Actually you can use induction motors as generators without any 
controller, in fact many people do this for micro-hydro generators.

Basically any magnetism in the  stators, even residual magnetism, will 
induce a current in the rotor.  This current creats a magnetic field 
which (because it's moving) induces a current in the stator which creats 
more magnetism...the tricky part is controlling the voltage produced.

Patchem, Eric EM2 wrote:

>How is an AC induction motor (in the case of AC drive) used for
>regenenerative braking? I understand how the motor works, but I cannot see
>how it would work as a generator. I know the inverter plays a major role in
>this, but I cannot see how an induction motor with no field connections, can
>produce an output.
>
>Eric.
>
>
>

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