**Design of Permanent Magnet (PM) DC Motors MCQs ( Design Of Electrical Machines ) MCQs – ****Design Of Electrical Machines MCQs**

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**Design Of Electrical Machines MCQs – ****Design of Permanent Magnet (PM) DC Motors ****MCQs ( Design Of Electrical Machines ) MCQs**

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# 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**Answer: c****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**Answer: b****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 * 10 ^{4}Â * number of parallel paths in the armature^{2}**

b) armature resistance = (Diameter + length)*total number of armature conductors*1.2 * 10

^{4}Â * number of parallel paths in the armature

^{2}

c) armature resistance = (Diameter + length)*total number of armature conductors/1.2 * 10

^{4}Â + number of parallel paths in the armature

^{2}

d) armature resistance = (Diameter + length)+total number of armature conductors/1.2 * 10

^{4}Â * number of parallel paths in the armature

^{2}

**Answer: a**

**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)

**Answer: b**

**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

**Answer: b**

**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.**

**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

**Answer: c**

**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

**Answer: c**

**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 conductorâ€™s acts as demagnetizing force on the fraction tips of the magnet.**

a) true

**b) false**

**Answer: b**

**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 10**

^{7}?a) d = angle/360

b) d = angle/240

c) d = angle/540

**d) d = angle/720**

**Answer: d**

**Explanation: If the total number of teeth is greater than 10**

^{7}Â 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

**Answer: c**

**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

**Answer: a**

**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

**Answer: b**

**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**

**Answer: b**

**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)

**Answer: a**

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

# Design Steps and Considerations â€“ 1

**1. How many design steps are present in the design of PMDC motors?**

a) 8

b) 9

c) 10**d) 11****Answer: d****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****Answer: d****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.**

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Three Phase Induction Motors MCQs

**3. What should be the range of the product of the magnetic field and magnetic flux density?**

a) 4-4.5 * 10^{6}

b) 4-4.3 * 10^{6}**c) 4.3-4.6 * 10 ^{6}**

d) 4.2-4.5 * 10

^{6}

**Answer: c**

**Explanation: The product of the magnetic field and magnetic flux density has a minimum value of 4.3 * 10**

^{6}. The product of the magnetic field and magnetic flux density has a minimum value of 4.6 * 10^{6}.**4. What should be the minimum value of the ratio of the magnetic to electric loading?**

a) 40

b) 30

**c) 50**

d) 60

**Answer: c**

**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

**Answer: b**

**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

**Answer: a**

**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 â€“ 0.06**

d) length of the magnet = sum of the volume of air gap and magnet * Area of the magnet â€“ 0.06

**Answer: c**

**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

**Answer: b**

**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

**Answer: a**

**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**

**Answer: d**

**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

**Answer: a**

**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

**Answer: a**

**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

**Answer: a**

**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**

**Answer: b**

**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

**Answer: b**

**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.**

# Design Steps and Considerations â€“ 2

**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**Answer: b****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**Answer: a****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**Answer: c****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**Answer: a****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**Answer: a****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**Answer: c****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**Answer: b****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**Answer: b****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**Answer: a****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**Answer: c****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**Answer: a****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**Answer: c****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**Answer: b****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**Answer: a****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**Answer: b****Explanation: There are 3 principle types of lubricants in use. They are oil, dry film lubricants and grease.**

**Design of Permanent Magnet (PM) DC Motors MCQs ( Design Of Electrical Machines ) MCQs – ****Design Of Electrical Machines MCQs**

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