centrifugal switch not open effect

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IG88

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Hey, Iam looking for an awsner on some trouble shooting theory.
- If the centrifugal switch of a start split phase motor does not open after reaching its approx 75% of max rmp, what would this do too the motor?

My hypothosis is that it would never reach rated RPM obiously.
because RPM= freq X 120/ Number of poles
and since the centrifugal swith does not open it remains to have 4 poles
causing Less RPM then rated!? correct?
And if what iam saying is true... ..then how can we ever reach 75% or even 100%(minus slip) of rated RPM to ever get the centrifugal switch to open in the first place???
 
First off, welcome to the forum. :)

To give a simplistic reply, the main thing that will happen is the motor will not come up to speed and it will rapidly overheat and burn up. :)

The theory doesn't matter much after the magic smoke is let out. :)

But you're pretty much right in you supposition.
 
090213-0628

Why won't the motor come to full speed? The start winding does not change the number of poles but it does provide a phase shifted magnetic field so that for starting you have a two phase motor. The start winding is made to have a relatively high resistance to produce the phase shift and is very inefficient, thus lots of heat, and if left on too long will burn up. From Bailey and Gault, p 295, "Ordinary commerical motors of this type are guaranteed to produce at least 150% of full-load torque when starting." --- "the starting current will be approximately 10 times full-load current.". From p 298 "In general, the starting efficiency of a split-phase motor is about 35 per cent of a polyphase motor.".

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gar, torque and speed aren't the same thing.... speed is rate of rotation, torque is force of rotation...

A very careful observation of a split-phase motor starting will reveal that the start cycle rpms are lower that the running rpms, you can even hear the difference in speed.

A carefully conducted experiment would be to put an external switch on the motor to bypass the start switch, and using that switch grab an rpm measurement within a few seconds, then compare that to running rpm.

I have a meter and a lot of split-phase motors around, I'll try to set up that experiment this weekend...
 
090213-1623 EST

mxslick:

It is correct that torque and speed are not the same thing.

If you can find a copy of Bailey and Gault --- "Alternating -Current Machinery", by Bailey and Gault, 1951, McGraw-Hill --- you will find an entire chapter on single phase motors. A single phase motor without some form of phase shifted field will not self start. This is because there is no inherent rotating magnetic field. It can be mechanically started by spinning it. A synchronous motor runs at synchronous speed with a shaft angular shift proportional to torque load. An induction motor runs near synchronous speed but at a slip speed less than synchronous proportional to torque.

To make a single phase motor self starting there has to be another magnetic field phase shifted from the main field to produce some form of rotating magnetic field. In some small motors this is done with a shading coil. The same shading coil technique is used to make AC relays with a single excited coil.

Suppose you build a two phase motor with identical coils. If both coils were supplied with the same excitation, then their currents would be in phase and no rotating field. Add series resistance to one of the coils and the current phase angle will shift and this creates a low grade rotating field. By changing the turns and the resistance of the starting winding one can improve the rotating field. The starting torque of the motor is dependent upon on how the inductance and resistance of the starting winding are adjusted.

This starting winding has not changed the basic number of poles on the motor.

I should expect you will hear a different noise from the motor because of the high current in the phase shifted winding when this winding is active.

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