starting means that would warrant the inverted configuration, but it might be a measure to circumvent a patent on the (later generic) stationary starting switch arrangement that toggles out of mechanical contact with moving parts when running. If you can see any of the starting switch, please can we have a pic?

I think the end-bell castings show it to be genuine Metropolitan Vickers Trafford Park construction rather than BT-H who had the much larger market share of FHP inductions. It might be early enough to be pre-AEI.

Phil G4SPZ 3rd Dec 2021 8:57 am

Re: Vintage 1/4 horsepower induction motor
Hi Lucien. Thanks for that interesting information. The starting switch is mounted at the back (cable-entry) end of the rotor, and is just visible through a ventilation slot beneath the motor, though I doubt I can get a good photo of it. I’ll try. The switch is of solid construction and has a bowed steel-strip spring which creates a very positive snap action.

I have asked my friend what his plans are for the lathe from which this motor originates.

Phil G4SPZ 3rd Dec 2021 9:52 pm

Re: Vintage 1/4 horsepower induction motor
5 Attachment(s)
Some photos as requested. First, the centrifugal switch assembly, viewed from beneath the motor. I rotated the shaft between shots. Incidentally there are four bundles of rotor windings at 90 degree intervals.

Phil G4SPZ 3rd Dec 2021 10:07 pm

Re: Vintage 1/4 horsepower induction motor
4 Attachment(s)
Secondly, views of the stator. Contrary to my original posting, which I made after inspecting the motor under relatively poor lighting levels, the stator does not contain copper windings but appears to consist of rods embedded in the stator, clamped between shorting rings at each end. The stator itself seems to be made up of thin laminations, each 1/128” or 0.2mm thick.

This does confirm that the design of the motor is effectively a split-phase centrifugal switch-start wound-rotor, fed via slip rings and rotating within a squirrel-cage stator, or as I originally surmised: "a squirrel-cage motor inside-out"!

My friend has decided to make the lathe available for use by the preserved steam railway for which we both volunteer. I shall be restoring the lathe in due course.


Phil G4SPZ 3rd Dec 2021 10:15 pm

Re: Vintage 1/4 horsepower induction motor
1 Attachment(s)
The overall dimensions of the motor are roughly 6” diameter and 7” long.


Lucien Nunes 3rd Dec 2021 10:31 pm

Re: Vintage 1/4 horsepower induction motor
Thanks for those Phil, I will dig around when I have a chance. I have seen a lot of motors but never one like this. Perhaps I have not been looking carefully enough.

There might be a patent for the combined radial-action switch integrated with the pancake rings, which minimises the length and allows it to fit in a short end bell. That partially explains why the rings are so large, which is detrimental to brush life. Also seems to need a lot of weight to balance it.

ex 2 Base 4th Dec 2021 5:06 pm

Re: Vintage 1/4 horsepower induction motor
Thanks Phil for the update. I've seen a lot of motors but never one like that. I have a copy of "Electric Motor Repair" by Robert Rosenberg and it's not mentioned in there, very unusual, The casting is typical of BTH, and AEI motors, they must have made millions. Ted

Lucien Nunes 4th Dec 2021 7:32 pm

Re: Vintage 1/4 horsepower induction motor
I thought that was a Metrovick end bell on the slipring end, and you think it's BT-H. At first I assumed the motor was likely to be BT-H manufacture, but now not sure. Must get the books out.

mark_in_manc 4th Dec 2021 8:31 pm

Re: Vintage 1/4 horsepower induction motor
That's very interesting. I wonder if the missing corner of the block which holds the brushes can be made up with Milliput. I've used it for similar kinds of things; it's useful stuff, and I think a black version exists.

1/4 hp sounds a bit small unless it's quite a small lathe. If you want something bigger, you might like to PM me - I have some single phase motors of various types.

Phil G4SPZ 4th Dec 2021 9:07 pm

Re: Vintage 1/4 horsepower induction motor
Hi Mark. It’s only driving a 2” model engineer’s lathe, so 1/4 HP is probably more than adequate. I’m thinking of using a plastic terminal box, cut to shape to surround the terminal block and held by the two existing bolts, to provide a secure fixing for a cable gland.

mark_in_manc 4th Dec 2021 9:22 pm

Re: Vintage 1/4 horsepower induction motor
That is a small one! If you are able (and the mods are OK with it) it would be interesting to see it alongside its motor on this thread.

Phil G4SPZ 4th Dec 2021 9:52 pm

Re: Vintage 1/4 horsepower induction motor
The lathe is a Flexispeed Meteor 2 from the 1960s. There are lots of images of similar lathes available on line as this seems to have been a popular model. It is currently detached from its motor, but as soon as I can marry the two up I’ll post some pictures.

Phil G4SPZ 28th Dec 2021 7:12 am

Re: Vintage 1/4 horsepower induction motor
Update: I made up a pair of ring terminals to fit the spring-loaded brush holders, wired it up adding an earth connection to the frame, and slowly applied mains power via my variac. At about 40 volts it slowly started to rotate - it works! The centrifugal starting switch cuts out at an estimated 700 RPM and the motor runs up to full speed at anything from 100 volts upwards.

I still haven’t found any references to this design of motor. There can’t be many still around.

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