AC machines Alternator

low dc voltage

The voltage applied to the field of a rotating cylindrical rotor type alternators

Definite Frequencies

. Alterators are usually designed to generate

. Forge steel

In case of Turbo-alternators the rotor is usually made of

load

The power factor of an alternator is determined by its

Non- Salient pole type

What kind if rotor is most suitable for turbo-altemators which are designed to run at high speed

full pitch winding to distributed winding

Distribution factor K is defined as the ratio of emf's of

Increase frequency

Increasing the speed of alternator, what will happen?

A

Out of the following conditions, the one which does not have to be met by alternators working parallel is

A. the machine must have equal rating
B. terminal voltage of each machine must be the same C. the machines must have the same phase
D. they must have the same frequency

It will get short circuited

It is never advisable to connect a stationary alternator to live bus-bars because

decrease the excitation

The power factor of an alternator is The operator is ordered to increase the power factor to 80%. What shall he do

prime mover power input is increased

In order to transfer the load from one alternator to another alternator, when they are operating in parallel

Cool the machine

. Hydrogen is used tin large alternators mainly to

Low and medium speed prime

Salient pole type alternators are generally used on

Governor

It plays an important role in over-speed protection of a generator

improve voltage waveforms

One of the advantages of distributing the winding in alternator is

poles

In huge alternators, the moving part is

as a return path for the flux

In case of alternator, the stator frame serves

work on Faraday's law

Alternators mostly work on rotating field type principle which is different from d.c. machines where the field is stationary. Therefore the alternators

of non-salient type

High speed alternators have a rotorconstruction

rotating field

The standard practice, now a days in alternators is to have

smaller because speed is high

The diameter of high speed turbo alternators as compared to hydroelectric ones are

low voltage

. The slip rings employed in a three-phase alternator in hydrostation are insulated for
In extra high tension voltage

Steam-turbo-alternators run at high speed

Steam-turbo-alternators are much smaller in size than water-turbine alternators for a given output. This is so because

reduction of irregularities produced in the waveform

Distributing the armature winding of alternator is more than one number of slots per pole per phase results

kVA will be lagging

If the input of the prime moyer of an alternator is kept constant but the excitation is increased then

to be kept constant but input torque should be increased

. For two alternators operating in parallel, if the load shared by one of them is to be increased, its field excita tion is

unity

The armature reaction of an alternator will be cross- magnetising of the power factor of the load is

Potier triangle method

The best and the most accurate method for determining the voltage regulation is

leading p.f.

An alternator is said to be over-excited if it is operating

from an auxillary generator driven by main generator
shaft.

From what source does a rotating field type alternator receive its exciter voltage

directly to load

High voltage alternator's are usually of the rotating field." type: This means the generated voltage is connected

strength of the magnetic field

. The maximum current that can be supplied by alternator depends upon the

improve voltage wave form

. One of advantages of distributing the winding in alternator is to

distributed winding to concentrated winding

Distribution factor Ka is defined as the ratio of emf's of

0.955

For a uniformly distributed winding the value of distribution factor is

elimination and suppression of harmonies

One of the advantages of a short pitch winding lies in

short pitch coil to full pitch coil

Pitch factor is defined as the ratio of the emf's of

open circuit voltage to short circuit current for same value of excitation

. Synchronous impedance of an alternator is defined as the ratio of

leakage reactance and fictitious reactance

synchronous reactance consists of two components

voltage

. Regulation of an alternator is defined as the rise in terminal... when full load is thrown off expressed as percentage of the terminal........... for the same

connecting alterators in parallel

Synchronization of alternators means

same voltage, same frequency and voltages, should be in phase opposition with respect to local series circuit

To synchronize alternator, three conditions must be fulfilled, namely

bright lamp method and a synchroscope

The method most commonly used for synchronizing the. alternators in a power house is:

prime mover power

ln order to transfer the load from one alternator to another, alternator, when they are operating in parallel

change its power factor

Adjustment of field excitation on one of the alternators operating in parallel will

causes its power factor to become lagging

Each of the two alternators operating in parallel are adjusted for unity power factor. Increasing the field current of one

salient pole type

What kind of roter is most suitable for turbo-alternators which are designed to run at high speed

increases stability

. Dampers in large generators

to reduce the harmonics from the generated e.m.f.

Short-pitch coils in the alternator are used

the reactive component of the output is changed

If the input to the prime-mover of an alternator is kept constant but the excitation is changed then

behave as a synchronous motor-and-will rotate in the same direction.

The driving power from the prime mover driving an alternator is lost but the alternator remains connected to the supply network and field supply is on. The alternator will