This set of Design of Electrical Machines Assessment Questions and Answers focuses on “Design Steps and Considerations”.
1. What is the relation between number of poles and total volume of magnet?
a) number of poles is directly proportional to the total volume of the magnet
b) number of poles is indirectly proportional to the total volume of the magnet
c) number of poles is directly proportional to the square of the total volume of the magnet
d) number of poles is indirectly proportional to the square of the total volume of the magnet
View Answer
Explanation: The first design consideration in the PMDC motor is the number of poles. The volume of the magnet is indirectly proportional to the number of poles.
2. What is the relation between number of poles and flux reversal in the armature?
a) number of poles is directly proportional to the flux reversal in the armature
b) number of poles is indirectly proportional to the flux reversal in the armature
c) number of poles is directly proportional to the square of the flux reversal in the armature
d) number of poles is indirectly proportional to the square of the flux reversal in the armature
View Answer
Explanation: The first design consideration in the PMDC motor is the number of poles. The flux reversal in the armature is directly proportional to the number of poles.
3. How many number of poles should be used for large motors of relatively low speed?
a) should be equal to 2
b) should be lesser than 2
c) should be greater than 2
d) should be more than 4
View Answer
Explanation: The number of poles should be greater than 2 for large motors with relatively low speed. The number of poles is equal to 2 for small motors.
4. In the PMDC motors the brush shift should be approached with considerable caution.
a) true
b) false
View Answer
Explanation: The brush shift in the PMDC motor should be approached with considerable caution. This is because as flux shift in the ceramic magnet will be found to be almost negligible.
5. What is the relation of the brush shift with the demagnetization effect?
a) brush shift is directly proportional to the demagnetization effect
b) brush shift is indirectly proportional to the demagnetization effect
c) brush shift is directly proportional to the square of the demagnetization effect
d) brush shift is indirectly proportional to the square of the demagnetization effect
View Answer
Explanation: The second design consideration is the brushes in the PMDC motor. The brush shift increases the demagnetizing effect.
6. How many primary reasons are present for the thermal failure?
a) 3
b) 4
c) 2
d) 5
View Answer
Explanation: There are 2 primary reasons for the thermal failure. The first one is an increase of the resistance of motor winding and second one is the inability of the motor to dissipate the heat generated.
7. What does the increase of the resistance of the motor winding cause?
a) high starting current
b) low motor torque
c) low starting current
d) high motor torque
View Answer
Explanation: The increase of the resistance of the motor winding is one of the cause of thermal failure. This produces lower motor torque in the machine.
8. What does the inability of the motor to dissipate the heat cause?
a) causes high starting current
b) insulation failure
c) causes low starting current
d) causes high starting current
View Answer
Explanation: The inability of the motor to dissipate the heat is one of the causes of thermal failure. The inability of the motor to dissipate heat causes insulation failure.
9. What is the solution to prevent the increase of resistance of motor windings?
a) higher ventilation arrangement
b) reduction in the number of poles
c) increase the coil windings
d) insulate the windings
View Answer
Explanation: The increase of resistance of the motor winding is one of the cause of thermal failure. They can be reduced by providing a higher ventilation arrangement in the machine.
10. How many types of gears are made use of in the PMDC motors?
a) 1
b) 2
c) 3
d) 4
View Answer
Explanation: Generally 3 types of gears are made use of in the PMDC motors. They are spur gears, helical gears, and worm gears.
11. What type of gears are used in the small loads and low inertia motors?
a) spur gears
b) helical gears
c) worm gears
d) worm gears and helical gears
View Answer
Explanation: Three types of gears are made use of in the PMDC motors. The spur gears are made use of in the low inertia and small load motors.
12. What type of gear is made use of in the high inertia motors?
a) spur gears
b) helical gears
c) worm gears
d) spur and helical gears
View Answer
Explanation: There are three types of gears made use of in the PMDC motors. The worm gears are used in the high inertia load.
13. What type of gear is made use for the silent operation?
a) spur gears
b) helical gears
c) worm gears
d) spur gears and worm gears
View Answer
Explanation: There are three types of gears made use of in the gearing system of the PMDC motors. The helical gears is made use for the silent operation.
14. How many types of bearings are made use of in the PMDC motors?
a) 2
b) 3
c) 4
d) 5
View Answer
Explanation: There are two types of bearings used in the PMDC motors. They are bell bearing and the journal bearing.
15. How many principle types of lubricants are available in the PMDC motor?
a) 2
b) 3
c) 5
d) 7
View Answer
Explanation: There are 3 principle types of lubricants in use. They are oil, dry film lubricants and grease.
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