3 phase ac motor question

[Njguy]

In a three phase ac induction motor, does it take roughly 3 phases to make a 360 degree rotation of the rotor? Or at least roughly. So if the motor was spinning at 1,000 rpm, a given electromagnet in the stator would be switching at roughly 50 hz?

 

[MikeMl]

In any induction motor, be it single or three phase, the rotor MUST slip relative to the rate at which the magnetic field rotates. That is the only way that the magnetic field will produce any current the rotor windings.

Practically, the rotor turns about 85% to 95% of the synchronous rate. The degree of slippage depends on load torque. Look at the rating plate on the motor. Its rating will show its rotation rate at the specified HP power output.

 

[cachehiker]

How many poles does the stator have?

1000 rpm = 16.67 Hz for a 2 pole.
1000 rpm = 33.33 Hz for a 4 pole.
1000 rpm = 50.00 Hz for a 6 pole.

That assumes absolutely positively zero load = no bearing friction or air resistance or anything.

The frequency would typically have to be about 5% higher at the motors rated load.

 

[dr pepper]

3 full phase waveforms will give you 1 electrical revolution.
As allready mentioned actual revolutions depends on the number of poles.
Some motors have more slip than others.

 

[MikeMl]

3 full phase waveforms will give you 1 electrical revolution.
As allready mentioned actual revolutions depends on the number of poles.
Some motors have more slip than others.

Induction Motor Rating Plates do not specify how many poles they have; they do specify what the loaded RPM will be. A typical rating may be 3450RPM @ 230V @ 60Hz. The synchronous speed for that motor would be 3600RPM, so the slippage would be -150RPM.

 

[dr pepper]

Yes, I'm a maintenance fitter I mess with these ugly things all the time.
Funny allthough some motors are marked with the no of poles most arent, but whenever you ring the rewind 'shop they allways talk about number of poles not rpm.