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

Answer: 1. Ward Leonard control

Explanation: 

• The Ward Leonard system has the ability to “regenerate,” that is, to return stored energy in the machine to the supply lines.
• For example, when a heavy load is first lifted and then lowered by the dc drive motor of a Ward Leonard system, the dc motor starts to act as a generator as the load is coming down, using its counter-torque as a brake.
• Under these conditions, the “generator” itself starts to operate in the motor mode, driving the synchronous machine as a generator and supplying power back to the ac system. This process is known as regeneration.

Answer: 1. 4.8 Ω

Explanation:

Given

V = 220 V

N₁ = 1000 rpm

I_a = 17.5 A

N₂ = 600rpm

R_a = 0.4 ohm

Eb₁ = V – Ia₁Ra

= 220 – 17.5 × 0.4

Eb₁ = 213 V

With constant flux,

E_b ∝ N

N2/N1 = E_B2/E_B1

E_B2 = 600 × 213/100

EB₂ = 127.8 V

To calculate External resistance value,

E_B2 = V− Ia(R + R V

127.8 = 200 − 17.5(R + 0.4)

R = 4.8Ω

Answer: 1. In both directions

Explanation: 

Advantages of the Ward-Leonard system:

• 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.
• No special starting gear is required. As the generator-induced voltage is gradually raised from zero, the motor starts up smoothly.
• The motor can be brought to a standstill quickly, simply by rapidly reducing the voltage of the generator
• It has inherent regenerative braking capacity.

Answer: 1. 275 RPM

Explanation: 

Given,

Output power P = VI_L = 100 kW

Terminal voltage (V) = 200 V

⇒ 200 × I_L = 100000

Load current (I_L) = 500 A

Shunt current (I_sh) = V / R_sh = 200/50 = 4 A

Brush drop voltage V_b = 2 V

Armature current (I_a1) = I_L + I_sh = 504 A

Speed (N₁) = 300 RPM

Armature resistance (R = 0.025 Ω

Generated emf, E_g = V + I_a1R_a + V_b

Eg = 200 + 504 × 0.025 + 2

Eg = 214.6 V

The machine acting as a motor: When the belt broke it behaves as a motor

Input power P = VIL = 10 kW

Terminal voltage (V) = 200 V

⇒ 200 × I_L = 10000

Load current (I_L) = 50 A

Armature current (I_a2) = I_L – I_sh = 46 A

Speed of motor = N₂

Back emf, E_b = V – I_a2R_a – V_b

Eb = 200 – 46 × 0.025 – 2

Eb = 196.85 V

Eg/Eb = N1/N2

214.6/196.85 = 300/N₂

N₂ = 275 RPM

Answer: 1. Residual magnetism of the generator

Explanation: 

In the ward Leonard method when the field excitation is controlled (i.e. capable of supplying current in one direction only), the minimum speed that can be obtained is 10% of the nominal speed. This is due to the presence of residual magnetism in the field circuit of the generator field. Thus, a lower limit of speed is imposed by the residual magnetism of the generator.

Due to the presence of residual magnetism, even with no field excitation enough voltage is generated to create crawling in the machine, especially at light load. To prevent crawling the following methods are adopted:

(i) The armature circuit is opened.

(ii) A differential field winding in the generator is connected across the armature terminals.

This field will oppose the residual flux and thereby reducing crawling.

Answer: 4. High cost, high maintenance and low efficiency

Explanation:

Drawbacks of Ward Leonard system:

1. Higher initial cost due to use of two additional machines (M-G set) of the same rating as the main dc motor.
2. Larger size and weight.
3. Require more floor area and costly foundation.
4. Lower efficiency due to higher losses.
5. Frequent maintenance is needed.

Since the efficiency, is essentially the product of the individual efficiencies of the two larger machines, the efficiency of this method is not as high as the rheostat speed control or field speed control method.

Answer: 4. 1.12

Explanation: 

Given armature resistance R_a = 0.1 Ω

Terminal voltage V_t = 200 V

Power delivered to load P = 4 kW

Armature current I_a = 4000 / 200 = 20 A

⇒ E_g = V_t + I_a R_a

Eg = 200 + 20 × 0.1

E₁ = Eg = 202 V

Given armature current I_a = 20 A

⇒ E_b = V_t – I_a R_a

Eb= 200 – 20 × 0.1

E₂ = Eb= 198 V

Concedering the flux per pole ϕ₂ = 1.1 ϕ₁ = 1.1ϕ Wb

Speed of the generator = N₂ rpm

The emf equation of the dc machine is given as

E = (ϕ Z N P) / (60

⇒ E ∝ ϕ N

$\frac{{{N_1}}}{{{N_2}}} = \frac{{{\phi _2}}}{{{\phi _1}}} \times \frac{{{E_1}}}{{{E_2}}}$

$\frac{{{N_1}}}{{{N_2}}} = \frac{{1.1\phi }}{\phi } \times \frac{{202}}{{198}}$

⇒ N₁ / N₂ = 1.12

Answer: 1. Easy and smooth speed control below normal is possible.

Explanation: 

In the rheostatic control method, a variable resistance (rheostat) is connected in series with the armature winding. This reduces the voltage across the armature and hence speed falls. Below the normal speed (not above) can be obtained.

Advantages Of rheostatic control Method

• Easy and smooth speed control below normal speed is possible.
• This is not so expensive method.

Disadvantages Of rheostatic control Method

1. A large amount of power is wasted in the controller resistance since it carries full armature current.
2. Speed above rated speed is not possible by this method.
3. Due to large power losses, the method is expensive and Wasted.
4. The output and efficiency of the motor are reduced.
5. This method results in poor speed regulation.

Answer: 2. Below rated speed

Explanation: 

For speed control of DC shunt motors using controlled rectifiers, armature voltage control gives Below rated speed

• In this method, the armature voltage is changed keeping the field flux constant.
• It is most economical when it is done at constant torque where the armature current remains constant and the motor output power varies linearly with the speed.
• In this method, the speed control is achieved in the downward direction from the base speed as the voltage applied to the armature is reduced from the rated value.
• If the motor is operated at a voltage higher than the rated value, the insulation of the armature winding may get damaged.
• Hence, the speed control in the upward direction from the base speed by increasing the armature voltage above its rated value is usually not obtained.

Answer: 1. Series motor

Explanation: 

• Series motor has poorest speed control i.e. with small variation in armature current there is a huge variation in speed.
• When the motor is in no-load condition, it runs at high RPM with little torque.
• As the motor is loaded, current increases in both windings. Increasing current in the stator increases torque while increasing back EMF which allows the motor to slow down. At max torque, RPM is minimum and current draw is high.

Answer: 2. In both direction

Explanation: 

Ward-Leonard method of speed control is achieved by varying the applied voltage to the armature. Hence it is basically a voltage control method.

Advantages of the Ward Leonard drive-

• 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

Answer: 3. Armature voltage

Explanation: 

Speed control of dc motors can also be obtained by varying the applied voltage to the armature. Ward Leonard’s system of speed control is based on this principle. In this method, the adjustable voltage for the armature is obtained from an adjustable voltage generator while the field circuit is supplied from a separate source.

Ward-Leonard method of speed control is achieved by varying the applied voltage to the armature. Hence it is basically a voltage control method. 

Answer: 4. All of the above

Explanation: 

In the rheostatic control method, a variable resistance (rheostat) is connected in series with the armature winding. This reduces the voltage across the armature and hence speed falls. Below the normal speed (not above) can be obtained.

Advantages Of rheostatic control Method

• Easy and smooth speed control below normal speed is possible.
• This is not so expensive method.

Disadvantages Of rheostatic control Method

1. A large amount of power is wasted in the controller resistance since it carries full armature current.
2. Speed above rated speed is not possible by this method.
3. Due to large power losses, the method is expensive and Wasted.
4. The output and efficiency of the motor are reduced.
5. This method results in poor speed regulation.

Answer: 4. All of the above

Explanation:

Advantages of field flux control method

• It provides relatively smooth and easy control.
• This is an easy and convenient method.
• Speed control above rated speed is possible.
• The speed control exercised by this method is independent of the load on the machine.
• It is an inexpensive method since very little power is wasted in the shunt field regulator due to the relatively small value of shunt current.

Answer: 4. All of the above

Explanation:

The ward-Leonard method is the basic adjustable armature voltage control method of controlling the speed of a DC shunt motor over the whole range from zero to normal speed in both directions, accomplished by means of an adjustable voltage generator.

Ward-Leonard method of speed control is employed where fine speed adjustment and smooth acceleration are required, as in driving rolling mills, ship propulsion, colliery winders, and elevators in both directions.