March 23, 2010

Basic principle of induction machine(motor and generator)- simple description

An induction machine is the most simple electrical machine from constructional point of view, in most of the cases. It can be classified into motor and generator. In this post, I want to discuss the characteristic common to both of these. Detailed description of each will be available soon in other posts.

Induction machines work on induction principle, in other words it depends on Faraday's law of induction (i.e. when a conductor moves in a magnetic field, it gets some voltage(induced voltage). this voltage can set up current if construction permits and can set up its own magnetic field.). In this case it should be noted that moving in a magnetic field actually makes the magnetic flux changing to the moving conductor(actually seems to be changing, from the view point of one who is moving), and this changing magnetic field causes voltage and current to be induced on the moving body.

But if the magnetic filed is itself changing in nature, then it can
induce voltage on a stationary conductor. This is the case for induction motor and generator. Motor remains stationary(rotor of the motor), a changing voltage(i.e. magnetic flux) is supplied to the stator and hence the rotor get some induced voltage because it remains stationary in changing magnetic field. This rotor voltage creates rotor current and rotor magnetic field(rotor flux), this rotor flux try to catch stator flux and thus rotor starts to rotate.

The case is not this simple in practice, but it is indeed the principle of rotation in induction machine.

When a voltage is supplied to the stationary coils(stators) , it creates a stator magnetic field. If the voltage is AC, then magnetic flux created by it is changing in nature. So stator produces a magnetic field variation and rotor get some induced voltage. For squirrel cage induction machine, end rings make the path for current flow and for wound rotor machine, external resistance or simply wire is used to provide current path. This current path allows rotor flux to be formed.

Stator flux can be thought of a man who is holding out his hand for someone to catch(and of course running because of changing(AC) stator voltage), and rotor flux can be thought of a lonely man in an island. When this lonely man see another man(stator flux) passing him with a stretched hand, he instantly reacts to catch the hand and starts running to the first man. But due to some causes(will be discussed in other posts), this lonely man can never reach the hand, but remains only a few inches behind the stretched out hand. So he can never lost the hope of catching the hand and continues to run as long as the hand remains stretched for him(i.e. as long as the stator is supplied with variable voltage) . So the rotor of the motor tries to catch stator flux and hence rotates. (This is the basic idea of slip in induction machines, you can read deails about the slip in the following post:

From the above discussion it should be noted that the stator flux (voltage also) must be AC for the rotor to rotate, so induction machine can run on AC only. And when acting as generator, it will generate AC directly.

In the case generator, rotor is rotated by external means(may be by turbine of some kind). If the rotor has some residual magnetism(i.e. some magnetic properties which stores magnet type properties inside the material, in a simple way to think about residual magnetism, but not describing it fully), then the rotor is actually providing a variable flux to the stationary coils in the housing(stator coils). So this stator coil will get some induced voltage on it by induction principle and we get some voltage to supply our load or to store it in a battery. Induction generators are used in small shops and households to provide extra power support and are less costly due to easy construction. In recent days, it is widely used by the people in those country where power authorities are bound to shed some load periodically due to supply shortage. Most of the time, rotor is rotated by a small diesel engine and the induction generator is coupled to it.

             Fig: A fractional horse power induction generator             

You may be interested about these:
1.Electrical braking,reason of direct current injection
2.Dynamic braking of electrical machines
3.Slip of Induction Machine: The Hidden Power

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