Replace electromechanical circuit

Discussion in 'General Electronics Chat' started by csfahey, Nov 2, 2015.

  1. csfahey

    Thread Starter New Member

    Nov 2, 2015
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    I have a 1/2 hp radial arm saw from the 1930's that uses a capactor and a magnetically driven mechanical switching arrangement to power the secondary windings of the electric motor. The capactor has burned out and I really don't want to deal with the mechanical switch as I am pretty sure it is going to be very noisy and heat up given how it looks like it works. Besides the thermal protection circuit breaker no longer works and the manufacturer cannot recommend a replacement that fits. So, what I want to do is replace the mechanical part with a solid state arrangement which I am pretty sure can be done using some diodes, resistors and a replacement capactor. What I am not sure of however what this type of circuit is called so I can research it and build it. My first thought was that it was an AC recitifier but that doesn't seem to be it nor does it seem to be a phase splitter or inverter. Can anyone point me in the right direction?
     
  2. Brownout

    Well-Known Member

    Jan 10, 2012
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    Your switch looks like a normal relay, and should work. After all, it's been working since the 1930's. But if you really hate it, you can replace it with a similarly spec'ed Solid-State relay.
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    There are external current relays for motor start windings, these replace the normal centrifugal switch version, if that is what the original function is.
    Max.
     
  4. PackratKing

    Well-Known Member

    Jul 13, 2008
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    There are good odds, that your original equipment, is only crusted with sawdust and wood-resins... and a thorough cleaning will likely put you back on track, with only a new capacitor...
     
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  5. Brownout

    Well-Known Member

    Jan 10, 2012
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    Don't know what we'd do without you.
     
  6. BillB3857

    Senior Member

    Feb 28, 2009
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    Do a Google search for "Potential Relay" for some info on the type system you probably have.
     
  7. csfahey

    Thread Starter New Member

    Nov 2, 2015
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    Yes, I am pretty sure that it only requires a new capacitor, but there is also a chance that the mechanical relay might be worn out as well after all it is 80 years old. In either case, this relay will be noisy and will probably get hot which is why I suspect that the thermal protection was in the circuit. Since I cannot keep it original which I had wanted to by replacing the circuit protection, I might as well make it work quieter, cooler and more reliably. So I would rather replace the mechanical relay than keep it. I have looked at relays and the configuration appears to be of the SPDT type. I will look into potential relays as well. If I decide to keep the original mechanical relay would a RC Snubber circuit help with the noise and heat?
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I have never noticed modern potential relays run noisy or hot?
    I have used the centrifugal type switch to turn off a Triac to save the contacts, but that is the internal switch.
    To do it electronically you would need something like the Honeywell current sensor modules to turn on/off a Triac.
    http://sensing.honeywell.com/products/current_sensors
    Max.
     
  9. csfahey

    Thread Starter New Member

    Nov 2, 2015
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    This does not appear to be a potential relay nor does it appear to have a centrifugal switch. I am having a hard time wrapping my head around how it works but I believe it works like this. During a positive current and voltage, the solenoid causes the contact to close the circuit to one set of windings as well as the capacitor. Then the current and voltage reverses which causes those contacts to open and the other circuit to close where the capacitor begins to discharge, charging the other windings until the current reverses again which switches the contacts on the relay again repeating cycle.

    Using the pictures I have included, can anyone confirm that this is how it works?
     
  10. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Can you trace the wiring out and post it?
    It appears to be a potential relay action looking at the files.
    I only see one relay contact.
    Max.
     
  11. csfahey

    Thread Starter New Member

    Nov 2, 2015
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    I will try. I need to take apart the relay at the wire connections to see if there is a hidden connection first.
     
  12. BillB3857

    Senior Member

    Feb 28, 2009
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    This is an interesting forum and post from someone that had the same problem as you. See post 27 for the success.

    http://forums.delphiforums.com/woodbutcher/messages/5903/10

    Sad thing is that you will need to register to see it. You can also use Google, Twitter and Facebook IDs to allow you in.
     
  13. csfahey

    Thread Starter New Member

    Nov 2, 2015
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    Thanks. The site had some good information. My machine is a little older than their but I think I can find the diagrams in there somewhere.
     
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