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.
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.
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.
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.
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.
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
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.
An old man would be better off never having been born.
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.
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.
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:
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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