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Up To Date DC Machines MCQs – New DC Motors ( DC Machines ) MCQs

Up To Date DC Machines MCQs – New DC Motors ( DC Machines ) MCQs

Latest DC Machines MCQs

By practicing these MCQs of DC Motors ( DC Machines ) MCQs – Latest Competitive MCQs , an individual for exams performs better than before. This post comprising of objective questions and answers related to DC Motors ( DC Machines ) Mcqs “. As wise people believe “Perfect Practice make a Man Perfect”. It is therefore practice these mcqs of DC Machines to approach the success. Tab this page to check ” DC Motors ( DC Machines )” for the preparation of competitive mcqs, FPSC mcqs, PPSC mcqs, SPSC mcqs, KPPSC mcqs, AJKPSC mcqs, BPSC mcqs, NTS mcqs, PTS mcqs, OTS mcqs, Atomic Energy mcqs, Pak Army mcqs, Pak Navy mcqs, CTS mcqs, ETEA mcqs and others.

DC Machines MCQs – DC Motors ( DC Machines ) MCQs

The most occurred mcqs of DC Motors ( ) in past papers. Past papers of DC Motors ( DC Machines ) Mcqs. Past papers of DC Motors ( DC Machines ) Mcqs . Mcqs are the necessary part of any competitive / job related exams. The Mcqs having specific numbers in any written test. It is therefore everyone have to learn / remember the related DC Motors ( DC Machines ) Mcqs. The Important series of DC Motors ( DC Machines ) Mcqs are given below:

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
Explanation: 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
Explanation: 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
Explanation: 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
Explanation: 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
Explanation: 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
Explanation: 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
Explanation: 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
Explanation: 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
Explanation: 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
Explanation: 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
Answer: c
Explanation: 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
Answer: a
Explanation: 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
Answer: d
Explanation: 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
Answer: a
Explanation: 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
Answer: b
Explanation: 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.

DC Motor Basics -2

1. Which of the following motor is used where high starting torque and wide speed range control is required?
a) All motors
b) Induction motor
c) Synchronous motor
d) DC motor
Answer: d
Explanation: DC series motor gives the maximum starting torque at starting. With the help of Ward Leonard speed control method various speeds are also available. It provides one mode to othe transmission also.

2. The armature voltage control of DC motor will provide ___________
a) Constant power drive
b) Constant voltage drive
c) Constant current drive
d) Constant torque drive
Answer: d
Explanation: By changing the armature voltage we can run motor to give various speeds at constant torque. While changing the field is also another idea to change speed but it’ll be variable torque constant power operation.

3. As there is no back emf at the instant of starting a DC motor, in order to prevent a heavy armature current from flowing though the armature circuit ___________
a) Series resistance is connected with armature
b) Parallel resistance is connected to the armature
c) armature is temporarily open circuited
d) a high value resistor is connected across the field winding
Answer: a
Explanation: To prevent large armature current from damaging the armature circuit we connect series resistance with armature and in order to avoid losses we reconnect the same resistance with field circuit once machine is started, i.e. the exact principle of starters.

4. What will happen to torque if back emf and speed of the DC motor is doubled?
a) Remain unchanged
b) Reduce to one-fourth value
c) Increase four folds
d) Be doubled
Answer: a
Explanation: Torque depends on armature current, so as long as armature current remains constant torque will not change. Speed depends directly with back emf thus, it will definitely become double when back emf is doubled.

5. At the instant of starting, when a DC motor is put on supply, it will behave like ______________
a) Highly resistive circuit
b) Low resistance circuit
c) Capacitive circuit
d) Inductive circuit
Answer: b
Explanation: At the starting when back emf is very low, machine will fetch large amount of armature current from supply acting as a low resistive circuit. In order to avoid such case, we use starters in DC machine which add some resistance to armature at the starting.

6. All others are advantages of DC motor over AC motor except _______________
a) Low cost
b) Wide speed range
c) Stability
d) High starting torque
Answer: a
Explanation: Dc machine is very useful over AC when operation is concerned but its superior speed control system itself requires 2 more machines. DC machine requires high maintenance due to commutator and brushes so it is an expensive machine.

 

DC Generator MCQs

 

7. If a DC motor designed for 45°C ambient temperature is to be used for 55°C ambient temperature, then the motor ___________
a) Of lower HP should be selected
b) Of higher HP should be selected
c) Can be used for 50°C ambient temperature also
d) Is to be de-rated by a factor recommended by manufacturer and select the next higher H.P. motor
Answer: d
Explanation: The ambient temperature is one of a factor in derating. If the ambient temperature given is high, the insulation may reach its maximum temperature limit quickly. Hence, the motor may have to be de-rated, and for application next motor must be used.

8. Torque developed by a DC motor depends upon _____________
a) magnetic field
b) active length of the conductor
c) current flow through the conductors
d) Current, active length, no. of conductors, magnetic field all
Answer: d
Explanation: Torque produced by a DC motor is directly proportional to an armature current flowing through the machine. While it also depends on some constant values like active length, no. od armature conductors and magnetic field which are constant for a given machine.

9. For very sensitive and wide speed control, which of the following method is preferred?
a) Armature control
b) Ward-Leonard control
c) Multiple voltage control
d) Field control
Answer: b
Explanation: In ward-Leonard method, by using big circuitry, very fine speed control over the whole range from zero to normal speed and that too in both directions can be obtained. The motor-generator set can provide speed both below and above the rated speed and in both direction, by this method.

10. Field weakening control method is used for the speed _________
a) Above rated speed
b) Below rated speed
c) Can be used anywhere
d) Can’t tell
Answer: a
Explanation: By reducing of field current, flux will also reduce, because flux is directly proportional to the field current passing through the winding. As speed is inversely proportional to flux, so this method can be used for only above rated speed.

11. Armature resistance control method is used for the speed _____________
a) Above rated speed
b) Below rated speed
c) Can be used anywhere
d) Can’t tell
Answer: b
Explanation: In armature circuit resistance method armature resistance can be only increased, by adding another resistance in series. As a result, the back emf will decrease. Speed is directly proportional to back emf so, it will also decrease. Therefore, this method is used for below rated speed.

12. Which function is performed by counter emf of a DC motor?
a) Exceeds supply voltage
b) Aids applied voltage
c) Helps in energy conversion
d) Regulates its armature voltage
Answer: c
Explanation: Back emf or counter emf makes the DC a self-regulating machine i.e. it makes the DC motor to draw as much armature current as it is just enough to develop the required load torque. Thus, it helps in energy conversion and it is always less than applied voltage.

13. The output power of any electrical (AC or DC) motor is taken from the ______________
a) Field
b) Coupling mounted on the shaft
c) Armature
d) Motor frame
Answer: b
Explanation: The output power of any electrical motor is always taken from the coupling mounted on the shaft because the motor converts input electrical power into output mechanical power. So, we get mechanical power at shaft coupling.

14. Why field winding of a DC series motor is provided with thick wire?
a) As it carries large load current
b) To provide large flux
c) In order to reduce eddy current to provide large flux
d) To reduce the of insulting materials
Answer: a
Explanation: In DC series motor, the cross-sectional area of wire used for field coils has to be large because the in this case field coils carry the load or armature current. As the cross-sectional area is large, the resistance is minimum and series voltage drop is also minimum.

Starting of DC Motors – 1

1. Why starters are required in a DC motor?
a) Back emf of these motors is zero initially
b) These motors are not self-starting
c) These motors have high starting torque
d) To restrict armature current as there is no back emf at starting
Answer: d
Explanation: At the time of starting (n=0), the induced emf of a motor is zero such that current drawn by armature, from rated voltage supply would be Ia= V/Ra. Since armature resistance is very low, armature current drawn is very high and will damage the machine.

2. For which motors DOL starter can be used?
a) Up to 5 H.P
b) Up to 10 H.P
c) Up to 15 H.P
d) Up to 20 H.P
Answer: a
Explanation: DOL starters are limited to the small rating motors where distribution system (mains supply) can withstand high starting currents without excessive voltage dips. For a large rating motor, ranging from 5 HP to 25 HP, oil immersed DOL starters are used which provides insulation against sparking on contact points, increases the life of starter.

3. A three-point starter is used for _________
a) Shunt motors
b) Shunt as well as compound motors
c) Shunt, compound and series motors
d) Not for DC motors
Answer: b
Explanation: 3-point starters are used only for shunt and compound motors, they are not used for series motors. Three-point starter is employed where motor field current can be varied in a narrow range and so does the motor speed.

4. The starting resistance of a DC shunt motor is generally ______
a) low
b) Around 0.5 kΩ
c) Around 5 kΩ
d) Infinitely large
Answer: a
Explanation: Starting resistance of a DC shunt motor and DC compound motor is low. Well, that’s the reason why we use starters in a DC motors, in order to limit the armature current flowing through the armature and to protect machine circuitry.

5. In a manual shunt motor starter relay positions are ________________________
a) Over load relay is connected in series and no volt relay in parallel with the load
b) Over load relay is connected in parallel and no volt relay in series with the load
c) Over load relay and no volt relay are both connected in series with the load
d) Over load relay and no volt relay are both connected in parallel with the load
Answer: a
Explanation: In simple manual shunt motor starter like 3-point starter over load relay coil is kept in series with DC mains while no volt coil is kept in parallel with DC mains. Both the coils are equally important in a motor starter circuit.

6. What will happen if DC motor is used without starter?
a) Heavy sparking at brushes
b) It’ll start smoothly
c) Will not start at all
d) Depends on load
Answer: a
Explanation: It would cause intolerably heavy sparking at the brushes which may destroy the commutator and brush-gear. Sudden development of large torque will cause mechanical shock to the shaft, reducing its life. Such heavy current cannot be generally permitted to be drawn from the source of supply.

7. Motor will start quickly when used without starter.
a) True
b) False
Answer: a
Explanation: The only thing in favour of direct starting must be mentioned here. Since the torque of the motor with direct start is much higher, the motor starts much more quickly. As a consequence, the Joule input per start is much less than that with resistance start.

8. In shunt and compound motor starting the shunt field should be made on with full starting resistance in ____________
a) Series with field
b) Parallel with field
c) Series with armature
d) Parallel with armature
Answer: c
Explanation: In shunt and compound motors starting the shunt field should be switched on with full starting resistance in armature circuit. A short time delay in this position allows the field current to build up to the steady value of the inductive field transients.

9. A starter is required for a 220-V shunt motor. The maximum allowable current is 55 A and the minimum current is about 35 A. The armature resistance of the motor is 0.4 Ω. What will be the number of sections of starter resistance required?
a) 5
b) 4
c) 6
d) 8
Answer: c
Explanation: I1=55 A, I2 =35 A
So, γ= Ratio of upper limit to the lower limit = 55/35 = 1.57, R1= 200/55= 4 Ω
Now, γn-1 = 4/0.4 = 10. By substituting γ value, we get n = 6.

10. γ is given as 1.585. Resistance at maximum allowable current is given equal to 4 Ω, what is the 5th step resistance?
a) 0.235
b) 0.370
c) 1.476
d) 2
Answer: a
Explanation: γ is given as 1.585. So γ-1 is equal to 0.631. R1 is provided and it is equal to 4 Ω.
r1= (1-0.631) * 4 = 1.476 Ω
r2= 1.476*0.631= 0.931 Ω, similarly calculating till r5= 0.235 Ω.

11. Four-point starter is used when ______________
a) Motor field current is varied in narrow range
b) Motor speed is varied in small range
c) Motor field current is varied over wide range
d) Can be used anywhere
Answer: c
Explanation: Three-point starter is employed where motor field current can be varied in a narrow range and so does the motor speed while four-point starter is used when motor field current can vary over a wide range and so does the motor speed.

Starting of DC Motors – 2

1. In three-point starter, as a starting handle is rotated __________
a) The resistance is added into armature circuit
b) The resistance is removed from field circuit
c) The resistance is added into field circuit
d) Resistance is neither added nor removed
Answer: c
Explanation: The starting resistance is arranged in steps between conducting raised studs. As the starting handle is rotated about its fulcrum, it moves from one stud to the next, one resistance step is cut out, and it gets added to the field circuit.

2. How much torque is ensured by resistance adding arrangement in 3-point starter?
a) Low and non-zero
b) Infinite
c) High finite
d) Zero
Answer: c
Explanation: As a starting handle is rotated one resistance step is added into field circuit. There is a short time wait at each stud for the motor to build up speed. This arrangement ensures a high average starting torque.

3. The resistance of NVC is _______
a) Small
b) Large
c) Infinite
d) Zero
Answer: a
Explanation: The resistance of no volt coil (NVC) is small. NVC resistance forms a part of field resistance when resistor rotating handle of three-point starter is moved from min. to max. position. Starting resistance is also added to the field circuit.

4. NVC will release the handle electromagnetically when ________
a) In the case of failure of field current
b) If the resistance is very high
c) At the end of each rotation
d) At the start of each rotation
Answer: a
Explanation: In case of failure of field current (due to accidental or otherwise open circuiting) NVC coil releases the handle (held electromagnetically), which goes back to the OFF position under the spring action.

5. Over-load coil performs the function when __________
a) In the case of failure of field current
b) If the resistance is very high
c) If armature current increases beyond certain value
d) At the start of each rotation
Answer: c
Explanation: The contact of this relay at armature current above a certain value (over load/ short circuit) closes the NVC ends, again bringing the handle to OFF position. NVC and OL release are protections incorporated in 3-point starter.

6. In 4-point starters the resistance is added in series with NVC because __________________
a) To increase field current
b) To increase armature current
c) To limit the NVC current
d) To limit armature current
Answer: c
Explanation: To overcome the problem caused when the field current is low, NVC is connected across the two lines, one line connected to F terminal through the starter and other directly to the second line from another L terminal of the starter. To limit the NVC current a protective resistance R is connected in series with it.

7. What will be the γ value for starter taking 4 steps, where ratio of resistance at maximum allowable current to armature resistance is equal to 1.8?
a) 1.2164
b) 1.8
c) 2.2468
d) 0.8220
Answer: a
Explanation: γ is defined as the ratio of upper current limit to the lower current limit in starters of DC machine. γ n-1 = ratio of resistance at maximum allowable current to the armature resistance. Substituting values for n=4, we get γ=1.2164.

8. Maximum allowable current for a 240-V DC shunt motor is equal to 65 A. Minimum allowable current for same is equal to 40 A. What will be the value of γ?
a) 0.6153
b) 1.265
c) 1.625
d) 2.652
Answer: c
Explanation: γ is defined as the ratio of upper limit current value to the lower limit current value. So, γ will be the ratio of 65/40. From calculations, we get γ= 1.625. Note that γ is unitless quantity.

9. For a certain machine having γ = 1.8, we are using starter with 5 steps. What will be the resistance at step 3 if step 2 resistance is equal to 2 Ω?
a) 2.111
b) 1.111
c) 3.6
d) 10.8
Answer: b
Explanation: γ is defined so as to calculate step resistance from the given maximum limit of current and minimum limit of the same. This γ = rn/rn-1. So, for calculating resistance at step 3 we’ll substitute the corresponding values in the equation, which will give step 3 resistance as 1.1111Ω.

10. Which of the following is the correct formula for calculating step resistance?
a) γ = rn/rn-1
b) γ = rn/rn+1
c) γ = rn*rn-1
d) γ = rn*rn+1
Answer: a
Explanation: For given values of armature currents upper and lower, corresponding equivalent resistances are calculated. So, by the induction formula we calculate the value of γ and the step resistance as well, where n denotes the number of steps.

11. Which of the following starter can sufficiently start the DC series motor?
a) 3-point starter
b) 4-point starter
c) 2-point starter
d) Cannot be determined
Answer: c
Explanation: 2-point starter is enough to start the DC series motor. Since armature and field winding are in series already high armature current will not flow. No requirement of 3 or 4-point starter in DC series motor.

Speed Control Using Field Control of Shunt Motor

1. The speed of a DC shunt motor can be increased by ______
a) Increasing the resistance in armature circuit
b) Increasing the resistance in field circuit
c) Reducing the resistance in the field circuit
d) Reducing the resistance in the armature circuit
Answer: b
Explanation: Speed of the DC motor is directly proportional to the back emf and inversely proportional to the flux produced by field. Where, flux produced is directly proportional to the current passing through the field winding (linear magnetization).

 

Armature Reaction MCQs

 

2. What will happen if excitation of DC shunt motor is changed?
a) Torque will remain constant
b) Torque and power both will change
c) Torque will change but power will remain constant
d) Torque, power and speed, all will change
Answer: c
Explanation: The motor will accelerate the mechanical load connected during this period but no increase in the mechanical load as Pload = T1W1 = T2W2 where W2 >W1. So, at the higher speed there is less electrical torque for the same mechanical load / power.

3. If the speed of a DC shunt motor is increased, the back emf of the motor will ___________
a) Increase
b) Decrease
c) Remain same
d) Become zero
Answer: a
Explanation: From, the speed-current characteristics of DC shunt motor we know that speed of the motor is directly proportional to the back emf and inversely proportional to the flux. So, for more speed there will be more back emf generated.

4. The speed of a DC shunt motor can be made more than full load speed by __________
a) Reducing the field current
b) Decreasing the armature current
c) Increasing the armature current
d) Increasing the excitation current
Answer: a
Explanation: Speed of the DC motor obtained from speed equation is inversely proportional to flux produced by the field. So, reducing the field current flux produced by armature will decrease, and speed will increase.

5. Speed regulation of DC shunt motor is calculated by ratio of difference of full load speed and no-load speed with full load speed.
a) True
b) False
Answer: b
Explanation: Speed regulation is defined as a ratio of difference of no-load speed with full load speed with no-load speed. Here, no-load speed is more than the full load speed. Thus, we divide difference by no-load value and not by full load value.

6. Which speeds can be obtained from field control of DC shunt motor?
a) Lower than rated speeds
b) Greater than rated speeds
c) Lower and greater than rated speeds
d) Neither lower nor greater than rated speeds
Answer: b
Explanation: Speeds greater than rated speeds can be obtained by lowering the flux of shunt field motor. Field cannot be made any stronger, it can only be weakened by this method. Thus, speed lower than the rated speed can’t be obtained.

7. No load speed of the DC shunt motor is 1322 rpm while full load speed is 1182 rpm. What will be the speed regulation?
a) 12.82 %
b) 11.8 %
c) 16.6 %
d) 14.2 %
Answer: b
Explanation: Speed regulation is equal to (No-load speed – Full load speed) / (Full load speed). By substituting all the values, speed regulation= (1322-1182)/ 1182. Speed regulation is given by 0.118. In percentage notation SR= 11.8 %.

8. Speed regulation of a DC shunt motor is equal to 10%, at no load speed of 1400 rpm. What is the full load speed?
a) 1233 rpm
b) 1273 rpm
c) 1173 rpm
d) 1123 rpm
Answer: b
Explanation: Speed regulation is equal to 0.1 which is also equal to (no-load – full load speed) divided by full load speed. Thus, by substituting all known quantities we get full load speed = 1400/1.1 = 1272.7 rpm so, speed equal to 1273 rpm.

9. Where will speed-torque characteristics will lie when armature reaction is considered?
a) Below the speed-torque characteristics when armature reaction is not considered
b) Above the speed-torque characteristics when armature reaction is not considered
c) On the speed-torque characteristics when armature reaction is not considered
d) Can be anywhere with the speed-torque characteristics when armature reaction is not considered
Answer: b
Explanation: The speed-torque characteristic which has a small linear drop due to the second term (Ra effect) and translates upwards as the field is weakened due to the armature reaction. The demagnetizing effect of the armature reaction causes the characteristics to somewhat bend upwards with increasing torque (increasing load current).

10. Working range of the speed-torque characteristic, with increasing speed will ___________
a) Reduce
b) Increase
c) Remain same
d) Cannot comment
Answer: a
Explanation: The working range of the speed-torque characteristic reduces with increasing speed in order for the armature current not to exceed the full-load value with a weakening field. Thus, armature current gives the bound limit for curve.

11. For speed x rpm, we get field current If1 and for speed y rpm, we get the field current If2. If y is greater than x then, ________________
a) If1 <if2
b) If1 >If2
c) If1 =If2
d) Cannot comment on If1, If2</i
Answer: b

Explanation: When speed-torque characteristic for different speeds is plotted on the same graph, we get the curve limited by armature currents also. For any value of field current flux through the field is directly proportional current, while flux is inversely proportional to speed.

12. 400-V dc shunt motor takes a current of 5.6 A on no-load and 68.3 A on full-load. Armature reaction weakens the field by 3%. What is the ratio of full-load speed to no-load speed? Given: Ra = 0.18 Ω, brush voltage drop= 2 V, Rf = 200 Ω.
a) 1.2
b) 0.8
c) 1.4
d) 1
Answer: d
Explanation: If = 400/200= 2 A
No-load:
Ia0 = 5.6 – 2 = 3.6 A
Ea0 = 400 – 0.18 \ 3.6 – 2 = 397.4 V
Full-load:
Ib>a (fl) = 68.3 – 2 = 66.3 A
Ea (fl) = 400 – 0.18 / 66.3 – 2 = 386.1 V
n (fl)/n (nl) = [386.1/397.4] [1/0.97] = 1.

13. In which of the following method, effect of armature reaction is more?
a) Field weakening method
b) Armature resistance control
c) Same in both methods
d) Cannot be determined
Answer: a
Explanation: In field weakening method we are reducing the working flux to increase the speed, by reducing the field current. Therefore, effect of armature flux on main field flux will increase in case of field weakening method.

Speed Control Using Field Control of Series Motor

1. Which of the following DC motor has the poorest speed control?
a) Differentially compounded motor
b) Cumulatively compounded motor
c) Shunt motor
d) Series motor
Answer: d
Explanation: DC series motor at no load condition gives infinite speed ideally. Practically it will damage all the armature circuit. Thus, as the load is reduced speed of the motor will go on increasing rapidly. So, speed control is very poor in series motor.

2. In variable speed motor ____________
a) Stronger commutating field is needed at low speed than at high speed
b) Weaker commutating field is needed at low speed than at high speed
c) Same commutating field is needed at low speed and at high speed
d) Cannot be determined
Answer: b
Explanation: According to commutating fields of machine, speed and overall performance of a machine is determined. For low speeds of motor in a variable speed motor we need weaker commutating field.

3. The speed of a motor falls from 1200 rpm at no-load to 1050 rpm at rated load. The speed regulation of the motor is ____________
a) 12.36%
b) 14.28%
c) 16.77%
d) 18.84%
Answer: b
Explanation: Speed of regulation of DC series motor is given by ratio of difference of no-load speed and full load speed with full load speed. Thus, in this case, speed regulation = (1200-1050)/1050= 14.28%.

4. Which of the following is not the method of speed control in DC series motor?
a) Diverter
b) Tapped-field control
c) Variable resistance in series with armature
d) Series- parallel control
Answer: c
Explanation: Diverter means adding a variable resistance in parallel with field winding. Tapped field control is the method where field ampere turns are adjusted in steps by varying the number of turns included in the circuit. In series parallel method field winding is connected so that it will form two parts.

5. For speed reversal, field control method is suitable.
a) True
b) False
Answer: b
Explanation: Field control method is not suited to applications needing speed reversal; since the only way to reverse speed is to disconnect the motor from the source and reverse the field/armature polarity. The field circuit being highly inductive, it is normally the armature which is reversed.

6. For large motors what is the ratio of compensating winding is required for increasing the speed of the motor?
a) 2:1
b) 4:1
c) 6:1
d) 8:1
Answer: a
Explanation: For motors requiring a wide range of speed control, the field ampere-turns are much smaller than the armature ampere-turns at high speeds causing extreme distortion of the flux density in the air-gap. This leads to poor commutation. Compensating winding can be used to increase the speed range which can be 2 to 1 for large motors.

7. In diverter resistor field control method of DC series motor, variable resistor is added ________
a) In parallel with field
b) In series with field
c) In parallel with armature
d) In parallel with load
Answer: a
Explanation: Diverter is a small variable resistor, which is connected in parallel with field winding. By varying this resistance value, the field current and consequently the field ampere-turns can be varied and speed can be controlled.

8. Why it is advisable to use inductively wound diverter resistor?
a) To make speed control on more range
b) For long-life of machine
c) Cost efficient
d) To avoid oscillations in speed
Answer: d
Explanation: One precaution to be taken in this method in order to avoid oscillations in speed initiated by load changes is to use an inductively wound diverter resistor. Thus, speed controlled output will vary smoothly when load is changed.

9. For higher diverter resistance, speed-torque characteristic will lie ____________
a) Above speed-torque characteristic of lower resistance
b) Below speed-torque characteristic of lower resistance
c) On the speed-torque characteristic of lower resistance
d) Cannot say
Answer: b
Explanation: Speed-torque characteristic for decreasing values of diverter resistance or decreasing value of kd will lie above of the previous or lower value of diverter resistance. Thus, for higher diverter value curve will lie below the previous one.

10. In tapped field control method ____________
a) A variable resistor is connected in parallel
b) A variable resistor is connected in series
c) Field winding is made with more out pins
d) Another field winding is added with the previous one
Answer: c
Explanation: In tapped field control of the DC series motor a field winding is simply tapped at various points and many out pins taken, so that any out pin can be connected and particular part of field winding will be selected in the circuit, while other part is made open.

11. Which of the following method will not give many speed values for a DC series motor?
a) Diverter
b) Series-parallel
c) Field tapped
d) All field control methods will give many speed values
Answer: b
Explanation: In series-parallel method, field winding is equally divided in two parts which are connected in series and parallel with respect to each other. Thus, in series connection we’ll get one speed and in parallel mode we’ll get one speed. These 2 speeds are only possible.

12. In series-parallel speed control method ______________
a) Speed given by parallel connection is more
b) Speed given by series connection is more
c) Both speeds can be equal
d) Will depend on other parameters
Answer: a
Explanation: In parallel case, total ampere turns will be exactly half of the ampere turns of the series case. Since speed of any DC motor is inversely proportional to the MMF or flux or ampere turns developed, speed of the parallel control method will always large than the series one.

13. Where diverters are used?
a) In shunt motors
b) In series motors
c) In both motors
d) All other motors except shunt and series motors
Answer: b
Explanation: Diverters are used in series motors for speed control, they are not used in shunt motors. Since, shunt field winding resistance is very high, if we connect a diverter across shunt field winding, total current will flow through diverter and it will almost short circuit the shunt field winding. This will increase the motor speed to a very high value.

14. What will happen to the speed of a series motor if the temperature of armature resistance is increased?
a) Not change
b) Decreases
c) Increases
d) Cannot be determined
Answer: b
Explanation: In a DC series motor, when the resistance increases due to rise in temperature, the IR drop in the circuit will increase, which reduces the effective voltage applied to the armature, hence speed decreases due to the direct proportionality.

Speed Control Using Armature Control in DC Motor

1. For which speed control method we get minimum efficiency?
a) Voltage control method
b) Field control method
c) Armature control method
d) Cannot be determined
Answer: c
Explanation: In armature control method, we vary armature voltage by adjusting variable resistance value. So, when we define armature efficiency, it is equal to η = [ (Vt-IaRe) Ia] / VtIa= 1-IaRe / Vt. Thus, efficiency is reduced drastically for large speed reductions.

2. The speed of a DC motor can be varied by changing ______________
a) Field current
b) Applied voltage
c) Resistance in series with armature
d) Field current, applied voltage or resistance in series with armature any method will work
Answer: d
Explanation: In field control method we change the flux produced by machine with the help of field current. If applied voltage is changed, definitely speed will change. By adding resistance in series with armature voltage of the armature can be varied to change the speed.

3. For constant torque drive which of the following speed control method is preferred?
a) Field control
b) Armature voltage control
c) Shunt armature control
d) Voltage control
Answer: b
Explanation: Armature control method provides a constant-torque drive. In the shunt motor case by keeping the field current at maximum value full motor torque can be obtained at full-load armature current at all speeds.

4. When the armature of a DC motor rotates, emf induced is in machine is called as ___________
a) Self-induced emf
b) Mutually induced emf
c) Back emf
d) Cannot be determined
Answer: c
Explanation: According to terminology used in DC machines, emf induced in a machine when armature rotates is called as back emf. This back emf value is directly proportional to the speed of the motor.

5. Which of the following method is used for DC motor with 12+ HP requiring frequent start, stop, speed reversal?
a) Drum type controller is used
b) Three-point starter is used
c) Four-point starter is used
d) Cannot be determined
Answer: a
Explanation: Drum controllers are used when an operator is controlling the motor directly/manually. The drum controller is used to start, stop, reverse, and vary the speed of a motor, by its mechanism. This type of controller is used on crane motors, elevators, machine tools, and other heavy applications.

 

Basics Of DC Machines MCQs

 

6. Flux density distribution is distorted by armature control method.
a) True
b) False
Answer: b
Explanation: Unlike field control method, the main field ampere-turns are maintained at a large value, flux density distortion caused by armature reaction is limited. Unlike field control scheme, speed reversal can be easily implemented here.

7. In rheostatic series control method of armature we add ____________
a) Variable resistor in parallel with armature
b) Variable resistor in series with armature
a) Fixed resistor in parallel with armature
b) Fixed resistor in series with armature
Answer: b
Explanation: In rheostatic series control method, we add variable resistance in series with the armature so that armature voltage can be varied, by varying the voltage drop at series resistor. By increasing value of series resistor speed can be decreased.

8. By series armature resistance method, we can get _________________________
a) Speed above rated speed
b) Speed equal to rated speed
c) Speed below rated speed
d) All speeds are possible
Answer: c
Explanation: The drawback of armature speed control method is we get only speeds which are below the rated speed. As the value of series resistance increases, back emf decreases which result in lowering of the speed.

9. Consider the armature control method, where 10 Ω resistance is connected in series with armature in first case. For second case resistance value is changed 100 Ω. Then ____________
a) Speed-torque characteristic of both will start from same point
b) For first case speed-torque characteristic will start below to the case 2
c) For first case speed-torque characteristic will start above to the case 2
d) Can’t say because other parameters are unavailable
Answer: a
Explanation: Speed-torque characteristic will start from same point because at zero torque presence of series resistance is not taken into account. Afterwards speed torque characteristic with higher series resistance value will lie below the other.

10. Speed regulation of armature series control method is __________
a) Very good
b) Zero
c) Poor
d) Cannot comment
Answer: c
Explanation: The speed regulation of the method is poor as for a fixed value of series armature resistance, the speed varies directly with load, being dependent upon the resistance voltage drop. In general, rheostatic control is economically feasible only for very small motors (fractional kW) or for short-time, intermittent show-downs for medium-sized motors.

11. In shunted rheostatic armature control method ______________
a) Variable resistor is added in parallel with armature
b) Variable resistor is added in series with armature
c) Variable resistor is added in parallel with armature and another variable resistor is added in series
d) Variable resistor is not added in whole circuit
Answer: c
Explanation: In shunted armature speed control method we add one variable resistance in series with armature and one variable resistance in parallel with armature, so by varying the ratio of this resistances we can get various different speeds.

12. Which of the following is correct statement?
a) Speed regulation of rheostatic armature control method is better than that of shunted armature control
a) Speed regulation of rheostatic armature control method is worse than that of shunted armature control
a) Speed regulation of rheostatic armature control method is almost equal to that of shunted armature control
a) Speed regulation of rheostatic armature control method and of shunted armature control are equally worst.
Answer: b
Explanation: Speed regulation of shunted armature control method is better than that of rheostatic armature control. External power loss in shunted armature control method is very high compare to that of rheostatic armature control method.

13. By series parallel method of armature control how many different speeds are possible?
a) 4
b) 8
c) 2
d) Infinite
Answer: c
Explanation: Here two identical motors are coupled together mechanically to a common load. Two speeds at constant torque are possible in this method—one by connecting the motors armatures in series and the other by connecting them in parallel.

14. Which method is best for series traction motors?
a) Rheostatic armature control
b) Shunted armature control
c) Series parallel control method
d) Any method can be employed
Answer: c
Explanation: Series-parallel control method is superior to the rheostatic control insofar as efficiency is concerned. However, it is limited to two speed steps. The method is commonly employed for speed control of series traction motors.

Ward Leonard Speed Control Method

1. Ward Leonard method is ___________
a) Armature control method
b) Field control method
c) Combination of armature control method and field control method
d) Totally different from armature and field control method
Answer: c
Explanation: Ward Leonard method is the combination of armature control method and field control method, which can also be called as voltage control method. This is the most efficient method of speed control over wide range.

2. Which of the following component is not used in Ward Leonard method?
a) AC motor
b) DC generator
c) DC motor
d) AC generator
Answer: d
Explanation: Whole unit of Ward Leonard speed control unit consists of various units like DC generator, DC motor, AC motor, exciter circuit and various pots which are used for carrying out smooth operation.

3. In Ward Leonard speed control method for lowering the speed of the motor ______________
a) Reduce armature voltage
b) Increase armature voltage
c) Increase field current
d) Decrease field current
Answer: a
Explanation: In Ward Leonard speed control method, speed can be reduced under base value by reducing armature voltage. By increasing field current speed can be reduced but this is not employed in Ward Leonard method.

4. Reducing the armature voltage will give us _______________
a) Variable torque speed control
b) Constant torque speed control
c) Variable and constant both can be achieved
d) Cannot comment on torque
Answer: b
Explanation: As seen from speed torque characteristics, reducing armature voltage will reduce the speed of the motor below base value but torque will remain same. Thus, reducing armature voltage will give constant torque speed control.

5. In Ward Leonard speed control method for increasing the speed of the motor ______________
a) Reduce armature voltage
b) Increase armature voltage
c) Increase field current
d) Decrease field current
Answer: d
Explanation: In Ward Leonard speed control method, speed can be increased above base value by weakening of the field, which can be done by lowering field current value. By increasing armature voltage speed can be increased but this is not employed in Ward Leonard method.

6. Reducing the field current will give us _______________
a) Constant torque and variable power speed control
b) Constant torque speed control
c) Variable power speed control
d) Constant power speed control
Answer: b
Explanation: As seen from speed torque characteristics, reducing field current will increase the speed of the motor above base value but power will remain same. Thus, reducing armature voltage will give constant power speed control, with variable torque.

7. Speed-power characteristic for Ward Leonard speed control method _________________
a) Will start from origin
b) Will start from some positive value on power axis
c) Will start from some positive value on speed axis
d) Depends on other parameters
Answer: a
Explanation: Speed power characteristic of DC motor is plotted when, Ward Leonard speed control method is employed. For speed equal to zero, which is less than base speed, we get constant torque but variable power operation. Thus, power will start increasing from origin.

8. Efficiency of Ward Leonard method is ____________
a) Higher than rheostatic control method but lower than shunted field control method
b) Lower than rheostatic control method
c) Higher than rheostatic control method and shunted field control method
d) Depends on load
Answer: c
Explanation: Unlike all other methods, external resistance is not added in the circuit of control system. Thus, efficiency of Ward Leonard control method is always highest at various different speeds.

9. Ward Leonard method is an ideal choice for motor which undergoes frequent starting, stopping, speed reversal.
a) True
b) False
Answer: a
Explanation: Absence of external resistance increases efficiency. Also, when the generator emf becomes less than the back emf of the motor, electrical power flows back from motor to generator, is converted to mechanical form and is returned to the mains via the driving ac motor. This aspect makes Ward Leonard method perfect for given application.

10. Starting gear used in Ward Leonard method___________
a) Is of small size
b) Is of large size
c) Size depends on application
d) Is absent
Answer: d
Explanation: No special starting gear is required in Ward Leonard method of speed control. As the induced voltage by generator is gradually raised from zero, the motor starts up smoothly. Speed reversal is smoothly carried out.

11. To get the speed of DC motor below the normal speed without wastage of electrical energy we use __________________
a) Ward Leonard control
b) Rheostatic control
c) Any of the Ward Leonard or rheostatic method can be used
d) Not possible
Answer: a
Explanation: Ward Leonard method of speed control is most efficient method of speed control in all aspects. We can get constant torque operation and constant power operations as well, with this method.

12. Speed control by Ward Leonard method, can give uniform speed variation _______________
a) In both directions
b) In one direction
c) Below normal speed only
d) Above normal speed only.
Answer: a
Explanation: Speed control by Ward Leonard method, gives uniform speed variation in both the directions and above and below of normal speed as well. Speed reversal is carried out smoothly by this control method.

13. Ward Leonard control is basically a _____________
a) Voltage control method
b) Field diverter method
c) Field control method
d) Armature resistance control method
Answer: a
Explanation: Ward Leonard speed control method is combination of rheostatic series control method and shunted armature control method with field control as well. Thus, it can be called as voltage control method also.

14. In Ward Leonard control of DC motor, the lower limit of speed is imposed by ____________
a) Residual magnetism of the generator
b) Core losses of motor
c) Mechanical losses of motor and generator together
d) Cannot be determined
Answer: a
Explanation: We get the speed below the base value by reducing armature voltage, which is the simple method of reducing back emf which is proportional to the residual magnetism. Thus, lower limit of speed is imposed by residual magnetism of the generator.

15. The disadvantage of the Ward Leonard control method is ___________
a) High initial cost
b) High maintenance cost
c) Low efficiency at high loads
d) High cost, high maintenance and low efficiency
Answer: d
Explanation: Ward Leonard speed control method requires large number of building blocks like generators, motors. Thus, installing cost and maintenance cost of the whole unit is very high. Apart from cost, it gives low efficiency at very high loads.

Braking of DC Motors – 1

1. Which of the following is the best braking method?
a) Friction
b) Electromechanical action
c) Eddy-currents
d) Electric braking
Answer: d
Explanation: Braking methods based on friction, electromechanical action, eddy-currents, etc. are independent of the motor but sometimes electric braking is better justified owing to its greater economy and absence of brake wear.

2. DC motor is still widely used in tractions due to its excellent braking properties.
a) True
b) False
Answer: b
Explanation: Dc motor is used in tractions because of its excellent braking characteristics and ability of smooth transition from the motor to the generator mode and vice versa. Also, characteristics suit perfectly for traction application.

3. Which of the following is not the method of electrical braking?
a) Plugging or counter-current
b) Dynamic or rheostatic
c) Regenerative
d) Eddy current
Answer: d
Explanation: Eddy current is the electrical effect or response of the system, which is reflected mechanically at brakes to reduce the speed of the motor. Thus, eddy current is not an electrical brake, it is mechanical one.

4. Which of the following is the plugging method of braking?
a) Reversal of field connections
b) Reversal of armature connections
c) Addition of equal and opposite field
d) Removal of field circuit from current machine circuit
Answer: b
Explanation: Plugging is method where connections are reversed at a given instant. Because of the problem of interrupting highly inductive field current and the time needed for the field current to build up in opposite direction, it is a common practice to reverse armature connections.

5. Which of the following is correct formula for braking torque in plugging?
a) n (ka2/Rb)
b) n2 (ka2/Rb)
c) n-1 (ka2/Rb)
d) (ka2/Rb)
Answer: a
Explanation: Braking torque is equal to braking power divided by speed of the motor.
T= [(nka )2/Rb]/n. As, Braking power is equal to Ea2/Rb/n. By solving for the braking torque from the above equation, we get n (ka2/Rb).

6. Electrical braking of any variety becomes less effective as ________________
a) Speed increases
b) Speed decreases
c) Independent of speed
d) Depends on supply voltage
Answer: b
Explanation: Braking torque of the DC machine is given by n (ka2/Rb). Here, braking torque is directly proportional to the speed of the motor, so as the speed decreases the efficiency of electrical brakes which is dependent on braking torque decreases.

7. Plugging is applied in a motor, if we don’t make the switch OFF what will happen?
a) Motor will come to rest as a result of plugging
b) Motor will come to rest and will start rotating in another direction
c) Motor will burn
d) Nothing will happen
Answer: b
Explanation: If the switch is kept ON near to zero speed, motor will have braking torque acting in opposite direction greater than the electromechanical torque. Thus, motor will come to rest and for the next instant motor will start rotating in opposite direction.

8. Plugging is used in ____________
a) Small motors only
b) Small and medium powered
c) Only in large heavy machines
d) Everywhere
Answer: a
Explanation: Plugging is used in small scale applications only. The large initial current and high mechanical stress restrict the application of plugging in large machines. So, in order to balance stress this method is used in small machines only.

9. Which of the following is dynamic braking?
a) Reversal of field connections
b) Reversal of armature connections
c) Addition of equal and opposite field
d) Removal of armature circuit from current machine circuit
Answer: d
Explanation: Reversal of the connections of armature is the method called plugging. In dynamic braking we remove the armature circuit and connect it to different resistor, with field circuit still connected to the external supply.

10. Braking time in the dynamic braking is the function of _____________
a) System inertia
b) Load torque
c) Motor rating
d) All- system inertia, load torque and motor rating
Answer: d
Explanation: In dynamic braking, when brakes are applied the armature is disconnected from machine circuit and connected to the braking resistor. Now, at this point motor is driven by kinetic energy gained earlier, dissipating power in braking resistor.

11. In dynamic braking, when braking is applied system acts as ___________
a) Freely running machine
b) Motor with slow speed
c) Generator
d) Motor with same speed in opposite direction
Answer: c
Explanation: The armature is disconnected from the supply and then a braking resistor Rb is immediately connected across it. The motor acts as a generator, driven by the inertia and stored kinetic energy dissipating power in Rb. This is a simple method of bringing a motor nearly to a standstill.

12. In which of the following electrical braking method, energy is supplied back to the supply?
a) Plugging
b) Dynamic braking
c) Regenerative braking
d) In all electrical braking
Answer: c
Explanation: In plugging energy is wasted in braking resistance which is equal to starting resistance while running as a motor. In dynamic braking energy is generated but it is not fed back to supply. In regenerative method energy is sent back for reuse.

Braking of DC Motors – 2

1. Regenerative braking is used when duty cycle ____________
a) Requires braking of machine
b) Requires accelerating of machine
c) Requires constancy of machine
d) Cannot comment on duty cycle
Answer: a
Explanation: Regenerative braking is used specially where the duty cycle requires the braking or slowing of the machine more frequently and is most useful in holding a descending load of high potential energy at a constant speed.

2. Regeneration is not easily possible for ____________
a) DC shunt motor
b) Separately excited motor
c) Compounding motor with weak series compounding
d) DC series motor
Answer: d
Explanation: Regeneration is possible with a shunt and separately excited motors and with compound motors with weak series compounding. Series motors need a reversal of either the field or the armature connections.

3. Which of the following method is not used for regeneration?
a) Increasing field current
b) Increasing armature speed
c) Increasing supply voltage
d) Reducing supply voltage
Answer: c
Explanation: Regeneration is achieved by either increasing field current, increasing armature speed, or by reducing supply voltage. Increasing supply voltage is not the method which is employed in regeneration process.

4. If the terminals of armature of DC motor are interchanged, this action will offer following kind of electrical braking ______________
a) Regenerative
b) Plugging
c) Dynamic braking
d) Depends on other parameters
Answer: b
Explanation: Plugging is electrical braking method, where field or armature connections are reversed technically. But field reversal is not employed as results obtained from field reversal are not good compare to armature reversal.

5. The plugging braking gives the _____________
a) Zero torque braking
b) Smallest torque braking
c) Highest torque braking
d) Variable torque braking
Answer: c
Explanation: In electrical braking called plugging direct reversal of connections is done, which causes maximum torque to act on shaft but in opposite direction. As the speed decreases this torque also starts decreasing.

6. Regenerative method of braking is based on ___________
a) Back emf is less than the applied voltage
b) Back emf is equal to the applied voltage
c) Back emf of rotor is more than the applied voltage
d) Cannot be determined
Answer: c
Explanation: The condition for regeneration is that the rotational emf is more than the applied voltage so that the current is reversed and the mode of operation changes from motoring to generating.

7. During regenerative braking of DC motors ____________
a) Motor will run as a generator
b) Motor will reverse in direction
c) Motor will run at reduced speed
d) Motor will run as free rotating shaft
Answer: a
Explanation: In regenerative method of electrical braking, motor is suddenly forced to act as a generator, all the energy then obtained is pushed back into the supply unlike in dynamic braking this energy is wasted.

8. Where dynamic braking is used?
a) Shunt motors
b) Series motors
c) Compound motors
d) All DC motors
Answer: d
Explanation: Dynamic braking is used in all DC motors though its implantation in series DC motor requires one more additional step of reversal of connections. Only care taken is, addition of braking resistance, armature resistance and series field resistance is lower than the critical resistance at that speed.

9. Which method of braking is generally used in elevators?
a) Plugging
b) Regenerative braking
c) Rheostatic braking
d) Mechanical braking
Answer: a
Explanation: Plugging braking provides maximum torque in opposite direction at the instant of braking, this characteristic of braking suits perfectly with the application that is in elevators. If switch is kept ON, we get reverse rotation also.

10. For which of the following motor dynamic braking is very effective?
a) Shunt motors
b) Separately excited motors
c) Series motors
d) Differential compound motors
Answer: b
Explanation: Dynamic braking is very effective for separately excited DC motors. As in separately excited motors the direction of field can be very easily altered by altering the terminals of the field, which is the condition in dynamic braking.

11. When is the dynamic braking is employed?
a) Non-reversing drive
b) Reversing drive
c) Both Reversing and Non-reversing
d) Cannot tell
Answer: c
Explanation: Dynamic braking is employed to brake both reversing drives and non-reversing drives.
In dynamic braking, the electrical energy generated during stopping action is released as heat through a voltage regulated transistor and resistor.

Up To Date DC Machines MCQs – New DC Motors ( DC Machines ) MCQs