Shaded pole motors

[waggy]

Can our learned electrical friends help me out please?

I have a fairly old "Glowworm" model 45f gas central heating boiler in my garage, used to keep me warm in the winter months. The motor for the flue fan has burnt out, no air flow, no gas to the burner!

It is a shaded pole motor, two pole I'm sure. I got a new motor yesterday, same fixing but slightly smaller coil size (physical, no idea of the rating). Everything works well but the motor hasn't got the power to turn the fan fast enough to create enough pressure to close the air pressure sensor, enabling the gas to flow.

The coil dimension is nominally 30mm wide by 45mm diameter. What would be the approximate wattage of the motor? Does anybody think a washing machine water pump motor would be powerful enough? I changed one of these for a friend some time ago and the motor was similar size.

All the information I can find is in Jim Cox's motor books, thats why I think it's a two pole machine.

Yours with a cold snap looming,

Waggy.

[baggo]

Hi Waggy,

just a thought but does the fan mount straight on to the motor shaft or does it have it's own bearings? If it has it's own bearings they may have dried up as the fans tend to run very hot. I have to strip the fan down in my boiler every now and again to lubricate the bearings.

Have no idea about the wattage of the motor but if it has a serial number you may be able to get some info off the web. The water pump motor may work if you can get it to fit but you won't know till you try! You'll have to check that it runs in the same direction as you can't reverse them!

John

PS a new fan will only cost you £100 + VAT !!

[modeng2000]

It is sometimes possible to reverse this type of motor by turning the laminated stator the other way up and re-assembling.
As an aside, I have done this for a quartz clock driving a chart recorder. The clock ran backwards to avoid one extra stage of gearing.

John

[JohnP]

Waggy,
There's nothing like a simple question to show up the holes in your knowledge! I was taught that shaded pole motors run at synchronous speed, unlike say an induction motor which needs a degree of slip in order to produce torque. That's all very well, but the shaded pole motor must produce torque at non synchronous speeds, otherwise it would never actually get up to speed. The motor's torque/speed characteristic is going to be important here, and I wonder if this is why your replacement motor isn't doing the job, because it can't get up to synchronous speed.

Alternatively, maybe it's a 4 pole machine and its synchronous speed is only 1500rpm. Is there any way you can run the motor off load and measure its speed?

The washing machine pump motor may be a good idea, they certainly see a varying load as the water sloshes back and forth.

I think some ex-equipment 240V AC fans have shaded pole motors, but these are getting rarer as low voltage DC units become more common.

I realise this may not be of much help, but if I post what I know, maybe someone else can add to or otherwise use it.

Cheers,

JohnP

[jackrae]

At this stage, apart from a burned out motor, you need to ask yourself if the pressure switch is working OK.

By way of example , I recently had to replace the "draught" pressure sensor on my tubo-flue fire . Although the capsule worked perfectly and operated the micro-switch at the correct pressure (0.5"wg) the switch contacts were dodgy.

I put this down to the switch probably having silver contacts (as against gold) and because of the relatively low current drawn by the sensor circuit, any oxide layer wasn't being cleaned off during switch operation.

Changed capsule c/w switch and all now OK.

Hope this helps
Jack

[houstonceng]

If it's the usual form of shaded pole motor (record players, washing machine pumps, etc, etc) - and it's only got one coils mounted on the laminations with the latter ending in pole pieces around a rotor consisting of wires bedded into aluminium & steel laminations - and it looks the same as your old motor (except for coil size) - then it's got the same number of poles and the true synchronous speed is going to be the same.

I suspect that it's not got the power and, consequently, isn't reaching synchronous speed with the fan mounted on it. Try it without the fan and see if it speeds up.

The ex-washing M/C pump motor may do the trick - assuming it rotates in the right direction. If not, you may be able (as John modeng2000 says) to take off the bearing blocks and, effectively reverse the position of the "shadings" wrt the output shaft. The last few motors I've had the displeasure of replacing (One W-M/C pump and one condensing boiler air pressure fan) have both been riveted together so you would have to drill them out and use bolts/new rivets in re-build.

Most of the failures I've come across are due to bearing seizure in the motor causing overheating (locked rotor) and coil burn-out OR "pennies in the pump" causing locked rotor, etc. If you catch either fault early, a good oil-around can sometimes free up the rotor until the next time. Usually it's too late by the time you notice "nil rotatum".

Anyway SP Motors are relatively cheap and I've usually been able to source genuine or pattern replacements from boiler spares/W-M/C spares dealers. That way the repair has some guarantee of working and I save my time for Mod.Eng. rather than doh-mess-tech repairs.

[waggy]

Many thanks for all your help and tips. The matter is now sorted out, found a boiler spares shop about two miles from home. Visited yesterday, my request for help produced the usual sharp intake of breath and the reply "I can't get you one of those, not until tomorrow lunch, anyway!" New fan assembly purchased, fitted and domestic bliss restored!

Such a nice change to find a shop where the person behind the counter knows what they are about!

Waggy.

[havoc]

Such a nice change to find a shop where the person behind the counter knows what they are about!

I rarely have that experience when going shopping for this hobby...

[waggy]

My dear Havoc, how right you are! I once called in a local DIY shop on the off chance they may have had 1/32" drills in stock. I was advised by the lad behind the counter "No mate, they don't make 'em that small!"

Interesting to note, the same shop now sells other delights, beer, wine, etc! I wonder why?

Waggy.

[waggy]

More help from our electrıcal frıends, please?

I have pulled apart the coıl from the old motor, there appears to be two coıls wound on the same former. Couldn't really say ıf there were two as the burn damage stopped me from completely unwındıng the coıl. If there were two wındıngs, why?

Yours wıth an overload,

Waggy.

[baggo]

Hi Waggy,

did the 'two' coils have seperate connections? It is possible the motor was a dual voltage i.e. 220 or 110. For 220 (240) the coils would have been in series, for 110 the coils would have been in parallel.

John

[Tony K]

Just a little off-topic, but has anyone else noticed Baggo's nocturnal tendency?
Regards, Tony. ;D

[waggy]

Aye up Baggo,

The motor only had two connectıons. No sıgn of any ınternal connectıons, only the two taıls. One of lıfes onsolved mısterıes I dare say!

Waggy.

[spurley]

Mickleover........................land of the midnight sun!! ;D ;D

[baggo]

Hi Waggy,

It could be that it was wound with two coils of thinner wire rather than one of larger gauge. This is sometimes done to get the correct resistance for the coil or to fit the right number of turns on the bobbin.

Anyway, must fly back to my rafter in the loft as the daylight is beginning to hurt my eyes

John

[Phil Sutton]

Hey Baggo,can it be that you can't see your reflection in the mirror? ;D ;D
(If so how come Dracula was always clean shaven............?? ;D)

Phil