Design Of Electrical Machines MCQs – Design of Small Electrical Machines, Starters and Field Regulators MCQs ( Design Of Electrical Machines ) MCQs
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Design Of Electrical Machines MCQs – Design of Small Electrical Machines, Starters and Field Regulators MCQs ( Design Of Electrical Machines ) MCQs
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Design of a Small Reluctance Motor
1. How is the reluctance motor with respect to a synchronous motor and are the field windings?
a) small synchronous motor with field windings
b) small synchronous motor without field windings
c) large synchronous motor with field windings
d) large synchronous motor without field windings
Answer: b
Explanation: Reluctance motor is nothing but a simple small synchronous motor with salient pole rotor. They are without field windings in which the field flux is produced.
2. Why is the three phase reluctance motor preferred over single phase reluctance motor?
a) single phase reluctance motors have the phenomenon of hunting
b) single phase reluctance motors have the phenomenon of over voltage
c) single phase reluctance motors have high losses
d) single phase reluctance motors have low output
Answer: a
Explanation: The reluctance motor is a small synchronous motor with salient pole rotor. The single phase reluctance motors have the phenomenon of hunting.
3. What is the relation of the input voltage with the magnetic flux?
a) if the input voltage is constant, the magnetic flux increases
b) if the input voltage is constant, the magnetic flux decreases
c) if the input voltage is constant, the magnetic flux is constant
d) if the input voltage is constant, the magnetic flux is zero
Answer: c
Explanation: The input voltage is given constant, which results in the constant magnetic flux. The magnetic flux is independent of the excitation.
4. What is the power factor in the reluctance motor and the range of efficiency?
a) leading power factor, 60-75%
b) lagging power factor, 50-75%
c) zero power factor, 55-80%
d) lagging power factor, 55-75%
Answer: d
Explanation: The power factor in the reluctance motor is lagging power factor. The efficiency of the machine is about 55-75%.
5. What is the angle at which the electromagnetic torque is maximum?
a) 30°
b) 45°
c) 60°
d) 90°
Answer: b
Explanation: The electromagnetic torque is maximum at the angle of 45°. The range of operation of the reluctance motor lies in the range of 0-45°.
6. What is the range of the ratio of the direct axis reactance to the quadrature axis reactance?
a) 1.5-2.3
b) 1.6-2.7
c) 1.6-2.2
d) 1.2-2.0
Answer: c
Explanation: The minimum value of the ratio of the direct axis reactance to the quadrature axis reactance is 1.6. The maximum value of the ratio of the direct axis reactance to the quadrature axis reactance is 2.2.
7. How many design dimension are present in the design of the small reluctance motor?
a) 3
b) 4
c) 5
d) 6
Answer: c
Explanation: There are 5 design dimensions present in the design of the small reluctance motors. They are the design of the main dimensions, design of stator windings, design of the rotor of the reluctance motor, design of performance parameters, design of losses and efficiency.
8. What is the range of the constant used in the calculation of the active power of reluctance motor?
a) 0.3-0.4
b) 0.35-0.55
c) 0.40-0.50
d) 0.35-0.60
Answer: b
Explanation: The minimum value of the range of the constant used in the calculation of the active power of reluctance motor is 0.35. The maximum value of the range of the constant used in the calculation of the active power of reluctance motor is 0.55.
9. How many steps are present in the calculation of the determination of main dimensions?
a) 5
b) 4
c) 3
d) 2
Answer: a
Explanation: There are 5 steps present in the calculation of the determination of main dimensions. They are electromagnetic power of reluctance motor, output coefficient, pole pitch, pole arc, peripheral velocity.
10. How many steps are present in the calculation of the design of stator windings?
a) 10
b) 11
c) 9
d) 12
Answer: b
Explanation: There are 11 steps involved in the calculation of the design of stator windings. They are input current to motor, number of stator slots, stator winding pitch, winding factor, useful flux, number of turns per stator winding, cross sectional area of the stator winding, slot area, mean length for conductor, active resistance of stator winding, specific permeance of leakage flux.
11. How many steps are present in the calculation of the design of rotor of reluctance motors?
a) 4
b) 5
c) 3
d) 2
Answer: a
Explanation: There are 4 steps involved in the design of rotor of reluctance motor. They are rotor diameter calculation, height of rotor core, mmf for magnetic circuit, saturation coefficient of motor.
12. How many steps are involved in the design of performance parameters?
a) 6
b) 5
c) 7
d) 8
Answer: c
Explanation: There are 7 steps involved in the design of the performance parameters. They are no load current, height of steel stator teeth, weight of steel in the stator core, copper loss in the stator winding under no load, active resistance and leakage reactance, active component of no load current, starting torque of 3 phase reluctance motor.
13. How many design steps are involved in the determination of the losses and efficiency?
a) 2
b) 3
c) 4
d) 5
Answer: b
Explanation: There are 3 steps involved in the determination of the losses and efficiency. They are copper loss in stator winding, iron loss in stator steel, mechanical loss in the motor.
14. What is the formula for the slot pitch factor in design of rotors?
a) slot pitch factor = 3.14*rotor diameter*number of rotor slots
b) slot pitch factor = 3.14/rotor diameter*number of rotor slots
c) slot pitch factor = 3.14*rotor diameter/number of rotor slots
d) slot pitch factor = 1/3.14*rotor diameter*number of rotor slots
Answer: c
Explanation: First the rotor diameter and the number of rotor slots are first calculated. On substitution the slot pitch factor can be obtained.
15. The active resistance of the stator winding is calculated at the temperature of 45° C.
a) true
b) false
Answer: b
Explanation: The active resistance of the stator winding determination is one of the steps in the design of stator windings. The value is calculated at the temperature of 45° C.
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
Answer: d
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
Answer: b
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
Answer: d
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
Answer: c
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
Answer: b
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.
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
Answer: a
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
Answer: b
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
Answer: b
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
Answer: c
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
Answer: b
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
Answer: a
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
Answer: b
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
Answer: d
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.
Principles Of Magnetic Circuit Designs MCQs
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
Answer: c
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
Answer: b
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.
Motor Starters, Calculation of Resistance Steps & Design of Field Regulators
1. What is the function of the motor starter with respect to current?
a) to slow the low current flow
b) to prevent the low current flow
c) to allow the large current flow
d) to prevent the large current flow
Answer: d
Explanation: There are varied type of starters which vary among themselves according to the function. The main function is to prevent the excessive current at the starting.
2. What is the work of the starter with respect to the mechanical stress?
a) to allow large mechanical stress
b) to restrict large mechanical stress
c) to allow small mechanical stress
d) to restrict small mechanical stress
Answer: b
Explanation: The main function of the starter is to prevent the excessive current at the starting. The other work of the starter is to restrict the large mechanical stress from acting on the machines.
3. What is the relation of the current with the starting torque in the starter concept?
a) the starter should restrict current to prevent low starting torque
b) the starter should restrict current to produce high starting torque
c) the starter should send current to prevent low starting torque
d) the starter should send current to produce high starting torque
Answer: d
Explanation: The main function of the starter is to prevent excessive current flow. At the same time it should allow current to produce good or high starting torque.
4. When does the starter take up liquid rheostat?
a) when the resistance can be varied heavily
b) when the resistance can be varied gradually
c) when the resistance cannot be varied
d) when the resistance should not be varied
Answer: b
Explanation: The starter actually picks up either liquid rheostat or metallic resistance during its operation. The starter takes up liquid rheostat during the situation where the resistance is gradually varied.
5. The starter take up metallic resistance when the resistance should not be varied in steps.
a) true
b) false
Answer: b
Explanation: The starter actually picks up either liquid rheostat or metallic resistance during its operation. The starter takes up metallic resistance when the resistance is to be varied in steps.
6. What happens when the starter takes up metallic resistance?
a) voltage fluctuates from high to low
b) voltage fluctuates between fixed upper and lower limits
c) current fluctuates from high to low
d) current fluctuates between fixed upper and lower limits
Answer: d
Explanation: The starter actually picks up either liquid rheostat or metallic resistance during its operation. The starter takes up metallic resistance when the resistance is to be varied in steps. When taken with resistance steps, the current fluctuated between upper and lower limits.
7. What is the product of the ratio of the current and the useful flux per pole?
a) product of ratio of current and useful per pole = (useful flux per pole due to upper limit current * upper limit current) / (useful flux per pole due to lower limit current * lower limit current)
b) product of ratio of current and useful per pole = (useful flux per pole due to upper limit current + upper limit current) / (useful flux per pole due to lower limit current + lower limit current)
c) product of ratio of current and useful per pole = (useful flux per pole due to upper limit current – upper limit current) / (useful flux per pole due to lower limit current – lower limit current)
d) product of ratio of current and useful per pole = (useful flux per pole due to upper limit current * lower limit current) / (useful flux per pole due to lower limit current * upper limit current)
Answer: d
Explanation: First the product of useful flux per pole due to upper limit current and lower limit current and then the product of useful flux per pole due to lower limit current and upper limit current. On substitution the product of ratio of current and useful per pole is obtained.
8. How many machines are considered in the calculation of the resistance steps?
a) 2
b) 3
c) 4
d) 5
Answer: b
Explanation: There are 3 machines considered in the calculation of the resistance steps. They are starters for dc shunt motors, starters for dc series motors, starters for three phase slip ring induction motor.
9. What is the concept of notching operation?
a) process of decreasing the voltage
b) process of increasing the efficiency
c) process of cutting out the resistance
d) process of adding on the resistance
Answer: c
Explanation: The process of notching operation occurs In the dc shunt motors. The concept of notching operation means the cutting out the resistance.
10. What happens in the dc shunt motor when the notching process occurs?
a) flux remains constant
b) speed remains constant
c) voltage remains constant
d) current remains constant
Answer: b
Explanation: Notching operation means cutting out the resistance. During the process of notching speed remains constant.
11. What is the formula of the ratio of the lower limit to upper limit of current with respect to the resistance?
a) lower limit of current / upper limit of current = motor resistance / resistance to limit the starting current
b) lower limit of current / upper limit of current = (motor resistance / resistance to limit the starting current)1/number of resistance
c) lower limit of current / upper limit of current = (motor resistance * resistance to limit the starting current)1/number of resistance
d) lower limit of current / upper limit of current = motor resistance * resistance to limit the starting current
Answer: b
Explanation: The motor resistance, resistance to limit the starting current and number of resistance is calculated. On substitution the ratio of lower limit to upper limit of current is obtained.
12. What is the relation of the ratio of rotor current limits and the ratio of lower limit to upper limit current?
a) ratio of rotor current limits = ratio of lower limit to upper limit current
b) ratio of rotor current limits > ratio of lower limit to upper limit current
c) ratio of rotor current limits < ratio of lower limit to upper limit current
d) no relation between ratio of rotor current limits and ratio of lower limit to upper limit current
Answer: a
Explanation: The ratio of rotor current limits and the ratio of lower limit to upper limit current are first calculated. The ratio of rotor current limits is approximately equal to the ratio of lower limit to upper limit current.
13. How many machines are considered for the design of field regulators for dc machines?
a) 2
b) 3
c) 4
d) 5
Answer: a
Explanation: Two machines are considered for the design of field regulators for dc machines. They are the shunt generators and shunt motor.
14. What is the first step in the design of the field regulators for dc machines?
a) calculation of the resistance of section
b) calculation of the total field circuit resistance
c) calculation of the field circuit resistance
d) resistance to be inserted
Answer: b
Explanation: 2 machines are considered in the design of field regulators for dc machines. They are the shunt generators and shunt motor. The first step is the calculation of the field circuit resistance.
15. What is the second step in the design of the field regulators for dc machines?
a) calculation of the resistance of section
b) calculation of the total field circuit resistance
c) calculation of the field circuit resistance
d) resistance to be inserted
Answer: d
Explanation: 2 machines are considered in the design of field regulators for dc machines. They are the shunt generators and shunt motor. The second step is the calculation of the resistance to be inserted.
16. What is the last step involved in the design of field regulators for shunt generators?
a) calculation of the resistance of section
b) calculation of the total field circuit resistance
c) calculation of resistance of section
d) resistance to be inserted
Answer: c
Explanation: 2 machines are considered in the design of field regulators for dc machines. They are the shunt generators and shunt motor. The last step is the calculation of resistance of section.
17. What is the first step in the design of the field regulators for dc motor?
a) number of sections
b) shunt field circuit resistance
c) shunt field resistance
d) resistance of step
Answer: a
Explanation: 2 machines are considered in the design of field regulators for dc machines. They are the shunt generators and shunt motor. The first step is the calculation of number of sections.
18. What is the plot of the magnetization curve?
a) field current in the y axis vs voltage in x axis
b) field current in the x axis vs voltage in y axis
c) armature current in the y axis vs voltage in x axis
d) armature current in the x axis vs voltage in y axis
Answer: b
Explanation: The magnetization curve is the curve which is used to obtain the various values required in the design of field regulators. The curve is the plot of field current in x axis and voltage in y axis.