Page 131 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

The output in kW is proportional to power factor for the alternator of a given kVA. For example output of 1000 kVA
alternator on full load will be 200, 500, 800, 1000 kW at power factor 0.2, 0.5, 0.8 and unity respectively but copper
losses in armature will remain the same regard less of power factor.
For the above reasons alternators are usually rated in kVA (kilo Volt Ampere).

Hunting
Hunting is a phenomenon in alternator which is caused by continuous fluctuation in load. When the load on the
alternator is frequently changing, then the rotor of the alternator runs unsteadily making a noise of a whistle due
to oscillations, or vibrations set up in the rotor. This phenomenon is called as hunting of alternators.
Hunting is prevented by the Damper Windings provided in the field pole core.
Generation of 3-phase voltage and general test on

alternator

Objectives: At the end of this lesson you shall be able to:
• xplain the method of generating 3-phase voltage wave-forms by a 3-phase alternator
• state the phase sequence of 3f supply
• state the method of testing an alternator for continuity insulation and earth connection
• state e.m.f. equation of the alternator
• state the I.E.E. regulations and B.I.S. recommendations pertaining to earthing of the alternator

Generation of three-phase voltage: Basically, the principle of a three-phase alternator (generator) is the same
as that of a single phase alternator (generator), except that there are three equally spaced coils or windings which
produce three output voltages which are out of phase by 120° with each other.
A simple rotating-loop, three-phase generator with its output voltage wave-forms is shown in Fig 1c.

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CITS : Power - Electrician & Wireman - Lesson 76-85