BEL Trainee Engineer Electrical Mock Test - 5 - Electrical Engineering (EE) MCQ

Large high quality single crystals of arsenic have been grown from the melt in a high pressure bomb made from synthetic quartz. Since arsenic sublimes before it melts, it is necessary to prepare crystals from the vapor phase or grow them from the melt under high pressure (melting point 813°C at about 30 atm). It has been shown [l-5] that both techniques can be used to prepare large single crystals. To learn about intrinsic properties of arsenic, it is essential that the purity of crystals be as high as possible.

A large number of optoelectronic devices consist of a p-type and n-type region, just like a regular p-n diode. The key difference is that there is an additional interaction between the electrons and holes in the semiconductor and light. This interaction is not restricted to optoelectronic devices. Regular diodes are also known to be light sensitive and in some cases also emit light. The key difference is that optoelectronic devices such as photodiodes, solar cells, LEDs and laser diodes are specifically designed to optimize the light absorption and emission, resulting in a high conversion efficiency.

Gallium Arsenide has the advantage that it has higher electron and hole mobilities compared to silicon. However, the stoichiometric growth is difficult in case of GaAs. GaAs can also be used in optoelectronics due to its high optoelectronic conversion efficiency.

Absolute Error: The deviation of the measured value from the true value (or) actual value is called error.

Absolute error (E) = A_m – A_t

A_m = Measured value

A_t = True value

Relative Static Error: The ratio of absolute error to the true value is called relative static error.

Limiting Error:

The maximum allowable error in the measurement is specified in terms of true value, is known as limiting error. It will give a range of errors. It is always with respect to true value, so it is a variable error.

Guaranteed Accuracy Error:

The allowable error in measurement is specified in terms of full-scale value is known as a guaranteed accuracy error. It is a variable error seen by the instrument since it is with respect to full-scale value.

Concept:

Resonant frequency for a series RLC circuit is given by:

Bandwidth for a series RLC circuit is given by:

Also, the quality factor is given by:

Calculations:

Given that, R = 10 Ω, L = 25 mH, C = 100 μF

For the given RLC network, the resonant frequency will be:

Qulaity factor, Q = 632.45/400 = 1.58

Power electronic circuits can be classified as follows.

1. Diode rectifiers:

• A diode rectifier circuit converts ac input voltage into a fixed dc voltage.
• The input voltage may be single phase or three phase.
• They find use in electric traction, battery charging, electroplating, electrochemical processing, power supplies, welding and UPS systems.

2. AC to DC converters (Phase controlled rectifiers):

• These convert ac voltage to variable dc output voltage.
• They may be fed from single phase or three phase.
• These are used in dc drives, metallurgical and chemical industries, excitation systems for synchronous machines.

3. DC to DC converters (DC Choppers):

• A dc chopper converts dc input voltage to a controllable dc output voltage.
• For lower power circuits, thyristors are replaced by power transistors.
• Choppers find wide applications in dc drives, subway cars, trolley trucks, battery driven vehicles, etc.

4. DC to AC converters (Inverters):

• An inverter converts fixed dc voltage to a variable ac voltage. The output may be a variable voltage and variable frequency.
• In inverter circuits, we would like the inverter output to be sinusoidal with magnitude and frequency controllable. In order to produce a sinusoidal output voltage waveform at a desired frequency, a sinusoidal control signal at the desired frequency is compared with a triangular waveform.
• These find wide use in induction motor and synchronous motor drives, induction heating, UPS, HVDC transmission etc.

5. AC to AC converters: These convert fixed ac input voltage into variable ac output voltage. These are two types as given below.

i. AC voltage controllers:

• These converter circuits convert fixed ac voltage directly to a variable ac voltage at the same frequency.
• These are widely used for lighting control, speed control of fans, pumps, etc.

ii. Cycloconverters:

• These circuits convert input power at one frequency to output power at a different frequency through a one stage conversion.
• These are primarily used for slow speed large ac drives like rotary kiln etc.

6. Static switches:

• The power semiconductor devices can operate as static switches or contactors.
• Depending upon the input supply, the static switches are called ac static switches or dc static switches.

Concept:

Let f(x) is a periodic function defined in (C, C + 2L) with period 2L, then the Fourier series of f(x) is

Where the Fourier series coefficients a0, an, and bn are given by

• If f(x) is an odd function, then only bn exists where a0 and bn are zero.
• If f(x) is an even function, then both a0 and an exists where bn is zero.

Calculation:

= 0

Concept:

Low power factor meter:

The instrument that measures the low value of power factor accurately is known as the Low Power Factor Wattmeter (LPFW). The low power factor meter is used for measuring the power of the highly inductive circuit.

The ordinary Wattmeter used for measuring the low power factor gives the inaccurate result. This happens because of two reasons.

• In low power factor meter, the magnitude of deflecting torque on moving coil is small even after the full excitation of the pressure and current coil.
• The error occurs in the reading because of the pressure coil inductance.

Application:

As the LPF is most suitable for inductive loads, it is suitable for the measurement of power in inductive loads, open circuit test on single phase transformer and load test on induction motor.

It is not suitable for the measurement of power in resistive loads.

• In ac drives, stable operating point is when load torque is equal to electromagnetic torque
• Criteria for the steady state stability is for a decrease in the speed the motor torque must exceeds the load torque and for increase in speed the motor torque must be less than the load torque.
• To check the stability at an operating point of the motor, if an increase in speed brings greater increase in load torque than the motor torque, the speed will tend to decrease and return to its original value, so operating point will be a stable point else operating point will be an unstable point.

The given circuit is a three-phase half-wave diode rectifier. The output voltage characteristics are shown below.

The average output voltage is given by

Where V_l­ is the rms line voltage

Concept:

The gauge factor is defined as the ratio of per unit change in resistance to per unit change in length. It is a measure of the sensitivity of the gauge.

Gauge factor,

Where ε = strain = ΔL/L

The gauge factor can be written as:

= Resistance change due to change of length + Resistance change due to change in the area + Resistance change due to the piezoresistive effect

If the change in the value of resistivity of a material when strained is neglected, the gauge factor is:

The above equation is valid only when the Piezoresistive effect that changes in resistivity due to strain is almost neglected.

For wire-wound strain gauges, the Piezoresistive effect is almost negligible.

Calculation:

Given that,

Resistance (R) = 130 Ω

Gauge factor of strain gauge (G_f) = 2

Strain (ε) = 1 × 10^-6

Laws of Boolean Algebra:

Application:

f(A, B) = ∑ m(0, 1, 2, 3)

= A̅ B̅ + A̅ B + A B̅ + AB

= A̅ ( B + B̅) + A (B̅ + B)

= A̅ + A = 1

State transition matrix:

It is defined as inverse Laplace transform of |sI - A|-1

⇒ L-1 |sI - A|-1 = eAt = ϕ(t)

Properties:

State transition matrix, ϕ(t) = eAt

• ϕ(0) = e(A0) = I, Identity matrix
• 
• ϕ(t1 + t2) = ϕ(t1) ϕ(t2)
• [ϕ(t)]n = ϕ(nt)
• ϕ(t2 – t1) ϕ (t2 – t0) = ϕ (t2 – t0) = ϕ (t1 – t0) ϕ (t2 – t1)

Application:

Given as state model, ẋ = A x(t)

Comparing with standard equation ẋ = A x(t) + B u(t)

, B = 0

Shortcut TrickShortcut Method to finding Adj (A)

Let A =

Adj A =

Note: Interchange the diagonal elements and change the sign of remaining elements.

Concept:

Circuit diagram for the resistance switching of 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 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.
• The value of R for critical damping is
• Resistance Switching also reduces the oscillatory growth of Re-striking Voltage

• Voltage appearing across the electrodes of circuit breaker or switching voltage or prospective voltage

Calculation:

Given that V = 132 kV, C = 0.05 μF, L = 50 mH

Critical resistance,   

Concept:

Base impedance is given by,

Where V_b is the base voltage and S_b is the base MVA

Actual value = Base value × Per unit value

Z_a = Z_b × Z_pu

Desired voltage(V₂) = (Reactive power(Q) × Actual Impedance)/(Actual voltage(V₁))

Calculation:

Z_pu = 0.2 p.u., V₁ = 400 kV = V_b, V₂ = 5 kV, S_b = 500 MVA

Z_a = 320 × 0.2 = 64 ohm

⇒ Q = 31.25 MVAR

Concept:

Final value theorem:

• A final value theorem allows the time domain behavior to be directly calculated by taking a limit of a frequency domain expression
• Final value theorem states that the final value of a system can be calculated by

Where F(s) is the Laplace transform of the function.

• For the final value theorem to be applicable system should be stable in steady-state and for that real part of poles should lie in the left side of s plane.

Initial value theorem:

It is applicable only when the number of poles of C(s) is more than the number of zeros of C(s).

Calculation:

Given that,

We can’t apply the initial value theorem as the number of poles equal to the number of zeros.

We can write the H(s) as,

By applying the inverse Laplace transform,

⇒ h(t) = 2δ(t) + 1.5 e^-0.5t

The initial value = h(0) = ∞

Final value,

Concept:

For the Step-Down, DC chopper-

V₀ = α V_s

Where V₀ = Average output Voltage

V_s = Input Voltage

And the duty ratio is α.

Ripple factor is given by-

Voltage Ripple factor

Where form factor (FF) =

So, V.R.F. =

Calculation:

α = 1/3

V_s = 200 Volt

V₀ = (1/3) × 200 = 66.66 Volt

V.R.F. = ≈ 1.5

Concept:

In a transformer,

Turns ratio (n)

Where,

N₁ = Number of turns in primary side

N₂ = Number of turns in secondary side

V₁ = Primary side voltage

V₂ = Secondary side voltage

Calculation:

V₁ = 250 V = LV side voltage,

V₂ = 3000 V = HV side voltage,

EMF or voltage per turn = 8 V

Number of LV turns of the transformer is

Number of HV turns of the transformer is

feasible : that is possible and likely to be achieved

The correct answer is A - iii, B - iv, C - i, D - ii

Key Points

• Task view allows you to organize the applications across multiple virtual desktops. 
• This feature allows you to resume previous activities, perform multitasking operations etc.
• In Windows 10, you can find the option to view tasks on the taskbar. The shortcut to use it is Win + Tab.
• Sometimes you want to hide the desktop icons temporarily rather than deleting them. To do this, right-click on the desktop, go to view and select show desktop icons.
• For displaying or hiding the icons you can use the shortcut Win + D.

Additional Information

• To take the current page screenshot in windows, use the shortcut key Alt + PrntSc.
• To add a virtual desktop, go to the task view, and select new desktop. Or you can use the shortcut Win + Ctrl + D. Hence option 2 is correct.

The correct answer is Align center.

Key Points

• Alignment is how the text in the document shows up to the user.
• By default, Excel provides left alignment of the text. But you can change the alignment format.
• For center alignment, place the cursor anywhere in the text and use the shortcut Ctrl + E. Hence option 4 is correct.
• To go back to the default alignment or left-align, place the cursor in the text and use Ctrl + L shortcut. Left alignment allows the user to align the text to the left margin of the page.

Additional Information

• Use Ctrl + R shortcut for the right alignment. Right alignment allows the user to align text to the right margin of the page.
• Justify text allows the user to align the text to both the margins of the document or page.

• Dragging: This is the correct term used when you press and hold the left mouse key and move the mouse around the slide. It refers to the action of moving an object or selecting multiple objects by holding down the mouse button and moving the cursor.


• Highlighting: This term is not applicable in this context. Highlighting usually refers to the action of selecting text or a portion of text by changing its background color.


• Selecting: This term is also not specifically used to describe the action of pressing and holding the left mouse key and moving the mouse around the slide. Selecting typically refers to choosing or picking something from a list or group of items.


• Both (b) and (c): This option is incorrect because the correct term for pressing and holding the left mouse key and moving the mouse around the slide is only dragging. The term "selecting" is not accurate in this context.

The correct answer is Alt+H.

Key Points

Additional Information

Some other important keys and functions are:

The correct option is (4)

Shift + Delete

Key Points

• On your keyboard, hold down the Shift key while pressing the Delete key. You will be prompted to confirm that you wish to remove the file or folder because you cannot undo this.
• Send files to the Recycle Bin on Windows, then empty the Recycle Bin to permanently erase the files. Without data or file recovery software, you cannot recover the files when the recycle bin is empty.
• Because Windows 11 is being used to display this screenshot, you must additionally click Show more options after right-clicking the file.
• The Permanently Delete option in the context menu won't appear until after that. Also, you will be asked to confirm the deletion after clicking "Permanently Remove."

Additional InformationDelete:- Eclipse offers a very straightforward method to recover a mistakenly deleted file and retains the deleted files in its local history.

F2:- When an icon, file, or folder is highlighted or selected in Microsoft Windows and Windows programs, pressing F2 renames the file.

Backspace:- A keyboard key called Backspace or Backspace deletes any character that comes before the cursor's current position.

The synonym of arid, parched and droughty is dry.

As position of M and N in Eq. alphabets are 13 and 14 respectively.Same position of F and R in Eq. alphabets are 6 and 18 respectively.

As 'Calf' is the youngest of 'Cow' similarly 'Kitten' is the young one of 'Cat'.

From the common explanation, we can say that there are at least 16 boys sitting in the row.
Hence, Option B is correct.
Final Arrangement:

Common Explanation:
References:
Only two boys sit to the left of J.
Eight boys sit between R and J.
Only three boys sit between R and M.
Inferences:
From the above references, we get two different cases:

References:
P sits fourth to the right of R.
O sits third to the left of M.
Inferences:
So, from this case 2 will be eliminated as P sits fourth to the right of R.

References:
Only three boys sit between O and H.
T sits third to the right of M.
V sits fourth to the right of O.
More than 12 boys sit between P and H.
More than seven boys sit between P and J.
Inferences:
From the above reference, we get the final arrangement: