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Old 30th Apr 2020, 11:59 am   #1
andynoiseboy
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Default Smiths Sectric very weak motor

Hello All
I'm new here and was hoping to find advice regarding a poorly Smiths clock.
The mechanism and motor all move freely but the motor frequently stalls, even when the next gear spindle is removed - it can barely drive itself.
Inspecting the rotor, I find that the magnetic poles are very weak and variable.

Does anyone know the pole layout and an effective method of re-gaussing the rotor, or is it destined for the dreaded quartz swap?

TIA
Andy
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Old 30th Apr 2020, 3:20 pm   #2
TrevorG3VLF
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Default Re: Smiths Sectric very weak motor

I am not familiar with this version of clock but those I have seen have a rotor with teeth which step one position for each cycle. The rotor is not magnetised, just pulled into position by the pole pieces.

I would dismantle and clean the bearings, lubricate with thin oil and then remantle. Will the rotor spin freely?
Check that there is a reliable connection between the mains plug pins.
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Old 30th Apr 2020, 5:47 pm   #3
andynoiseboy
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Default Re: Smiths Sectric very weak motor

Thanks for the reply Trevor.
The rotor does spin quite freely, as does the rest of the mechanism.
I have cleaned and lightly oiled everything but it hasn't helped.
Electrically, it's also fine - no poor joints etc.
If what you say about the rotor having no permanent magnets is the case, then perhaps the fact that there is some uneven residual magnetism in the rotor is what's causing the stalling??
I'd assumed that the rotor would have NSNSNS poles like synchronous motors tend to...
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Old 3rd May 2020, 8:48 am   #4
Mike Phelan
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Default Re: Smiths Sectric very weak motor

I thought ALL rotors on synchronous motors were magnetised

Yours has just lost its magnetism over the years - not uncommon.
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Old 3rd May 2020, 5:15 pm   #5
Nickthedentist
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Default Re: Smiths Sectric very weak motor

Quote:
Originally Posted by Mike Phelan View Post
I thought ALL rotors on synchronous motors were magnetised
You are the clock expert as far as I am concerned, Mike, but Les Pook says in chapter 3 of his 2015 book;

Quote:
3.2.1. RELUCTANCE MOTORS
A reluctance synchronous motor is a synchronous motor in which the rotor is a temporary magnet made from a magnetically soft material...


3.2.2. MAGENTISED SYNCHRONOUS MOTORS
As the name... implies, the rotor is permanently magnetised. Hence, the rotor poles do not change polarity as the AC voltage on the stator coil changes sign. This means that the stator poles have to be of alternate polarity... The drive to the rotor is smoother than in a reluctance motor so flywheels are not used.
The clock in question here is a Smiths Deluxe AKA BM7 or BM39. Les describes it as having a "magnetised rotor".

Temco MkI and II movements (and a few others), in contrast, are described as having reluctance motors.

Hope that helps,

Nick.

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Old 3rd May 2020, 5:19 pm   #6
gingpeakin
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Default Re: Smiths Sectric very weak motor

I have a Rega Planar turntable that has sat idle for maybe 15 years and the same thing has happened to its synchronous motor. Sadly the replacement is about 60.

Ging
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Old 3rd May 2020, 5:23 pm   #7
Mike Phelan
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Default Re: Smiths Sectric very weak motor

Quote:
Originally Posted by Nickthedentist View Post
Quote:
Originally Posted by Mike Phelan View Post
I thought ALL rotors on synchronous motors were magnetised
You are the clock expert as far as I am concerned, Mike, but Les Pook says in chapter 3 of his 2015 book;

Quote:
3.2.1. RELUCTANCE MOTORS
A reluctance synchronous motor is a synchronous motor in which the rotor is a temporary magnet made from a magnetically soft material...


3.2.2. MAGENTISED SYNCHRONOUS MOTORS
As the name... implies, the rotor is permanently magnetised. Hence, the rotor poles do not change polarity as the AC voltage on the stator coil changes sign. This means that the stator poles have to be of alternate polarity... The drive to the rotor is smoother than in a reluctance motor so flywheels are not used.
The clock in question here is a Smiths Deluxe AKA BM7 or BM39. Les describes it as having a "magnetised rotor".

In general, only the later designs of clock had reluctance motors as far as I can see.

Hope that helps,

Nick.
I've learnt something again, Nick!
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Old 4th May 2020, 8:45 am   #8
Nickthedentist
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Default Re: Smiths Sectric very weak motor

Andy,

Your best bet might be to buy a cheap tatty clock with the same movement on eBay and use that to repair yours. I recently did this to get a beautifully-art deco mantle clock with a butchered movement going properly again. The donor cost about 15 incl. postage.

Alternatively, Smiths published designs for a remagnetiser, but it's not trivial to build one and I don't know anyone who has. I can post the details if you want.

Nick.
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Old 5th May 2020, 12:05 pm   #9
andynoiseboy
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Default Re: Smiths Sectric very weak motor

Many thanks Nick for the info.

I'm hoping to keep it as original as I can so would certainly be interested in regaussing the rotor poles. I'm an electronics engineer so may be able to construct such a contraption..!

I'll also keep an eye out for a replacement in the meantime.

Cheers
Andy

Quote:
Originally Posted by gingpeakin View Post
I have a Rega Planar turntable that has sat idle for maybe 15 years and the same thing has happened to its synchronous motor. Sadly the replacement is about 60.
Ging
If I have any luck remagnetising I'll let you know...
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Old 5th May 2020, 2:46 pm   #10
Nickthedentist
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Default Re: Smiths Sectric very weak motor

Here's the details, though this one is for the later Bijou movement:
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Old 5th May 2020, 8:04 pm   #11
andynoiseboy
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Default Re: Smiths Sectric very weak motor

Thanks again Nick

Great document.
It's very close to what I imagined would be required.
I'll gather some materials and see what I can achieve...

Cheers
Andy
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Old 22nd Aug 2020, 7:29 pm   #12
bremhillbob
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Default Re: Smiths Sectric very weak motor

Hi Andy, I have something similar but am fortunate in that the magnets in the rotor are strong enough (even though they seem weak to me). My rotor has (what seems to be) 3 sets of magnets - 6 in all. I'd have thought that some molybdenum bar magnets placed on the end of each rotor would be enough to re-energise the magnets, or is that too simplistic?
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Old 24th Dec 2020, 11:57 am   #13
John_Dw
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Default Re: Smiths Sectric very weak motor

Having just started to'get interested' in vintage synchronous clocks I have quickly run into the 'Smiths Bijou' weakly magnetised rotor problem. I recently bought, on eBay, a Smiths Sectric Bakelite mantel clock, that was 'not tested'...

A quick strip down quickly showed that the stator winding was showing the correct DC resistance at about 11.5 Kohms but the fibre wheel that the rotor gear drove was suffering from a distinct lack of teeth in a couple of areas.Being a glutton for punishment I bought a second identical 'un-tested' clock for spares.Well, like the first clock this one also was not working, but the fault this time was an open circuit coil but the fibre gear was in good condition.: thumbsup:

I stripped both movements and replaced the fibre gear in the first clock, so now I , theoretically, had a complete working clock. Sadly the rotor in the first movement had no discernible residual magnetism and while it was just about possible to get the motor running the torque was almost non-existent, the clock only running when placed on its face, in any other position it stopped.

My immediate thought was to try the rotor from the other movement but even though the motors externally looked identical, the rotor was of a different design, the first running on a fixed spindle in the motor housing, while the second having an integral shaft that ran in a bronze bearing in the top of the motor housing.

Searching the internet I was able to find out that the rotor had a one piece iron 'disc' with six, alternatively magnetised poles (North, South).

Having had success in the past magnetising screwdrivers etc by wiping with a strong permanent magnet I thought that I would give the principle a try with the rotor. I have some very strong 4mm dia by 23mm long rod magnets from one of the magnetic sculpture games so removed the rotor and went round the rotor wiping across the end of a pole with the magnet, changing the magnetic polarity for each set of adjacent poles as I rotated the rotor. I did this for several rotations of the rotor then repeated it wiping the magnet on the top of the end of each pole of the rotor.

When I placed the rotor back in the motor, straight away I could see that there had been a very marked change as when turning it you could feel the pull as each pole passed one of the fingers of the stator, and on turning the motor upside down the rotor no longer fell out. .

Re-assembling the clock and manually starting the motor showed a massive difference in the torque produced and the motor now runs happily in any orientation. I am still going to have to re-visit it however as, while the 'start finger' that moves to give the rotor a kick when releasing the hand set knob appears to be operating correctly I think that it will have to be manipulated slightly to get a positive engagement with the rotor gear to work.

Having previously only played with Ferranti synchronous clocks, which appear to be 'bomb proof', the movements apparently being designed in 'the style of IKB', I am much less enamoured with these Smiths Bijou movements and will probably steer clear of them in future...
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Old 24th Dec 2020, 8:54 pm   #14
stevenkw2t
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Default Re: Smiths Sectric very weak motor

Thanks John, I think you've provided a Christmas present for those of us with 'Smiths Sectric very weak motors'. I'm not capable of building a proper remagnetiser (as outlined in Nick's pdf) but I could certainly find a neodymium permanent magnet to try out.
Cheers and have a Merry Christmas!
-Steven
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Old 24th Dec 2020, 10:52 pm   #15
PJL
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Default Re: Smiths Sectric very weak motor

John, that is very helpful, must try it. I have a few sectric clocks and all but one are weak but once you manage to get them started they do seem to keep going.
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Old 25th Dec 2020, 11:29 am   #16
John_Dw
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Default Re: Smiths Sectric very weak motor

Glad to have been of help. My 'Smith' (1946 manufacture I believe) Sectric mantle clock is still running strongly since starting it yesterday... Still prefer the Ferranti offerings, in true 'Blue Peter' fashion, here's some I prepared earlier along with the obligatory Smiths alarm.
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Old 7th Jan 2021, 12:21 am   #17
John_Dw
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Default Re: Smiths Sectric very weak motor

Despite my preference for the Ferranti synchronous clocks over the Smiths Sectric ones I couldn't resist getting this job lot of 'New Old Stock' Smiths spares......
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