Induction Generators Explanation, Working, Types, and Applications

 

What is an Induction Generator?

 

Like other electric generators, an induction generator converts mechanical energy into electrical energy. As the name suggests, it is a type of induction machine and works on Faraday's Law of Electromagnetic Induction. 

 

An induction machine can work as either a motor or a generator, meaning an induction motor can be operated as a generator. As generators, induction machines are not so popular because of their reactive power demand. A source of reactive power is required all the time at the stator side to establish its magnetic field (discussed further in this article).

 

Induction generators are commonly known as “asynchronous generators” because they never run at synchronous speed. In fact, they generate electricity when running at a speed greater than synchronous speed, which is typically achieved with the help of a prime mover. 

 

 

Construction / Parts of Induction Generators

 

Like any other electrical rotating machines, an induction generator consists of two major parts: stator (stationary part) and rotor (rotatory part). Generally, a squirrel cage rotor is used. The construction of an induction machine is identical for both induction motors and generators. (refer construction of induction motors)

 

Working of Induction Motor as a Generator 

 

As discussed above, the basic working principle for both the induction machine i.e. induction generator and the induction motor are common, which is based on Faraday's Law of Electromagnetic Induction. 

 

We have already published an article on the working of induction motors. We recommend you to first go through that article, so that you gain a solid understanding of how an induction motor works, which will help in easily understanding the concept discussed here.

 

In induction motors, we explained that the induction motor runs at a speed N which is always slightly less than the synchronous speed Ns (N < N_s), because if the motor runs at synchronous speed Ns (N = N_s), then the relative speed between the rotor and synchronously rotate magnetic field would be zero, resulting in no torque developed in the motor. Therefore, the slip in induction motors is always positive (can be observed with the help of the given expression of slip).

If the speed of the induction motor (N) is increased to super synchronous speed (N > N_s) while it remains connected to AC mains, then the induction motor works as an induction generator. In this condition, it generates active power to mains while drawing the reactive power from the AC mains.

 

The super synchronous speed for the induction motor can be achieved with the help of a prime mover. Due to the super synchronous speed of the rotor (N > N_s), the operating slip of induction generators is negative (can be verified with the help of the expression of slip given above). Generally, the operating slip for the induction generators is (-0.01 to -0.05).

The reactive power burden of induction generators on the grid can be relieved by installing shunt capacitors (connected in delta configuration) across the terminals of induction generators. These shunt capacitors deliver the lagging vars or reactive power demand by the generators and the rest reactive power demand by the load is fulfilled by the transmission line.

Since, they absorb reactive power or lagging vars, in other words, we can say that they deliver leading vars and in case of generators, the operating power factor is what they deliver. Therefore, an induction generator operates at a leading power factor.

 

The frequency of active power supplied is equal to the frequency of the reactive power drawn by it from the AC mains.

 

Torque Slip Characteristics of Induction Generators

 

The torque slip characteristics of the induction motor is shown in the given figure. In these characteristics, the region in which N > N_s or slip is negative is the generating zone, representing the torque slip characteristics of induction generators.

 

In the generating zone, the torque slip curve is linear in between the zero slip (s=0) and the slip (s_m) at which maximum torque (pushout torque) occurs. This is the recommended operating region for the induction generators, as discussed above the operating slip for induction generators is (-0.01 to -0.05).

Advantages & Applications of Induction Generators

A major advantage of the induction generator is its frequency regulation, meaning in induction generators the frequency of output voltage is independent of speed variation unlike synchronous generators, where the speed has to be tightly controlled to match the synchronous speed, so that efficiency of output voltage does not deviate from the line frequency. As we discussed above the frequency of the active power supply will depend on the frequency of the reactive power source.

 

This self-regulation property makes induction generators best suitable for variable speed power generation like wind-mills, micro hydro power plants etc. when connected to an infinite bus.

Disadvantages of using Induction Generators

 

Induction generators require a reactive power source all the time to generate active power. Hence, they will act as a reactive power burden on the existing system. This is the major drawback of using induction generators.