Induction motors

[mbrooke]

https://www.youtube.com/watch?v=3GI-biC3aP8

I think this does an excellent job of explaining 2 phase induction motors and AC induction motors in general. However I am having sme trouble understanding what cause the motor to never be able to turn at the same speed as line frequency, ie 60hz=1800 rpm, but what cause the motor in reality to turn at 1,725 rpm even without load?

 

[LMAO]

https://www.youtube.com/watch?v=3GI-biC3aP8

I think this does an excellent job of explaining 2 phase induction motors and AC induction motors in general. However I am having sme trouble understanding what cause the motor to never be able to turn at the same speed as line frequency, ie 60hz=1800 rpm, but what cause the motor in reality to turn at 1,725 rpm even without load?

Think of an induction motor as a transformer. Induction motor with no load running at zero torque and slip is like a transformer with open secondary. In reality though, there is some leaking current so transformer always draws small current even when with open secondary. In case of induction motor, there is always some load (air resistance, angular momentum of rotor, etc...) so realistically, slip is never zero.
Other extreme is when transformer secondary is shorted; that's like an induction motor with locked rotor.

 

[Electric-Light]

Slip is needed to produce torque. It's the same with train wheels or tires. If you mark your tires with lines pointing down on all four corners, they won't stay aligned no matter how careful you're to not spin out.
Synchronous motor is an exception, which behaves like a toothed belt. When you overload a synchronous motor enough, it skips a few teeth.

 

[templdl]

https://www.youtube.com/watch?v=3GI-biC3aP8

I think this does an excellent job of explaining 2 phase induction motors and AC induction motors in general. However I am having sme trouble understanding what cause the motor to never be able to turn at the same speed as line frequency, ie 60hz=1800 rpm, but what cause the motor in reality to turn at 1,725 rpm even without load?

2 phase motor? I'm not familiar with a 2 phose moter, just 1ph and 3ph. Can you clarify?

 

[Jraef]

I like to explain it this way to my students.

Magnets repel each other. The stator windings and steel form electromagnets that have magnetic fields rotating around in a circle as the phases wax and wane. But there are no magnets in the rotor of an induction motor, yet. If a rotor was just steel, it would not rotate. So there is steel AND conductors called rotor bars, which together form ELETROmagnets in the rotor too. But unlike the stator where the electricity is directly connected to the coils, the electricity for the rotor electromagnets must come FROM the magnetic field lines of force of the stator cutting across the conductors (rotor bars) in the rotor and causing current to flow. So before the rotor gets a magnetic field, the stator must have one FIRST. The time lag in creating the rotor current by induction then the magnetic fields around the rotor bars means there is a slight delay in the rotor field establishment. In the mean time, the field strength of the stator fields is already waning a bit, so the repulsion expressed on the rotor fields makes them run slightly slower, at the slip speed.

In a synchronous motor or a permanent magnet motor, the rotor fields are already there, so no lag, no slip.

 

[mbrooke]

Slip is needed to produce torque. It's the same with train wheels or tires. If you mark your tires with lines pointing down on all four corners, they won't stay aligned no matter how careful you're to not spin out.
Synchronous motor is an exception, which behaves like a toothed belt. When you overload a synchronous motor enough, it skips a few teeth.

But then how does the motor start spinning at all? The video makes it sound like the actual moment of the rotor induces current and in turn counter EMF in the squirrel cage. Isnt it the 60HZ AC field from the stator doing it?

 

[mbrooke]

2 phase motor? I'm not familiar with a 2 phose moter, just 1ph and 3ph. Can you clarify?

The video shows a 2 phase motor, or at least the model with the arrow. The 90* waves would be 2 phase.

 

[GoldDigger]

But then how does the motor start spinning at all? The video makes it sound like the actual moment of the rotor induces current and in turn counter EMF in the squirrel cage. Isnt it the 60HZ AC field from the stator doing it?

The motor starts spinning because of the torque at zero rpm produced by the two (for single phase) or three (for three phase) coils which are spaced out in angle around the rotational axis of the motor. The magnetic field induced by one coil is attracted to the next pole around the circle.
What makes this all possible, avoiding philosophical tail chasing, is that the induced field lags in time behind the applied field.
An alternative to a capacitor shifted voltage on the starting winding for single phase is the shaded pole coil, which also introduces a delay. The mechanism is related to the way a shaded pole section can keep an AC relay from chattering as the current periodically passes through zero.

Another way of looking at the induction motor is that the rotating field lines exert a drag on the conductive rotor, creating a torque which causes the rotor to try to follow the rotating field.
If the rotor is spinning at the same speed as the rotating field, there will be no drag and therefore no torque.

Both methods of analysis, properly applied, lead to the same result.

 

[templdl]

The video shows a 2 phase motor, or at least the model with the arrow. The 90* waves would be 2 phase.

Thanks for the clarification. If this an educational vidio it would be theoretical as the common power sources are either 1 or 3 ph except for some older systems in the Philadelphia where they do have some 2ph still around. 2ph transformed themselves have either 4 or 5 wire secondaries which is interesting which often makes difficult to get ones mind tuned into.
If in fact infact you are teaching a 2phase motor design, that design may not be used much in real life.
Does this make any sense?