Chuck Yung
EASA Technical Support Specialist
We have had several calls like this, due to renewed interest in using induction generators for wind generation. With induction wind generators, there are some critical factors that mean the difference between success and failure. These are application issues, and the ultimate responsibility rests squarely with the end-user. By the way, most of these issues are applicable to all induction generators, regardless of the prime mover.
The purpose of this article is to provide a simplified explanation of how induction generators work, and what problems we might expect when repairing them. We owe it to our customers to help them avoid these pitfalls. If not, a generator may come back as an unjustified warranty claim, and no one wants that.
How does an induction generator work?
The principle is simple enough – drive an induction motor at (synchronous speed + slip rpm) to produce power. Consider the example in Table 1, a 6-pole induction motor:
Operating from 60 Hz power, the synchronous speed of a 6-pole motor is 1200 rpm. We know that the motor speed is lower when loaded, and that the difference between synchronous speed and actual speed is slip. In the Table 1 example, running speed is 1140 rpm, so the slip is 60 rpm (1200 – 1140 = 60). By driving the motor at 1260 rpm (synchronous speed of 1200 rpm + 60 rpm slip = 1260), it becomes an induction generator when connected to the line.
Pitfalls to keep in mind
The simplest way to understand the potential problems is to review some induction motor basics. Figure 1 illustrates the following basic points:
- “Peak” or “breakdown” torque (“A”) occurs a bit below full-load operating speed (“B”).
- Actual load affects both speed and current. Without a load, an induction motor operates very near synchronous speed and the magnetizing current (“no-load” amps) is usually about 1/3 of full-load amps (FLA) for 4-pole machines. (The lower the speed, the larger the magnetizing current as a percentage of total current.)
There are a couple of cautions about most of the graphs you might see published elsewhere similar to Figure 1. First, the starting current can vary widely; the “6-7 times FLA” rule of thumb often used by contractors is shown with the blue line. But the instantaneous motor amps at the moment of starting can be more than double the expected current based on the kva code letter. And the kva code may be higher than the usual E-F-G-H.
Second, most similar graphs show torque and current going to zero at 100% of synchronous speed. That’s misleading. Current doesn’t drop below the no-load current unless we have an over-hauling load. An example of an over-hauling load is a conveyor moving material downhill, where too much load can cause the motor to turn faster than synchronous speed. When that happens, it becomes an induction generator.
The results of an overloaded motor are familiar to our industry: melted ties, symmetrical overheating of the entire winding, charred insulation, and sometimes a melted rotor cage. The same thing can happen to an induction generator, under similar conditions.
Induction generator
Now consider what happens when that same induction motor is driven as an induction generator. In Figure 2, we’ve added a mirror image of Figure 1. Just as the driven load can prove too much for an induction motor and stall it, the prime mover (wind) driving the generator can force the rotor to turn faster, past the “pushover torque” (“C” in Figure 2) the reflection of the breakdown torque (“A” in Figure 1) of the motor curve.
The symptoms are familiar: charred windings, melted aluminum rotor cage, etc.
As the load decelerates the motor along the torque curve in Figure 1, the current keeps pace along the steep current curve in the same figure. Operating as a motor, the stalling motor loses speed. Operating as an induction generator, if the prime mover (e.g., wind) drives the rotor overspeed, the increased “negative slip” has the same effect on the rotor and stator windings (Figure 1).
The greater the overspeed at which the rotor is driven, the more “negative slip,” and the higher the current in both stator and rotor. Expect physical evidence of heat in the stator windings and rotor if the generator operates much above the “synchronous rpm + slip” speed.
If the induction generator is metered, another customer “complaint” offers further proof: low power factor.
Metering is critical because of the need to monitor output from the generator. While excessive heating may be an indication of overload, low power factor can also indicate an overload. Just as a lightly loaded induction motor has low (lagging) power factor, induction generator power factor is affected by load. Figure 1 illustrates the relationship between speed and load. For the induction generator, the torque, load and slip curves are mirror images of an induction motor.
Other considerations for wind generators
Ventilation is a major consideration to keep in mind. Most wind generators mount the generator within a nacelle (Figure 3) – an aerodynamic enclosure used to minimize airflow disruption that would reduce the effectiveness of the blades. Airflow may be obstructed if the nacelle has been modified or poorly designed.
Metering, speed control, and ventilation are 3 areas where problems are most often encountered.
Recommended protection:
- Speed control: overspeed = overload
- Shaft currents: for a variety of reasons, shaft currents are a concern for many wind generators.
- Thermal device to monitor winding temperature (RTD, thermistor, etc.)
- Phase balance monitor
Summary
The bottom line is that induction generator repair should be done with a great deal of caution. One of the most critical factors in the success or failure is the customer’s understanding of their responsibility. By being proactive in helping our customers better understand the unique aspects of induction generators, we can save them grief and head off unjustified warranty claims.
Armed with an understanding of how an induction generator works, wind generation offers a real niche opportunity for EASA members in high-wind areas and areas where tax incentives are strong. If single-phased, even though it’s being driven, expect a failure pattern similar to a single-phased motor. Finally, the same cautions apply to both single-and three-phase induction generators.
Suggested reading:
- “Induction Generators: What can go Wrong?” in the October 2000 issue of Electrical Apparatus magazine.
- NorthAmerican WindPower magazine.
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