Design of Electrical Machines Questions and Answers – Losses and Temperature Rise

This set of Design of Electrical Machines Problems focuses on “Losses and Temperature Rise”.

1. How many types of losses are present in synchronous machines?
a) 7
b) 3
c) 4
d) 5

Explanation: There are 7 losses in the synchronous machines. They are i) iron loss due to main field, ii) iron loss due to parasitic field, iii) I2R loss in the armature winding, iv) eddy current loss in armature conductors, v) stray load loss, vi) loss in field windings, vii) friction and windage loss.

2. What is the classification of the iron loss due to the main field?
a) hysteresis loss
b) eddy current loss
c) hysteresis loss or eddy current loss
d) hysteresis loss and eddy current loss

Explanation: The iron loss due to main field is due to the hysteresis loss. The eddy current loss also contribute to the iron losses due to main field.

3. What are the factors the pole face loss depends upon?
a) slot opening
b) air gap length
c) number of slots and speed of machines
d) slot opening, air gap length, number of slots and speed of machines

Explanation: The pole face loss depends upon the slot opening and the air gap length. It also depends on the number of slots and speed of machines.

4. What is the range of the pole face loss in the synchronous machines?
a) 40-60 % of iron loss
b) 20-60 % of iron loss
c) 25-70 % of iron loss
d) 40-80 % of iron loss

Explanation: The pole face loss has a minimum value of 25% of the iron loss. The pole face loss has a maximum value of 70% of the iron loss.

5. What is the formula for the copper loss in the synchronous machine?
a) copper loss per phase = current per phase * dc resistance
b) copper loss per phase = current per phase2 * dc resistance2
c) copper loss per phase = current per phase2 * dc resistance
d) copper loss per phase = current per phase * dc resistance2

Explanation: First the current per phase is calculated and the value is squared. Next the dc resistance is calculated and the sum of the square of the current per phase and dc resistance gives the copper loss per phase.
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6. What is the formula for the total eddy current loss in conductors?
a) total copper loss = 3 * average value of the eddy current constant * current per phase2 * dc resistance
b) total copper loss = 3 / average value of the eddy current constant * current per phase2 * dc resistance
c) total copper loss = 3 * average value of the eddy current constant / current per phase2 * dc resistance
d) total copper loss = 3 * average value of the eddy current constant * current per phase2 / dc resistance

Explanation: The average value of the eddy current constant is obtained. Next the I2R loss values are calculated and multiplying with 3 gives the total copper loss.

7. What is the cause of the stray load losses in the synchronous machine?
a) stray field
b) stray armature
c) stray field and stray armature
d) stray field or stray armature

Explanation: The stray load loss occurs due to stray fields. They are formed when the machine is being loaded.

8. What is the voltage drop in the carbon and graphite brushes?
a) 1 V
b) 0.3 V
c) 0.6 V
d) 0.75 V

Explanation: The voltage drop in the carbon and graphite brushes is 1 V. The voltage drop in the brushes containing metal is 0.3 V.

9. What factors does the friction and windage loss depend upon?
a) construction of the machine
b) speed of the machine
c) rating of the machine
d) construction, speed, rating of the machine

Explanation: This loss consists of the bearing friction and rotor windage loss. The loss depends upon the type of construction, speed and ratings of the machines.

10. What is the reduction in the total friction loss with the hydrogen cooling?
a) 0.3-0.5 % of kVA rating
b) 0.2-0.3 % of kVA rating
c) 0.3-0.4 % of kVA rating
d) 0.3-0.6 % of kVA rating

Explanation: The friction loss depends upon the type of construction, speed and ratings of the machines. The hydrogen cooling reduces the total friction loss by 0.3-0.4% of the kVA rating.

11. What is the formula to obtain the temperature rise of the surface?
a) temperature rise of the surface = Surface area * cooling coefficient * dissipating surface
b) temperature rise of the surface = Surface area / cooling coefficient * dissipating surface
c) temperature rise of the surface = Surface area * cooling coefficient / dissipating surface
d) temperature rise of the surface =1 / Surface area * cooling coefficient * dissipating surface

Explanation: The surface area is first calculated from its formula. Next, the cooling coefficient and the dissipating surface are obtained and on substitution gives the temperature rise of the surface.

12. What factor/s does the cooling coefficient depend upon?
a) speed of the cooling medium
b) configuration of the surface
c) speed of the machine and configuration of the surface
d) speed of the machine or configuration of the surface

Explanation: The cooling coefficient depends upon the speed of the machine. The cooling coefficient also depends upon the configuration of the surface.

13. The value of the cooling coefficient varies from 0.025 to 0.04 in the back of the stator core.
a) true
b) false

Explanation: The cooling coefficient value is required in the calculation of the temperature rise of the surface. The value varies from 0.025 to 0.04 for the back of the stator core.

14. The peripheral speed is the armature peripheral speed in the stationary field coils.
a) true
b) false

Explanation: There are various peripheral speeds in various parts of the machine. In the stationary field coils the peripheral speed is nothing but the armature peripheral speed.

15. What all factors does the heat to be dissipated by cooling surfaces depend upon?
a) hysteresis loss
b) eddy current loss
c) heating loss
d) hysteresis, eddy and heating losses

Explanation: The heat to be dissipated by the cooling surface of the armature core would consist of the hysteresis loss and the eddy current loss. It also consists of the heating loss or the I2R in the active part of the armature.

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