Basics of DC Machine MCQ || DC machine Questions and Answers

1. The direction of rotation of a DC motor can be determined by:

  1. Ampere’s law
  2. Fleming’s left-hand rule
  3. Fleming’s right-hand rule
  4. Lenz’s law

Answer: 2. Fleming’s left-hand rule


Fleming’s left hand is used to find the direction of force or motion acting on the moving coil in the DC motor.

According to  Fleming’s left-hand Rule when a current-carrying conductor is placed in an external magnetic field, the conductor experiences a force perpendicular to both the field and to the direction of the current flow.


2. What will happen if DC shunt motor is connected across AC supply?

  1. Will run at normal speed
  2. Will not run
  3. Will Run at a lower speed
  4. Burn due to heat produced in the field winding

Answer: 2. Will not run


In this type of motor, the field winding is connected in shunt or in parallel with the armature, both being supplied from a common power supply.

In the case of a parallel field connection, it won’t rotate at all since the shunt fields normally have high reluctance so the field current will be drastically reduced, which may not be able to create the requisite torque.

The motor will start humming and will create vibrations, as a torque produced by positive and negative cycles will cancel out each other. DC motor will be heated up and it may burn.

Note:- It depends on whether the field winding of the dc motor is connected in series or in shunt with the armature winding. If it is connected in series, the motor will rotate since the torque, which varies as the product of the armature and field current is always positive. Thus, a positive average torque causes the motor to rotate, however, the pulsating nature may cause the commutator segments and brushes to wear out. Thus only small-sized dc motors may be used with ac supply.


3. Which of the following motor is used where high starting torque and wide speed range control is required?

  1. All motors
  2. Induction motor
  3. Synchronous motor
  4. DC motor

Answer: 4. DC motor


The motor used for high starting torque and wide speed range control requirement is dc motor. In dc motor wide speed range control by using the Ward Leonard speed control method.

The main advantages of the Ward Leonard drive are as follows:-

  • Smooth speed control of DC motor over a wide range in both the direction is possible
  • It has an inherent braking capacity
  • The lagging reactive volt-amperes are compensated by using an overexcited synchronous motor as the drive and thus, the overall power factor improves
  • When the load is intermittent as in rolling mills, the drive motor is an induction motor with a flywheel mounted to smooth out the intermittent loading to a low value


4. The function of a commutator in a D.C. machine is to _____

  1. Prevent sparking
  2. Reduce iron losses
  3. Reduce friction
  4. Change alternating voltage to direct voltage

Answer: 4. Change alternating voltage to direct voltage



  • In the case of the DC generator, the commutator is used to convert generated AC in armature into DC.
  • In the case of the DC motor, the commutator is used to convert DC to A.C.
  • Due to the limitation of the commutator dc generators are not usually designed beyond 650 V
  • The physical connection to the armature winding is made through a commutator-brush arrangement.
  • The function of a commutator in a dc generator is to collect the current generated in armature conductors.
  • While in the case of a dc motor, the commutator helps in providing current to the armature conductors.
  • A commutator consists of a set of copper segments that are insulated from each other.
  • The number of segments is equal to the number of armature coils. Each segment is connected to an armature coil and the commutator is keyed to the shaft.


5. What will happen if the back emf of a DC motor vanishes suddenly?

  1. The motor will stop
  2. The motor will continue to run
  3. The armature may burn
  4. The motor will run noisy

Answer:3. The armature may burn


If the back emf of a dc motor vanishes suddenly, the motor circuit will try to retain the back emf by drawing more current from the supply.

The voltage equation of DC motor is, Eb = V – IaRa

As the back emf vanishes zero, the whole supply voltage appears across armature and heavy current flows.

If supplying unit didn’t trip down by this time, excess current in the armature may heat up the armature and it may cause burning of armature winding.


6. The armature voltage control of DC motor will provide _______

  1. Constant power drive
  2. Constant voltage drive
  3. Constant current drive
  4. Constant torque drive

Answer:4. Constant torque drive


Armature control is a closed-loop system, while field control is an open-loop system. Closed-loop systems are often the preferred choice for operators and business leaders who are looking for stability and the convenience of an automated process. Armature-controlled motors offer nearly unmatched accuracy and control, along with a wide speed variation range.

Additional advantages of armature controlled DC motors include:

  • Constant field current and torque: With the armature control method, the field current and torque levels remain constant throughout the application. Regardless of the speed of the motor.
  • Fast and simple speed variation: Armature-controlled DC motors are known for their exceptional speed control, which allows operators to vary the speed as necessary in both directions.

By changing the armature voltage we can run the 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.


7. A 4-pole, lap-connected, separately excited dc motor is drawing a steady current of 40 A running at 600 RPM. What is the frequency of DC motor?

  1. 50 Hz
  2. 25 Hz
  3. 20 Hz
  4. 10 Hz

Answer:3. 2o Hz


Speed of the Motor

Ns = 120f/P

600 = 120f/4

f = 20 Hz


8. What will happen when the speed of the DC motor increases?

  1. Back emf increases but line current falls.
  2. Back emf falls and line current increase.
  3. Both back emf as well as line current increase.
  4. Both back emf as well as line current fall.

Answer:1. Back emf increases but line current falls.


The speed of the DC motor is directly proportional to the back emf generated inside it and is inversely proportional to the flux produced in the machine.

If the speed of a DC motor increases, there will be an increase in back emf also. When the speed of a DC motor increases, emf will get an increase, and hence the current drawn will get decrease.


9. As there is no back emf at the instant of starting a DC motor, in order to prevent a heavy armature current from flowing through the armature circuit ______

  1. Series resistance is connected with armature
  2. Parallel resistance is connected to the armature
  3. Armature is temporarily open circuited
  4. High-value resistor is connected across the field winding

Answer:1. Series resistance is connected with armature


  • As there is no back emf at the instant of starting a DC motor, in order to prevent a heavy armature current from flowing through the armature circuit a series resistance is connected with the armature.
  • The starter resistance 1s connected n series with the armature resistance to limit the starting current, while the regulator is connected in series with the field winding to control the speed of the motor.
  • The regulator consists of only resistance, but the starter consists of one no-volt release and one overload release in addition to starting resistance. A regulator has two terminals, whereas a starter has three or four terminals.


10. The torque of a D. C. Shunt Motor is directly proportional to

  1. Armature current
  2. Field current
  3. Impressed voltage
  4. Number of poles

Answer:1. Armature current


For a shunt motor, flux ϕ is practically constant

∴ Ta ∝ ϕ Ia

(ii) For a series motor, flux ϕ is directly proportional to armature current Ia provided magnetic saturation does not take place.

∴ Ta ∝ I2

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