Rating of Alternator

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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