# DC Motor Speed Control MCQ || DC Motor Speed Control Questions and Answers

1. The PWM control of DC motor varies _________

1. Linearly with speed
2. Inversely with speed
3. Parabolically with speed
4. Exponentially with speed

Explanation:

• PWM (Pulse Width Modulation)  is often used for DC motor control using a microcontroller is Pulse Width Modulation (PWM) method. The speed of the electric motor depends on the modulator voltage. The greater the voltage, the faster the rotation of an electric motor.
• The method is to apply a pulse train to the power terminals of the motor. The average voltage obtained at the terminals is then proportional to the duty cycle of the pulse train, which is proportional to the speed of rotation (rpm) of the motor.
• Thus, as the duty cycle is increased, the motor rpm increases and vice versa. When the power supply is constant, it runs at 100% of its power rating (at no load.)
• As the duty cycle reduces, the speed and the power reduce. In the PWM technique duty ratio is a linear function with respect to speed.

2. In which of the following method, the effect of armature reaction is more?

1. Field weakening method
2. Armature resistance control
3. Same in both methods
4. Cannot be determined

Explanation:

Field control method:

By adding external resistance in the field circuit, we can control the speed of the DC shunt motor above the normal speed.

• In this method, speed variation is accomplished by means of a variable resistance inserted in series with the shunt field.
• An increase in controlling resistances reduces the field current with a reduction in flux and an increase in speed. Therefore, the effect of armature flux on main field flux will increase in the case of the field weakening method.
• This method of speed control is independent of the load on the motor. Power wasted in controlling resistance is very less as field current is a small value.
• In the field weakening method, we are reducing the working flux to increase the speed, by reducing the field current.

We know that,

N ∝ Eb

N ∝ (V − Ia R/φ

ϕ ↓  → N ↑

By varying flux, we can increase the speed more than its base speed. This method is a constant power and variable torque drive.

3. Ward-Leonard system of speed control is not recommended for _______

1. Constant speed operation
2. Wide speed
3. Frequent-motor reversed
4. Very slow speed

Explanation:

Ward Leonard’s method of speed control is used for controlling the speed of a DC motor.

• Smooth speed control of dc motor over a wide range in both directions is possible.
• This method is used for the speed control of large motors.
• The speed can be controlled in both the direction of rotation of the motor easily.
• The motor can be brought to a standstill quickly, simply by rapidly reducing the voltage of the generator
• It has inherent regenerative braking capacity.

Note:- Since Ward Leonard’s method is used for a wide range of speed control therefore it is not practically used in constant speed drive.

4. The most economical method of electrical braking is:

1. Plugging
2. Dynamic braking with self-excitation
3. Regenerative braking
4. Dynamic braking with separate excitation

Explanation:

The most economical method of electrical braking is regenerative braking. Regenerative braking is mostly employed in electric traction. The kinetic energy of the motor is converted into electrical energy, which is fed back to the supply lines instead of wasting it in a rheostat. This method can be applied to DC motors and induction motors.

5. In Ward Leonard speed control method for lowering the speed of the motor ______

1. Reduce armature voltage
2. Increase armature voltage
3. Increase field current
4. Decrease field current

Explanation:

Ward Leonard’s method of speed control is used for controlling the speed of a DC motor. It is a basic armature control method. The speed control of the D.C. motor can be obtained by varying the applied voltage to the armature.

At the base speed, the motor armature is fed at rated voltage and its field current is adjusted to the maximum value, i.e. the field is excited at rated voltage. Reducing the armature voltage provides a constant- torque speed control where the speed can be reduced below the base value.

6. A 50 kW, DC shunt motor is loaded to draw rated armature current at any given speed. When driven at 1.5 times the rated speed by field control, the output power delivered by the motor is approximately

1. 75 kW
2. 50 kW
3. 100 kW
4. 33 kW

Explanation:

The field control method is constant power and variable torque drive. The given shunt motor is driven at 1.5 times the base speed or rated speed by using field control.

In this condition, it behaves as constant power & variable torque drive. Hence the output power delivered by the motor is 50 kW.

7. Reducing the armature voltage will give us ______

1. Variable torque speed control
2. Constant torque speed control
3. Variable and constant both can be achieved
4. Cannot comment on torque

Answer: 2. Constant torque speed control

Explanation:

Ward Leonard’s method of speed control is used for controlling the speed of a DC motor. It is a basic armature control method. The speed control of the D.C. motor can be obtained by varying the applied voltage to the armature.

At the base speed, the motor armature is fed at rated voltage and its field current is adjusted to the maximum value, i.e. the field is excited at rated voltage. Reducing the armature voltage provides a constant- torque speed control where the speed can be reduced below the base value.

8. A 200-V Dc generator supplies 4 kW at a terminal voltage of 200 V, the armature resistance being 0.5 Ω. If the machine is operated as a motor at the same terminal voltage with the same armature current, find the ratio of the generator speed Ng, to the motor speed Nm.

1. Ng/Nm = 1.25
2. Ng/Nm = 1.105
3. Ng/Nm = 0.905
4. Ng/Nm = 0.833

Explanation:

Given-

V = 200

Ra = 0.5 Ω

Ia = constant

ϕ2 = ϕ1

Let ϕ1 = Generator flux

ϕ2 = Motor flux

Ng = Generator speed

Nm = Motor speed

In a dc generator, induced emf is

Eg = V + IaRa

Eg=200 + 20 × 0.5=210 V

In a dc motor, the back emf is

Eb = V – IaRa

Eb = 200 − 20 × 0.5 = 190

$\frac{{{N_m}}}{{{N_g}}} = \frac{{{E_b}}}{{{E_g}}} \times \frac{{{\phi _1}}}{{{\phi _2}}} = \frac{{190}}{{210}} = 0.904$

Ng/Nm = 1.105

9. In Ward Leonard speed control method for increasing the speed of the motor ______

1. Reduce armature voltage
2. Increase armature voltage
3. Increase field current
4. Decrease field current

Explanation:

Ward Leonard’s method of speed control is used for controlling the speed of a DC motor. It is a basic armature control method. The speed control of the D.C. motor can be obtained by varying the applied voltage to the armature. For obtaining speeds above base speed the field is gradually weakened maintaining armature voltage at rated value.

10. The drawbacks of the ‘Armature Resistance Control’ method of speed control of a dc motor are

1. High Power Loss
2. Speed Above base speed not possible
3. Speed Reduction is not possible
4. All of the above

Explanation:

In the Armature series resistance control method a variable resistance is connected in series with the armature or motor. If the load and torque developed by the machine is constant, the speed of the motor depends upon the back emf.

Drawbacks of armature resistance control method:

• A large amount of power is wasted in the external resistance R and hence less efficient.
• It is restricted to keep the speed below the normal speed of the motor and an increase in the speed above normal level is not possible.
• For a given value of variable resistance, the speed reduction is not constant but varies with the motor load.
• This speed control method is used only for small motors.

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