Overspeeding an Asynchrone Electric Engine

It works as an Induction generator. Wind generators are usually such.

Puzzles me too. Maybe at just the right speed it will become a perpetual motion nothing.

It will regenerate, and it will also do a certain amount of braking, up to the point of lowering the negative slip speed to where it stops doing so. In other words they can act as a form of slight braking, and often are used that way on things like long overland conveyors, but only within the limits of the negative torque capacity of the motor itself, and if you exceed that you overload the motor. This works as long as the load is "overhauling", meaning the load is putting kinetic energy INTO the motor. But if you think it through, it CANNOT brake a motor to a stop ... as soon as it slows the motor down it ceases to brake!

Thank you.

I was thinking about this problem as a potential way to use an asynchrone motor as generator.

It is logic that the rotor will be braked till the speed is equal to the field speed. The main puzzle was: will there be energy returned to the grid or will all be consumed in the rotor and just be heat.

Well, i did say it will work as a generator...called an induction generator...perhaps this link will convince you ? http://en.wikipedia.org/wiki/Induction_generator

Simply put to you:

It will work as a generator with the same frequency as the grid. And because of that, it is resisting the force that is pulling it to overspeed... that is why it generates power: Converts the mechanical power into electric power.

To be used as a breaking device only, then you load it electrically...that is consume the electric power for example into a set of resistances etc.

Very well put. GA.

What many others fail to understand is that it CANNOT be speeded up beyond the speed of the mains. To do so would need an infinite amout of mechanical energy as input and the motor would have to supply most of the energy/power needed by the grid itself.....which of course is only possible in ones head, not in real life!!

One must think in a practical manner!!

i am a little confused here, perhaps more than the OP. Do you mean that the machine CANNOT be run faster than synchronous speed at all ?

Here is an extract from the Wiki link i gave earlier:

An induction generator is a type of electrical generator that is mechanically and electrically similar to a polyphase induction motor. Induction generators produce electrical power when their shaft is rotated faster than the synchronous frequency of the equivalent induction motor. Induction generators are often used in wind turbines and some micro hydro installations due to their ability to produce useful power at varying rotor speeds. Induction generators are mechanically and electrically simpler than other generator types. They are also more rugged, requiring no brushes or commutators.

Induction generators are not self-exciting, meaning they require an external supply to produce a rotating magnetic flux. The external supply can be supplied from the electrical grid or from the generator itself, once it starts producing power. The rotating magnetic flux from the stator induces currents in the rotor, which also produces a magnetic field. If the rotor turns slower than the rate of the rotating flux, the machine acts like an induction motor. If the rotor is turned faster, it acts like a generator, producing power at the synchronous frequency.

In induction generators the magnetising flux is established by a capacitor bank connected to the machine in case of stand alone system and in case of grid connection it draws magnetising current from the grid. It is mostly suitable for wind generating stations as in this case speed is always a variable factor.

It should have been written:-

"...if it tries to turn faster....."

I think it was right;

"If the rotor is turned faster..." because the only way it can turn faster is by the outside rotational force applied by the prime mover, i.e. wind, water, engine etc.

On its own "it" itself cannot "try" to turn at all except as a motor, and then it cannot do so faster than synchronous (or even synchronous) on its own (leaving VFDs out of this discussion for now).

Yes, perhaps "braking" was a poor choice of words, I should have said it can be used as a "retarding" device what serves to prevent an overhauling load from running away.

In keeping with your line of questioning now, yes it returns energy back to the grid, up to the limits of the unit design. In other words if you have a 100kW asynchronous motor that is given 100kW of external super-synchronous rotational force, it will put (roughly*) 100kW back onto the grid.

(* less efficiency)