The induction motor I am using has a dead current relay and finding the part is proving quite difficult. The motor has neither a run nor a start capacitor. I am considering the idea of switching the dead relay with a capacitor from a pancake induction motor salvaged from an old bread making machine. My guess is that it should work considering a bread machine requires quite a bit of torque.

Is there issues with doing this? Will the motor burn out? The motor has a main winding and a start winding which is connected to the current relay. If I replace the current relay with a capacitor, I'm afraid that it will burn out the start winding. If I did this, will I have to also add a relay to switch the capacitor off after a second or two?

  • \$\begingroup\$ I don't think I understand. Are you saying that there is only one winding, the main winding, and it is switched with a relay? Or are you saying that the relay controls a start winding? If it controls a start winding, what turns it on and off? And if there is no capacitor, how is the start winding phase-shifted to control direction of rotation? \$\endgroup\$ – mkeith Dec 24 '15 at 18:29
  • \$\begingroup\$ I clarified my question. There is a single start winding and a main winding. \$\endgroup\$ – user148298 Dec 24 '15 at 19:00

A split-phase motor that does not have a capacitor, is designed so that the starting winding has a higher resistance to inductance ratio than the main winding. That is usually accomplished by using a smaller wire size for the starting winding. With that design, it is essential to disconnect the starting winding after a few seconds to avoid overheating the winding. If a sufficiently small value capacitor is inserted in series with the starting winding, the overall impedance of the winding could conceivably be raised to the point that the current would be sufficiently low to permit operation as a permanent split capacitor motor. However, that small current in the starting winding would likely not be sufficient to start the motor. The normal current in the starting winding during starting would be several times the rated full-load current of the motor. A safe current in the starting winding for continuous operation would be a fraction of the rated full-load current.

The easiest way to use the motor without the starting relay would be to use a momentary-contact start switch that would connect the start winding only as long as you hold the switch closed. I have operated a machine that had a manual starting arrangement like that.

Re comment question:

The momentary-contact start switch would connect the start winding in the same way as the starting relay. It would connect the winding directly to the power supply without a capacitor in series. Both windings draw high current during starting and the current in both windings is reduced as the motor approaches full speed. However, the start winding is not large enough to continuously carry even the reduced current that it would draw at full speed. Here is a diagram:

enter image description here

  • \$\begingroup\$ I seem to remember that older clothes dryers had this type of arrangement, too. \$\endgroup\$ – mkeith Dec 24 '15 at 20:05
  • \$\begingroup\$ Hmmm. Confused. How does a momentary contact switch amplify the current? \$\endgroup\$ – user148298 Dec 24 '15 at 20:10
  • \$\begingroup\$ Oops. I understand now. The momentary switch just turns on the winding. It doesn't give it a starting jolt. \$\endgroup\$ – user148298 Jan 15 '16 at 13:39

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