Small refrigerator compressors in the US generally all have exactly the same design and I am trying to understand their starting mechanism better. How exactly does it work?

What I generally understand is that the motor is a squirrel cage motor which has two sets of windings in a "star delta" configuration. On the outside of the motor is a small black box with a two holes that connect to two pins coming out of the compressor called the "positive temperature coefficient relay". One pin is the hot for the "star" windings and the other is for the "delta" windings. When the compressor starts the PTC directs house current to the first pin very briefly, then due to the wire getting hot, quickly switches over the current over to the other pin (the "run" pin). Is this basically correct?

If so, my main question is what kind of star delta motor is being used? I believe there are 5 different types: Normal, Enhanced, Combined, Partially Wound, and Uninterrupted.

Since there are two sets of windings in star delta motor, that would seem to increase the weight of the motor a lot, but makes it cheap to start it, since you only need a very cheap, small relay to do it. Is that the tradeoff being made from a design perspective? I assume that in operation only the run windings are used, so the start windings essentially are dead weight at that point, is that right?

  • $\begingroup$ There is only one set of windings in a star delta motor. They are connected as a star when starting to reduce the initial current, and switched to a delta connection when running. The reasons for replacing a mechanical switch assembled from several components and with moving parts (i.e. a relay) by a solid state electronic device are "cheaper, smaller, and more reliable". $\endgroup$ – alephzero Mar 19 '17 at 1:10

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