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Rating of Alternator

By Ashik

Power rating of alternator is defined as the power which can be delivered by an alternator safely and efficiently under some specific conditions. Increasing load, increases losses in alternator, which leads to temperature rise of the machine. The conductor and insulator parts of the machine have some specific over heating withstand limits. The power rating of an alternator is so specified, that at that maximum load, the temperature rise of different parts of the machine does not cross their specified safe limit.
The copper losses i.e. I2R loss varies with armature current and core losses vary with voltage. The temperature rise or heating of alternator depends upon cumulative effect of copper losses and core losses. As there is no role of power factor upon these losses, the rating of alternator generally given in VA or KVA or MVA.
In other word, as the losses of alternator are independent of electrical power factor, hence power factor does not come into picture while power rating of an alternator is calculated or estimated. Although losses of alternator depends upon its KVA or MVA rating but actual output varies with electrical power factor.
The electrical output of an alternator is product of power factor and VA and output is expressed in KW.
Some times alternators are also rated by its power instead of VA rating. That time electrical power factor of the alternator must be specified too.
In addition to KVA rating, an alternator is also rated with voltage, electric current, frequency, speed, number of phase, number of poles, field ampere, excitation voltage, maximum temperature rise limit etc.
The table below shown a standard rating plate of alternator. 


Make ……………
Kilo Watt Rating 5000
Power Factor 0.85 lag
KVA Rating 5,88,000
Stator Volt 21,000
Stator Ampere 16,200
Rotor Volt 340
Rotor Ampere 4040
R.P.M 3000
Hz 50
Phase 3
Armature Connection Double Star
Coolant Water & Hydrogen (Forced)
Gas Pressure 3.5 bar
Insulation Type + F
Type ……………
Specification IS5422 & IEC34
Product Serial Number ……………
Manufacturing Year ……………

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