Category: DC Machines Objective Questions

DC Machines Multiple Choice Questions & Answers on “Operating Characteristics of DC Compound Motor – 2”.

1. Net field current in a compound motor is given by ______________
a) If (net)= I_f + (N_se/N_f) I_a
b) If (net)= I_f – (N_se/N_f) I_a
c) If (net)= I_f + (N_f/N_se) I_a
d) If (net)= I_f – (N_f/N_se) I_a
Answer: a
Clarification: Net flux in a compound motor is determined by addition of shunt field flux and series field flux. Thus, for calculating effective flux, net field current is find out. If is the constant value given for shunt field, while If net takes the presence of series field also.

2. With increase in armature current, speed of the cumulative compound motor _________
a) Reduces slowly compare to shunt motor
b) Reduces sharply compare to shunt motor
c) Remains constant
d) Increases in proportionality with armature current
Answer: b
Clarification: As armature current increases, the back emf decreases similar to shunt motor. By the same time series field resistance carries more armature current producing more amount of flux which is inversely proportional speed.

3. Speed current characteristic of cumulative DC compound motor at larger values of armature current is similar to that of _____________
a) DC shunt motor
b) Induction motor
c) DC differential compound motor
d) DC series motor
Answer: d
Clarification: At higher values of current speed of the DC cumulative compound motor starts decreasing due to effect of series field flux, which is also seen in DC series motor. Thus series motor and DC cumulative motor follow same characteristics.

4. Differential compound motor is used in large number of applications.
a) True
b) False
Answer: b
Clarification: A differentially compound motor has flux/pole φ = (φse – φf). It is seen that on over-load φ reduces sharply and so the motor acts like a series motor on no load. This is why the differential compound motor is not used in practice.

5. Where speed-current characteristic of DC shunt motor lies with respect to DC cumulative compound motor (assume current greater than full load current)?
a) Can be represented by same curve
b) Above
c) Below
d) Cannot be determined
Answer: b
Clarification: Because of φse increasing with Ia, the speed of the compound motor falls much more sharply than the shunt motor. Therefore, the n – armature current characteristic of the compound motor lies above that of the shunt motor for I_a < I_a ( fl) and lies below for I_a > I_a ( fl).

6. Where speed-current characteristic of DC shunt motor lies with respect to DC cumulative compound motor (assume current smaller than full load current)?
a) Can be represented by same curve
b) Above
c) Below
d) Cannot be determined
Answer: c
Clarification: At start, speed of DC cumulative motor is more than the speed of shunt motor because of series characteristic effect. Because of φ_se increasing with Ia beyond full load current, the speed of the compound motor falls much more sharply than the shunt motor.

7. Only motor listed below whose speed increases with increase in armature current is ___________
a) DC series motor
b) DC cumulative compound motor
c) DC shunt motor
d) DC differential compound motor
Answer: d
Clarification: Shunt motor speed though decreases by slight value as armature current increases, it is assumed to be constant. But series motor and cumulative motor speed decreases as armature current increases. Only differential motor flux reduces with increase in armature current thus, speed increases.

8. Torque of the cumulative compound DC motor is _____________
a) Addition of Ia and Ia² term
b) Subtraction of Ia from Ia² term
c) Subtraction of Ia² from Ia term
d) Addition of -Ia and -Ia² term
Answer: a
Clarification: DC cumulative compound motor is the addition of best of series and best of shunt field motor. For shunt type torque is proportional to armature current while in series motor torque is proportional to square of armature current.

9. Torque-current characteristic of DC cumulative compound motor _____________
a) Lies above shunt motor characteristic
b) Lies below shunt motor characteristic
c) Lies above of series and shunt motor characteristic both
d) Lies between of series and shunt motor characteristic both
Answer: c
Clarification: Starting from zero armature current the characteristic of cumulative motor lies above the series characteristic and shunt characteristic, because it is the sum of series and shunt characteristic.

10. For strong series field, speed-torque characteristic of cumulative compound motor lies __________
a) Above series motor characteristic if current is less than full load current
b) Above shunt motor characteristic if current is less than full load current
c) Below series motor characteristic if current is more than full load current
d) Below series motor characteristic if current is less than full load current
Answer: b
Clarification: Due to strong series field, speed-torque characteristic lies above of shunt field characteristic but below of series field characteristic, as no-load speed is finite for cumulative compound motor, at armature current less than full load current.

11. Which of the following DC motor has negative speed regulation?
a) Shunt motors
b) Series motors
c) Differential compound motors
d) Series and differential compound motors
Answer: c
Clarification: In differential compound motors flux will reduce as load is increased because series flux opposes shunt field flux. This will increase speed upon addition of load. Therefore, it has negative speed regulation means speed will increase due to increase in load.

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DC Machines Multiple Choice Questions on “DC Motor Control Through Converters”.

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
Answer: a
Clarification: 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
Answer: a
Clarification: 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
Answer: a
Clarification: 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
Answer: b
Clarification: 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
Answer: b
Clarification: 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
Answer: b
Clarification: 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
Answer: c
Clarification: 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
Answer: d
Clarification: 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
Answer: c
Clarification: 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
Answer: d
Clarification: 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
Answer: d
Clarification: 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
Answer: a
Clarification: 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.

DC Machines, .

DC Machines Questions and Answers for Experienced people on “Ways to Reduce Effects of Armature Reaction – 2”.

1. Axis undergo shifting as a result of armature reaction, can be balanced by ______
a) Increase in armature current
b) Decrease in armature current
c) Introducing interpoles
d) Removing interpoles
Answer: c
Clarification: Apart from distortion of the resultant flux density wave, its MNA also gets shifted from its GNA by a small angle α so that the brushes placed in GNA are no longer in MNA as is the case in the absence of armature current, due to armature reaction. This effect is countered by the interpoles placed in GNA.

2. The choice of average coil voltage determines the minimum number of commutator segments for its design.
a) True
b) False
Answer: a
Clarification: The maximum allowable voltage between adjacent segments is 30–40 V, limiting the average voltage between them to much less than this figure. The choice of the average coil voltage determines the minimum number of commutator segments for its design, to avoid any flashover and ultimately short circuit.

3. Compensating winding will provide incomplete neutralization ____________
a) Under pole region
b) In interpolar region
c) Everywhere
d) Complete neutralization
Answer: b
Clarification: The compensating winding neutralizes the armature mmf directly under the pole while in the interpolar region, there is incomplete neutralization. Further, the effect of the resultant armature mmf in interpolar region is rendered insignificant because of large interpolar gap.

4. Cross-magnetizing effect of armature reaction can be reduced by __________
a) Removing saturation in teeth and pole-shoe
b) Making smooth pole shoes
c) Introducing saturation in teeth and pole-shoe
d) Cannot be determined
Answer: c
Clarification: The cross-magnetizing effect of the armature reaction can be reduced by making the main field ampere-turns larger compared to the armature ampere-turns such that the main field mmf exerts predominant control over the air-gap flux. This is achieved by introducing saturation in the teeth and pole-shoe.

5. Cross-magnetizing effect of armature reaction can be reduced by __________
a) Removing saturation in teeth and pole-shoe
b) Making smooth pole shoes
c) Chamfering the pole shoes
d) Cannot be determined
Answer: c
Clarification: By chamfering the pole-shoes which increases the air-gap at the pole tips. This method increases the reluctance to the path of main flux in a DC machine but its influence on the cross-flux is much greater. This is because the cross flux has to cross the air-gap twice.

6. To counter the effect of shift in MNA due to armature reaction, which of the following component can be shifted?
a) Poles
b) Commutator
c) Brushes
d) Cannot be determined
Answer: c
Clarification: To counter the effect of shift in MNA due to armature reaction, the brushes could be shifted. A small brush shift in appropriate direction, in the direction of rotation for generator and in opposite direction for motor, also helps in commutation.

7. Calculate the number of conductors on each pole piece required in a compensating winding for a 6-pole lap-wound dc armature containing 286 conductors. The compensating winding carries full armature current. Assume ratio of pole arc/ pole pitch = 0.7.
a) 6
b) 8
c) 9
d) 7
Answer: a
Clarification: The number of ampere-turns required for compensating winding is AT_cw /pole = AT_a (peak) *(pole arc/pole pitch) = [IaZ/(AP2)] * (pole arc/pole pitch).
N_cw/pole = (Z/2AP) * (pole arc/pole pitch) = [286 / (2*6*6)] 0.7 = 2.78.
Compensating conductors/pole = 2 * 2.78 = 6 (nearest integer).

8. A compensating winding with ampere-turns greater than peak ampere turns is required in order to neutralize the effect of armature reaction because _____________
a) Pole arc = Pole pitch
b) Pole arc > Pole pitch
c) Pole arc < Pole pitch
d) Can’t be determined using pole arc, pole pitch
Answer: b
Clarification: The number of ampere-turns required for compensating winding in a DC machine is AT_cw /pole = AT_a (peak) *(pole arc/pole pitch) = [IaZ/(AP2)] * (pole arc/pole pitch). Thus, if compensating winding ampere turns are more then, pole arc is definitely greater than pole pitch.

9. If pole arc is less than pole pitch, a compensating winding will have ampere-turns _________ (compare to peak ampere turns).
a) Less
b) Equal
c) More
d) Can’t be specified
Answer: a
Clarification: The ampere-turns required for compensating winding in a DC machine is AT_cw /pole = AT_a (peak) *(pole arc/pole pitch) = [IaZ/(AP2)] * (pole arc/pole pitch). Thus, if compensating winding ampere turns are less then, pole arc is smaller than pole pitch and vice-versa.

10. What is the pole arc/pitch ratio, if 360 AT compensating winding is used where 1960AT is peak value?
a) 0.7
b) 0.8
c) 0.9
d) 0.6
Answer: d
Clarification: The ampere-turns required for compensating winding in a DC machine is AT_cw /pole = AT_a (peak) *(pole arc/pole pitch). If compensating winding of 360AT is used the, 360/1960 will give ratio of pole pitch /pole arc, equal to 0.6.

11. Only drawback of compensating winding is _______
a) Cost
b) Unavailability of material
c) Construction
d) Not a single drawback
Answer: a
Clarification: Compensating winding is the best method in order to prevent the effect of armature reaction and its consequences. Only problem is compensating winding is expensive, but it is must to use them in machines with heavy overloads occur.

DC Machines for Experienced people, .

DC Machines Multiple Choice Questions on “DC Motor Basics -1”.

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
Answer: d
Clarification: 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
Answer: c
Clarification: 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.
Answer: a
Clarification: 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
Answer: c
Clarification: 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
Answer: a
Clarification: 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
Answer: d
Clarification: 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
Answer: a
Clarification: 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
Answer: d
Clarification: 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
Answer: d
Clarification: 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
Answer: c
Clarification: 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) E_a = 2 x supply voltage
b) E_a = supply voltage
c) Supply voltage = 2 x E_a
d) supply voltage = 4 x E_a
Answer: c
Clarification: For a motor, from power equation it is known that,