Design of Electrical Machines Questions and Answers – Design Steps and Considerations – 1

This set of Design of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Design Steps and Considerations – 1”.

1. How many design steps are present in the design of PMDC motors?
a) 8
b) 9
c) 10
d) 11

Explanation: There are 11 steps involves in the design of the PMDC motors. They are minimum sum of air gap volume and magnet volume, ratio of magnetic to electric loading, area of magnet, length of magnet, value of flux, number of turns per coil, running armature resistance, armature diameter, axial dimensions, wire cross section and radial thickness.

2. What happens to the armature diameter and the volume of air gap and magnet when the angle is lower in value?
a) volume of air gap and magnet increases, armature diameter increases
b) volume of air gap and magnet increases, armature diameter decreases
c) volume of air gap and magnet decreases, armature diameter decreases
d) volume of air gap and magnet decreases, armature diameter increases

Explanation: The lower values of angle, reduces the volume of air gap and magnet. The reduction of volume of air gap and magnet, increases the armature diameter.

3. What should be the range of the product of the magnetic field and magnetic flux density?
a) 4-4.5 * 106
b) 4-4.3 * 106
c) 4.3-4.6 * 106
d) 4.2-4.5 * 106

Explanation: The product of the magnetic field and magnetic flux density has a minimum value of 4.3 * 106. The product of the magnetic field and magnetic flux density has a minimum value of 4.6 * 106.

4. What should be the minimum value of the ratio of the magnetic to electric loading?
a) 40
b) 30
c) 50
d) 60

Explanation: The calculation of the ratio of the magnetic to electric loading is the second step in the design of the PMDC motors. It should have a minimum value of 50.

5. What is the formula for the area of the magnet in the design of PMDC motors?
a) area of magnet = flux * 4.95 * residual flux density
b) area of magnet = flux / 4.95 * residual flux density
c) area of magnet = flux * 4.95 / residual flux density
d) area of magnet = 1/flux * 4.95 * residual flux density

Explanation: First the residual flux density is calculated. Next, the flux is calculated and substitution in the formula gives the area of magnet.

6. What is the range of length of the magnet in the PMDC motors?
a) 2.5-4 cm
b) 2-3 cm
c) 2.5-3 cm
d) 1.5-4 cm

Explanation: The minimum value of the length of the magnet in the PMDC motor is 2.5 cm. The maximum value of the length of the magnet in the PMDC motor is 4 cm.

7. What is the formula of the length of the magnet?
a) length of the magnet = sum of the volume of air gap and magnet * Area of the magnet + 0.06
b) length of the magnet = sum of the volume of air gap and magnet / Area of the magnet + 0.06
c) length of the magnet = sum of the volume of air gap and magnet / Area of the magnet – 0.06
d) length of the magnet = sum of the volume of air gap and magnet * Area of the magnet – 0.06

Explanation: The sum of the volume of the air gap and magnet is first calculated. Next, the area of the magnet is calculated from its formula and on substitution gives the length of the magnet.

8. What is the relation between the flux and the no local speed?
a) flux is directly proportional to the no local speed
b) flux is indirectly proportional to the no local speed
c) flux is directly proportional to the square of the no local speed
d) flux is indirectly proportional to the square of the no local speed

Explanation: The calculation of the flux value is one of the design steps. The flux is indirectly proportional to the no local speed calculated.

9. What is the formula of the number of turns per coil?
a) number of turns per coil = number of conductors/2*coils/slot*number of armature teeth
b) number of turns per coil = number of conductors*2*coils/slot*number of armature teeth
c) number of turns per coil = number of conductors*2*coils/slot/number of armature teeth
d) number of turns per coil = number of conductors/2*coils/slot/number of armature teeth

Explanation: The number of conductors is calculated along with the coils per slot is calculated. Next, the number of armature teeth is calculated, and on substitution gives the number of turns per coil.

10. What is the formula for the armature resistance?
a) armature resistance = running armature resistance / 1.0 to 1.0
b) armature resistance = running armature resistance * 1.3 to 1.5
c) armature resistance = running armature resistance * 1.4 to 1.5
d) armature resistance = running armature resistance / 1.3 to 1.3

Explanation: The running armature resistance is first calculated in the PMDC motor. It is divided by 1.3 and that gives the armature resistance of the machine.

11. What is the relation between axial dimension and the area of the magnet?
a) area of the magnet is directly proportional to the axial dimension
b) area of the magnet is indirectly proportional to the axial dimension
c) area of the magnet is directly proportional to the square of the axial dimension
d) area of the magnet is indirectly proportional to the square of the axial dimension

Explanation: The calculation of the axial dimension is one of the steps in the PMDC motors. The axial dimension is directly proportional to area of the magnet.

12. What is the relation of the wire cross-section with respect to the armature resistance?
a) wire section is directly proportional to the armature resistance
b) wire section is indirectly proportional to the armature resistance
c) wire section is directly proportional to the square of the armature resistance
d) wire section is indirectly proportional to the square of the armature resistance

Explanation: The 10th design step of the PMDC motor is the calculation of the wire cross section. The wire cross section is directly proportional to the armature resistance.

13. The radial thickness of the joke directly proportional to the flux.
a) true
b) false

Explanation: The last design step in the PMDC motor is the calculation of the radial thickness of the joke. The flux value is directly proportional to the radial thickness of the joke.

14. The radial thickness of the joke is directly proportional to the length of the stator slots.
a) true
b) false

Explanation: The last design step in the PMDC motor is the calculation of the radial thickness of the joke. The radial thickness of the joke is indirectly proportional to the length of the stator slots.

15. What is the formula for the length of the stator slots?
a) length of the stator slots = 2 * perimeter of one magnet
b) length of the stator slots = 1/2 * perimeter of one magnet
c) length of the stator slots = 1/3 * perimeter of one magnet
d) length of the stator slots = 3 * perimeter of one magnet

Explanation: The length of the stator slots is required in the calculation of the radial thickness of the joke. The length of the stator slots is equal to half the perimeter of one magnet.

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