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

Concept

The circuit of the DC motor is given by:

\(E_b=V-I_aR_a\)

where, E_b = Back EMF

V = Supply voltage

I_a = Armature current

R_a = 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.

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.

Universal motors

• 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.

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 (I_a) when the magnetic circuit is unsaturated, i.e. (T α I_a²)
• 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.

Explanation:

Shunt Motor Current Calculation

Definition: A shunt motor is a type of DC motor where the field windings are connected in parallel (or shunt) with the armature windings. It is widely used for applications requiring constant speed under varying load conditions.

Given Data:

• Motor speed = 1500 r/min
• Source voltage (V_s) = 120 V
• Line current (I_L) = 51 A
• Shunt field resistance (R_sh) = 120 Ω
• Armature resistance (R_a) = 0.1 Ω

Objective: To calculate the armature current (I_a).

Solution:

In a shunt motor, the line current (I_L) is the sum of the armature current (I_a) and the shunt field current (I_sh).

Step 1: Calculate the shunt field current (I_sh)

The shunt field current is given by Ohm's law:

I_sh = V_s / R_sh

Substitute the values:

I_sh = 120 V / 120 Ω

I_sh = 1 A

Step 2: Calculate the armature current (I_a)

The armature current is given by:

I_a = I_L - I_sh

Substitute the values:

I_a = 51 A - 1 A

I_a = 50 A

Conclusion:

The armature current in the motor is 50 A.

Additional Information

To further analyze the options, let’s evaluate why the other choices are incorrect:

Option Analysis:

Option 1: 1 A

This option is incorrect because 1 A is the shunt field current (I_sh), not the armature current (I_a). The armature current is calculated by subtracting the shunt field current from the line current, resulting in 50 A.

Option 2: 51 A

This option is incorrect because 51 A is the line current (I_L), which is the sum of the armature current and the shunt field current. The armature current alone is 50 A.

Option 3: 50 A

This is the correct answer. The armature current is calculated as the difference between the line current and the shunt field current, which is 50 A.

Option 4: 12 A

This option is incorrect and does not correspond to any calculation or parameter in this problem. It might be a distractor or an arbitrary value.

Conclusion:

In a shunt motor, the line current is the sum of the armature current and the shunt field current. By understanding this relationship and applying Ohm's law, the armature current is determined to be 50 A, making Option 3 the correct choice.

In a DC motor, T ∝ ϕI_a

In series motor, ϕ ∝ I_a

⇒ T ∝ (I_a)²

In shunt motor, ϕ is constant

⇒ T ∝ I_a

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

Characteristics of DC series motor:

Characteristics of DC shunt motor:

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.

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

• 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

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 P_m = E_b × I_a
• E_b is EMF developed and I_a is armature current
• In terms of torque and speed mechanical power developed is given by P_m = 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.

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, E_b = V – I_aR_a

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

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.

Concept:

Voltage equation for dc machine working as the motor is

V_t = E_b + I_aR_a

Where,

V_t is the terminal voltage

E_b is induced emf or back emf

I_a is armature current

R_a is armature resistance

Calculation:

Given that,

V_t = 200 V

I_a = 20 A

R_a = 0.5 Ω

Therefore, induced emf in the dc motor is

200 = E_b + (20 × 0.5)

⇒ E_b = 200 – 10 = 190 V

Power flow in the motor

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 = P_d - 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

• 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 )