Using Induction Motors as Generators

Using induction motors as generators is a very cost effective way of providing a generator for a turbine system. It especially works well with single phase or three phase systems that are interconnected to the utility, as an induction system requires no governor controls. The induction motor, instead of consuming energy, is driven at 50 RPM over its rated speed and the motor becomes a generator. Induction generators are much less expensive than other types of generators, but require excitation to operate. This is why they are ideally suited to interconnected utility applications. It is possible to utilize induction motors as generators in stand alone applications, utilizing the residual magnetism in the windings, as well connecting capacitors to supply continual excitation. Research is being done by Morehead Valley Hydro Inc. and Thomson and Howe Energy Systems for larger stand alone induction systems, utilizing a synchronous generator to supply constant excitation.

In single phase operations, it is possible to utilize induction motors as generators and get near three phase efficiency by connecting capacitors to the other unused leg of the motor. This can result in a very smooth running generator, operating at 100% Power Factor (PF). The extra efficiency is gained by the motor (generator) running balanced on all three legs, which is actually less heat (friction) output. A tachometer is mounted to the end of the generator shaft, which sends a signal to the computer to maintain 60 Hz. As more water is put through the turbine, the generator tries to speed up but is locked in at 1850 RPM, and the net result is more power output.