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Multiple choice question for engineering

Set 1

1. Where is field winding mounted in a DC machine?
a) Stator
b) Rotor
c) Absent
d) Anywhere on stator or rotor

View Answer

Answer: a [Reason:] The field winding (concentrated type) is mounted on salient-poles on the stator and the armature winding (distributed type) is wound in slots on a cylindrical rotor. In AC machines field winding is mounted on rotor.

2. What are the materials used for brushes in dc machines?
a) Iron
b) Carbon
c) Aluminum
d) Steel

View Answer

Answer: b [Reason:] On some extent carbon brush can act as a self-lubricating brush. On moment, polishes the commutator segments. Damage to the commutators is less when copper brushes are used on occurrence of sparkover.

3. Function of yoke is to provide the return path for magnetic flux.
a) True
b) false

View Answer

Answer: a [Reason:] The function of yoke is that it protects the entire machine from dust and dirt. It also provides mechanical support for the magnetic poles. It acts as the return path for the magnetic flux.

4. The angle (electrical) made by brushes with axes of adjoining filed poles is ______________
a) 450
b) 1800
c) 900
d) 300

View Answer

Answer: c [Reason:] Brushes in a DC machine are normally placed electrically in the interpolar regions and therefore make an angle of 900 electrical with the axes of adjoining filed poles.

5. In a DC machine, rectification process is carried out in order to get unidirectional output (DC). This rectification process is carried out by ______________
a) Half wave rectifier
b) Full wave rectifier
c) Mechanical rectification
d) Centre tapped rectifier

View Answer

Answer: c [Reason:] In a DC machine electronic rectification is not used. Instead they use mechanical rectification with the help of commutator-brush assembly.

6.Which of the following part is used in construction of DC machine but not in AC machine?
a) Armature Winding
b) Field winding
c) Commutator
d) Shaft

View Answer

Answer: c [Reason:] Commutator is used in mechanical rectification process, to convert induced AC to output DC. In AC machine, we don’t need rectification process.

7. In a DC machine fractional pitch winding is used to _______________
a) To improve cooling
b) To reduce sparking
c) To reduce copper losses
d) To increase generated EMF

View Answer

Answer: b [Reason:] Due to poor performance of brush, poor undercutting of commutator, incorrect spring pressure sparking at brush faces happen. To overcome this sparking fractional pitch winding is used.

8. In normal dc machines operating at full-load conditions, the most powerful electromagnet is _________
a) Field winding
b) Interpole Winding
c) Interpole and compensating winding together
d) Armature winding

View Answer

Answer: a [Reason:] Electromagnet is more powerful when its MMF is high. At full-load condition, field winding contains maximum ampere turns, hence it is most powerful electromagnet in a DC machine.

9. If a DC motor is connected to AC supply what will happen then?
a) Not run
b) Burn
c) Run at normal speed
d) Run at extremely low speed

View Answer

Answer: b [Reason:] If a DC motor is connected to AC supply, an alternating current pass through the brushes and commutator to the armature winding, while it passes through the commutator it is converted into DC so that the group of conductors under successive field poles carry current in same direction. So, the flux per pole will remain constant and not vary. There will be production of heat due to flow of eddy current in field winding and the motor will be burned.

10. The armature of DC motor is laminated to ____________
a) To reduce mass
b) To reduce hysteresis loss
c) To reduce eddy current loss
d) To reduce inductance

View Answer

Answer: c [Reason:] The armature is built up in a cylindrical or drum shape high grade silicon steel in form of lamination. By using laminations, the circular path of eddy currents is terminated. Hence heating and ultimately damage to the armature can be reduced by lamination.

Set 2

1. What will happen if DC shunt motor is connected across AC supply?
a) Will run at normal speed
b) Will not run
c) Will Run at lower speed
d) Burn due to heat produced in the field winding

View Answer

Answer: d [Reason:] In case of parallel field connection, it won’t rotate at all and will start humming and will create vibrations, as a torque produced by positive and negative cycle will cancel out each other. DC motor will be heated up and it may burn.

2. What will happen if the back emf of a DC motor vanishes suddenly?
a) The motor will stop
b) The motor will continue to run
c) The armature may burn
d) The motor will run noisy

View Answer

Answer: c [Reason:] If back emf vanishes suddenly, motor circuit will try to retain back emf by drawing more current from supply. If supplying unit didn’t trip down by this time, excess current in armature may heat up the armature.

3. What will happen, with the increase in speed of a DC motor?
a) Back emf increase but line current falls.
b) Back emf falls and line current increase.
c) Both back emf as well as line current increase.
d) Both back emf as well as line current fall.

View Answer

Answer: a [Reason:] In case of DC motor, the speed is proportional to the back emf (Ea ∝ N). So, with the increase in speed, the back emf also increases. Therefore, armature current is also decreased, in case of series motor, armature current is equal to the line or load current.

4. Which part will surely tell that given motor is DC motor and not an AC type?
a) Winding
b) Shaft
c) Commutator
d) Stator

View Answer

Answer: c [Reason:] All other parts except brushes and commutator are same in AC machine when outer looks are only taken in consideration. Commutator is used only in DC machine for providing mechanical rectification and not in AC machine.

5. In DC motor, which of the following part can sustain the maximum temperature rise?
a) Field winding
b) Commutator
c) Slip rings
d) Armature winding

View Answer

Answer: a [Reason:] Maximum temperature rise can be sustained by field winding, as it is not involved in rotary parts. Field winding is present away from rotary parts of the machine, so temperature rise in the machine will not produce any effect in machine rotations.

6. Direction of rotation of motor is determined by ____________
a) Faraday’s law
b) Lenz’s law
c) Coulomb’s law
d) Fleming’s left-hand rule

View Answer

Answer: d [Reason:] Flemings laws can be summarized as whenever, a current carrying conductor comes under a magnetic field, there will be a force acting on the conductor and on the other hand, if a conductor is forcefully brought under a magnetic field, there will be an induced current in that conductor.

7. The current drawn by the armature of DC motor is directly proportional to ________________
a) Torque
b) Speed
c) The voltage across the terminals
d) Cannot be determined

View Answer

Answer: a [Reason:] From the equation of torque generated in a DC machine, we know that in both DC motor and DC generator, current drawn is directly proportional to the torque required by the machine.

8. Which power is mentioned on a name plate of a motor?
a) Gross power
b) Power drawn in kVA
c) Power drawn in kW
d) Output power available at the shaft

View Answer

Answer: d [Reason:] Name plate of the motor shows rated values i.e. rated speed, rated current, rated voltage. It also shows output power available at shaft when all other quantities are set to rated values.

9. An electric motor is having constant output power. So, motor will have a torque speed characteristic _______________________
a) Circle about the origin.
b) Straight line parallel to the speed axis.
c) Straight line through the origin.
d) Rectangular hyperbola

View Answer

Answer: d [Reason:] In case of DC motor for the constant output power, Ea×Ia = T×ω. As T×ω = Constant, so the torque speed characteristic is in the form of rectangular hyperbola, which represented as xy= constant.

10. Which of the following quantity will decrease if supply voltage is increased?
a) Starting torque
b) Operating speed
c) Full-load current
d) Cannot be determined

View Answer

Answer: c [Reason:] When supply voltage is increased full load current will decrease in order to keep output power constant, which will decrease torque at that moment, while starting torque will remain as it is, irrespective of any change in supply voltage.

11. In which of the following case we will get maximum power?
a) Ea = 2 x supply voltage
b) Ea = supply voltage
c) Supply voltage = 2 x Ea
d) supply voltage = 4 x Ea

View Answer

Answer: c [Reason:] For a motor, from power equation it is known that, dc-machines-questions-answers-dc-motors-basics-1-q11

12. Sometimes motor has to be de-rated.
a) True
b) False

View Answer

Answer: a [Reason:] Derating refers to the operation of equipment at reduced capacity/power or speed. Derating in motors can be caused due to the following reasons- Frequency, Voltage, Ambient temperature, Altitude.

13. The armature shaft of a DC motor must be able to withstand ______________
a) Bending moment due to weight of the armature.
b) Any unbalanced magnetic pull on the armature core.
c) Twisting stains due to transmission of torque.
d) Bending moment, unbalanced magnetic pull and twisting stains

View Answer

Answer: d [Reason:] The armature shaft must be able to withstand any unbalanced magnetic pull on the armature core, bending moment due to weight of the armature and commutator, twisting stains due to transmission of torque, for a good and long run application of motor.

14. In DC machines the residual magnetism is present. The order of residual magnetism is ___________
a) 2 to 3 per cent
b) 10 to 15 per cent
c) 20 to 25 per cent
d) 50 to 75 per cent

View Answer

Answer: a [Reason:] In a DC machine residual magnetism is present and it plays very important role in starting of any DC machine. It’s present because of the previous application on the same motor, it also provides some no-load voltage.

15. Sparking is discouraged in a DC motor.
a) True
b) False

View Answer

Answer: b [Reason:] Sparking at brushes and commutator segments lead to damage of commutators, which is the main distinguishable component in a DC machine. So, it is advisable to fasten the commutation speed and avoid sparking.

Set 3

1. Half wave converters are used for controlling DC motor of ________________
a) Below 400 W
b) 400 W – 4000W
c) More than 4000W
d) Anywhere

View Answer

Answer: a [Reason:] Half wave converter requires a single thyristor and a free-wheeling diode. In half wave converter circuit the motor current will always be discontinuous. Ultimately it results in poor motor performance. This type of converter is employed only for motors below 400W.

2. How many thyristors does we need in half wave converter?
a) 1
b) 2
c) 3
d) Many

View Answer

Answer: a [Reason:] Only one thyristor is sufficient for the half wave converter purpose, this is employed for small motors only. This thyristor is connected to the single-phase AC supply and with parallel combination of free-wheeling diode with motor armature.

3.Semi-converters can’t be used when regeneration is required.
a) True
b) False

View Answer

Answer: a [Reason:] According to the one-quadrant converter, it gives voltage and current of one polarity at DC terminals. Therefore, it does not provide support for regenerative braking, i.e. power flow from DC motor to the AC supply.

4. How many thyristors does we need in semi-converter?
a) 1
b) 2
c) 3
d) Many

View Answer

Answer: b [Reason:] Semi-converter requires two thyristors. Thyristor Th1 is fired at angle of α and Th2 at angle π+α with respect to the supply voltage v and the process is repeated continuously. Circuit also requires two diodes and one free-wheeling diode.

5. When armature current becomes discontinuous?
a) Small firing angles
b) Large firing angles
c) Infinite firing angle
d) Does not depend on firing angle

View Answer

Answer: b [Reason:] There are various reasons when armature current becomes discontinuous armature current, prominent reasons are large values of firing angle, high speed and sometimes low torque can also affect.

6. Which of the following is the effect of non-uniform armature current?
a) Ratio of peak to average and rms to average armature current decreases
b) Ratio of peak to average and rms to average armature current increases
c) Ratio of peak to average increases and rms to average armature current decreases
d) Ratio of peak to average decreases and rms to average armature current increases

View Answer

Answer: b [Reason:] The armature current sometimes due to some reasons may become non-uniform. In this conditions motor performance deteriorates with discontinuous armature current. So, the ratio of peak current to average current increases. Also, rms to average armature current increases.

7. How many quadrants does full-converter work?
a) One
b) Half
c) Two
d) Can be any

View Answer

Answer: c [Reason:] A full-converter is a two-quadrant converter in which the voltage polarity of the output can reverse, but the current remains unidirectional because of the unidirectional thyristors. It requires no free-wheeling diode.

8. How many thyristors are required for full converter?
a) 1
b) 2
c) 3
d) 4

View Answer

Answer: d [Reason:] Full converter requires 4 thyristors. The only different thing about the full converter is, it does not require free-wheeling diode. A full converter can be used to feed a separately excited DC motor. Each thyristor is fired accordingly.

9. Full-converter can be used in DC motor for regenerative braking in ______________
a) Constant operation
b) Variable operation
c) Inversion operation
d) Opposite operation

View Answer

Answer: c [Reason:] When α is greater than 900, the average motor voltage becomes negative. If the motor terminals are reversed, then the circuit will act as a generator in order to feed power back to the AC supply. This is the inversion operation of the converter and used in regenerative braking.

10. Dual converter operates in _________________
a) 1
b) 2
c) 3
d) 4

View Answer

Answer: d [Reason:] The dual-converter is the modified one and can operate in all the four quadrants. Its circuit contains two full-converters converting to the DC in either direction. The dual converter provides virtually instantaneous reversal of voltage at DC terminals.

11. Which converter/s can be used for DC series motor control?
a) Semi-converters
b) Half-wave converter
c) Full-converter
d) Semi converters and full converter

View Answer

Answer: d [Reason:] Semi-converters and full-converters are also employed for control of a DC series motor. The total circuit inductance is high because of the series field, as a consequence of which the armature current is mostly continuous.

12. Three phase converters are employed for _______________
a) Large kW motors
b) Small kW motors
c) In all motors
d) Never used

View Answer

Answer: a [Reason:] Three phase converters are employed when DC motor of very high ratings is used. In three phase converters, the advantage is the ripple frequency of the motor terminal voltage is generally higher than that in single-phase converters.

Set 4

1. The efficiency of the DC motor at maximum power will be ___________________
a) 100%
b) Around 90%
c) Anywhere between 75% and 90%
d) Less than 50%

View Answer

Answer: d [Reason:] For getting maximum power, derivative of power with respect to current is equal to 0. This is practically impossible to achieve as, current required is much more than its normal rated value. Large heat will be produced in a machine and efficiency of motor will be less than 50 %.

2. The hysteresis loss in a DC machine least depends on _____________
a) Frequency of magnetic reversals
b) Maximum value of flux density
c) Volume and grade of iron
d) Rate of flow of ventilating air

View Answer

Answer: d [Reason:] As iron core of the armature is rotating in magnetic field, some losses occurs in the core which is called core losses. These losses are categorized as Hysteresis loss and Eddy current loss. They depend on all quantities listed above.

3. Which of the following is not the effect of iron loss?
a) Loss of efficiency
b) Excessive heating of core
c) Increase in terminal voltage
d) Rise in temperature of ventilating air

View Answer

Answer: c [Reason:] Iron loss causes excessive heat production in the core of a machine, which will rise the temperature of ventilating air, as it acts as heat exchanger. Thus, terminal voltage rise is not an effect of any loss.

4. Which of the following loss is likely to have highest proportion at rated load of the DC generator?
a) Hysteresis loss
b) Field copper loss
c) Armature copper loss
d) Eddy current loss

View Answer

Answer: c [Reason:] Armature copper loss is directly proportional to the square of armature current multiplied by the armature resistance and also the series field resistance if present any. As, at loaded condition armature current is very high.

5. Which of the following loss in a DC generator varies significantly with the load current?
a) Field copper loss
b) Windage loss
c) Armature copper loss
d) Cannot be determined

View Answer

Answer: c [Reason:] Armature copper loss is directly proportional to the square of armature current, as load current varies armature current varies, which is reflected significantly in loss as a square of it. Thus, armature copper loss can be detected.

6. Which of the following methods is likely to result in reduction of hysteresis loss in a DC generator?
a) Providing laminations in armature core
b) Providing laminations in stator
c) Using non-magnetic material for frame
d) Using material of low hysteresis co-efficient for armature core material

View Answer

Answer: d [Reason:] Providing laminations will work for reducing eddy current losses, but hysteresis loss is dependent on the material chosen. Thus, using different material for core of armature will definitely work.

7. Which of the following loss/losses in a DC generator is dissipated in the form of heat?
a) Mechanical loss
b) Core loss
c) Copper loss
d) Mechanical, Copper and Core

View Answer

Answer: d [Reason:] All the losses listed above dissipate the heat. This, dissipated heat due to various losses results in increasing the temperature of ventilating air. These losses are dangerous in long running of a machine, can reduce efficiency also.

8. Which of the following losses are significantly reduced by laminating the core of a DC generator?
a) Hysteresis losses
b) Eddy current losses
c) Copper losses
d) Windage losses

View Answer

Answer: b [Reason:] Hysteresis losses can be minimized by using material with lower hysteresis coefficient. Eddy current losses can be minimized by using laminated sheets of core structed together. To reduce other losses, current should be minimized, can’t be reduced as it also has lower limit.

9. The total losses in a well-designed DC generator of 10 kW will be nearly equal to ________
a) 100 W
b) 500 W
c) 1000 W
d) 1500 W

View Answer

Answer: b [Reason:] Total losses in a DC machine can be approximated to 4-5% of its rating from the experimental observations. Thus, 5% of 10 kW is equal to 500 W. It’s an approximation formed on various observations.

Set 5

1. Product of torque and mechanical angular velocity ω is_____
a) Ea/ω
b) Ea*Ia
c) ω/Ea
d) Can’t tell

View Answer

Answer: b [Reason:] According to statement of energy conversion, electrical and mechanical power of the machine must balance in a machine. Ea*Ia is referred to as electromagnetic power. Thus, torque = (Electromagnetic power)/ ω.

2. Condition for linear magnetization is______
a) φ α If
b) φ α Ia
c) φ α 1/If
d) φ α 1/Ia

View Answer

Answer: a [Reason:] For a linear magnestization to take place, flux produced by the current flowing through particaluar flux producing coil must vay in direct proportion. Here, φ is produced due to field winding which carried current If.

3. A 4-pole Dc wound machine is lap wound with 400 conductors. The pole shore is 20 cm long and average flux density over one-pole pitch is 0.4 T, the armature diameter being 30 cm. Here, motor is drawing 25 A current at 1500 rpm.
What is the available torque (N-m) at shaft?
a) 29.9
b) 59.8
c) 14.95
d) 44.85

View Answer

Answer: a [Reason:] Gross mechanical power developed is equal to Ea*Ia. Ea*Ia is referred to as electromagnetic power. Thus, torque = (Electromagnetic power)/ ω. Substituting all the values we get torque = 29.9 Nm.

4.When Ea>Vt machine is said to be operating in which of the following mode?
a) Depends on the Shaft speed
b) Generating
c) Motoring
d) Inducing

View Answer

Answer: b [Reason:] The machine operates in generating mode (puts out electrical power) when Ia is in the direction of induced emf Ea. For the given armature circuit Vt = armature terminal voltage= Ea – Ia * Ra; which implies Vt < Ea.

5. When Ea<Vt machine is said to be operating in which of the following mode?
a) Depends on the Shaft speed
b) Generating
c) Motoring
d) Inducing

View Answer

Answer: c [Reason:] The machine operates in motoring mode (puts in electrical power) when Ia is in the direction opposite of induced emf Ea. For the given armature circuit Vt = armature terminal voltage= Ea + Ia * Ra; which implies Vt> Ea.

6. Simple equation of DC machine operating in generating mode, with non-zero Ra value is________
a) Vt = Ea – Ia/Ra
b) Vt = Ea + Ia*Ra
c) Vt = – Ea + Ia*Ra
d) Vt = Ea – Ia*Ra

View Answer

Answer: d [Reason:] The machine operates in generating mode (puts out electrical power) when Ia is in the direction of induced emf Ea, also Ea>Vt. For the given armature circuit, with non-zero value of Ra, Vt= armature terminal voltage= Ea- Ia*Ra.

7. Simple equation of DC machine operating in motoring mode, with non-zero Ra value is________
a) Vt = Ea – Ia/Ra
b) Vt = Ea + Ia*Ra
c) Vt = – Ea + Ia*Ra
d) Vt = Ea – Ia*Ra

View Answer

Answer: b [Reason:] The machine operates in motoring mode (puts in electrical power) when Ia is in the direction opposite of induced emf Ea, also Ea<Vt. For the given armature circuit, with non-zero value of Ra, Vt= armature terminal voltage= Ea+ Ia*Ra.

8. When torque of the electromagnetic origin is in the opposite direction of rotation of armature, machine is said to be operating in which of the following mode?
a) Depends on other parameters
b) Generating
c) Motoring
d) Inducing

View Answer

Answer: b [Reason:] In a generating mode, torque of the electromagnetic origin is in the opposite direction of rotation of armature, implying mechanical power is absorbed and a prime mover is needed to run the machine. Ea and Ia are in the same directions.

9. When torque of the electromagnetic origin is in the direction of rotation of armature, machine is said to be operating in which of the following mode?
a) Depends on other parameters
b) Generating
c) Motoring
d) Inducing

View Answer

Answer: c [Reason:] In a motoring mode, torque of the electromagnetic origin is in the direction of rotation of armature, implying electrical power is absorbed and a prime mover is not needed to run the machine. Ea and Ia are in the opposite directions.

10. What is the armature current for a DC motor if terminal voltage is 255V and open circuit voltage is equal to 250V, where armature resistance is 0.05Ω?
a) 10A
b) 100A
c) 1KA
d) 1A

View Answer

Answer: b [Reason:] Open circuit voltage means the voltage at armature current equal to 0, i.e. Vt=Ea= 250V. Actual Vt at loaded condition is 255V. For motoring mode, Vt – Ea = Ia*Ra. By substituting all the given values, we get Ia= 100A.

11. For a DC generator feeding 100kW power into 230V mains, having armature resistance and field resistance equal to 0.08Ω and 115Ω resp. The value of armature current is_____
a) 436.8 A
b) 434.8 a
c) 432.8 A
d) Data insufficient

View Answer

Answer: a [Reason:] Field current is equal to 230/115 = 2A, by Ohm’s law. When running as a generator line current IL= 100k/230= 434.8A. Since power is supplied to 230V mains, Ia=Il+If = 434.8+2 =436.8A. For motoring mode Ia= IL- If.

12. What is the armature current for a DC generator if terminal voltage is observed to be 245V and open circuit voltage is equal to 250V, where armature resistance is 0.05Ω?
a) 10A
b) 100A
c) 1KA
d) 1A

View Answer

Answer: b [Reason:] Open circuit voltage means the voltage at armature current equal to 0, i.e. Vt=Ea= 250V. Actual Vt at loaded condition is 245V. For motoring mode, Ea – Vt= Ia*Ra. By substituting all the given values, we get Ia= 100A.

13. For a DC motor taking 10kW power from 230V mains, having armature resistance and field resistance equal to 0.08Ω and 115Ω resp. The value of armature current is_____
a) 45.68 A
b) 43.48 a
c) 41.48 A
d) Data insufficient

View Answer

Answer: c [Reason:] Field current is equal to 230/115 = 2A, by Ohm’s law. When running as a motor line current IL= 10k/230= 43.48A. Since power is taken from 230V mains, Ia=IL- If = 43.48-2 =41.48A. For generating mode Ia= IL+ If.