1. Which test is used to determine the efficiency of a traction motor?

Field’s test

Hopkinson’s test

Retardation test

Swinburne’s test

Answer: 1. Field test

Explanation:

Small series motors can be tested by the brake test, while large series motors cannot be tested by this method. Field’s test is applicable to two similar series machines which are used for traction purposes.

Field test for series motor:

This test is applicable to two similar series motors.

One of the machines is run as a generator while the other as a motor.

2. A 460 V series motor runs at 500 rpm taking a current of 40 A. Calculate the percentage change in torque if the load is changed so that the motor takes of 30 A.

(Negative sign indicates that the torque is reduced from the original value.)

3. Which of the following are the advantages of the Field Test of DC Motor?

Easy determination of stray losses

Uniform Load Distribution

Same Iron losses

All of the above

Answer: 4. All of the above

Explanation:

Advantages of Field Test in DC Motor

Since the excitation and speed of the two machines are the same, determination of stray losses is easy.

The generator armature is connected to load resistance R directly without any switch so that the load is not thrown off accidentally.

Since the motor and generator fields are in series, the iron losses in both machines are the same.

4. Which of the following are the disadvantages of DC motor Field test.

Bulky Setup

High Energy Consumption

Both 1 and 2

None of the above

Answer: 2. Fleming’s left-hand rule

Explanation:

Disadvantages of Field Test in DC Motor

The setup is bulky, which requires a lot of space.

Since two machines are used, there will be an increase in the consumption of energy.

The electrical output from the generator is dissipated through load resistance in the form of heat. In the case of Hopkinson’s Test, it is used as feedback to the motor.

5. In a DC series motor, the ratio of back emf (Eb) to supply voltage (V) indicates:

Running torque

Efficiency

Speed regulation

Starting torque

Answer: 2. Efficiency

Explanation:

Electrical motor efficiency is defined that the ratio of mechanical power developed to the total electrical power input.

In the case of DC motor, total electrical power input = VI_{L}

Where V is the supply voltage and I_{L} is the load current.

Mechanical power developed = E_{b}I_{a}

Where E_{b} is back emf and I_{a} is armature current.

So, efficiency is given by = (E_{b}I_{a})/VI_{L}

In dc series motor, armature current = load current.

Efficiency = Eb/V

Therefore, the ratio of back emf (E_{b}) to supply voltage (V) indicates efficiency.

6. What is the relationship between the speed (N) and armature current (Ia) in the case of a dc-series motor?

N ∝ 1/I_{a}

N ∝ √(I_{a})

N ∝ I^{2}_{a}

N = I_{a}

Answer: 1. N ∝ 1/Ia

Explanation:

DC series motor has high starting torque.

DC Series motor speed-torque characteristics are such that as torque increases speed decreases.

DC series motor has develops high torque at low speeds, low torque at high speed, this is the basic requirement of the traction unit.

In DC series motor, the current passing through the field winding is the same as that in the armature.

If the mechanical load on the motor increases, the armature current also increases. Hence, the flux in a series motor increases with the increase in armature current and vice-versa.

(i) Speed (N) vs Current (I_{a}) Characteristic:

The speed of the series motor is given by,

N ∝ E_{b}/φ

Where E_{b} is back EMF & given by,

E_{b} = V – I_{a}(R_{a} + R_{se})

When the armature current increases, the back EMF decreases due to I_{a}(R_{a} + R_{se}) drop while the flux increases.

However, Ia(Ra + Rse) drop is quite small under normal conditions and may be neglected.

N ∝ 1/φ ∝ 1 / Ia

7. For a DC series motor, the torque is ________ to the speed.

Directly Proportional

Inversely Proportional

Equal to

Twice

Answer: 2. Inversely Proportional

Explanation:

From the speed equation of DC series motor, the speed is directly proportional to the back emf and is inversely proportional to flux:

N ∝ E_{b}/φ

Since in DC series Motor the torque is directly proportional to the flux, therefore, the torque is inversely proportional to the speed. The torque sped curve is, therefore, a rectangular parabola.

8. A ________ motor runs at the highest speed when the load is removed.

Cumulatively compound

Series

Shunt

Differentially compound

Answer: 2. Series

Explanation:

From the characteristics of the DC series motor is clear that the torque is inversely proportional to the speed.

At less load torque the speed of the motor is high and vice versa.

Hence DC series motor will achieve the highest speed when the load becomes removed.

9. A d.c. series motor has linear magnetization characteristics and negligible armature resistance. The motor speed is

directly proportional to √T

inversely proportional to √T

directly proportional to T

inversely proportional to T

Answer: 2. inversely proportional to √T

Explanation:

In a DC series machine, both field and armature winding is connected in series.

Therefore, field current will be equal to armature current.

Torque produced ∝ I_{f} × I_{a}

i.e., T ∝ ϕ . I_{a}

∵ ϕ ∝ I_{a} (in DC series motor)

∴ T ∝ I^{2}_{a}

or, √T ∝ I_{a}

we also know that,

N ∝ 1/φ ∝ 1/Ia ∝ 1/√T

N ∝ 1/√T

Therefore, the curve will be a rectangular hyperbola.

10. What happens if an ordinary DC series motor is connected to an AC supply?

The motor is at standstill condition.

It will rotate and exert unidirectional torque.

It will oscillate and exert unidirectional torque

It will rotate and exert bidirectional torque

Answer: 2. It will rotate and exert unidirectional torque.

Explanation:

An AC supply will exert unidirectional torque because the direction of armature current and field current reverses at the same time and the direction of rotation will as it is.

Due to the presence of alternating current, eddy currents are induced in the yoke and field cores which causes excessive heating of the yoke and field cores.

The power factor becomes low because of the high inductance formed by the field and armature circuit.

There is sparking at the brushes of the DC series motor