# Design of Electrical Machines Questions and Answers – Rating of Machines

This set of Design of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Rating of Machines”.

1. What is the concept of power rating of machines with respect to voltage?
a) the required supply voltage for smooth running of the machine
b) the required supply voltage for stopping the machine
c) the required supply voltage for speeding the machine
d) the required supply voltage for slowing up the machine

Explanation: Power rating is nothing but the standard value at which the machine is said to be safe in operation. Rating determines the voltage which allows the smooth running of the machine.

2. What is the concept of power rating of machines with respect to current?
a) Maximum permissible amount of current that can easily flow
b) Minimum permissible amount of current that can easily flow
c) Maximum permissible amount of current that can stop the machine
d) Maximum permissible amount of current that can stop the machine

Explanation: Power rating always deals with two variables, one is current and the other is voltage. When it comes to the current, it is the maximum permitted current that can be allowed into the machine.

3. What happens if there is insufficient rating of the machine?
a) The efficiency of the machine increases
b) The efficiency of the machine improves
c) Damage and shutdown occurs

Explanation: When there is insufficient rating, it can lead to the damage of the windings of the machine. It indirectly, leads to the shutdown of the machine to avoid more hazards.

4. What happens if the power ratings of the machine are decided liberally?
a) Damage occurs to the machine
b) Efficiency of the machine improves
c) Long life of the machine
d) Uneconomical usage of the machine

Explanation: If the power rating becomes very liberal, then it causes a high initial cost. Along with the high initial cost, loss of energy also occurs and leads to uneconomical usage.

5. If the power ratings are crossed, machine breakdown occurs.
a) True
b) False

Explanation: Every machine has a permissible limit for both voltage and current, for its efficient operation. If the limit is crossed, it will lead to the breakdown of the machine.

6. Which are the important criteria related to the power ratings of the machine?
a) Heat should be prevented from generation
b) Heat should be dissipated through power ventilation, irrespective of the time
c) Heat should be prevented through power ventilation within a short time period
d) Heat should be converted to some useful form

Explanation: Due to the components present in the machine, I2R losses occur in the machine. Due to this, heat is produced, and proper thermal ventilation should be provided to prevent the machine from breakdown.

a) Output power is indirectly proportional to the temperature rise
b) Output power is indirectly proportional to the square of temperature rise
c) Output power is directly proportional to the temperature rise
d) Output power is directly proportional to the square of temperature rise

Explanation: Thermal loading is nothing, but the increase of the output power with respect to the temperature rise. It can lead to the power rating levels being crossed.

8. What is the ideal condition for thermal dissipation?
a) Heat generated > Heat Dissipated
b) Heat generated < Heat Dissipated
c) Heat generated = Heat Dissipated
d) Heat generated = 0

Explanation: For ideal thermal dissipation, the heat dissipated should be equal to the heat generated. In that case, there will be very less power loss and high efficiency.

9. What is the main objective of power ratings of machines?
a) helps in building a suitable thermal model of machines
b) helps in building a suitable physical model of machines
c) helps in classifying the machines into different types
d) helps to improve the machine efficiency

Explanation: The power ratings, mainly help in building the thermal model. It helps in reducing the heat losses and helps in bringing out a smooth and efficient operation of the machines.

10. Power ratings help in classifying machines to different classes of duties.
a) true
b) false

Explanation: The classification of the machines into different classes of duties depends on the power ratings. The ratings also helps in identifying the different types of machines available under different classes.

Sanfoundry Global Education & Learning Series – Design of Electrical Machines.

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