Poly-phase Induction generator

[nand arora]

What will happen if the rotor is made to rotate at double the synchronous speed i.e slip becomes less than -1. I have never seen any diagram showing torque slip curve with slip less than -1.

 

[subtech]

At what speed does the rotor operate under normal conditions?

 

[nand arora]

At what speed does the rotor operate under normal conditions?

normally induction motor is used in motoring mode and thus speed of rotor is less than synchronous speed.

when we want to use induction machine as an induction generator or say use it in generating mode then rotor speed should be more than the synchronous speed.
But I don"t know how the induction machine will behave if rotor speed is kept more than twice the synchronous speed.

http://electricalstudy.net/lesson/poly-phase-induction-generator-or-asynchronous-generator-2/

 

[nand arora]

At what speed does the rotor operate under normal conditions?

speed control of induction motor can be studied from the following link:

http://electricalstudy.net/lesson/speed-control-of-poly-phase-induction-motor/

motoring, generating and breaking action and their curve can be studied from the following link:

http://electricalstudy.net/lesson/torque-slip-character-sticks/

But there is not much mention of the part of the curve in generating mode with rotor speed above double the synchronous speed. will there be any breaking action in that part. How will the machine behave??

 

[jim hardy]

http://people.ucalgary.ca/~aknigh/electrical_machines/induction/basics/trq_speed.html

Have you ever thrown the reversing switch on an induction motor that's still spinning?

As the folks above say, the speed-torque curve is pretty much symmetric about synchronous speed.

 

[nand arora]

Thanks for the reply

 

[nand arora]

for reference see the character-stick curve on the 4th figure of the page displayed by the following link:
http://electricalstudy.net/lesson/torque-slip-character-sticks/
Why the portion of curve with speed above twice the synchronous speed not been shown in generating action?
Will there be something similar to braking that will take place here as this part of the curve is mirror image of that part.

 

[jim hardy]

Why the portion of curve with speed above twice the synchronous speed not been shown in generating action?
Will there be something similar to braking that will take place here as this part of the curve is mirror image of that part.

here's what they said at your link:

This can be obtained by plugging i.e. say initially stator magnetic field was rotating in clockwise direction and for breaking we reversed the connections. Thus stator magnetic field starts rotating in anti-clockwise direction while the rotor still rotates in clockwise direction. Thus we get slip greater than one.

The case they considered was reversing the applied field while rotor is still spinning from motor's having been recently being run forward.. Same thing i proposed, throw the reversing switch while it's running...
For that case you couldn't get slip>2.

Look at the formula they derived

I don't see slip limited to 2.

Torque continues to fall off because rotor frequency continues to increase , so rotor reactance x₂ still limits rotor current.

For me it would be counter-intuitive to expect some discontinuity to appear. Rotor is just a coil.

What do you think ?

old jim

 

[nand arora]

yes, I believe you are right about the braking mode.

But I want to ask about generating mode. Will there be some action similar to braking action in slip between -1 to -2 as this part of the curve is mirror image of the part with slip between 1 to 2 (braking action). Probably nothing should happen as such.

I also want to know that can we take the slip to any negative value say -3 or even -4 (theoretically) as we can make the rotor to rotate at 4 to 5 times the synchronous speed in direction of the rotating magnetic field.

For reference see the link mentioned below:
http://electricalstudy.net/lesson/poly-phase-induction-generator-or-asynchronous-generator-2/

 

[jim hardy]

But I want to ask about generating mode. Will there be some action similar to braking action in slip between -1 to -2 as this part of the curve is mirror image of the part with slip between 1 to 2 (braking action). Probably nothing should happen as such.

It might be helpful to imagine yourself very small and riding on the surface of the rotor with a magnetometer.
Torque reverses when slip changes sign. That's the RPM where your magnetometer would show flux reversing its direction of apparent motion, ie North ceases to overtake you and you commence to overtake North, passing it once per slip cycle.
The only place where slip changes sign is at zero slip, ie synchronous speed.
So torque ought to asymptotically approach zero. Well, discounting friction and windage. Make that electromagnetic torque .

I also want to know that can we take the slip to any negative value say -3 or even -4 (theoretically) as we can make the rotor to rotate at 4 to 5 times the synchronous speed in direction of the rotating magnetic field.

Why not ?
Caveat: I would be very nervous around an 1800 RPM rotor that's oversped to 9,000 RPM. Centrifugal force is 25X design , so i'd want a stout "Scatter Shield" around it.
As a kid i oversped a bronze fan blade assembly.
Chunks of it buried themselves in the hardwood floor and walls, and chopped up some furniture. Miraculously they all missed my buddy and me.

 

[nand arora]

It might be helpful to imagine yourself very small and riding on the surface of the rotor with a magnetometer.
Torque reverses when slip changes sign. That's the RPM where your magnetometer would show flux reversing its direction of apparent motion, ie North ceases to overtake you and you commence to overtake North, passing it once per slip cycle.
The only place where slip changes sign is at zero slip, ie synchronous speed.
So torque ought to asymptotically approach zero. Well, discounting friction and windage. Make that electromagnetic torque .

Thanks for the reply...now i completely understand it. similar description has been given in the explanation of
http://electricalstudy.net/lesson/poly-phase-induction-generator-or-asynchronous-generator-2/
where it has been discussed about looking from the view point of rotor conductor.