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Question

How much energy is used by an 80% efficient, 2kW motor running at a peak load for 5 hours?

This question was previously asked in
RRB ALP/Technician 2018 CBT 2 Electronics Mechanic Question Paper (23-Jan-2019) (Shift 1)
The correct answer is
8 kWh

Calculating Motor Energy Consumption

This solution calculates the energy output of a motor based on its input power rating, efficiency, and operating time.

Given Parameters

  • Motor Power Rating (Input, \(P_{in}\)): 2 kW
  • Motor Efficiency (\(\eta\)): 80% or 0.8
  • Time (\(t\)): 5 hours

Calculating Useful Output Power

The efficiency (\(\eta\)) relates the useful output power (\(P_{out}\)) to the input power (\(P_{in}\)):

\( \eta = \frac{P_{out}}{P_{in}} \)

To find the useful output power, rearrange the formula:

\( P_{out} = P_{in} \times \eta \)

Substitute the given values:

\( P_{out} = 2 \text{ kW} \times 0.8 = 1.6 \text{ kW} \)

This means the motor delivers 1.6 kW of useful power.

Calculating Energy Used (Output)

Energy used (in this context, the useful work done) is calculated by multiplying power by time:

\( \text{Energy} = P_{out} \times t \)

Substitute the calculated output power and the given time:

\( \text{Energy} = 1.6 \text{ kW} \times 5 \text{ hours} \)

\( \text{Energy} = 8 \text{ kWh} \)

Conclusion

The motor utilizes its input power and efficiency to deliver 8 kWh of energy over 5 hours.

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Important Questions from Work Power and Energy

  1. Which of the following is an example of potential energy?

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