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Longer explanation of why the speed of the motor isn¡¯t 1800 or 1500 rpm.
¿ªÔÆÌåÓýWalter Wrote
¡°Motor speed is dependant on the number of poles in the motor and the frequency of the AC voltage being supplied to it.?
North America 60 Hz, therefore a four pole motor will run at 1800 RPM less line losses (friction, drag, etc) ?so usually about 1725 RPM,
Europe and some other countries 50 Hz, so 1500 RPM less loses so about 1425 RPM.¡±
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Largely correct, but it isn¡¯t line losses or friction and drag that causes the reduction in speed (and line losses are totally different than drag or friction in any case.)
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An induction motor operates by creating a rotating magnetic field. The magnetic field induces currents in the rotor¡¯s conductors.
This creates a magnetic field in the rotor.
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Until the rotor is actually rotating, the induced field is in sync with the stator field, and produces no torque.
Essentially the rotor does not know which way to rotate, both ways ¡°look¡± equally good,
so it just sits there getting hot, because there is a lot of current flowing.
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Imagine it as there being a south pole of a magnet on the stator, lined up with a north pole on the rotor. It doesn¡¯t want to move.
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To get it moving you can spin it by hand, and it will start going whichever way you turn it.
That isn¡¯t really useful, so you add a start circuit.
The capacitor and inductance of the motor in the start winding phase shift a little bit of the current that the rotor sees,
and all of the sudden there is torque, and the motor ¡°knows ¡° which way to go.
The magnet on the stator has moved ¡°ahead¡± a little, and rotor turns to line them back up.
This very turning action induces more current into the rotor, but it lags further behind. So the rotor turns more.
More current gets induced in the rotor and the process dogpiles on.
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The speed rises and eventually the start circuit cuts out. But because the rotor is now producing torque it doesn¡¯t care.
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Now if you come up to full synchronous speed somehow (The 1800 rpm or 1500 rpm above)
the rotor is no longer cutting through the field created by the stator, and there is no current induced in the rotor.
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With no field in the rotor, there is nothing for the stator field to ¡°pull¡± on so the torque drops to 0.
If the torque drops to 0 the rotor slows down, and a little current is induced, creating torque.
This difference in speed is called the ¡°slip¡±.
At any given speed the slip is what generates the torque required by the load.
As your load increases, the rotor speed drops, so the slip gets bigger, as the slip increases so does the induced current, and hence the torque.
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This need for slip is why the ¡°no load¡± speed of the motor is less than the synchronous speed.
The ¡°full load¡± speed of the motor is even lower.
As the load increases further you reach a point where the stator poles can no longer pull ¡°hard¡± enough, and you slip poles, and the motor stalls.
The start circuit cuts back in, but can not produce much torque. So the motor soon starts humming the ¡°death song of its people¡±.
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Different motor designs will have different speeds where this breakdown occurs,
which is why differing motors of the same nominal ¡°Hp¡± rating may not work for a particular application.
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R James (Jim) Klessig P.E. | Senior Power Systems Engineer |
Electrical Reliability Services, VERTIV jim.klessig@..., jim_klessig@... 1876 Gwin Rd, Mckinleyville | CA | 95519 | USA | Cell (707) 497-9611 | eFax 614-410-0653 ?
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