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shaded pole motor

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Lynne27/01/2016 22:47:01
62 forum posts
23 photos

I have an extractor fan motor (not working) where one shaded ring is wired in parallel with the stator windings. An internet search of wiring diagrams does'nt show this, but there are so many I may have missed it. Is this wiring arrangement unusual. I would be interested to know a bit more about it.

Regards Lynne

john fletcher 128/01/2016 08:56:50
527 forum posts

Shaded pole motors are cheap as chips. Lots of washing machine pumps and fan heater and as you say extractor fans have shaded pole motors. They have poor starting torque. On some it is possible to dismantle the motor and reassemble it so that it run in the reverse direction, not all though. Usually the shaded pole is a piece of copper (ring) inserted in the main laminations.John

John Haine28/01/2016 09:33:17
2604 forum posts
133 photos

Wikipedia tells you all that is probably worth knowing:

**LINK**

If the shaded pole winding isn't just a shorted turn, then it may be connected to things like capacitors in various ways to control e.g. starting torque; of there may be two that can be shorted (one or the other) to reverse the motor. I don't think they would be connected in parallel with the stator winding - as they have much lower inductance this would just blow the fuse!

KWIL28/01/2016 10:26:08
3117 forum posts
56 photos

I have never seen a shaded pole motor where the shading ring is connected to anything else. Try removing any linking wires so that you are left with the coil only connectd to the power.

Reversal can be achieved where you can disemble the motor completely and invert the laminated stator block.

Lynne28/01/2016 22:09:54
62 forum posts
23 photos

Thanks to all who have responded to my post. Having looked again one lead is is attached to a stator coil, however

the other lead is not , I had thought that there was a link from where the other wire of the coil was soldered, to the tag where the other lead shaded ring was attached , and there is'nt. deteriorating eyesight, age related that is my excuse. There was once a speed control system consisting of potentiometer triac resistor and capacitor, which overheated in a serious fashion, burnt out infact. I am not into electronics sufficiently to put together a replacement.. The resistance through the stator windings is 8 ohms, which seems very low.

It is going to be an interesting exercise to find a replacement which is dimensionally similar. The extractor assembly is German , as are the rest of the kitchen units, and fitted over 25 years ago

Regards, Lynne

KWIL29/01/2016 09:41:21
3117 forum posts
56 photos

If product is German in origin, start by looking at Papst motors as a starting point.

Lynne30/01/2016 04:23:18
62 forum posts
23 photos

KWIL, Thanks for the lead.

Colin Whittaker30/01/2016 09:09:11
99 forum posts
12 photos

A shaded pole motor is an induction motor. Induction motors use a rotating magnetic field from the stator to drag the rotor around. Once it starts spinning an induction motor no longer needs a rotating field.

The simplest induction motor is the three phase where the rotating magnetic field comes from three or six windings and the 120 degree phase difference between phases.

Unfortunately the majority of us don't have three phases and some slight of hand is needed to make an induction motor work with a single phase.

The most common solution is to use one set of windings connected to the AC supply and a second set of windings connected via a phase shifting capacitor. Usually the capacitor is used just for starting and then disconnected with a centrifugal switch but sometimes the capacitor is left permanently connected.

A shaded pole motor saves on the sets of windings, it only uses one. The phase change is obtained by a copper ring circling half of each pole piece. As the flux starts to increase through the pole a current is induced in the copper ring that opposes and delays the build up of flux. This causes the magnetic field to rise first in the unshaded pole and then move to the shaded pole. It's not a high torque from rest so it probably won't work for a compressor but it does find many applications such as a fan.

However, if you have two sets of windings then this sounds like a variation on the capacitive start motor where one winding is normal but the second has a significantly higher resistivity. This higher resistivity delays the build up of the current in this coil and therefore delivers a delayed field hence a rotating field. (But please don't call it a shaded pole motor.)

Lynne31/01/2016 11:19:05
62 forum posts
23 photos

Colin, thanks for your informative response.The motor does indeed have 2 stator winding, and 2 shading rings, one of which has 2 leads from it which I now am aware was probably connected to a capacitor. There was once a speed controller which gave variable speed (not stepped) and a capacitor which had seen better days, just a sticky mess, , so I could'nt determine any values the rest of the controller had burnt out .The extractor hood assy is German, and a label on the casing states 'motor 230v, 50hz,230w.

I have readings through the 2 stator windings, the rotor looks ok , but how would I check?. Capacitor type and values? and wiring it in would be speculative. I would be quite pleased to have the motor run at one speed.

The motor has all the characteristics of a ______ from what I have seen of pictures, so what should I call it. I could come up with a few non electrical terms. Regards Lynne

John Haine31/01/2016 11:36:31
2604 forum posts
133 photos

Rotors are essentially solid steel and copper, if it rotates freely it is almost certainly OK.

As it has 2 shaded poles it will probably run quite happily without a capacitor. How are the stator windings arranged? Can you post some photos?

Lynne01/02/2016 23:22:31
62 forum posts
23 photos

John, I have created an album,and there should be a photo of the motor

Martin W01/02/2016 23:53:49
792 forum posts
29 photos

Just a quick thought, could that be a connection to a thermal switch/fuse that is bedded in the assembly to protect the windings/motor from overheating?

Martin

Nicholas Farr02/02/2016 05:39:51
avatar
1975 forum posts
936 photos
Posted by Colin Whittaker on 30/01/2016 09:09:11:

A shaded pole motor is an induction motor.........................................................................................................

(But please don't call it a shaded pole motor.)

Hi Colin, I beg to differ, a shading coil can be clearly seen on each pole of Lynne's motor, therefore it is a shaded pole motor.

Regards Nick.

Nicholas Farr02/02/2016 05:49:08
avatar
1975 forum posts
936 photos
Posted by John Haine on 31/01/2016 11:36:31:

Rotors are essentially solid steel and copper, ....................................................

Hi John, the majority are steel laminations and an aluminium embedded squirrel cage, but copper can also be used for the squirrel cage.

Regards Nick.

Les Jones 102/02/2016 09:01:26
2092 forum posts
144 photos

Hi Lynne,
I have never seen any connections made to the shading rings on a motor. I can not see any connections to the shading rings on the side of the motor in your photo. can you post a picture of the other side of the motor showing the connection to the shading rings. Also can you post the schematic of how the motor was originally wired. Was the capacitor you refer to just the low value capacitor (In the region of 100 nF) used in the phase angle speed controller or was it a larger value capacitor.

image020216a.jpg

I have copied your picture into this post so that people do not have to go to your album to see it.

Les.

Colin Whittaker02/02/2016 09:35:12
99 forum posts
12 photos

Thanks for posting the picture.

Nick, without the picture it had sounded as though there were two sets of coils. I can see now that there is just one set of coils connected in series or parallel so they both see the same phase.

The shading rings will delay the field causing it to rotate anti clockwise as viewed in the picture.

Lynne, I can't see any reason for a capacitor to be included unless it was for power factor correction but that seems a little over the top for a small cooling fan motor.

Variable speed control? Well it will almost certainly be a variable frequency drive with some way of limiting the current as the frequency is dropped. Getting this motor to work at a fixed "semi-synchronous" speed looks achievable. Variable speed operations sounds ambitious.

Nick again, I see your comment about aluminium wired squirrel cages. I had thought all squirrel cages used copper wiring to maximise the rotor volume/area available to the magnetic flux of the steel. However I see from an internet search that cast aluminium is being used. Is this common? I've never seen it and I'm frequently at the electric motor rewinders here in Phuket.

Cheers, Colin

P.S. I was taught electric motor theory by Prof. Eric Laithwaite. That may or may not have been an advantage when hypothesising on motors like this.

John Haine02/02/2016 10:43:34
2604 forum posts
133 photos

Colin is correct, this is a basic single winding shaded pole motor where the winding is split between the two opposing pole pieces. The extra black wire is odd and if I had to guess it's an extra winding that used to generate a low voltage to do with the crude triac speed control. The two "shade" windings are entirely conventional.

Lynne, I'd suggest stripping off the thick black wire and the motor should work at a single speed off the mains. No need for any capacitors or anything like that. This should get your extractor back on the road.

This motor probably has quite a high resistance rotor and would qualify as a "torque motor". These run at high slip ratio and you can vary their speed by varying the winding current. Used for example as spool drives in reel-to-reel tape decks, where a low current is used when playing or recording, a higher current for fast-forward or rewind. I used to have a Magnavox deck which had two of these for the spools, plus some big resistors switched in and out to vary the torque depending on what you were doing. There was a third more conventional motor driving the capstan at constant speed.

The triac control would just be working like a lamp dimmer, the capacitor probably as a phase shifting component in the gate drive. Horrid.

Modern higher power induction motors use cast-in aluminium windings for their squirrel cage, but all the cheap shaded pole motors I have ever seen have their rotors made of copper rods rivetted through the laminations to copper end rings - the kind of thing that can be done in a jobbing engineering shop.

Edited By John Haine on 02/02/2016 10:44:51

Muzzer02/02/2016 12:48:38
avatar
2904 forum posts
448 photos

Can't help thinking the black wires simply go to a thermal cutout buried down the side of the winding. It also looks as if the free black wire was originally soldered to the spare tag (second from left on top row). I bet if you measure between the black wires you will find an open circuit (non-resettable thermal trip blown) and if you bypass the black wires the motor will run again.

Of course, even if this "fixes" the motor, the cause of the overheating may still remain. Generally the lifetime of a motor like this will be down to the bearings (not shown). Both porous bushes and ball bearings dry out with time, at which point the motor load, winding current and temperature will increase. Unless this is an impedance protected motor, you need a thermal cutout to protect against stalled or overloaded conditions. You might bypass the cutout but if you don't rejuvenate the bearings, you may simply cause a fire next week / month / year.

For motors of this quality, you may be best to throw it in the skip and buy a new one. They aren't really intended to be repaired.

Lynne03/02/2016 17:09:25
62 forum posts
23 photos

Muzzer. You are correct, the free black lead did go to the tag. Your suggestion that the two were from a thermal cutout buried down the side of the windings, encouraged me to try and extract it, and it did relatively easily, together with the protective shroud. I have put a picture in my album, for as yet I have not sussed attaching pictures to my posts. However I did get a reading of 8 ohms across the two leads, and 18 ohms through the two stator windings, so what does that tell you?

Reasons for overheating. The motor and cyl. type fan, were seriously gunged up with fat, despite renewing filters from time to time. It would be a shame to bin it, as the bracketry and bearings are robust, and incorporate rubber mtgs.

Les. Thanks for the picture upgrade. Wish I were able to do that. No info on wiring arrangement. Capacitor value, no idea. The innards had extruded themselves from the casing, which was a sticky mess, and attempts to clean, resulted in removal of any data that existed. Picture of the other side in my album Sorry.

John & Colin. Thanks for all your input. I guess that if the device is indeed a thermal cut out, it should be retained in the circuitry , that is if the motor ever runs again .

Muzzer03/02/2016 17:27:20
avatar
2904 forum posts
448 photos

Sounds as if the cutout didn't blow - or that it may be some form of PTC (resetting) device if your meter is accurate - have you checked what reading you get with the probes shorted together?

If the bearings are still OK but the fan simply bunged up, it may be possible to reuse it but I'd suggest you keep the thermal protection thing in place. Sounds as if it should work again if you free it up.

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