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I have an induction motor, I'm very sure its a permanent split capacitor motor. One winding is 16 ohms, the other is 4 ohms. I have seen that the higher resistance winding, the 16 ohm one, is the auxillary winding connected through the capacitor, but someone also told me otherwise. How should I connect it?

This is an expansion of Unknown induction motor wiring?

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  • \$\begingroup\$ If you have no documentation or a valid part number that would lead to a manufacturer's specification it's guesswork. Do you feel lucky? \$\endgroup\$ – Andy aka May 17 '20 at 13:28
  • \$\begingroup\$ I've usually assumed the higher resistance is the start (aux) winding, but I haven't seen one with such a large difference between windings. Makes me wonder if one of the windings might be faulty. \$\endgroup\$ – user_1818839 May 17 '20 at 14:47
  • \$\begingroup\$ @Brian Drummond you may be correct. How can I test it? It came from an old dishwasher. I assumed it worked, as I think the dishwasher had a problem in the control curcuit, and I wouldnt expect a problem with both, but it could be. \$\endgroup\$ – Randomaker May 17 '20 at 14:58
  • \$\begingroup\$ It may be useful to revise the question to state whether this is a 120 V, 60 Hz motor or a 220 V, 50 Hz motor. I believe design practices may be quite different between the two. There may be differences among countries of origin as well. \$\endgroup\$ – Charles Cowie May 17 '20 at 16:24
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I have started to doubt my thought that the lower resistance winding would be the one with the additional capacitive impedance in series. I have seen a couple of trouble-shooting guides that say the opposite. However the guides I saw seemed to refer to specific purpose motors, heating and A/C fan motors, I think. Appliance and equipment motors have undergone many years of optimization to produce motors that are exactly suited to the load with minimum cost. All dishwasher motors are likely to be similar, but not necessarily the same as furnace blower motors.

I know that there are PSC motors that are designed with two identical windings to make it simple to reverse the direction of rotation. That is obviously not the case here, but that illustrates the extent of variability PSC motor designs.

The best way to determine the connection remains identification by examination of the original wiring. Another reasonable alternative would be to examine or find documentation for a similar appliance.

Because of the water heater in most dishwashers, it is difficult to determine from sales literature how much current the pump motor draws. If it is as little as 2 amps, the lost heat in the 16 ohm winding would be 48 watts. That seems like a lot for a motor that might require only 400 watts in normal operation, but motors of that size and type may have only 50% or 60% efficiency. If the motor takes 5 amps and most of it goes through the 16 ohm winding, that would mean 400 watts dissipated in the winding. That would burn it up pretty fast. On the other hand, if the 4 ohm winding is assumed to be the main winding and the self inductance is not as high as it would be for a main winding, the current could be too high.

If you want to proceed on the basis of someone's guest guess, have an ammeter in place during the test. Clamp-on ammeters are available at a reasonable price and very good to have for anyone who does electrical DIY projects etc. It will be easier to instantly see a problem with an analog meter. However you do need to be aware that for the first fraction of a second, the current will normally be quite high. Connect the motor through a switch that is easy to operate and close to the ammeter. Switch off immediately if either the ammeter or the sound of the motor is not what you expect.

If you can, look at the size of the wire used for the motor windings. If some windings use larger windings, you can be sure those are part of the main winding. That also indicates the relative current in each winding.

If you determine which connection seems to be better, check the temperature of the motor carefully during initial operation. Disconnect if it seems to be heating up to much or too quickly. Measure the current in each winding. It is possible the either connection may appear to work. Either connection may be viable with no-load operation. If that is the case, you won't know if you have the correct connection until you put a load on the motor.

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  • \$\begingroup\$ I will just wait and look at the original winding and see what I can find and maybe keep looking online for a circuit diagram. If I cant find anything, I will try it as you suggest. Thanks! \$\endgroup\$ – Randomaker May 17 '20 at 15:07
  • \$\begingroup\$ I agree with this. The aux winding only has to provide direction, and with a series capacitance the current through it will be lower, so it can better tolerate the loss of a higher resistance winding. The less I^2*R loss in the main winding, the better. But 4:1 seems high. Any hints on testing for shorted turns in a winding? \$\endgroup\$ – user_1818839 May 17 '20 at 15:09
  • \$\begingroup\$ There may be a method for testing for shorted turns. It may involve applying DC and looking for an anomaly in the voltage waveform upon disconnection. I may have heard about that somewhere or it may be thinking of something completely different. At any rate, if such a test exists it may not be easy to perform. \$\endgroup\$ – Charles Cowie May 17 '20 at 16:15
  • \$\begingroup\$ The L/R ratio of each winding ought to be the same for making a Quadrature phase current at max RPM minus Line f RPM. 4:1 Impedance ratio becomes a 2:1 resonant frequency between PRF:SRF thus to reach optimum 90 deg phase differential between windings, with both being above line f. Thus to get a similar impedance on Aux , it must use the smaller L and series Cap. \$\endgroup\$ – Tony Stewart EE75 May 17 '20 at 17:18
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Ok, I just looked at the dishwasher. The 16 ohm coil is the aux winding and the 4 ohm is the main as I expected. I also found the motor current draw in a sticker on the motor. As far as the shorted windings, I'm guessing there is no way to find out other than wiring it up and hoping nothing blows up. I will closely monitor the amperage and temperature when testing. Thanks for all the help! I will keep you updated as to if it works or not.

EDIT:

I tested it and it works great! It only uses about 1.4A with no load, and I let it run for a few minutes and it isnt even warm. Just barely above room temperature. I cant wait to use it in a project! Thanks again for all the help!

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