AC motor wiring question

IIRC there are some motors, like the ones inside a sealed refrigeration compressor that use a relay to switch out the start winding when the motor started. I'm remembering that they were current operated and connected the start winding when the run winding's current was above some threshold.

Google is your friend, maybe you can find info on those kind of start relays.

I'd say the lower resistance winding is likely to be the run winding.

HTH,

Jeff

There are indeed some compressor motors which use an external relay. I cut one of those sealed refrigeration compressors open once just out of curiosity. It took about half an hour and four angle grinder discs. The pump actually looked much more robust than those found on little el-cheapo direct drive air compressors. I still have it somewhere.

Chris

Eric,

One of my favourite electric motor books - Electric Motors in the Home Workshop by Jim Cox - is written for people like us. On identifying four-lead motors it says the following:

"The capacitor-run motor will usually have two approximately equal resistance windings."

...and...

"In a split-phase motor the resistance of the start winding will be higher than that of the main winding."

So it looks like it is a split-phase motor (i.e., a motor which does not need a capacitor) which uses an external relay to switch the start winding. You might be able to get a suitable relay from a motor repair shop or McMaster.

Good luck!

Chris

just use a short time delay relay,or salvage one from an appliance with a simlar motor, or add a "push during starting" switch

With Dayton AC gearmotors (non-capacitor) I've found operation using run windings will make the motor run warm/hot, but not burning hot. On the other hand if the start winding does not disengage the motor will rapidly burn out because of the relativley large current passing through the start windings because they were designed only to allow a large jolt of current to start the motor and not for continous operation.

If your two windings are the same gauge wire I would suspect the higher resistance winding would be the run winding and the lower resistance winding the temporary or start winding.

A call to Grainger's tech line should rapidly clarify the matter;

1-888-361-8649

Good luck.

dennis in nca

Yes Chris, that is a good book and I have it. Along with Electric Motors and Control Techniques. Both books lead me to believe the low resistance winding is the run winding, but since the motor runs the same (apparently) with either I'm looking for a little guidance. ERS

Greetings Jeff, I have a 4CR relay starter from Klixon that came with the motor. It was not connected though. And It can be wired either way with the relay and work. ERS

Well, if it was a Grainger sold motor I'd call them. But it wasn't. I will check the wire gauge and see if they are the same. ERS

The higher resistance winding is the start winding. The higher resistance comes from more turns of smaller wire. More turns to get a larger inductance than the run winding, which is how they get the phase shift that results in the rotational component of the magnetic field.

The Klixon components I am familiar with are thermal devices, which makes me believe that your Klixon is either an overtemperature protection device or a thermal time delay device to control a relay that cuts off the start winding.

Running on the start winding for long will result in burnout, as the start winding is not designed to operate for more than a few seconds under normal conditions.

You can get a current sensing motor starting relay, but it has to be fairly well matched with the run winding current under normal conditions. Typically, motor start relays pull in on the high current through the run winding during the first few milliseconds after voltage is applied, then drop out, disconnecting the start winding, when the run winding current drops below some threshold the the engineers determine during development. Try an appliance parts dealer after first determining the start and run currents The counter people MAY be able to help select a relay from your measured current, but most of them are primarily skilled in tracking down manufacturer's part numbers, not in electrical design.

Eric, You can check into a "Sinpac" electronic switch for this motor. I've installed a lot of them, and they seem to be bulletproof. It takes the place of a centrifugal switch for the start windings. It's only about 1" x 1-1/4" x 3/4" in size, epoxied into a metal can for mounting. PV-16 is the model (IIRC). Ken.

It's almost certain that you've got a split phase start motor that is normally started via a current relay that switches in the high resistance start winding for a couple of seconds while the motor runs up to speed.

If you look carefully at the winding you should find two sets of windings at 90deg both with a roughly similar number of turns. However the start winding is wound with thinner wire - about half the diameter of the main winding wire.

This gives the start winding about the same or a bit less inductance than the run winding but about four times the resistance. It is this 4:1 difference in L/R ratio that gives the necessary phase shift to provide the starting torque.

Once started the motor will run alone on either the start winding or the run winding but running on the start winding will result in reduced power and overheating.

Jim

Greetings Anne, Thanks for the reply. The Klixon device is a motor starting relay. It does use heat to drop out the starting winding. Search for the 4CR device on the Klixon website and you'll see how it works. Eric

Thanks Ken. The motor works fine with the Klixon device. I just wasn't sure which winding is the start winding. I am now pretty sure it's the high resistance winding. However, I have repaired a number of centrifugal starting switches. Sometimes with a lot of hassle. That Sinpac switch seems like it would be a great option. Eric

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Permanent capacitor run is a possibility but less likely than split phase. The winding resistance ratio is typically rather smaller - about 2:1 with assymetric machines down to 1:1 for two or three phase derivatives.

The basic type is true 2 phase with two equal windings at

90deg. These run fine with a capacitor feeding the second phase but, with equal winding inductance, a rather large capacitor is needed.

With a moderate increase in turns a smaller and cheaper capacitor can be used. However, if too large a number of turns is used for this winding, the flux density and hence torque drops.

The two types can be differentiated by looking carefully at the windings. With capacitor run machines the capacitor phase occupies roughly the same slot space as the lower resistance main winding. With split phase start the slot space occupied by the start winding is a lot less than the main winding.

Jim

Interestingly, this motor has the slot space filled completely. The ratio of resistance is 4:1. It runs fine with either a cap or without and using the high resistance winding only long enough to start it. I think I will use it with the Klixon relay and forget about the cap. If this doesn't supply enough torque then I'll wire in the cap. ERS

Greetings Ken, I had a little time so I checked out the SINPAC starting switches. They sure seem to be the cat's meow.A year ago I had to repair a centrifugal starting switch because a plastic part which held the springs had broken. The repair part consisted of a shallow phenolic cup shaped piece with three slots machined through for the springs. This cup held all the broken pieces together in their proper places and was glued to the plastic parts. I would have just pitched the motor except it was part of a custom made (by me) buffer. The time required to machine the cup, fixture the pieces, and epoxy the whole mess together was about two hours. That Sinpac switch would have been much easier and faster. Thanks again for the heads up. Cheers, Eric

Glad I could be of assistance..... Ken.