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Old 24th Aug 2018, 9:42 pm   #1
TrevorG3VLF
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Default Westclox direction

I have one working Westclock which is driven by a synchronous motor. When the mains goes off, it always starts in reverse. To overcome this, I leave it running for a day or so when it will then run forwards.

What is the mechanism behind this?
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Old 24th Aug 2018, 11:51 pm   #2
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Default Re: Westclock direction

Don't leave it running for a day going in the wrong direction. Just switch it off and on again until it catches the right half cycle of the mains and goes in the right direction. There should be a pawl that makes it always go in the correct direction, but it's often normal for these to become stuck or broken.
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Old 25th Aug 2018, 1:28 pm   #3
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Default Re: Westclock direction

There is no pawl and never has been, either on this one or on other worn out ones.

A dozen struggles to get at the switch behind the washing machine is too much with my faulty spine so I just leave it running in reverse for a while.

Why not run it in reverse?
Why does it go backwards? It is not just a matter of chance.
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Old 25th Aug 2018, 5:40 pm   #4
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Default Re: Westclock direction

The simplest kind of synchronous motor has a stator coil with two pole pieces, and an armature consisting of a simple permanent magnet with a North pole on one side of the spindle and a South pole the other side. When power is applied to the coil, one pole piece becomes a North pole -- so attracting the South pole of the armature and repelling the North pole -- and the other becomes a South pole, so attracting the North pole of the armature and repelling the South pole. The armature rotates accordingly so the opposite poles line up. Then the supply reverses, and so do the poles of the stator. The armature has enough inertia to carry on in the direction it is already rotating, so it carries on rotating in the same direction. But there is no saying which way it will go when it gets that first initial kick from rest.

This also means that if something stops the armature from going round, it will automatically reverse itself to clear the obstruction -- a property exploited in old lighting effects, which used various combinations of moving lamps, filters and mirrors to scatter beams of light. In some applications, such as revolving display stands and microwave oven turntables, it does not even matter which way it is turning. (In a shop window, it can even add a bit of variety if the figures revolve in different directions on different days. Or even used by spies to pass on yes/no or binary-coded messages to one another; with three revolving figures in a shop window, one could signal one of eight possible rendezvous points to a confederate .....)

So in a clock which you want always to go forwards, you can force the direction of rotation if you have some kind of pawl mechanism which slips in one direction, but catches in the opposite direction. (Or you could have a manual starter which gives the motor an initial kick before power is applied, so it continues in that direction. Or, if you are really cheap, you simply rely on the clock hands being turned forwards to force the motor around in that direction; the odds are in your favour that after a power failure, the clock will need to be advanced.)

It sounds as though your clock does have an anti-reverse pawl, but it has become a bit gummed up with dried lubricant or something, or perhaps a weakened or broken spring; which is causing it to take some while before it catches and causes the motor to reverse.

Another way to make a permanent magnet synchronous motor always go in the same direction is to have two windings, with one fed via a capacitor so as to introduce a phase shift, and the pole pieces spaced at 90° intervals. Now the pattern at the first pole piece goes off - weak N - strong N - weak N - off - weak S - strong S - weak S - off, while at the second goes strong N - weak N - off - weak S - strong S - weak S - off - weak N. The third pole piece is of S whenever the first is N and vice versa, and of the same strength; likewise the fourth and second pole pieces. This pattern of two interacting fields will always attract and repels the armature in the same direction, because the weak states still cause the armature to favour a particular position. If the capacitor is wired permanently between the two windings, we can use a single pole changeover switch to select forward or reverse by app.ying power directly to one or the other end of the capacitor. This method also makes the motor more powerful, since there are two coils helping to push and pull the armature around; but it is considered somewhat overkill for a clock.
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Old 25th Aug 2018, 7:09 pm   #5
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Default Re: Westclock direction

This is only information I've Googled, not from experience, but Westclox have at times used a Sangamo combined induction/synchronous unit.

At start up the induction part runs in a particular direction carrying the synchronous part with it until it reaches a speed where the synchronous part pulls into lock which then over-rides the weaker induction part.

Such a motor relies on some 'lost motion' between the induction rotor (an outer copper cup) and the permanent magnet rotor on the same shaft. If the lubrication between these two rotors has dried up and they rotate together as one, then I'm not sure what the effect might be.

Perhaps the 'phase' relationship between the stuck parts might tend to favour one direction, but that's just pure speculation.

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Old 25th Aug 2018, 7:51 pm   #6
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Default Re: Westclock direction

That's the same as the principle of some of Garrard's posher turntable motors; which were were four-pole types (hence with a deterministic starting direction) and used an armature having both a permanent magnet (for synchronous operation) and a shorted turn (for operation as an induction motor).

When power is first applied, the motor runs as an induction motor and attains its maximum sun-synchronous speed of about 1400 rpm; it is then close enough to synchronous speed for the motor to finish the last bit of acceleration running as a synchronous motor, and thereafter runs synchronously at 1500 rpm. (No power is transferred into the shorted turn at synchronous speed, since the armature is effectively stationary with respect to the stator.)
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Old 25th Aug 2018, 8:46 pm   #7
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Default Re: Westclock direction

>>No power is transferred into the shorted turn at synchronous speed, since the armature is effectively stationary with respect to the stator<<

- Not the case with the Sangamo (Holtz) motor though, since there are often (usually ?) different numbers of poles for the two sections. Hence why the inductive part is delberately made weaker than the synchronous part.

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Old 26th Aug 2018, 2:21 am   #8
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Default Re: Westclock direction

Surely the two sections would need to have the same number of poles A two-pole synchronous motor will run at 3000 rpm on 50Hz. A two-pole induction motor might manage 2800, 2900 with the wind behind it, but there has to be some slip or else there would be no induction. And surely you would want the induction section to take the motor as near as possible to synchronous speed for reliable handover to the synchronous section? That would require the same pole count; a 4-pole induction motor would only manage 1400-odd rpm, not enough for a 2-pole synchronous section to catch up from.
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Old 26th Aug 2018, 8:38 am   #9
John M0GLN
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Default Re: Westclock direction

Microwave ovens are the same, or at least ours is, it changes rotation every time it's used for the same reasons as Julie says in #4, the motor won't be able to turn in the same direction again as there is no free movement in that direction so it reverses.

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Old 26th Aug 2018, 8:59 am   #10
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Default Re: Westclock direction

The clock rotor is a disc with many teeth and the two poles also have teeth to match. When running in synchronism, the teeth will line up with the supply and the rotor will run slowly, perhaps 1 rev/sec. This then drives a plastic worm for further speed reduction which wears out.

None of this explains why the clock has a preference to start backwards. Techman says not to let it run backwards but no reason was given.
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Old 26th Aug 2018, 9:11 am   #11
John M0GLN
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Default Re: Westclock direction

Quote:
Originally Posted by TrevorG3VLF View Post
This then drives a plastic worm for further speed reduction which wears out.

None of this explains why the clock has a preference to start backwards.
As you said the plastic worm wears out, as all the moving parts will have worn to some extent over the years it may just be easier to start in one direction, the one with least wear.

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Old 26th Aug 2018, 10:28 am   #12
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Default Re: Westclock direction

Maybe we need to see a photo of the clock in question's movement to determine if it is just a self starting synchronous or a hybrid induction/synchronous.
But as others have said, it will have reverser pawl to correct running in the wrong direction, but these do wear/break/lube dries out etc, all of which stop their intended function. As for running it in reverse, this will not have any adverse effect, though I don't know why you would let it run backwards?
It is however nice to know these synchronous clocks are still used and enjoyed, I run far too many!
Rob
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Old 26th Aug 2018, 10:35 am   #13
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Default Re: Westclock direction

If the OP's clock does have the type of motor I described earlier, then here's a very relevant discussion on a clock forum, where a reverse start is explained:-

https://mb.nawcc.org/threads/help-sa...acment.100723/

I've annotated one of the pictures from that thread to clarify the operation:

Click image for larger version

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Trevor: Has the motor been reassembled at some time ?

Cheers
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Old 26th Aug 2018, 2:19 pm   #14
julie_m
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Default Re: Westclock direction

Well, that's really shooting cannons at sparrows, as they say in Düsseldorf!

So assuming I have counted right and estimated right where I could not see to count, it starts as a 12-pole shaded pole induction motor, which would max out at about 470 rpm (just shy of 3000 / (12/2) = 500) and always starts in the same direction by dint of the shading. But there is also a 30-pole synchronous element which will take over as soon as the spindle speed gets up to 400 rpm (the permanent magnet has 4 poles, so we are dividing our 3000 cycles/minute by 30/4 here).

It's all very clever, but still leaves me thinking there must be a simpler way of doing it!
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Old 26th Aug 2018, 3:44 pm   #15
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Default Re: Westclock direction

They are well and truly over engineered but work beautifully, I have a couple of clocks that use the Sangamo motor, it quite good to watch it spin up on induction and then sync lock.
The simpler (and cheaper) way, was to make it a manual start motor thus motor direction is guaranteed correct or self start with a direction corrector pawl, which was how most achieved it.
All good stuff though!
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Old 26th Aug 2018, 7:17 pm   #16
julie_m
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Default Re: Westclock direction

Or even use a split-phase motor, which would always start in the same direction -- though might require a new capacitor every 25 years or thereabouts!
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Old 31st Aug 2018, 8:30 am   #17
Mike Phelan
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Default Re: Westclock direction

Quote:
Originally Posted by Techman View Post
Don't leave it running for a day going in the wrong direction.
Trevor,
There's no reason why it can't run backwards for ever!

The Sangamo mentioned in the link is a completely different motor.

If it is starting automatically (in either direction) then it must be self starting, therefore there will be some means for it to start in the correct direction and that must have failed. Often because of dried up oil or someone putting WD40, 3-in-1 or other goop all over the movement.
We really need some pictures.
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Old 31st Aug 2018, 12:18 pm   #18
TrevorG3VLF
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Default Re: Westclock direction

Correction The clock is a Westclox, not Westclock.

Thanks Mike

I do not want to get the clock down again and possibly damage it when opening it. I have asked where the old faulty clocks have gone but noone will own up. Will have a brivet if I get some energy. If I find them then I will take a photo.

My father bought a clock (Smiths?) at an auction when we had electricity installed just after WWII. It was placed in the cowshed as farm time. I have washed the thrips out a couple of times, I have not checked if it is still there and working.

There is no mechanism to start in the right direction. Turn it off enough times until it runs forwards. I think there must be retained magnetism in the rotor which is offset from the 'teeth' to give a preference to the reverse direction. Such offset would be derived from the necessary torque when running forwards.
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Old 31st Aug 2018, 7:36 pm   #19
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Default Re: Westclox direction

Perhaps it's an 'M' series motor ? (Yet another self starting synchronous motor with no reversing pawl... )

Patent description here:-

http://pdfpiw.uspto.gov/.piw?Docid=2015042

Click image for larger version

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Perhaps one of the shading rings has been assembled in the wrong orientation? The Sangamo motor only has one so the effect is to simply reverse the start, but in the 'M' there are two, and it's not obvious what would be the effect of assembling with one in the correct position and the other offset.

Cheers.
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Old 30th Sep 2018, 1:05 am   #20
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Default Re: Westclox direction

I have found a motor with a reversing pawl while liberating the transformer from an old microwave.
I have takes some photos because it is quite easy to see how it works.
https://www.vintage-radio.net/forum/...13#post1079313
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