The squirrel cage Induction Motor makes an excellent generator when it is driven above its synchronous speed. The same features that make this motor desirable over the other types of motors makes the Induction Generator desirable over other types of generators, namely the inherent ruggedness of the squirrel cage design and the simplicity of the control systems.
The Induction Motor becomes a generator when it is connected to an electrical power system and then driven above its synchronous speed by some prime mover. The prime mover may be a turbine, an engine, a windmill, or anything that is capable of supplying the torque and speed needed to drive the motor into the over-speed condition.
The performance characteristics as a generator will vary slightly from those as a motor. In general, the slip RPM and power factor will be lower and the efficiency will be higher. The differences may be so slight as to be undetectable by normal field measuring methods.
A major advantage of the Induction Generator is frequency regulation. The speed has to be tightly controlled with a synchronous generator so that its frequency doesn’t deviate from line frequency. The output frequency and volts are regulated by the power system in the Induction Generators and are independent of speed variations. The self-regulation effect minimizes control system complexity.
Induction Generator controls are very much like those of an Induction Motor, with some exceptions:
- The system must be equipped with a speed limiting control. In the event that electrical load is lost, the torque of the prime mover will rapidly accelerate the system to potentially dangerous speeds. A brake, governor, or throttle shutoff is required to avoid dangerous speeds.
- The electrical breaker must be equipped to limit fault current. In the event of a short circuit fault in the power system, the generator is supplying the fault current. Current limiting fuses are usually adequate.
- The torque output of the prime mover must be limited to prevent overloading the generator. This control may be inherent in the design of the prime mover, or may be based on feedback signals from the output of the generator. In the extreme case, the prime mover could push past the pushover (breakdown) torque of the generator, causing runaway speed.
- In some cases, the prime mover speed may drop below the generator’s synchronous speed. If this happens, the generator will motorize to drive the system. If such response is undesirable, then power could be shut off with a reverse power relay, or an over-running clutch could be used to allow the motor to run at no load.
The Induction Generator may be used as a motor to accelerate the system to running speed, or the prime mover may be used to provide the acceleration. In the latter case, it is not necessary to consider starting torque and current in the machine design. This leaves the designer free to maximize the full load running performance characteristics.
The Induction Generator is being increasingly used as a means of recovering energy that would otherwise be lost. The generated power may be consumed on site or sold to the utility system supplying the site (the Public Utilities Regulatory Act requires that the utility buy the power). Wind and water driven generators are being used to convert that energy into electrical energy.
Some typical applications of Induction Generators are:
- A paper mill has a significant supply of available fuel in the bark and wood scrap. Used in a boiler, this can generate 4000 HP of excess steam. The largest single load is a 2000 HP, 3600 RPM pump. By mechanically connecting a 4000 HP turbine and a 2000 HP Induction Generator to the pump, the fuel can be used to drive the pump and generate 2000 HP of electricity. In the event of steam failure, the generator can be used as a motor to drive the pump. Further, the pump will help limit the system over-speed in event of electrical load loss.
- A Water Company finds that it can buy electrical power at low rates at night and sell power at high rates during the daytime peak load period. It builds low and high storage basins and installs several pumps. At night it pumps water from the low basin to the high basin, buying power from the utility. At peak periods, the water flows back down through the pumps, driving the motors as generators. The power is sold to the utilities. The arrangement is so simple that it can be remotely operated.
- The wind blows constantly between the desert and mountains of California. An enterprising individual set up some towers with windmills driving Induction Generators through gearboxes. Power is generated proportional to the wind velocity and sold to the local utility. The operation of the "Wind Farm" is virtually automatic with the proper equipment.
There are some definite differences in Induction Generator use that should be accounted for in the application:
- The indiscriminate use of Induction Motors as generators should be avoided. It is possible that a particular motor would not work well as a generator due to internal magnetic saturation. The internal voltage as a generator can be higher than it would be as a motor with the same terminal voltage. The magnetic densities in the machine are determined by the voltage at the equivalent circuit air gap. For a motor, the air gap voltage is typically 85-95 percent of the terminal voltage. For a generator, the air gap voltage is typically 100-105 percent of the terminal voltage. This higher air gap voltage may make the machine over-saturate magnetically, and have high core losses and draw high magnetizing currents. It is conceivable that the machine could overheat at very low load output. If an Induction Motor is to be used as a generator then this information should be known by the designer so he can make appropriate allowances in the magnetic densities.
- Induction Motors are usually rated 460 volts for use on a 480 volt system. Induction Generators should be rated for the nominal system voltage, or slightly higher rather than lower, because the generator is now the power source rather than being a load on the power system.
- Power factor correction capacitors can be used to correct the generator power factor in the same manner as for an Induction Motor. However, if there is any chance that the generator may over-speed, whether connected to the power system or not, the capacitors should be connected to the system through a separate breaker so that when the generator breaker is opened, the capacitors will not be connected to the generator. Under overspeed conditions, the capacitors can overexcite the generator and cause uncontrolled high voltages to occur. These voltages can destroy generator insulation systems and also can be hazardous to other equipment and to personnel.
Induction generators are designed for specific applications and not for general purpose use. Contact your local distributor or sales representative to place a technical inquiry.