Sizing the series capacitor for an AC Synchronous motor

Discussion in 'General Electronics Chat' started by wingit, Dec 12, 2014.

  1. wingit

    Thread Starter New Member

    Dec 12, 2014
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    I am new to this forum and electronics .. so please type slowly!


    I have an old record deck (Systemdek) with a Phillips synchronous motor that is defunct and no longer manufactured. I know it is unserviceable as a multi meter shows open circuit between all three wires; the grey common feed and red and blue out. There is a very simple PCB that goes with the motor that consists of a power resistor, 6K8, to drop voltage to 110V from 220/240V for the motor and four capacitors. A friend lent me another multi meter which confirmed that the capacitors are as specified and I had already confirmed the resistor was fine. I can purchase a near identical (looking) motor from Farnell and few other sources all designed for turntable use and the same voltage and r.p.m .. BUT .. they specify a different capacitor capacitance from that which is on the present board.


    I will not pretend to understand how this works but there is a capacitor on the blue outlet wire of the motor. This makes it rotate clockwise (apparently). Here is my question .. the original motor has a capacity of 0.82 micro farads (400V) on the PCBbut the replacement motor I can find specifies 0.22 micro farads. Of the three other capacitors, 2 seem to be spark suppressors for the main switch and the other links the non-motor side of the above directional capacitor to mains neutral.

    If I buy the new motor and put it into the existing circuit to check that I've at least found the right solution to getting the table turning could I wreck the motor or would it just run less efficiently or perhaps at the wrong speed? There was a hint on the specification sheet for the new motor that a higher capacitance produced greater torque but could end up with a noisier motor .. so I guess a lower capacitance might be better for a turntable.. but perhaps the original manufacturers, who have moved on, took this into account? The motor will cost around 70 UK Pounds Sterling so I want to avoid damaging it before I have even started.


    Grateful for any help.
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    For optimum motor performance you should be able to replace the cap quite easily as a .22μf should be easily available, use a high quality AC rated cap.
    It would probably run on the .82μf just for test purposes.
    Max.
     
  3. wingit

    Thread Starter New Member

    Dec 12, 2014
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    Thanks Max. The present capacitor just says 400v, no reference to ac. I see 450VDC and 440VAc capacitors on Farnell site. As we're dealing with 240VAC supply will the 440VAC be overkill being able to cope with an actual 440V ac supply? Thanks and apologies if I am asking stupid questions.
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The price and size difference will not be that great.
    Use high quality Plastic film or paper dielectric.
    Max.
     
  5. wingit

    Thread Starter New Member

    Dec 12, 2014
    3
    0
    Thanks Max. The present capacitor just says 400v, no reference to ac. I see 450VDC and 440VAc capacitors on Farnell site. As we're dealing with 240VAC supply will the 440VAC be overkill being able to cope with an actual 440V ac supply? Thanks and apologies if I am asking stupid questions.
    OK I'll look for those. One last thing .. this is my pathetic attempt to shoe the circuit. I do not understand what the lower capacitor is for and have not seen anything similar mentioned on spec. sheets. Could this in any way be altering the "apparent" capacitance of the 0.82uF unit so that 0.2 uF'ish might be the overall result?
    thank you for your patience.
    upload_2014-12-12_17-35-57.png
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,500
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    Appears to be an Active low pass filter, the 6.8k is in parallel with the 1μf cap.
    If correct would filter out all but the AC mains frequency
    Max.
     
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