# Design of Electrical Machines Questions and Answers – Design of Small Universal Commutator Motors

This set of Design of Electrical Machines Interview Questions and Answers for Experienced people focuses on “Design of Small Universal Commutator Motors”.

1. What are the applications of the small universal commutator motors?
a) industry
b) medicine
c) domestic sector
d) industry, medicine and domestic sector

Explanation: Small universal commutator motors have power outputs varying from few watts to hundreds of watts. They have lots of application in industry, medicine, domestic sector.

2. What type of excitation is used in the small universal commutator motors and what type of supply is provided?
a) parallel excitation, dc supply
b) series excitation, dc or ac supply
c) series excitation, ac supply
d) parallel excitation, dc supply

Explanation: The excitation which is provided is the series excitation in the small universal commutator motors. The type of supply provided is dc or ac supply.

3. What is the material used in the lamination of the magnetic poles of small universal commutator motor?
a) copper
b) aluminium
c) gold
d) sheet steel

Explanation: Unlike dc motors universal commutator motors are having laminated poles. The laminated poles are laminated using the sheet steel.

4. What is the thickness of the laminations of magnetic poles?
a) 0.3-0.5 mm
b) 0.2-0.4 mm
c) 0.35-0.5 mm
d) 0.4-0.5 mm

Explanation: The minimum value of the thickness of the laminations of magnetic poles is 0.35 mm. The maximum value of the thickness of the laminations of magnetic poles is 0.5 mm.

5. How many type of excitations does the universal commutator motors have?
a) 1
b) 2
c) 3
d) 4

Explanation: There are two kinds of excitation present for the universal commutator motors. One winding is intended to run the motor to ac supply voltage. The other winding is connected in series with the first winding when the motor is operated from dc supply voltage.
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6. The number of turns of field winding in the motor must be considerably less than the number of turns in the armature winding.
a) true
b) false

Explanation: The number of turns of field winding in the motor must be considerably less than the number of turns in the armature winding. The speed of the machine is irrespective of the supply.

7. What is the relation of the copper loss and brush contacts with the total loss in small dc motors?
a) copper loss and brush contacts = 2 * total loss
b) copper loss and brush contacts = 2/3 * total loss
c) copper loss and brush contacts = 1/3 * total loss
d) copper loss and brush contacts = total loss

Explanation: The copper loss and brush contact loss is being compared with the total loss in order to deduce an equation. The copper loss and brush contact loss is 2/3 times the total loss.

8. What is the relation of the copper loss and brush contacts with the total loss in universal commutator motors?
a) copper loss and brush contacts = total loss
b) copper loss and brush contacts = total loss/2
c) copper loss and brush contacts = total loss * 2
d) copper loss and brush contacts = total loss * 3

Explanation: The copper loss and brush contact loss is being compared with the total loss in order to deduce equations. The copper loss and brush contact loss is half the total loss.

9. What is the range of the transformation ratio in the pole machines?
a) 0.05-0.1
b) 0.1-0.2
c) 0.1-0.25
d) 0.3-0.4

Explanation: The transformation ratio in 2 pole motors is 0.1-0.25. The transformation ratio in the 4 pole motors is 0.05-0.1.

10. For what outputs are the 2 pole machines made use of?
a) output > 200 W
b) output < 200 W
c) output > 300 W
d) output < 300 W

Explanation: The 2 pole machines are made use of when the output is below 200 W. The 4 pole machines are made use of when the output is above 200 W.

11. What is the range of the power factor for the 4 pole motors?
a) 0.6-0.85
b) 0.75-0.95
c) 0.6-0.8
d) 0.7-0.9

Explanation: The power factor for the 4 pole motors is 0.6-0.85. The power factor for the 2 pole motors is 0.75-0.95.

12. What is the value of the specific electric loading for the continuous duty motor type?
a) 8000-11000 A per m
b) 6000-9000 A per m
c) 12000-20000 A per m
d) 15000-25000 A per m

Explanation: The specific electric loading for continuous duty motor type is 6000-9000 A per m. The specific electric loading for power rating above 100 W but below 200 W is 8000-11000 A per m and the specific electric loading for power rating above 200 W but below 750 W is 12000-20000 A per m.

13. What is the specific magnetic loading for the motors having output less than 100 W?
a) 0.25-0.30 T
b) 0.3-0.4 T
c) 1.3-1.5 T
d) 0.25-0.35 T

Explanation: The specific magnetic loading for the output below 100 W is 0.25-0.35 T and the specific magnetic loading for the continuous duty motors is 0.3-0.4 T and the specific magnetic loading for the short time duty motors is 1.3-1.5 T.

14. What is the formula for the pole pitch in the universal commutator motor?
a) pole pitch = 3.14 * diameter * 2 * no. of poles
b) pole pitch = 3.14 / diameter * 2 * no. of poles
c) pole pitch = 3.14 * diameter / 2 * no. of poles
d) pole pitch = 3.14 * diameter * 2 / no. of poles

Explanation: The diameter and the number of poles are calculated. On substitution, the pole pitch of the universal commutator motor is obtained.

15. What is the formula of the pole arc of the universal commutator motor?
a) pole arc = ratio of armature axial length to armature diameter * pole pitch
b) pole arc = ratio of armature axial length to armature diameter + pole pitch
c) pole arc = ratio of armature axial length to armature diameter – pole pitch
d) pole arc = ratio of armature axial length to armature diameter / pole pitch

Explanation: Firstly the ratio of armature axial length to armature diameter is calculated. Next, the pole pitch is calculated and on addition of both the terms the pole arc is obtained.

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