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In the last article we discussed, what is alternator. And how does it work? Today we will see, about the excitation system of alternator or Generator.

The DC voltage supplied to the alternator field to create a permanent magnetic field is called the excitation of the alternator. Different types of alternators provide different levels of voltage excitation.

Since the value of the power factor depends on the load, again it is not possible for the supplier to know the value of this power factor, so the rating of the alternator is written in KVA.

How many types of excitation system of alternator?

There are two methods for field excitation of alternator. Namely:

(i) Brush connected excitation system and

(ii) Brushless excitation system of alternator

1. Brush connected excitation system

Each alternator requires a separate DC supply to excite the field. The supplied DC voltage can usually range from 125Volt to 250Volt. The source from which DC supply is given is called exciter. The exciter is usually a DC shunt generator.

A shunt generator in a powerhouse is driven by a motor or a small diesel engine. The two terminals of the main exciter are connected to the alternator field through two slip rings and brushes. The exciter of small alternators is self-excited and the exciter of large alternators is separately excited.

The exciter required to excite the field of the main exciter of a large alternator is called a pilot exciter. The pilot exciter is a self-excited DC shunt generator and is connected to the shaft of the main exciter.

Also, the exciter is often operated using pulleys and belts along the shaft of the alternator. The connection diagram of two brush-connected exciters with the alternator is shown below.

Brush connected excitation system

2. Brushless excitation system of alternator

The brushless excitation system is used for self-excitation of the rotor field of small and medium-sized alternators. No external DC voltage is supplied to it. Excitation work is done by the E.M.F generated in the rotor in the same manner as in a three-phase alternator.

In a brushless excitation system – connected with a rectifier – current flows internally in the windings to create electromagnets, which perform the field excitation work. As a result, slip rings and brushes are not required in this method. The connection diagram of the brushless exciter is shown below.

Brushless excitation system of alternator

Function of excitor

The functions of the exciter are three, namely:

1. Exciting the rotor field,

2. Controlling the terminal voltage and

3. Controlling reactive power factor.

Why is the rating of alternator expressed in KVA?

Generally, it is reasonable to rate the alternator in KVA. It is explained below:

Alternator carries a rated load at rated voltage. If the rated load is expressed in watts (W) or kilowatts (kW) then the power factor (P.t) of the load needs to be known. Because, P = √3V_LI_Lcos_θ . If the alternator supplies rated current at rated voltage, its power (P) may vary according to the value of the power factor (P.f.).

For example, a 20KVA single-phase alternator has a rated current of 100 Amp. And the rated voltage is 200 Volt.

So kW of the alternator at unity power factor  = VIcosθ  = \frac{200\times100\times1}{1000}   =20kW

But if the value of power factor is reduced to 0.8 then output of alternator will be = \frac{200\times100\times0.8}{1000} = 16kV 

But in both cases the apparent power of the alternator will be VI = \frac{200\times100}{1000} =20kV. That is, 20KVA will remain unchanged.

If 0.8 Pf. Instead of 20kW, 16kW load has to be supplied but the alternator has to supply 125 Amp more. In this case, the alternator has to supply 25 Amp more than the rated current to supply the rated power which is harmful to the alternator. Even harmful to transmission lines.

So if an alternator has kW on its nameplate, its rated current is not known; If its power factor is not known.

P.f. Regardless the rated KVA remains the same at all times. So it is advisable to have a KVA rating instead of an alternator KW rating.

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