DC Motor MCQ Quiz - Objective Question with Answer for DC Motor - Download Free PDF

Last updated on May 13, 2025

Latest DC Motor MCQ Objective Questions

DC Motor Question 1:

What is the relationship between back EMF and the supply voltage in a DC motor?

  1. Back EMF is directly proportional to the supply voltage.
  2. Back EMF is equal to the supply voltage.
  3. Back EMF is independent of the supply voltage. 
  4. Back EMF opposes the supply voltage. 

Answer (Detailed Solution Below)

Option 4 : Back EMF opposes the supply voltage. 

DC Motor Question 1 Detailed Solution

Concept

The circuit of the DC motor is given by:

qImage6817752dcf8c7fa0e48b97c3

\(E_b=V-I_aR_a\)

where, Eb = Back EMF

V = Supply voltage

Ia = Armature current

Ra = Armature resistance

From the above expression, we observed that on increasing the supply voltage, the back EMF decreases.

Hence, we can say that the back EMF opposes the supply voltage.

DC Motor Question 2:

Why is the simple brake test generally used for small motors only?

  1. The simple brake test does not work with large motors.
  2. Large motors have lower efficiency. 
  3. Large motors produce excessive heat that is difficult to dissipate. 
  4. Small motors do not require cooling. 

Answer (Detailed Solution Below)

Option 3 : Large motors produce excessive heat that is difficult to dissipate. 

DC Motor Question 2 Detailed Solution

Concept:

The brake test is a direct method used to determine the output power of a motor by applying a braking force and measuring torque. It is typically used only for small motors because the heat generated during the test is manageable. In large motors, excessive heat is produced during braking, which is difficult to dissipate and may damage the motor or distort results.

Limitation of Brake Test:

For large motors, alternative testing methods like Prony brake or dynamometer-based methods are preferred due to better heat handling and accuracy.

Evaluation of Options:

Option 1: The simple brake test does not work with large motors –  Incorrect
It can technically work, but it's impractical due to heat issues.

Option 2: Large motors have lower efficiency –  Incorrect
Efficiency is not the limiting factor for brake test usage.

Option 3: Large motors produce excessive heat that is difficult to dissipate –  Correct
This is the main reason brake tests are limited to small motors.

Option 4: Small motors do not require cooling –  Incorrect
Even small motors need cooling, but the heat generated during testing is manageable.

DC Motor Question 3:

Universal motors exhibit high starting torque due to _______.

  1. series connection of stator and rotor windings
  2. permanent magnet stators
  3. shunt winding configuration
  4. use of squirrel-cage rotors

Answer (Detailed Solution Below)

Option 1 : series connection of stator and rotor windings

DC Motor Question 3 Detailed Solution

Universal motors

qImage6813465d676c090a671f577b

  • Universal motors are a type of motor that can operate on both AC and DC supplies. They are structurally similar to a DC series motor. In these motors, the stator (field) winding and the rotor (armature) winding are connected in series.
  • This series connection causes the same current to flow through both windings. Since both the magnetic field (produced by the stator) and the armature current reverse simultaneously with AC, the direction of torque remains the same, enabling the motor to run on an AC supply.
  • The key point is that in a series configuration, torque is proportional to the square of the armature current, which results in very high starting torque, making universal motors ideal for appliances like mixers, drills, and vacuum cleaners.

DC Motor Question 4:

In the torque-load characteristic curve of a DC series motor, what happens as the load increases?

  1. The torque becomes constant. 
  2. The torque decreases proportionally
  3. The torque increases proportionally to the square of the armature current.
  4. The armature current decreases.

Answer (Detailed Solution Below)

Option 3 : The torque increases proportionally to the square of the armature current.

DC Motor Question 4 Detailed Solution

Torque-load characteristic curve of a DC series motor

  • In a DC series motor, the torque (T)  is approximately proportional to the square of the armature current (Ia) when the magnetic circuit is unsaturated, i.e. (T α Ia2)
  • As load increases, the motor slows down, increasing the armature current.
  • Since the field winding is in series with the armature, an increase in current also strengthens the magnetic field.
  • This results in a rapid increase in torque, making the motor well-suited for applications requiring high starting torque (e.g., electric trains, cranes).
  • At very low or no load, the motor can overspeed dangerously because the torque is low and there's insufficient counterforce — this is why DC series motors must never run without load.

DC Motor Question 5:

It is intended to measure the resistance of the series field winding of a DC series motor accurately. Which is the most suitable method?

  1. Kelvin double bridge method
  2. Fall of potential method
  3. Wheatstone bridge method
  4. Substitution method

Answer (Detailed Solution Below)

Option 1 : Kelvin double bridge method

DC Motor Question 5 Detailed Solution

The most suitable method to measure the resistance of the series field winding of a DC series motor is the Kelvin double bridge method.

Kelvin Double Bridge

  • The series field winding of a DC motor has low resistance, typically in the milliohm range.
  • The Kelvin double bridge is specifically designed to measure low resistances accurately.

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Measurement of resistance, inductance, and capacitance

 

Measurement of resistance

High resistance

Megger circuit, Loss of charge method

Medium resistance

Substitution method, Ohmmeter, Wheatstone bridge

Low resistance

Kelvin double bridge, Potentiometer, Ammeter-Voltmeter method

 

Measurement of inductance

High Q value

Hay’s bridge

Medium Q value

Maxwell Inductance Capacitance bridge

Low Q value

Owen’s bridge, Anderson bridge

 

Measurement of capacitance

Lossy Capacitor

Schering bridge

Lossless Capacitor

De-Sauty’s bridge

Measurement of frequency

 

Wien Bridge

Top DC Motor MCQ Objective Questions

F1 U.B Madhu 29.01.20 D11

The three characteristics shown in the given graphs, represent which of the following motors?

  1. Three-phase induction motor
  2. Three-phase synchronous motor
  3. DC series motor
  4. DC shunt motor

Answer (Detailed Solution Below)

Option 3 : DC series motor

DC Motor Question 6 Detailed Solution

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In a DC motor, T ∝ ϕIa

In series motor, ϕ ∝ Ia

⇒ T ∝ (Ia)2

In shunt motor, ϕ is constant

⇒ T ∝ Ia

The characteristic of DC series and shunt motor are shown below.

Characteristics of DC series motor:

SSC JE Electrical 77 15Q Full Test 2 Part4 Electrical Machines Part1 Hindi images q7a

Characteristics of DC shunt motor:

SSC JE Electrical 77 15Q Full Test 2 Part4 Electrical Machines Part1 Hindi images q7b

In case of dc shunt motors, the regenerative braking is employed when the load _________.

  1. has an overhauling characteristic
  2. is variable
  3. is constant
  4. also acts as braking force

Answer (Detailed Solution Below)

Option 1 : has an overhauling characteristic

DC Motor Question 7 Detailed Solution

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Regenerative braking: In this type braking back emf Eb is greater than the supply voltage V, which reverses the direction of the motor armature current. The motor begins to operate as an electric generator.

Overhauling motor: A motoring motor is converting electrical energy into mechanical energy. An overhauling motor is being driven by the load and is converting mechanical energy into electricity, acting as a generator.

In case of regenerative braking, dc shunt motor acts a generator and hence regenerative braking is employed when the load has an overhauling characteristic.

Which of the following motor has the poorest speed regulation?

  1. Shunt motor
  2. Series motor
  3. Differential compound motor
  4. Cumulative compound motor

Answer (Detailed Solution Below)

Option 2 : Series motor

DC Motor Question 8 Detailed Solution

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The speed regulation of a DC motor is defined as the change in speed from no load to full load. It is expressed as a fraction or a percentage of the full load speed.

Percentage speed regulation \(= \frac{{{N_{nl}} - {N_{fl}}}}{{{N_{fl}}}} \times 100\)

No load speed of a DC series motor is very high. So, it has poorest speed regulation.

When the direction of power flow reverses, a differentially compounded motor becomes

  1. Differentially compounded generator
  2. A shunt generator
  3. Cumulatively compounded generator
  4. A series generator

Answer (Detailed Solution Below)

Option 3 : Cumulatively compounded generator

DC Motor Question 9 Detailed Solution

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  • The direction of rotation of a DC compound motor may be conveniently reversed by reversing the connection of both series and shunt field winding or may be reversing the armature connection but not both at the same time
  • When the direction of power flow reverses, a differentially compounded motor becomes a

           cumulatively compounded generator

  • When the direction of power flow reverses, a cumulatively compounded motor becomes a

           differentially compounded generator

A DC shunt machine develops an AC EMF of 250 V at 1500 rpm. Find the torque developed for an armature current of 50 A.

  1. 59.6 N-m
  2. 79.6 N-m
  3. 69.6 N-m
  4. 49.6 N-m

Answer (Detailed Solution Below)

Option 2 : 79.6 N-m

DC Motor Question 10 Detailed Solution

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Concept:

Torque:

  • When armature conductors of a DC motor carry current in the presence of stator field flux, a mechanical torque is developed between the armature and the stator.
  •  Torque is given by the product of the force and the radius at which this force acts.

Power:

  • Mechanical power developed by the motor is given by Pm = Eb × Ia
  • Eb is EMF developed and Ia is armature current
  • In terms of torque and speed mechanical power developed is given by Pm = torque × speed.
  • Speed is in radians per second (ω).
  • \({\rm{\omega }} = 2\;\pi \frac{N}{{60}}\) radians per sec.

Calculations:

\({\rm{\omega }} = \frac{{2\; \times \;{\rm{\pi }}\; \times \;1500}}{{60}}\)

= 157.08 radians/sec.

Power = generated emf × armature current

= 250 × 50

= 12500 watts.

Torque = power/speed.

Torque = \(\frac{{12500}}{{157.08}}\)

Torque = 79.577 Nm.

What will happen if the back emf of a D.C. 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 (Detailed Solution Below)

Option 3 : The armature may burn

DC Motor Question 11 Detailed Solution

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If back emf of a dc motor vanishes suddenly, motor circuit will try to retain back emf by drawing more current from 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 armature may heat up the armature and it may cause burning of armature winding.

Which of the following motor is suitable for driving cranes, hoists, traction, large lifts, air compressors....etc.?

  1. DC series motor
  2. DC shunt motor
  3. DC compound wound motor
  4. Any of these

Answer (Detailed Solution Below)

Option 1 : DC series motor

DC Motor Question 12 Detailed Solution

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DC series motor:

It has a very high starting torque. Hence it is used for heavy-duty applications such as traction, driving cranes, hoists, large lifts, air compressors......etc.

DC shunt motor:

It is almost a constant speed motor. Hence it is used for driving constant speed like shafts, lathes, centrifugal pumps, small printing presses, paper making machines, blowers, conveyor belts,.....etc.

DC cumulative compound motor:

It is used for moderately high starting torque applications with intermittent loadings.

Ex:- Rolling mills, escalators, elevators, punching machines, cutting tools, planar machines.....etc.

A 200 V d.c machine has Ra = 0.5 Ω and its full-load la = 20 A. Determine the induced e.m.f when the machine acts as a motor.

  1. 210 V
  2. 190 V
  3. 200 V
  4. 215 V

Answer (Detailed Solution Below)

Option 2 : 190 V

DC Motor Question 13 Detailed Solution

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Concept:

Voltage equation for dc machine working as the motor is

Vt = Eb + IaRa

Where,

Vt is the terminal voltage

Eb is induced emf or back emf

Ia is armature current

Ra is armature resistance

Calculation:

Given that,

Vt = 200 V

Ia = 20 A

Ra = 0.5 Ω

Therefore, induced emf in the dc motor is

200 = Eb + (20 × 0.5)

Eb = 200 – 10 = 190 V

The torque developed by a motor while running at 1500 rpm is 206 N-m, and the shaft torque available is 200 N-m. The iron and mechanical losses, in watts, are:

  1. 300 π
  2. 250 π
  3. 150 π
  4. 500 π

Answer (Detailed Solution Below)

Option 1 : 300 π

DC Motor Question 14 Detailed Solution

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Power flow in the motor

F1 Vinanti Engineering 26.12.22 D10

Input power - Armature loss = Developed power

Developed power - (Iron + Mechanical losses) = Shaft power

The relationship between torque and power is:

\(τ= P / ω\)

where, τ = Torque

P = Power

ω = Speed in rad/sec

Calculation

Given, τd = 206 Nm

τshaft = 200 Nm

Iron and mechanical loss = Pd - Pshaft

Iron and mechanical loss = \((\tau_{d}-\tau_{shaft})× \omega_r\)

\(\omega_r={2π N_s\over 60}\)

\(\omega_s={2× π× 1500\over 60}=157\space rad/s=50π \space rad/s\)

Iron and mechanical loss = (206 - 200) × 50 × π  

Iron and mechanical loss = 300 π

Which part of the motor confirms that it is a DC motor?

  1. Frame
  2. Commutator
  3. Shaft
  4. Stator

Answer (Detailed Solution Below)

Option 2 : Commutator

DC Motor Question 15 Detailed Solution

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  • Based on construction, the basic difference between DC & AC motor is the commutator.
  • The commutator is only present in DC motor, which performs the commutation phenomenon.
  • Commutation can be defined as the reversal of current (AC to DC or DC to AC).
  • In DC motor commutator converts DC to AC(Inverter function). 
  • In DC generator commutator converts AC to DC (Rectifier function )
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