induction motor as generator


Thread Starter


how can generate 3 phase voltage with induction
motor someone saying using capacitor to give reative power to motor and running it above it
synchronous speed ?
Induction motor cannot be used as a generator. You would use a synchronous motor as a generator.

Peter Cleaveland

As I recall, if you hook a 3-phase motor to the mains and then use external means to spin it faster than synchronous speed it automatically becomes an induction generator, with no need to use a capacitor. The faster you spin it (within limits) the more it generates. Kind of gives you some ideas for alternate energy, doesn't it?
If you drive an induction motor beyond synchronous speed, you will generate rather than absorb power. The amount of overspeed (slip speed) is typically 1% of synch speed.

1. An induction motor receives excitation from connected bus (for example, electric utility), therefore can only parallel with power source; you cannot run isolated from it.

2. You must have prime mover with shaft power greater than power you intend to generate.

If you require emergency power or voltage amplitude different from your utility, then you must use a synchronous generator.

Tim James
Ewing Controls, Inc.
You must be connected to a 3-phase system to "excite" the motor/generator. When the motor/generator is turning below synchronous RPM, it acts as a motor, and is a load on the system. When the motor/generator is turning
above synchronous RPM, it acts as a generator, and supplies power to the system. The induction generator, like an induction motor, has a lagging power factor. Capacitors can be used to improve the power factor if the reactive power is not available from the system.

That's about all there is to it. You parallel the induction motor with a live bus (utility) at synchronous speed, then run it a few RPM above it's synchronous speed, and it will generate power into the grid.

It makes for a simpler system, in that you can get along without some of the protective relaying, synchronizing circuits and logic that you'd normally need to parallel with an existing source.

The manufacturers of the engine and the electronic governor you have on the engine should have more specific information.

Paul Baker
We manage several hydroelectric facilities with various differing generation units. One is an induction generation plant. To bring the generator online ( in a very oversimplified explanation) the unit is brought to +/- 5% of synchronous speed and a capacitor bank acts a reactor for the generator, thus allowing the unit to be brought online and acting as PF correction. As the load increases the speed will increase very slightly from the synchronous speed.

Several Books are available on the subject if you search the local library or book retailers.

John Morris
Upstate Testing & Controls
http://www.utcvolt.comMail to:[email protected]

Phil Corso, PE

Virtually any multi-phase induction (asynchronous) motor, if driven above its synchronous speed, can generate power. Called an Induction Generator, certain precautions are necessary:

a) The circuit it is connected to must be capable of supplying the reactive power required for excitation (magnetism) purposes.

b) The supply circuit must also be capable of supplying the losses of the connecting circuitry.

c) Protective devices must at least address overspeed, and the induction generator's response to external electrical faults.

Negative comments.
For the reasons in a) and b) the induction generator can only be operated in parallel with an electrical power system, or with an electrical load supplemented with power capacitors. Asymmetrical electrical faults could have substantial negative impact on the induction generator's mechanical integrity.

Positive Comments.
An induction generator is auto-synchronizing. Also, its initial short-circuit current contribution is of short duration, while for
sustained symmetrical faults it is zero.

Alternate Power Comments.
Induction generators have been around for almost a century mostly on low-head hydraulic applications. Larger sizes (MW) started to regularly appear in the 60's. Today they are widely used in wind farms and
industrial cogeneration facilities.

Phil Corso, PE
(Boca Raton, FL)
In fact most wind turbine manufacturers use asynchronous generators on their turbines because it is the cheapest and easiest way to connect them to the grid (no synchronisation required etc.). They also actually use them to motor the turbine up to speed (turbines without pitch control) as at 0 rpm the turbine blades are
aerodynamically stalled.

best Regards::

Rolf Gerste
PRISMA Automation Pty Ltd
PO Box 35867
Winnellie NT 0821
Phone 08-8984 4385
Mobile 0417 837 933
Fax 08-8984 4001
mailto:[email protected]
ACN 097 718 342

Robert Dannenfelser

This is what we use in our current version of the PowerWorks microturbine product. This is a 70 kW nominal output multi-fuel fired turbine, and hooked to the grid, once the turbine spins it up to 3600 rpm, we start producing energy. At 3660 rpm, we have a 70 kW generator, at ISO 59 degF air temperature. No synchronization is required. As such, it's main use is peak shaving and co-generated hot water.

Bob Dannenfelser ----- PowerWorks Project Manager
(V) 603-430-7084 (F) 603-431-9575 (C) 410-227-6718 (H)

"What the Mind can conceive and believe, it can achieve, thus, we become what we think about!"