# Design of Electrical Machines Questions and Answers – Design Preliminaries

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

1. What does the copper factor in PMDC motors represent?
a) it represents the armature circular area for conductors
b) it represents the field circular area for conductors
c) it represents the fraction of the armature circular area for conductors
d) it represents the fraction of the field circular area for conductors

Explanation: The copper factor represents the fraction of the armature circular area for conductors. It is represented by the letter K.

2. What is the range of the copper factor in PMDC motors?
a) 0.1-0.3
b) 0.1-0.2
c) 0.1-0.4
d) 0.2-0.4

Explanation: The copper factor represents the fraction of the armature circular area for conductors. The range of the copper factor is between 0.1-0.2.

3. What is the formula for the armature resistance in PMDC motor?
a) armature resistance = (Diameter + length)*total number of armature conductors/1.2 * 104 * number of parallel paths in the armature2
b) armature resistance = (Diameter + length)*total number of armature conductors*1.2 * 104 * number of parallel paths in the armature2
c) armature resistance = (Diameter + length)*total number of armature conductors/1.2 * 104 + number of parallel paths in the armature2
d) armature resistance = (Diameter + length)+total number of armature conductors/1.2 * 104 * number of parallel paths in the armature2

Explanation: First the diameter, length and the total number of armature conductors are obtained. Next the number of parallel paths in the armature is calculated and on substitution it provides the armature resistance.

4. What happens to the diameter when the poles are more than 2?
a) diameter = 2 * diameter * (number of armature teeth embraced by one coil/total number of armature teeth)
b) diameter = 2.32 * diameter * (number of armature teeth embraced by one coil/total number of armature teeth)
c) diameter = 2.32 * diameter * (number of armature teeth embraced by one coil * total number of armature teeth)
d) diameter = 2 * diameter / (number of armature teeth embraced by one coil/total number of armature teeth)

Explanation: The diameter is the exact calculated value for 2 pole motors. But when the poles are more than 2, the above formula is made use of to calculate the armature resistance.

5. What factor does the permeance coefficient depend upon?
a) geometry of the magnet
b) geometry of the magnet, airgap, associated non-portions of the magnetic circuit
c) airgap
d) associated non-portions of the magnetic circuit

Explanation: The permeance coefficient depends upon the geometry of the magnet and the airgap. It also depends on the associated non-portions of the magnetic circuit.
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6. What is the range of the permeance coefficient in the PMDC motors?
a) 3-5
b) 4-9
c) 4-8
d) 3-9

Explanation: The minimum value of the permeance coefficient used in the PMDC motors is 4. The maximum value of the permeance coefficient used in the PMDC motor is 8.

7. What is the usual value of the permeance coefficient of the PMDC motor?
a) 4
b) 5
c) 6
d) 7

Explanation: The range of the permeance coefficient for the PMDC motor is 4-8. The value is usually around 6 for most of the applications.

8. The field current flowing in the conductor’s acts as demagnetizing force on the fraction tips of the magnet.
a) true
b) false

Explanation: The armature current flowing in the conductors acts as demagnetizing force. Its acts on the fraction tips of the magnets present.

9. What is the value of the demagnetizing coefficient if the total number of teeth is greater than 107?
a) d = angle/360
b) d = angle/240
c) d = angle/540
d) d = angle/720

Explanation: If the total number of teeth is greater than 107 then the demagnetizing coefficient become the ratio of the angle and 720. Otherwise d is one half the ratio of the maximum number of teeth that can be situated within the angle to the total number of teeth.

10. What is the value of the reluctance factor in the calculation of the intensity of magnetic field?
a) 1
b) 2
c) 1.15
d) 1,45

Explanation: The reluctance factor is one of the factors made use of in the calculation of the intensity of magnetic field. The value of the reluctance factor is around 1.15 generally.

11. What is the formula of the magnetic to electrical boarding ratio?
a) magnetic to electrical boarding ratio = number of poles * permeance coefficient * flux per pole/number of conductors * armature current
b) magnetic to electrical boarding ratio = number of poles / permeance coefficient * flux per pole*number of conductors * armature current
c) magnetic to electrical boarding ratio = number of poles + permeance coefficient * flux per pole/number of conductors * armature current
d) magnetic to electrical boarding ratio = number of poles * permeance coefficient / flux per pole*number of conductors * armature current

Explanation: The permeance coefficient is first calculated along with the number of poles and the flux per pole. Then the number of conductors are noted and the armature current is calculated to give the magnetic to electrical boarding ratio.

12. How is the value of the magnetic to electrical boarding ratio related with the volume of iron and volume of copper?
a) high magnetic to electrical boarding ratio gives high copper volume and high iron volume
b) high magnetic to electrical boarding ratio gives low copper volume and high iron volume
c) low magnetic to electrical boarding ratio gives low copper volume and low iron volume
d) low magnetic to electrical boarding ratio gives low copper volume and high iron volume

Explanation: The high value of magnetic to electrical boarding ratio gives a high volume of iron. But the high value of magnetic to electrical boarding ratio gives low copper volume.

13. For good performance the small dc motor should have magnetic to electrical boarding ratio greater than 70.
a) true
b) false

Explanation: The performance of the small DC motor depends on the magnetic to electrical boarding ratio. The magnetic to electrical boarding ratio should be greater than 50 for good performance.

14. What is the formula for the flux density for the PM motors?
a) flux density = residual flux density / 1 + (1.11/permeance coefficient)
b) flux density = residual flux density * 1 + (1.11/permeance coefficient)
c) flux density = residual flux density / 1 + (1.11*permeance coefficient)
d) flux density = residual flux density * 1 + (1.11*permeance coefficient)

Explanation: The residual flux density is calculated first along with the permeance coefficient to obtain the flux density of the PMDC motor. The flux density is 0.85 times the residual flux density.

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