The power rating of an alternator is defined as the power that can be safely and efficiently supplied by the alternator under certain conditions. As the load increases, the losses in the alternator increase, which increases the temperature of the machine. The conductor and insulator parts of the machine have certain thermal tolerance limits. The manufacturer specifies the power rating of the alternator in such a way that at this maximum load the temperature rise of the various parts of the machine does not exceed their specified safe limits.
Copper losses i.e. We know that the I2R loss varies with armature current and core losses vary with voltage. The temperature rise or overheating of the alternator depends on the combined effect of copper losses and core losses. Since power factor has no role on these losses, alternator ratings are usually given in the order of VA or KVA or MVA.
In other words, as alternator losses are independent of electrical power factor, power factor does not come into the picture while we calculate and estimate the power rating of an alternator. Although the losses of an alternator depend on its KVA or MVA rating, the actual output varies with the electrical power factor.
The electrical output of the alternator is the product of the power factor and VA. We express the output in KW.
Sometimes alternatives are also rated by their strength rather than VA rating. At this point the electrical power factor of the alternator should also be specified.
Apart from KVA rating, an alternator also has voltage, current, frequency, speed, number of phases, number of poles, field amperage, excitation voltage, maximum temperature rise limit rating etc.