centrifugal switch in a 3 phase motor?

I picked up a fractional HP 3 phase induction motor and was surprised to find that it contains a centrifugal switch. It looks just like the sort
used in single phase motors to control the start winding but it doesn't seem to be connected to the windings in any way. Is the purpose to tell a controller that the motor has been stalled? Perhaps it came from a rollup door drive or something? Maybe this sort of thing is common but this is the first I've encountered it.
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Rich. wrote:

Sure, I just took a few. It's a 1/3HP motor with dual shafts and a flange mount on one end. The black wire from the switch that appears to go to the windings is actually just tied into them physically, apparently to keep it out of harm's way.
http://mysite.ncnetwork.net/resnvzjy/motor/motor1.jpg
http://mysite.ncnetwork.net/resnvzjy/motor/motor2.jpg
http://mysite.ncnetwork.net/resnvzjy/motor/motor3.jpg
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That looks like the bi-metal thermal overloads that 3 phase motors use for burnout protection.
BTW, want a new one? http://www.surpluscenter.com/item.asp?item-1225
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Rich. wrote:

There are thermal protectors glued in with the windings but there is very definitely a centrifugal switch there. Note the brass flyweights on the ring around the shaft in the first picture, and the switch contacts are mounted on the beige boards attached to the inside of the end bell. The switch contacts come out on separate wires and are closed at rest, and open when the motor exceeds a few hundred RPM as the brass weights fly out and retract the ring that presses them closed. This stuff is not electrically connected to the windings in any way.
LOL that's exactly where I got this one. It works perfectly, I was just curious if anyone could confirm the purpose of the switch.
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From your description of the centrifugal switch and brass flyweights being in there, the only thing it can be is a startup switch. It must run to an extra startup winding that gives the motor a kick in the pants to get it going. Once up to a certain RPM the switch kicks out the startup winding.
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Rich. wrote:

There is no startup winding, the windings are exactly like any other 3 phase motor I've seen, three separate windings that can be wired delta or wye for 240 or 480V use. The switch functions exactly like a starting switch but is wired completely independent to separate wires exiting the motor, the switch contacts close when the motor slows. One of the wires to the switch is physically but *not* electrically attached to the windings, I thought at first it was connected but it's just tied there to keep it from contacting moving parts.
The more I think about it, the more I think it must be a safety device, either to reverse a door or other machinery if it jams on a foreign object, or as someone else suggested, to shut off fuel to a burner of some sort if the blower fails, or to trigger an alarm.
At any rate I removed it as I'm running the motor from an inverter drive (those have sure gotten cheap and compact) and found the switch would drag and make noise at low RPM so it's purely curiosity at this point. Odd motor, but it was cheap and I think it will nicely replace the single speed PSC motor and clumsy stepped pulleys on my little drill press.
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I bet it is a clothes dryer motor, if it stops switch shuts off power to heater, almost all clothes dryers are set up this way.
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jim mac wrote:

A 3 phase clothes dryer? I guess there might be big commercial machines that are. This is not like any domestic clothes dryer motor I've seen.
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Could this be a wound rotor machine with resistors to increase starting torque? That would be pretty fancy for a fractional horsepower three phase motor.
I couldn't see much from the pictures.
Bill
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Salmon Egg wrote:

No, it's quite an ordinary induction motor. I found nothing unusual about it aside from the centrifugal switch. It operates just fine with that stuff removed.
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Could it be that it is cheaper to just have the centrifugal switched installed whether needed or not? I can picture saving money on manufacturing facilities and inventory by minimizing unnecessary variation.
I remember several early electrobic calculators where the cheap model had the same electronics as a more expensive version. The cheap model just did not have the switches to use everything on the board.
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Salmon Egg wrote:

I'm quite certain it was intended as a stall sensor, although I may never know the actual end use. There is no way they would have installed a switch and wired it up to the connector with all the other wires from the motor if it were not intended to be used. I can think of numerous applications for which it would have been useful, I'd just never seen one.
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That is how some capacitor start motors worked. The capacitor connected to increase torque on starting. Then disconnected (by centrifugal force) once the motor is spinning; when no longer required.
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In article

The trouble with this explanation is that a three phase motor DOES NOT need a method to produce a rotating field.
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wrote:

...because three-phase power is a rotating field.
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It is not the way to do it, but a "three phase" motor could run on a single phase supply if it can only get started. Otherwise, what is the function of this CENTRIFUGAL switch?
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Salmon Egg wrote:

It's pretty obvious that it's to detect the motor has stopped rotating, I've said that from the beginning.
Whether the original intent for this particular motor was to tell the controller to reverse the rotation to back off whatever jammed the equipment the motor was running, cut the power, or to sound an alarm to alert somebody that the motor has jammed, failed, or otherwise stopped turning we may never know. I can think of numerous uses for it, but have never actually encountered any of them in practice. I haven't seen one of these in the field and apparently nobody else here has either.
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The capacitor is not about producing a rotating field. Starting torque can be maybe doubled with only half the current. Variations exist for three phase motors. Don't know why one would do this. But a late 1990s IEEE paper suggested it; maybe so that the three phase motor can be smaller:

Badr, et al Dec 1995.
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Salmon Egg wrote:

Well sure, I've got several things with capacitor start motors, the thing is they're all single phase, and in every one of them the capacitor is mounted either in a hump on the side of the motor or under a cap over the back end. I've never come across one that was external, and I've certainly never seen a 3 phase motor that used a capacitor of any sort for anything. The whole purpose of the capacitor is to provide a phase shift and with polyphase power you already have that.
If there is such a thing as a *three phase* capacitor start motor, I'd love to see it, but until then I don't believe they exist.
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