[Explained] Why Alternator rated in KVA but Motor rated in KW or HP?

Hey, today we are going to know why alternator rated in KVA or MVA but electric motors are rated in KW or MW, or HP. The electric motor and alternator both are electrical machines and have windings or coils inside them but why the rating is not the same? KVA, MVA, KW, and MW are used for AC supply purposes so remember that we are talking about AC motors and AC generators or alternators.


Before going to our main topic you should know the difference between KVA and KW

KVA and MVA are the large units of apparent power, and KW, and MW is the large units of active power.

(KVA = Kilo Volt X Kilo Ampere) that means it is the simple product of current and voltage.

(KW = Kilo Volt X Kilo Ampere X power factor) that means it is the product of current, voltage, and power factor.

Why Alternator rated in KVA or MVA?

(1) Power Factor is dependent on the type of load or power consumers, not on the power generators.

        Resistive load = unity power factor
        Inductive load = lagging power factor
        Capacitive load = leading power factor

As the alternator is not a load, no power factor concept comes. So the alternator is rated only based on current and voltage that is VA, KVA, or MVA. So while ratings are printed on the nameplate of the alternator the units VA, KVA, or MVA are used.

(2) When engineers design an alternator they do not know what type of load will be connected to the alternator so they cannot decide the power factor as the power factor depends on the type of load. They only know the voltage and current that can be delivered by the alternator. So this can be called as a reason why alternators rated in KVA.

(3) The losses that occur in an alternator such as copper loss and iron loss do not depend upon the power factor. The copper loss depends upon the flow of current through the windings, and the iron loss depends upon the voltage produced by the alternator.

These are the three main reasons why alternators are rated in KVA.

Why is an Electric Motor rated in KW or MW?

As the electric motor is a load or power consumer so the power factor comes in the rating. The electric motor is an inductive load so it has always a lagging power factor. The current drawn by the motor depends upon the power factor so the power factor must be taken in the power rating. So as the power consumed by the motor depends on the voltage, current, and power factor which is why the electric motor is rated in Watts, or KW, or MW.

HP or Horse Power is also a unit of power. In the case of electrical power,

1 HP = 746 watt (approx.)

So, basically, you may understand the difference between KVA and KW ratings mainly depends upon the power factor. So the devices or machines that are not associated with power factor will be rated in KVA and the devices or machines that are associated with power factor will be rated in KW.


Read Also:

• Why Transformer Size is reduced with the increase of Frequency?
• Why Current, Voltage changes But Power, Frequency constant in Transformer?
• Why DC is used not AC for excitation of Synchronous Alternator and Motor?
• Why Alternator Armature Winding always connected in STAR?

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