Induction motor - no load speed

[MrPacane]

Under no load, does an induction motor run exactly at its synchronous speed (i.e. relative field speed) or at a speed that is close to the synchronous speed but slightly less?

Thanks for your time.

 

[SirAskalot]

What happens when the rotor rotates at synchronous speed (or what does not happen) ?

If you can draw the torque vs. speed curve, or have the equation, you can get the torque at synchronous speed. This does not describe the physics behind what happens but will give you the answer.

Tell us what you think happens and your reasoning, then we can guide you to the right answer. Getting a yes or no answer yield no actual knowledge!

 

[MrPacane]

At synchronous speed, the slip is equal to 1 and so the torque is 0. If there's no torque, I think the motor's speed will decrease because of air friction. In order to have torque (and keep the motor running at a constant speed), there needs to be a small difference between the synchronous speed and the actual mechanical speed of the motor.

Am I right?

 

[SirAskalot]

You certainly are correct mr. Pacane. Even at no-load there will be a small load torque (wind, bearing friction). According to Newtons second law in rotational/cylindric form, steady state (constant speed) is achieved when the load torque and motor torque are equal. A difference in torque gives an (positive-/negative) -acceleration.
(How much this torque and speed difference is in a real case must be individual considered, some motors have large cooling fans connected to the shaft and may give considerably windage loss)

And how is torque produced in an induction motor? Why is there no torque production when slip is equal to 1 as you stated? What happen inside the rotor?

 

[MrPacane]

Oh, I just realized I made a mistake! At synchronous speed the slip is zero (slip is equal to 1 at starting because the mechanical speed is zero).

Torque is produced by the Lorentz forces (due to moving charges in a magnetic field). In an induction motor, there are moving charges in the rotor because of the induced voltage. In addition, these moving charges are in a magnetic field produced by the stator.

In other words, first the stator produces a changing magnetic field which induces a voltage in the rotor. Then, Lorentz forces act on the stator because of the induced current running through it and because of the stator's magnetic field.

 

[SirAskalot]

And how do the fields, induced voltage and current behave at rotor speed equal to synchronous speed?

And how is it that torque changes as the rotor speed changes? What factor in the torque equation changes and why does i change?

Slip is a measure of the relative speed difference between stator and rotor "speed".

 

[Mike_In_Plano]

Yes. you hace to have a frictionless machine and infinite time (due to the iniertia) to perfectly reach the synchronous speed. That's because the motor puts out less ans less torque as it approaches the speed and zero torque when it gets there.

A more interesting thing happens if you supply torque to the more and drive it OVER synchronous speed. Then, the induction motor behaves as a generator, supplying power back to the line. However, this doesn't work with a simple induction motor being turned. It requires that the line be connected to do this generator trick.