Electric Clocks (Smiths) coils?

Discussion in 'Electric Horology' started by berntd, Sep 10, 2009.

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  1. berntd

    berntd Registered User

    Jun 21, 2009
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    Hello

    someone just brought me an electric clock motor for repair.

    There is a problem with the coil in the motor.
    It gets enormously hot.

    I have a second one here but it is only the coil and it is completely damaged.

    Does anyone know where to get a replacement coil like this or how to best rewind it?

    The wire is only 0.08mm thick and from the dimensions I roughly calculated around 10000 turns of it!!

    That will be too difficult for me to wind on the lathe - I think.

    It is 250V 50Hz

    Kind regards
    Bernt
     
  2. Hayson

    Hayson Registered User

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    Try the electric clocks forum berntd. The guys over there are more likely to have a specific interest and knowledge about your problem.
     
  3. Scottie-TX

    Scottie-TX Registered User
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    #3 Scottie-TX, Sep 10, 2009
    Last edited: Sep 10, 2009
    BERND, this clock was designed to run on 250 V at 50 hz.
    It will not run on 125V, 60 hz
    Most motor field coils do get hot. Some get very hot.
    Whoops! My bad. AUstralia! You DO have 250/50 mains.
    Then; Does motor run and if it does - how hot does it get and how long does it take to get that hot?
     
  4. Hayson

    Hayson Registered User

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    Berntd, You are in Aussie I believe? Don't you guys use 240/50 same as here in NZ?
     
  5. Steven Thornberry

    Steven Thornberry User Administrator
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    This is now in electric clocks for greater exposure. Re-direct left in clock repair.
     
  6. berntd

    berntd Registered User

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    Hello

    Oops, I didn't know we had an electric clock forum - sorryo:)

    Anyway, yes, 250V -50Hz is what it is rated for and that is ok for here in Oz and all countries with 220 -240V 50Hz.

    the motor I have here runs if started by hand but if the slightest load is applied, it starts oscillating in the same spot, even after the load is removed.
    It gets too hot to hold after about 1 minute.

    I measured 460kOhm on its coil. This should result in 0.5Watt.

    Since no wattage is specified, I have no idea what the resistance of such a coil should be when working correctly. Maybe this has a partial short?

    The second coil I have here for repair was physically damaged by whoever removed it from its motor.

    Regards
    Bernt
     
  7. Scottie-TX

    Scottie-TX Registered User
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    #7 Scottie-TX, Sep 10, 2009
    Last edited: Sep 10, 2009
    Welll, I don't wanna discourage ya BERND - and I won't but D.C. RESISTANCE measurements are useful for little more than test of continuity on coils. Coils are measured in inductance and the coil's INDUCTANCE is a function of the frequency and voltage applied - a formula - and measured in henrys.
    Yeah; One minute - not good. I doubt a short altho remotely possible.
    I also however question, 460ku. That's a VERY high resistance. I'd expect it to measure more like a fraction of an ohm - like .8 for example.
    No matter. This ain't gettin' her fixed. Let's await the experts.
    C'mon in LES! Door's open.
     
  8. eskmill

    eskmill Registered User
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    #8 eskmill, Sep 11, 2009
    Last edited: Sep 11, 2009
    Thank you for the invitation Scottie. Honestly, I've looked at this query/thread several times wondering which version of Smith's electric he has that won't run and then I realize it doesn't matter.

    The field coil isn't open but if there's some shorted turns, they'll get hot and finally melt one or more turns clean open, then there's infinity ohms continuity and a bad smell.

    Some clock rotors run hot....the 4 Watt plated steel Synchron rotors sold by several suppliers and eBay get too hot to handle but continue to go....on.....and.....on. Don't aske me why they don't fry. I'd be cautious about using them without lots of ventilation but that's an aside.

    The principal cause of heating in any AC induction motor is cheap iron in the field or core. Iron with excessive permeability that cannot change state rapidly will heat on AC; the higher the Hz the more the heat...period. Good, high quality iron is costly and isn't the easiest material to handle in terms of mass production.

    Unlike fractional horsepower and larger AC motors, itsy-bitsy AC clock motors or rotors have so much controlled reactance in the field that they don't care whether or not the rotor is free or locked. A simple test to show current increase per load on the GE/Telechron and the Synchron clock rotors revealed almost no measurable increase in current for locked rotor.

    That the motor will spin if helped but then stutters and stops is indicative of shorted turns causing an unevenness of magnetic distribution among the poles, a condition that gets worse with shaded pole self-starters.

    I suspect Bernt has a candidate for replacement with a like donor motor. Agreed they're not easy to find in the "Land Down Under" as Burt Munro would say.
     
  9. Mike Phelan

    Mike Phelan Registered User

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    They do get quite warm, but not exactly too hot to the touch.
    It might have one or more shorted turns. That will stop the clock if too many of these.
    You won't get a replacement coil, and it will have to be rewound. Depending on what sort of Smiths it is, wire is 44-46 SWG.
    It is possible, as I have done it before, but a coil winder is a better bet. Pity you are on the other side of the planet - there must be places near you that can rewind coils?
    Confirms shorted turn(s) :eek:
    Unlikely! About 10-15 ohms on those I've measured, but there is no way to find a shorted turn by resistance measurement.
    See above. Current, and therefore power (wattage) is irrelevant as it's inductive load consisting of both resistance and reactance.
    I'll resist (!) going into AC theory, but the upshot is that you need to get the coils rewound.
    HTH
     
  10. berntd

    berntd Registered User

    Jun 21, 2009
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    Hello all,

    No need to explain theory. I am an electronics engineer by trade :=

    Nobody here repairs these because "they" claim it is 240AC and thus a safety/liability issue - say no more. They convert the clocks to 16V AC by rewinding with thicker wire and adding a seperate transformer. I am against this pratcie at this stage as I deem it unnecessary because the original design is definetly safe enough.

    The resistance I measured is really that high.
    I calculated betwen 8600 and 12000 turns of wire based on the cross sectional dimensions of the coil (the stripped/damaged one).
    The wire is 0.08mm thick = ~3mils. Gauge AWG or SWG = :???:
    The wire is way too thin to wind on a lathe without breaking it.

    Based on this extremely high DC resistance, I ignored the reactance (just for now) in my power calculation as it will never be that high at such a low frequence (50Hx)

    Proper specs as for no of turns or current consumption would help me a lot.

    Here is a picture of the motor and seperate coil I have.

    101.jpg



    Kind regards
    Bernt
    (Chapter 72)
    -> posts merged by system <-
    Hello again,

    Small correction:
    DC resistance of the motor is 447Ohm NOT kOhm. What was I thinking...

    Bernt
     
  11. Mike Phelan

    Mike Phelan Registered User

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    You have my sympathy, as I used to be one!
    I've done that once, but agree with you as I don't think anyone has been killed or injured by a clock!
    Too high. This motor will have 44 SWG (0.081mm) and approximately 12 Ohms.
    Number of turns is about right, but like I said, not critical. The bobbin is almost filled.
    Nope - done it. The important bit is to use some sort of bearing for the reel, and speed control on the lathe. I did it with a variac connected to my Unimat 3. Uset a gloved hand to feed the wire.
    Took a couple of goes before I got it right.
    Still high!
     
  12. coldwar

    coldwar Registered User

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    Berndts' Smiths assembly resembles very closely one of the Sangamo designs, perhaps made by Smiths under Sangamo patents. Here are images of a Sangamo example for comparison. If so, he might utilize a donor field and coil designed for running voltages suiting needs, or perhaps use a Sangamo assy intact as retrofit. CW

    Sorry, this first image simply would not come in to focus.

    PICT0005-4.jpg
    PICT0006-3.jpg
    PICT0007-3.jpg
     
  13. Mike Phelan

    Mike Phelan Registered User

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    That does look quite a bit different from a Smiths, but as you say, the coil might fit, though the stator housing is quite deep.

    Surely, the best option is to find someone who rewinds things - motors, car magnetos et al? Must be some in every part of the world.

    Last time I had four clock stators wound for a total of £24, as I did not have my Avo-Douglas winder then.
     
  14. berntd

    berntd Registered User

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    Hello mike,

    Is there a bobbin inside this motor?
    I opened it and I can only see 2 metal "plates with teeth (poles)" that encase all the wire. It all seems to be potted together and I don;t want to break it apart as in yet. How does it best come apart?

    I will probably pass on this one and return it to the owner as I have sooo much other stuff to do here and finding a transformer winder should not be impossible.

    Kind regards
    Bernt
     
  15. Mike Phelan

    Mike Phelan Registered User

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    Hi Bernt
    The coil is the one you can see with the wire on, inside the two parts of the stator - the thing with teeth on it. There is no other bobbin.
    HTH
     
  16. berntd

    berntd Registered User

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    Hi Mike,

    Does one have to chisel / force that assembly out of the housing and then chisel the 2 stator halves apart?

    Are those 2 metal stator halves open on the outside circumference or do they enclose the coil all round?

    Does one have to make a bobbin from insulated material forst and then wind the coil onto that before putting it into the stator?
    What is the proven way?

    The spare coil I have here is just wire with some potting material inside it and insulation tape.

    For this to meet any sort of safety standard, it will have to have the correct insulation material for a bobbin (flammabuility rating) and it must meet minimum creepage distances.

    Kind regards
    Bernt
     
  17. berntd

    berntd Registered User

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    Hello again

    To make it worse, there is not a single supplier here in Australia who can deliver 40AWG or 44SWG wire (0.08).

    It has apparently been obsolete for many years here :-(

    Nearest would be 0.125mm wire but there is no way that one will get anywhere near the number of turns with such a thick wire.

    Kind regards
    Bernt
     
  18. Mike Phelan

    Mike Phelan Registered User

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    Hi Bernt
    The two halves are not fixed in any way, but it's possible that the coil has deteriorated to an extent that it has increased in size and is jamming the stator together - the two halves should just fall apart.
    They do enclose the enclose the coil all round as there has to be a complete magnetic circuit.
    It does need a bobbin for insulation, but that is made in one piece and is very thin - (2mm?), so making one would be difficult.
    Correct! My thoughts are if you are doing this professionally, i.e. it's not your clock, forget it - too many insurmountable problems. If it was your clock, then this might be of interest.
     
  19. berntd

    berntd Registered User

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    Hello all,

    The plot thickens :cool:

    doind some calculations, I came up with a absolute maximum DC resistance of around 47 Ohm , based on the theoreticla number or turns. - which sort of balances with what Mike recomenned.

    I also noticed that the soldering on the wire tabs seemed new and fresh.
    I subsequently grilled the owner last night on what is going here, telling him that there is somehting seriously wrong with whatever is inside this motor and I don't believe it could have gone wrong like that on its own.

    Lond story short, I spoke to his 'other' clock repair man who tol me he had the motor rewound and the spare coild (see picture) is the original one. Apparently, since the rewind, it has been behaving exactly like I described with overheating and stopping/oscillation.

    So, good thing I didn't base my rewind on what is in the motor now as it is obviously not right either.

    I am now really curious and will pull it apart after all to see what mess they made in there :bang:

    I especially want to know how they got a resistance of 450Ohm in there...


    Kind regards
    Bernt
     
  20. berntd

    berntd Registered User

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    Me again...

    While everyone was sleeping in Europe and the US :D, I took the motor apart.
    Nothing wrong with the coil. Looks like a good rewinding job BUT it has been done with 0,17mm wire instead of 0.081.
    That means half the number of turns and with that a far lower reactance and magnetic field strength. That is why it does not work.

    Does anyone have a source for the right wire (mailorder?) or details of someone who can wind a coild like that?

    Kind regards
    Bernt
     
  21. Mike Phelan

    Mike Phelan Registered User

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    Even with the right gauge of wire, there's enough of it to go to the moon and back to obtain that resistance!

    That is looking like someone has used a transformer with a lower voltage to the motor - an acceptable way to do it.
     
  22. berntd

    berntd Registered User

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    No, no transformer.

    It was rewound as is to work of 240V and the mains plug was fitted straight to it. I think whoever did it could also not get the right wire and used the thinnest available, probably telling the customer take it or leave it.

    BTW I triple checked the resitance of that coil today with a different meter and it was exactly 438 Ohm. There is no damage to that coil so with current information, I have to conclude that it came with this resitance. It is definately copper wire = Hmm...

    Still need source for 0.08mm wire....


    Kind regards
    Bernt
    -> posts merged by system <-
    Hello Mike and all,

    Just for interest:

    Resistance:

    I have here 0.17mm wire on same bobbin as 0.08mm wire should
    I can thus say if ~10000 turns of 0.08mm wire fit in, about 5000 or 0.17mm wire will fit in.

    Specific resistnace of 0.17mm (37SWG) wire is ~0.7Ohm/m
    from here:
    http://www.precisionwires.com/erw/EngData/AWR/AWRSWG.aspx


    I have a mean diameter of ~45mm for the coil.
    This results in:
    l = 3.14 * 0.045 * 5000 = ~706m.

    This results in 706 * 0.7 = 494 Ohm.

    Pretty close to what I measured actually = 438 Ohm.


    For the correct wire of 0.08 and say 10000 turns,
    Specific resistnace = 1.38 Ohm/m for 40 SWG
    l = 3.14 * 0.045 *10000 = 1413m

    This results in 1413 * 1.38 = ~1900 Ohm.

    I would expect to see roughly between 1500 and 2000 Ohm resistance on a correctly wound coil.

    Based on this, I can't ever see the resistnace for such a coil to be in the range you mentioned or did I get it wrong again ? :confused:


    Kind regards
    Bernt
     
  23. Mike Phelan

    Mike Phelan Registered User

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    #23 Mike Phelan, Sep 17, 2009
    Last edited: Sep 17, 2009
    I just measured a Smiths Bijou coil and got a value of 11.6k with a meter I usually use!
    I'll double-check with a different meter at some point, but I'll be off-forum for the weekend.

    PS: Just noticed by re-reading my posts that I said 12 ohms, and of course it should have been 12k. With your thicker wire, that might explain the reading you got; the number of turns on a synch motor is not too critical, but the limiting factor is heat. :(

    D'oh!
     
  24. RODALCO

    RODALCO Registered User

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    Hi , just reading through this interesting discussion re the Smiths clock motor.

    With the thicker gauge wire of 0.17 instead of 0.08 a larger current would pass through it with excessive heating and the risk of a coil burn out.

    I would suggest to try the coil with a lower voiltage like 110 or 48 Volts AC.
    If you have a variac it would be even better. see at which voltage the clock starts reliably and measure it with an AC voltmeter.
    That will set you motor voltage.

    You may have to run the clock of a small transformer or fit a capacitor in series with the motor.

    Most 240 Volt clock draw about 2 to 3 Watts of power.
     
  25. Mike Phelan

    Mike Phelan Registered User

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    Hi Rodalco
    Not seen you in The Other Place recently! A capacitor is probably the best idea - no wasted heat. Like we do for series valve heaters in radios.

    When our electricity meters were the induction disc type, it was insufficient to turn with judt a clock running, so free power!
     
  26. berntd

    berntd Registered User

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    #26 berntd, Sep 24, 2009
    Last edited: Sep 24, 2009

    Hello,

    I have since ordered a .250kg spool of the correct wire from the bay in Europe.
    When it arrives, I will try and rewind the coil properly.

    I do not think the motor will have enough power if there are not enough turns in it AND a lower voltage.
    The magnetic flux will only be a fraction of the original then.
    It did not even run properly with 240V applied so a lower voltage should make it even worse.


    Also: a series capacitor will not provide good results.
    What is good for radio valve heaters, it is not good in a motor.
    Valve radio heaters can be considered a resistive load but a motor like this is a highly inductive load.


    Kind regards
    Bernt
     
  27. Mike Phelan

    Mike Phelan Registered User

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    It's worked for me with a 12v wall-wart and something like 35 SWG wire (cannot remember the exact details). Surely, the current as well as the voltage must be taken into account?
     
  28. Al Schook

    Al Schook Registered User

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    #28 Al Schook, Sep 25, 2009
    Last edited: Sep 25, 2009
    I have had some success using wire out of old audio transformers like those used in sound systems to match load impedance to the source .. here they call them line transformers. The laminations can disassembled with a little persuading and the coils unwound. The trick will be to find one with the correct diameter wire.
    I found this web site but I am not able to check all of it's pages thoroughly but I see it offers to do small orders so it could be helpful. I think the smallest I ever had was around 40 awg. I used to get Belden at my local supplier but alas, no more.
    http://www.wiretron.com/magnet.html

    EDIT: Just remembered Small Parts .. here is a url:
    Small Parts
     
  29. berntd

    berntd Registered User

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    Hello Mike and group

    I have rewound / made a new coil for this motor.
    It tiook endless hours. Whoever has done this, will know what I mean.
    11560 turns of wire - give or take 100 :D

    Motor now rund and does not overheat. It does have a lot more torque than before, once it runs and it will not stop on its own.

    I still have another problem and need advise:

    It does not start reliably. When power is applied, it may or may not start or sit there and oscillate in the same psotion. I can then spin it either way and it will run that way.

    Maybe this is normal? If not where shoukd I look for the problem? Maybe there is some part missing?

    Kind regards
    Bernt
     
  30. Mike Phelan

    Mike Phelan Registered User

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    Excellent result, Berntd - well done!
    That's interesting; it seems as though the original had one or more shorted turns that only showed up by loss of power.
    Ah - I have encountered this before, if it's the same thing. The two halves of the stator; look at the "teeth", i.e. the internal poles that interact with the rotor. If they are not perfectly central, it will do what you are saying. It took me a long while to figure that out when I got one of these years ago.
    Let us know.
     
  31. berntd

    berntd Registered User

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    Hi Mike,

    Thank you for helping. I assume that it IS supposed to always start runnign then?

    With centre, do you mean equally spaced teeth (gaps between teeht equal)or the height of the whole assemble in the jousing??

    It currelty pulls the rotor slightly into the housing - vey slightly

    The 2 metal halves lock into the housing with a keyway so they can't be twisted off centre.

    I can thus say that the gaps between the teeth are as equal as I can get them.
    What should the gaps between them be o better, how deeply should they mesh?

    I also painstakingly bent all the teeth (they were pushed way in by the previous rewinder) so they are as close to the rotor as possible without touing it and an equal distance from the rotor. I specially turned a mandrel for this.

    If it isn't that, what else can it be?
    Would you have a pictore of a good working stator and rotor perhaps?

    Kind regards
    Bernt
     
  32. Mike Phelan

    Mike Phelan Registered User

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    Hi Berndt
    Yes!
    The spacing between the teeth (and the same with the three-armed "spider" on the rotor.
    Ah - in that case the stator is fine. Depth is not critical assuming everything is in place. There are two different sorts - some have two identical halves for the stator and can be rotated separately. Yours is the other sort where the coil goes inside completely, and the other part is just a flat plate with the teeth on it.
    Unfortunately, the only one I have in the house is working and in use. I wonder, because of your comment about the torque, if it's been wound with thicker wire and is too powerful to self-start? Just clutching at straws here!
     
  33. berntd

    berntd Registered User

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    #33 berntd, Oct 11, 2009
    Last edited: Oct 11, 2009
    Hi Mike,

    No, nothing is rewound with thicker wire. Remember, I specially went to all the trouble in getting the correct 0.081mmwire and all. What a pian that was...

    I do not understand how the motor is supposed to start - technically as I cannot see anything that would unbalance / misalign the field through the rotor so it has to try and straigthen itself?

    There is a 3 prong spider (3 poles) on the rotor and also a single 2mm thick piece of metal that is cut to have the teeth / poles of the rotor.

    Perhaps there is somethng else missing on the stator?

    I have since opened the motor again to have a better look.


    I JUST found a reference (www) which mentioned that the rotor has to be a permanent magnet. If so, mine currently isn't.
    I also found a reference that mentions that some clock motors from the 1930s are manual start?

    I am open to all suggestions.

    Kind regards
    Bernt
     
  34. eskmill

    eskmill Registered User
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    #34 eskmill, Oct 12, 2009
    Last edited: Oct 12, 2009
    Bernt. The rotor of the Smith's Bijou Sectric is composed of six permanent magnets, each having two tips. It should be self-starting and in the proper direction. A sketch of the rotor pole pieces is noted as figure 8 in Eliot Isaacs's "Electric Clocks and Watches - 1840- 1960."

    I'll try to attach a sketch from Isaac's book.
    http://home.earthlink.net/~lexmd/Smiths Bijou Rotor.jpg
    http://home.earthlink.net/~lexmd/Smiths Figure 12.jpg

    http://home.earthlink.net/~lexmd/Smiths figure 2.jpg
     
  35. berntd

    berntd Registered User

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    Hello Eckmill,

    Thank you sooo much for that.
    Wile the forum was down (before I read this), I did some tests with a compass and found the rotor exactly like your diagram.

    This motor does not start reliably.
    There is really nothing wrong with it electrically and I have since concluded that it is because the rotor has lost most of its magnetism, to the point where there is too little force for field of the poles on the stator to push the rotor away and thus starting.

    I concluded this by painstakingly adjusting the stator teeth (by bending)as close as 0.2mm to the rotor. This improved the starting of the motor somewhat but it is still unreliable.

    Since this is a very complex magnet (made of a single piece of steel) I see no way I could possible remagnetize it.

    Unless, I am actually missing another part (like a complete magnet) on the rotor?


    Kind regards
    Bernt
     
  36. eskmill

    eskmill Registered User
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    Bernt. Think of the rotor as a mass of steel with magnetic domains; six of them sharing the mass of the rotor.

    Each of the six "magnetic" poles should be first de-magnetized using alternating current.

    Then each should be re-magnetized alternating the North to South pattern using the sketch below as a pattern. After re-magnetizing the first poles, remove the temporary winding and reapply it to the next poles but reversing the DC current and so-on.

    I'd recommend some heavy wire such as #18 AWG. (about 1mm heavy enamel insulated) Use a strong source of current for both un and re-do operations and of course be quick to keep the smoke inside the wire.:p

    http://home.earthlink.net/~lexmd/Smiths Bijou Rotor Redo.jpg
     
  37. berntd

    berntd Registered User

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    #37 berntd, Oct 13, 2009
    Last edited: Oct 13, 2009
    Hello Eckmill

    Thank you so much for the kind ideas and all!

    By the way, I can also add that the motor shown in your diagrams will start in either direction with no preference, at random.

    I am still unsure how these motors are supposed to start in the correct direction but currenly I believe it has something to do with the reluctance of the 3 leg spider that is also on the rotor. (not shown in you diagrams).

    Re-magnetising:
    The method you show looks good in theory but in practice, apparently
    all the rotor legs will need to be magnetised at the same time or else
    the whole assembly will only have 1 north - south, right through the middle.

    Furthermore, I spoke to an 'expert' in magnetism today and he stated that it will need a current of between 5000 and 7000A (!) applied for 0.5 seconds and rampind down to 0 to semi-permanently magnetise something like this!

    That said, I could make coils for all the legs and try it with a lower current of say 100A or so (car battery) but the size of this rotor could be a problem.

    Hmm...

    Say, has anyone done this successfully?

    Kind regards
    Bernt
     
  38. eskmill

    eskmill Registered User
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    Bernt. I agree that certain versions of Smith's Bijou clock motor were equipped with a starting lever.

    It is important for this discussion to assure that we are both on the "same page" so-to-speak. (your only photo does not reveal the inside of the motor)

    However, the model in the sketches I copied from Isaac's book will self-start and in the intended direction of rotation. The rotor has six double tipped poles and the stator has sixteen pairs of poles split between the upper and lower area of the field coil winding.

    True, initially at the instant of circuit closure, the rotor will "hunt" or vibrate. But because of the inbalance or lack of eveness of rotor poles to stator poles the intended direction of rotation will be favored.
     
  39. berntd

    berntd Registered User

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    #39 berntd, Oct 13, 2009
    Last edited: Oct 13, 2009
    Hello Eckmill,

    i agree that this is VERY important.

    The motor I have (and the one in your figure) has 15 pole pairs (30 teeth).
    This results in 360/15 = 24 degrees per pair. (12 degrees per step)

    Since the rotor has essentially 6 spokes /poles, it has 60 degrees between spokes and we can thus deduct that ALL the poles will exactly align with the spokes on the rotor at any position. Nothing is unbalanced (unless I have it wrong).

    I think the direction secret is in the 3 prong spider on the rotor.
    Here is a sketch of the arrangement:
    81.jpg



    Kind regards
    Bernt
     
  40. eskmill

    eskmill Registered User
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    Bernt.

    Your motor with fifteen (thirty) stator poles combined with six rotor poles is a type that won't self-start dependably and in the intended rotation.

    The example I quoted had sixteen or thirty-two) stators combined with the six double tipped pole rotor; an unbalanced ratio.

    That you note that there is a three "star" shaped piece that augments the rotor poles, could be another means for self-starting but only if the tips of the three legs are interspersed between the other six poles.
     
  41. berntd

    berntd Registered User

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    Hello Eckmill,
    I drew the star's tips into the sketch (see last post). The tips of it are bent up and shaped as shown and they sit in between the main rotor poles as shown.

    Although this star provides a reluctance and "shorts" out the mangetic field on 2 adjacent stator poles when the rotor is stationary, it is still unclear how this could possibly make it start in a certain direction. :confused:


    Kind regards
    Bernt
     
  42. eskmill

    eskmill Registered User
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    Bernt.

    The three leg "star" is permeable. The adjacent stator poles induce a current into the tip of the star leg. This creates an instantenous and temporary magnetic field in the tip which is interspersed between the six rotor tips creating a favored direction of rotation.
     
  43. berntd

    berntd Registered User

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    #43 berntd, Oct 14, 2009
    Last edited: Oct 14, 2009
    Hi Eckmill,

    Now you have stumped me. Not easy to do but you have :Party::D

    I understand everthing you write except the part where it creates a favoured direction of movement.

    :???:At power up, the field will cut trough the tips of the star, inducing a voltage/current in the tip and in turn a reciprocal field. That is as far as I understand.
    Please explain it again so I can make sense of it.

    Kind regards
    Bernt
     
  44. Mike Phelan

    Mike Phelan Registered User

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    Only just come on with the forum being down. Those I've seen have a separate device to reverse the rotation if it starts the wrong way.

    There are various designs for this, but the usual one is a lever that presses on the fibre wheel and moves inward if the direction is wrong, and its end intercepts the pinion, making it "bounce" back.

    One thought is that there are a few different rotor designs, all of the same diameter, but it is possible that the three-armed spider is too high and does not go into the stator poles sufficiently.

    I fell foul of this once.
     
  45. berntd

    berntd Registered User

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    Hello all,

    Another evening spent with this @#$% motor :rolleyes:

    Mike,
    should there be a steel ball inside the rear bush the rear lid?

    I found that the rear pinion of the rotor is worn pretty badly from its end. There is a leaf spring on the rear lid that pushes a piece of metal into the bish onto the end of the pinion to remove all end shake. It
    no longer reaches the pinion or there is a ball missing in there.



    It now starts 8 out of 10 times but not necessarily in the same direction.

    How did I do this?
    Magnetic field strentgh and thus force increases quadratic relative to the distance educion (I seem to remember this as I stufdied this stuff many years ago but never since used it)

    I removed the stator cups again (!) and in the lathe bent the tabs to form a gap ~0.2mm with the rotor. (I previouosly set it to ~0.6)
    I then used engineering blue on the tabs to find where the rotor was rubbing on them. I fine tuned it over and over untill I had it as good as it can possibly be.

    Because of looseness on the rotor and bushes, it is not possible to make the gaps smaller without rubbing in some position or other.

    This reduction in the stator/rotor gaps makes it possible for the motor to start with the remaining magnetism.

    The spider does line up with the poles on the stator as it is bent up. (It sits underneath the main rotor plate).

    All in all, I am now pretty much done with it and if the owner is still unhappy, tough :mad:


    Kind regards
    Bernt
     
  46. Paul Biggs

    Paul Biggs New Member

    Nov 12, 2017
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    Hello,
    Apologies if this is an old link.
    I have a similar issue.
    A Smiths lantern clock. 240 volts 5Hz
    We have checked the coil and it is circa 12Henrys - so we believe good.
    We now suspect the magnet.
    We have a demagnetiser and magnetiser on site.
    But how do check - demagnetize it.
    Kind Regards, Paul
     

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