Rating of Alternator

Power rating of an 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 the order of VA or KVA or MVA.
In other words, 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
Kilo Watt Rating5000
Power Factor0.85 lag
KVA Rating5,88,000
Stator Volt21,000
Stator Ampere16,200
Rotor Volt340
Rotor Ampere4040
Armature ConnectionDouble Star
CoolantWater & Hydrogen (Forced)
Gas Pressure3.5 bar
Insulation Type+ F
SpecificationIS5422 & IEC34
Product Serial Number...............
Manufacturing Year...............

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