Induction Motor Repair - Start Circuit

Discussion in 'General Electronics Chat' started by davmacrat, Jun 23, 2009.

  1. davmacrat

    Thread Starter New Member

    May 18, 2009
    3
    0
    Ladies/Gents

    Need some help if possible? I am currently attempting to repair a single phase Induction motor (Squirrel Cage) for a crofter friend of mine. Repair and make do is often the only option I'm afraid.

    I am aware of the theory behind these motors, and when I opened it up, one of the 2 capacitors had blown, however, after also ascertaining that the centrifugal switch had also welded closed, I was surprised to find that it was what looked like the "Run" capacitor that had failed (300μF) rather than the intermittent duty Start capacitor (400 - 450μF).

    Further investigation into the wiring revealed that the permanent duty and intermittent duty capacitor were wired in parallel, in the start circuit. I know that the present capacitors are replacements due to an earlier failure. Due to the age of the motor I suspect that the start capacitors, since there have always been 2 I'm told by the owner were originally non-polarised electrolytic's back to back.

    By my calculations what we have here is an equivalent of 700 - 750μF. But I cannot understand why the last engineer would use an oil filled motor run capacitor with a fixed value in place of a variable start. Can anybody think of a reason why this would have been done? Further to this, can anyone please explain why two aluminium electrolytics wired in parallel are referred to as Back-to-back when they are already non-polarised and what the effect of doing this has; other that changing the phase of the current waveform of course?

    Kind Regards

    Dave
     
  2. davmacrat

    Thread Starter New Member

    May 18, 2009
    3
    0
    Please..... I ask againg, Is there anybody out there with any ideas at all?

    Kind Regards

    Dave
     
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