Motor Ratings in kW:
- Motors are devices that do mechanical work, like spinning a fan or driving a conveyor belt.
- The power they consume and produce is used for real work, like moving things.
- So, it makes sense to rate motors in kilowatts (kW) because kW measures real, useful power.
- The in-phase component of the current they draw, multiplied by the supply voltage, gives the true power in watts (kW).
- Also, the mechanical work done by the motor is measured in watts, so it's logical to express motor output power in kW.
Alternator and Transformer Ratings in kVA:
- Alternators generate electricity, and transformers change the voltage of that electricity.
- When they operate, they need to handle various types of electrical loads, like resistive, inductive, or capacitive.
- The designers of transformers and alternators don't know what kind of load will be connected to them in the future.
- To ensure safety and prevent overheating, they rate these devices in kilovolt-amperes (kVA), which measures apparent power.
- Apparent power considers both real power (useful work) and reactive power (like creating magnetic fields).
- By using kVA, the designer can account for the worst-case scenario without knowing the exact load type.
- The actual output power of these devices, in terms of useful work, is measured in kilowatts (kW).
- kW represents the real power output, while kVA accounts for the total power, including reactive power.
- The relationship between kW and kVA is determined by the power factor of the system. kW = kVA x power factor.
In simple terms, motors are rated in kW because they do real work, while alternators and transformers are rated in kVA because they have to handle different types of loads, and kVA accounts for both real and reactive power to ensure safe operation.