GENERATOR

1.                   What is mean by the excitation of a generator?

Excitation refers to the creation and strengthening of the magnetic field by the passage of direct current through the field coils. There are two methods by which a generator may be excited.

(a)    Self- excitation: The iron core of the field poles always retains a small amount of magnetism called residual magnetism. When a DC generator starts up from rest, the residual magnesium is sufficient to induce a small voltage in the amateur windings, and a correspondingly small current starts voltage is induced in the armature. The voltage thus goes on developing until it reaches the normal voltage.

(b)   Separate excitation: Here the field current is obtained from a separate source. An AC generator does not lend itself to self-excitation because the field windings require DC current, whole the amateur produces only AC. The most common arrangement employed top furnish DC supply for the field is to provide a separate excite which is nothing but a small self excited DC generator, coupled directly to the main generator shaft. This exciter furnishes the DC fields current of the main generator.

One of the modern methods employed in the case of small machines is to provide a metal rectifier to convert AC voltage generated by the amateur to DC to excite the field system instead f exciter.

2.       Are any special precautions necessary in paralleling of alternators?

Yes. The following conditions should be satisfied before they can be connected together.

(a)    The frequency of both should be identical.

(b)   The voltage of both must be the same.

(c)    The phase rotation in both machines should be the same.

(d)   Identical phase wire R.Y.B. should be connected together at the exact instant when there is zero phase different between them.

(e)   For satisfactory parallel operation the characteristics of the two machines should be similar.

3.       What is the effect for increasing or decreasing the excitation of an alternator when running in parallel with another?

As already stated excitation does not affect load sharing. Its only effect is to change the division of the reactive current. Consider two alternators running in parallel taking equal loads if excitation of No.1 machine is increased, it will take more of the lagging reactive current and No.2 machine will take less. Any inequality of excitation is equalized by a circulating reactive current passing from the over excited machine to the other. Therefore, the field regulator of the two machines should be adjusted, keeping an eye on the load current as well as the power factor meter on the alternator panel.

4.       What is an automatic voltage regulator and what is its function and why is it provided?

Due to rapid variations in the load from instant the voltage also fluctuates considerably because of the varying voltage drop in the armature circuit. To overcome this unsatisfactory feature automatic voltage regulators are usually provided to maintain the generator voltage reasonably constant in spite of the fluctuating load i.e., to increase the voltage when the load is high and reduce it when the load comes down.

There are several types of automatic voltage in use:

(a)    Brown Boveri regulator

(b)   Carbon pile regulator

(c)    Tirril voltage regulator

5.       If a generator fails to build up voltage when started or the voltage of the generator is too low what are the possible defects and how will you overcome them.

Some of the possible defects are given below.

1.)    Loss of residual magnesium.

2.)    Open circuit or loose connection in the exciter circuit also short circuited turns in field windings.

3.)    Reversed field windings.

4.)    Open or short-circuit in armature windings.

5.)    Excessive contact resistance on commentator due to incorrect grade of brushes or insufficient pressure or dirty commutator.

6.)    Incorrect position of brush.

6.       What indication and protective equipment is provided on the alternator panels?

A.)   Indicating instruments.

                                i.   Volt meter.

                              ii.   Ammeter with 3 position ammeter switch

                            iii.   Frequency meter

                             iv.   Power factor meter

                            v.   Synchorscope with synchronizing plug.

B.)    Protective device:

i.)      Overload relay

ii.)    Reverse power relay

iii.)  Earth leakage relay

C.)    Control devices

i.)      Excitor regulator

iv.)  Oil circuit breaker.