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Very old electromagnet advice needed

Discussion in 'Repairing Electronics' started by wombweller, Apr 16, 2010.

  1. Llamarama

    Llamarama Member

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    You could measure the diameter of the wire to find its gauge, then find the resistance of the coil. for example lets say the diameter is 0.122mm, or 40SWG. According to the datasheet a wire of 40SWG has a resistance of 1.48Ω per metre.

    From this you divide the coil's resistance by the resistance per metre to work out it's approximate length.

    I hope this is helpful. :)
     
  2. wombweller

    wombweller Member

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    Hi
    Actually the US. seems to have everything I need to either purchase a new old stock of have repaired.
    Mike's Clock Clinic's Electric Plug In Clocks and Telechron Electric Rotor and Other Electric Motor Service Charges Page

    The owner of that site contacted me to say he has the parts if I require them.
     
  3. Hero999

    Hero999 Banned

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    Who uses the Yellow Pages these days?

    I would think that the clock is definitely worth it, the question is whether replacing the motor with a more modern one would reduce its value enough more than it would cost to have the old one reqound?
     
  4. dave

    Dave New Member

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  5. RODALCO

    RODALCO Well-Known Member

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    clock motor

    Hi mark,

    You can also consider rewinding the clock motor coil with a thicker gauge of wire and run it off a small transformer which you can fit in the clock.
    e.g. 1100 turns and design it for 24 Volts ac.

    Is the actual geartrain still ok ?
    The clock motor looks very old too me, nice movement with heavy geartrain, defenitely worth getting back into life again.

    Regards

    Raymond
     
  6. Hero999

    Hero999 Banned

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    That's a good idea, but for safety's sake I think it would be better to use an AC wall wart, rather than an internal transformer.
     
  7. wombweller

    wombweller Member

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    Hi Ray

    Pheew we get about pal!!

    The gear train is ok.

    Ok this is where my knowledge stops, if by lowering the number of turns and using a thicker gauge will it effect something ?? just can help wondering 'why wouldn't it have been this way in the beginning'?

    What I do know is that the speed of the motor is related to the frequency of the mains- RPM = ( f * 60 ) / ( n / 2 ) (where f is mains frequency in Hertz, and n is the number of poles)
    so that's (50HZ*60=3000) / (2/2=1) =3000rpm with a gear inside to give 1rpm. if my observations are correct.

    I like your idea and if it works fine then that's something I can try,but why have an 11000ohm 240v coil in the first place, what am I missing?

    Hero I agree if this method is workable I prefer a mains adaptor!!

    Cheers Guys:)
     
  8. wombweller

    wombweller Member

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    I Quite agree,
    Cheers
     
  9. shortbus=

    shortbus= Well-Known Member

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  10. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi Mark,
    I think Raymond has misread your earlier post, you say, a 11,000 OHM coil, increasing the wire diameter and winding on 1100 Ohms of wire will not give a 240Vac to 24Vac reduction.
    Its the number of TURNS that have to be reduced by 10:1 and the wire gauge current capacity increased by a factor 10.

    I would suggest you count the turns as you unwind the former/bobbin, if you decide to do it this way. I think this is the best option.

    If you do buy a wall wart, get one with an AC output with a current rating to suit the coil.
     
  11. Hero999

    Hero999 Banned

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    Note that the wire gauge shoudn't be increased by a factor of 10, the cross-sectional area should be increased by a factor of 10.

    If you know the thickness of the wire, you can calculate the cross-sectional area by rearranging the formula for the are of a circle, you can then select a gauge with approximately 10 times the cross-sectional area.

    There's a table in Wikipedia which will help you.
    American wire gauge - Wikipedia, the free encyclopedia
     
  12. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    .

    Dear hero,
    Please read the damn posts correctly.........the wire gauge current capacity increased by a factor 10.:)
     
  13. Hero999

    Hero999 Banned

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    Please stop assuming that I'm contradicting you.

    I know that's what you said, I was merely clarifying that the cross-sectional area needs to be increased by a factor of 10.
     
  14. shortbus=

    shortbus= Well-Known Member

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    The only way that the voltage can be changed is to completely change the motor. The motor has a internal gear train in it. That gear train takes the output RPM of the actual motor, and converts it to the RPM that the clock gear train needs.

    To change the motor voltage from 240 to 24 entails much more than wire diameter. These motors use the frequency of the mains to regulate the speed. The rotor of the motor is expecting a certain amount and frequency of magnetism and current to operate. One of the links I posted gives the differing motor specs and RPM outputs.
     
  15. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    The frequency of the supply to motor when using a 240Vac to 24Vac step down transformer will be the same, also the current thru the winding, providing the number of turns is proportional to the original number of turns for 240V, will create the same magnetic flux in the rotor and hence it will run at the same synchronous speed as the original.

    This method of conversion is often used.
     
  16. wombweller

    wombweller Member

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    Hi Eric
    This is what I'd assumed as well.One of my last posts stating the the formula for revolutions is-

    RPM = ( f * 60 ) / ( n / 2 ) (where f is mains frequency in Hertz, and n is the number of poles) So that's 50*60=3000 / 2/2=3000 +any added gearing,No mention of Volts!! This brings me back to the question again of why did they go for 240v 11,000 ohms in the first place?

    I have just purchased a like motor in the hope I won't have to do anything with this one.When I've fitted the new one I will then do the damaged one to see I I can get it working, it's an interesting challenge.

    Thanks for all the imput everyone.
     
  17. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi Mark,
    I would guess the decision for a 240Vac coil is the cost, no additional transformer is required, I would have done the same if making a number of those systems.

    OT: have you been on your south coast safari yet.?:)
     
  18. Hero999

    Hero999 Banned

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    They chose 240V, presumably because it's the voltage in the country it's designed to run in?

    The coil resistance is merely down to the number of turns and the thin gauge used, if you rewound for 24V the resistance will drop to 110R.

    Just another thought: you should be able to estimate the number of turns pretty accurately.

    If you can accurately measure the thickness of the wire (use a micrometer) and the coil dimensions, you should be able to calculate the number of turns.

    Rewinding for a lower voltage should then be trivial, much easier than unwinding the 240V coil.
     
  19. wombweller

    wombweller Member

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    [QUOTE: have you been on your south coast safari yet.?:)[/QUOTE]

    I went to funtington Sussex a month or two ago so was not too far away from you!! Had a flying visit, did think about popping in at your's for coffee :eek: Very nice views about.The bad part was the M62 and M1 Murder!!

    Cheers Mark
     
    Last edited: Apr 23, 2010
  20. mneary

    mneary New Member

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    Ten AWG steps is approximately ten times the cross sectional area. (e.g. from AWG 40 0.00501mm² to AWG 30 0.0509mm²).

    When rewound on the same bobbin, there is room for 1/10 the number of turns, and requires 1/10 the wire length.
    The resistance is 1/100 of the original (10x cross section, 1/10 length).
    The inductance is also approximately 1/100 of the original, since on equal cores the inductance is as the square of the number of turns.
    (I have used 10:1 because it yields 22-24V, but the ratios can be calculated for any gauge wire.)
    When 1/10 the voltage is applied, the current is 10x, leaving the power dissipation approximately the same.

    You should be able to estimate the gauge of the existing wire by carefully measuring the bobbin dimensions and the DC resistance of the coil. You could alternatively measure the length of the wire (after unwinding) and either the DC resistance or the bobbin size. As an example, I estimate that an 11,000 ohm coil on a 25mm long, 10 mm inside diameter, 20 mm outside diameter bobbin is approximately 51400 turns using 3.1 km of AWG 40. (If that coil was originally 240V, AWG 30 would be used to convert it to 24V. It would use about 300 m of enamel AWG 30 insulated wire.)

    Sources of error:
    --Insulation thickness is not accounted for. Enamel insulation is negligible with larger wire but becomes significant where the wire diameter is a few micrometers.
    --DC resistance is a factor of temperature. Also assumes wire hasn't been stretched.
    --Heavier gauge wire may not pack as efficiently.
     
  21. shortbus=

    shortbus= Well-Known Member

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