1 Answer
Motors are rated in **kW (kilowatts)**, while transformers are rated in **kVA (kilovolt-amperes)** because of the different ways they handle electrical power and the factors affecting their performance. Here's the explanation:
### 1. **Motors Rated in kW (Kilowatts)**:
- **kW** measures **real power**, which is the actual power used by the motor to perform useful work (like turning a fan, pump, etc.).
- A motor consumes real power to produce mechanical output. The **efficiency** of a motor directly relates to the real power it uses.
- The **power factor** of the motor is usually considered while converting electrical power (in VA) to real power (in watts). This tells you how effectively the motor is using the electricity.
- Since the motor's purpose is to convert electrical energy into mechanical energy, the rating in **kW** gives a clearer picture of how much work it can do.
### 2. **Transformers Rated in kVA (Kilovolt-Amperes)**:
- **kVA** measures **apparent power**, which includes both the real power (kW) and reactive power (kVAR).
- A transformer’s role is to transfer electrical power from one circuit to another, without converting it into any mechanical work. It handles both **real power (kW)** and **reactive power (kVAR)**, which are needed to maintain the voltage and current in the system.
- The **reactive power** is related to magnetic fields, which the transformer needs to function. The power factor in a transformer can vary based on the load, and it is difficult to predict without knowing the specifics of the load it’s supplying.
- Since the transformer does not use the power (in terms of mechanical output), it is rated in **apparent power (kVA)** to cover all types of power.
### Summary:
- **Motor (kW)**: Rated based on the actual power (real power) it uses to perform work.
- **Transformer (kVA)**: Rated based on the total power (apparent power) it can handle, including both real and reactive components.
So, a motor's capacity is related to the real output, whereas a transformer's capacity is related to its ability to transfer both real and reactive power.
### 1. **Motors Rated in kW (Kilowatts)**:
- **kW** measures **real power**, which is the actual power used by the motor to perform useful work (like turning a fan, pump, etc.).
- A motor consumes real power to produce mechanical output. The **efficiency** of a motor directly relates to the real power it uses.
- The **power factor** of the motor is usually considered while converting electrical power (in VA) to real power (in watts). This tells you how effectively the motor is using the electricity.
- Since the motor's purpose is to convert electrical energy into mechanical energy, the rating in **kW** gives a clearer picture of how much work it can do.
### 2. **Transformers Rated in kVA (Kilovolt-Amperes)**:
- **kVA** measures **apparent power**, which includes both the real power (kW) and reactive power (kVAR).
- A transformer’s role is to transfer electrical power from one circuit to another, without converting it into any mechanical work. It handles both **real power (kW)** and **reactive power (kVAR)**, which are needed to maintain the voltage and current in the system.
- The **reactive power** is related to magnetic fields, which the transformer needs to function. The power factor in a transformer can vary based on the load, and it is difficult to predict without knowing the specifics of the load it’s supplying.
- Since the transformer does not use the power (in terms of mechanical output), it is rated in **apparent power (kVA)** to cover all types of power.
### Summary:
- **Motor (kW)**: Rated based on the actual power (real power) it uses to perform work.
- **Transformer (kVA)**: Rated based on the total power (apparent power) it can handle, including both real and reactive components.
So, a motor's capacity is related to the real output, whereas a transformer's capacity is related to its ability to transfer both real and reactive power.