Electrical Engineering Test

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Electrical Engineering Test - 2

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Question 1
2 / -0.66

The current and potential coil of a dynamometer type wattmeter were accidentally interchanged while connecting. After energizing the circuit, it was observed that the wattmeter did not show the reading. This could be due to the

A
Damage to potential coil

B
Damage to current coil

C
Damage to both the potential and current coil

D
Loose contacts

Solution

Electrodynamometer instrument has two fixed coils and one moving coil. For using this instrument as a wattmeter to measure the power, the fixed coils acts as a current coil and must be connected in series with a load. The moving coil acts as a pressure coil and must be connected across the supply terminals.

In the question given that, the current and potential coils of a dynamometer type wattmeter were accidentally interchanged while connecting. Hence total voltage applies across current coil which has very less resistance which leads to damage the current coil.
Question 2
2 / -0.66

In a 132 kV system, Phase to Ground capacitance is 0.01 μF and inductance is 4H. Calculate the critical resistance to be connected in order to eliminate restriking if a magnetizing current of 5 A is interrupted by the circuit

A
20 kΩ

B
10 kΩ

C
100 kΩ

D
200 kΩ
Solution
Concept:

The circuit diagram for the resistance switching of the circuit breaker is shown below.
- Resistance Switching in Circuit Breaker refers to a method adopted for dampening the over-voltage transients due to current chopping, capacitive current breaking, etc.
- In this method, a shunt resistance is connected across the contacts of the circuit breaker.
- The main role of shunt resistance R is to limit the growth of re-strike voltage and cause it to grow exponentially up to recovery voltage.
- R is so selected that the circuit becomes critically damped then re-strike voltage rises exponentially till recovery voltage is reached.
Question 3
2 / -0.66

In the figure shown below, the duty ratio of the buck converter is α and the duty ratio of the boost converter is β. The input voltage of the buck converter is 15 V and the output voltage of the boost converter is 45 V. Which of the following equation satisfy the correct relationship between is α and β?

A
3α + β = 3

B
α + 3β = 3

C
α = 3β

D
3α = β

Solution

Concept:

In a buck converter (step down chopper),

V₀ = α V_s

In a boost converter (step-up chopper),
Question 4
2 / -0.66

A
Removable singularity

B
Essential singularity

C
Pole of order 3

D
Pole of order 4

Solution
Question 5
2 / -0.66

In the system shown in the figure, r(t) = sin ωt

The steady-state response c(t) will exhibit a resonance peak at a frequency of:

A
4 rad/sec

B
2√2 rad/sec

C
2 rad/sec

D
√2 rad/sec

Solution

Concept:

The closed-loop transfer function for a standard 2^nd order system is defined as

The Resonant Peak is one of the frequency domain specifications for a 2^nd order system, which is defined at the peak (maximum) value of the magnitude of the CLTF and is calculated at resonant frequency ω_r, given by
Question 6
2 / -0.66

In a double cage and deep bar induction machine which effect/rule controls the rotor resistance & inductance?

A
Hysteresis effect

B
Skin effect

C
Fleming rule

D
None of the options
Solution
Double squirrel-cage induction motor:
- A double squirrel-cage induction motor has two parallel windings in the rotor
- The rotor of a double squirrel cage motor has two independent cages on the same rotor
- The figure below shows the cross-sectional diagram of a double squirrel cage rotor
- Bars of high resistance and low reactance are placed in the outer cage, and bars of low resistance and high reactance are placed in the inner cage
- The outer cage has a high ‘resistance to reactance ratio' whereas, the inner cage has a low 'resistance to reactance ratio'
- The outer cage has a smaller cross-sectional area than the lower cage.
Explanation:

Skin effect is the tendency of an alternating electric current (AC) to become distributed within a conductor such that the current density is largest near the surface of the conductor, and decreases with greater depths in the conductor.

In a double cage and deep bar induction machine, the skin effect controls the rotor resistance and inductance.

Important Point
- Higher starting torque
- Outer cage increases CU loss due to high resistance
- The inner cage reduces full load power factor
- Efficiency reduces
- Cost is higher
Application:
- Rolling Mills
- Cranes
- Compressors
- Conveyors
- Reciprocating pumps
Question 7
2 / -0.66

In a biased differential relay the bias is defined as ratio of

A
Number of turns of restraining and operating coil.

B
Operating coil current and restraining coil current.

C
Fault current and operating coil current.

D
Fault current and restraining coil current.

Solution

Where n_o and n_r are the number of turns on operating (or) relay coil and restrained coil respectively.
Question 8
2 / -0.66

Figure-1 shows a 3-phase inverter fed by a constant voltage source V_DC and connected to a balanced resistive load at the output. Each switching device may conduct for 120° (or) 180°. The wave form shown in the figure- II is the

A
line voltage with 120° firing

B
load phase voltage with 120° firing

C
line voltage with 180° firing

D
load phase voltage with 180° firing

Solution

Hence, the given waveform represents the load phase voltage when the inverter is operating in 120° conduction mode.
Question 9
2 / -0.66

When the sinusoidal steady state current is called the symmetrical short circuit current, then the unidirectional transient component is called

A
AC short circuit current

B
DC short circuit current

C
AC offset current

D
DC offset current
Solution
Transient on a transmission line:

Let us consider the short circuit current and the following assumptions are made.
- The transmission line is fed from a constant voltage source
- The short circuit takes place when the line is unloaded
- Line capacitance is negligible, and the line can be represented by a lumped RL series circuit
With the above assumptions, the line can be represented by the circuit model as shown.

The short circuit assumed to takes place at t = 0. The parameter α controls the instant on the voltage wave when a short circuit occurs.

The current after short circuit is composed of two parts.

i = i_s + i_t

i_s is the steady-state current and i_tis the transient current.

The sinusoidal steady-state current(i_s) is called the symmetrical short circuit current and the unidirectional transient component(i_t) is called the DC off-set current, which causes the total short circuit current to be unsymmetrical till the transient decays.

Question 10

2 / -0.66

As compared to power MOSFET, a BJT has

Higher switching losses but lower conduction losses

Higher switching losses and higher conduction losses

Lower switching losses and Lower conduction losses

Lower switching losses but higher conduction losses

Solution

Concept:

BJT	MOSFET
Bipolar device	Unipolar device
Low input impedance	High input impedance
Current controlled device	Voltage-controlled device
Low on-state voltage drop and low conduction loss	High on-state voltage drop and higher conduction loss
Higher power handling levels and hence preferred in high power applications	Lower power handling levels and hence preferred in low power applications
Secondary breakdown occurs and high switching losses	Free from the secondary breakdown and lower switching losses
Negative temperature coefficient	Positive temperature coefficient
Not advisable for parallel operation	advisable for parallel operation
Lower operating frequency(10kHz)	higher operating frequency(100kHz)
On state in the saturation region	On state in the ohmic region
Controlled turn on and turn off operation of the device	Controlled turn on and turn off operation of the device
Turn on and turn off time depend on junction capacitance	Smaller turn off time
Continuous Controlled signal requirement	Continuous Controlled signal requirement

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Electrical Engineering Test - 2

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Question 1

Damage to potential coil

Damage to current coil

Damage to both the potential and current coil

Loose contacts

Solution

Question 2

20 kΩ

10 kΩ

100 kΩ

200 kΩ

Solution

Question 3

3α + β = 3

α + 3β = 3

α = 3β

3α = β

Solution

Question 4

Removable singularity

Essential singularity

Pole of order 3

Pole of order 4

Solution

Question 5

4 rad/sec

2√2 rad/sec

2 rad/sec

√2 rad/sec

Solution

Question 6

Hysteresis effect

Skin effect

Fleming rule

None of the options

Solution

Question 7

Number of turns of restraining and operating coil.

Operating coil current and restraining coil current.

Fault current and operating coil current.

Fault current and restraining coil current.

Solution

Question 8

line voltage with 120° firing

load phase voltage with 120° firing

line voltage with 180° firing

load phase voltage with 180° firing

Solution

Question 9

AC short circuit current

DC short circuit current

AC offset current

DC offset current

Solution

Question 10

Higher switching losses but lower conduction losses

Higher switching losses and higher conduction losses

Lower switching losses and Lower conduction losses

Lower switching losses but higher conduction losses

Solution

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