ISRO HSFC Technician B Electrician Solved Question paper

Answer: "Bell "

Figure represents the symbol of Electrical Bell.

Answer: "2.67 kΩ "

The given circuit contains a 15V source, a 4kΩ resistor, and a 6V Zener diode in parallel with a variable resistor R. The task is to find the minimum value of R that will turn on the Zener diode.

Here’s how we can approach this problem:

Zener Diode Behavior:

• A Zener diode acts like a voltage regulator when it is reverse-biased and the voltage across it exceeds its breakdown voltage.
• Zener diode can be turned ON (conducts current) only if the voltage drop across it is equal to or greater than its Zener breakdown voltage.
• In this case, the Zener diode will start conducting when the voltage across it reaches 6V.

Circuit Analysis:

• To ensure that the Zener diode conducts, the voltage across it must be at least 6V.
• Since the Zener diode and resistor R are in parallel, they will have the same voltage across them.
• Therefore, we need to find the minimum value of R that will allow a 6V drop across it.

Calculating the Current:

• The total current flowing through the 4kΩ resistor and the Zener diode will be the same.
• We can calculate the current using Ohm’s law:
• I = (15V – 6V) / 4kΩ = 2.25 mA

Calculating the Minimum Value of R:

• Now that we know the current through R, we can use Ohm’s law again to find the minimum value of R:
• R = 6V / 2.25 mA = 2.67 kΩ

Therefore, the minimum value of R that will turn on the Zener diode is 2.67 kΩ.

Answer: "Provided on the pole face "

• In a salient pole alternator, the damper winding is primarily provided on the pole face. This design allows the damper winding to effectively dampen oscillations and improve stability during operation.
• Damper winding is made with low resistance copper, aluminum, or brass.
• When a disturbance, such as a sudden load change or a short circuit, occurs, it can cause the rotor to oscillate about its synchronous speed. These oscillations can lead to increased mechanical stress on the machine and, in severe cases, can even cause the rotor to lose synchronism.

Damper winding, particularly in salient pole synchronous machines, serves several important functions:

• Assisting in Starting: Damper windings play a crucial role during the starting of synchronous motors. When the motor starts, it behaves like an induction motor. The electromotive force (EMF) generated in the damper winding allows current to flow within the rotor, producing torque that helps the motor to start.
• Reducing Hunting: Damper windings help control hunting, which occurs when there are sudden changes in rotor speed, leading to instability in the machine. The currents generated in the damper winding counteract these fluctuations, helping to stabilize the rotor’s speed.
• Functioning Under Various Conditions: The damper winding is primarily active during starting and hunting situations. At other times, it remains inactive and essentially serves as a dummy.

Answer: "Becomes half "

Initial Condition:

• When a single capacitor is connected to a DC source, it charges to a voltage V and stores a charge Q given by:

Q = C * V.

Connecting a Second Capacitor in Series:

• Adding a second capacitor of the same capacitance C in series reduces the total capacitance to

 1/C_total = 1/C + 1/C = C/2.

• The total voltage across the series combination remains the same (V).
• The charge on each capacitor in the series combination is equal and given by

Q_series = (C/2) * V.

Comparison of Charges:

• Comparing the initial charge Q = C * V with the new charge Q_series = (C/2) * V, we see that the charge on each capacitor in the series combination is half of the original charge.

Key Takeaway:

When a second capacitor of the same capacitance is connected in series with the first capacitor, the total capacitance decreases, and as a result, the charge stored on each capacitor also decreases. This is because the total voltage across the series combination remains constant, but it is now shared between two capacitors.

Additional Insights:

• In a series connection, the voltage across each capacitor is inversely proportional to its capacitance.
• The total capacitance of capacitors in series is always less than the capacitance of the smallest individual capacitor.
• This concept is important in understanding the behavior of capacitors in various circuits, especially in filter circuits and timing circuits.

Answer: "400 W "

In a transformer, iron loss, also known as core loss, is primarily constant and does not depend on load conditions. This loss consists of two main components: hysteresis loss and eddy current loss, both of which are inherent to the material of the core.

Given Data:

• Iron loss at full load = 400 W

Iron Loss at Half Load:

Since iron losses are constant and do not change with the load, the iron loss at half load will remain the same as at full load. Therefore, the iron loss at half load is also 400 W.

Answer: "2⁵¹ "

The given sequence is a geometric progression where each term is obtained by multiplying the previous term by 2.

Identifying the Sequence

The first few terms of the progression are:

– 1st term: $ 2^1 = 2 $

– 2nd term: $ 2^2 = 4 $

– 3rd term: $ 2^3 = 8 $

In general, the $ n $-th term of this geometric progression can be expressed as:

$a_n = 2^n$

Finding the 51st Term

To find the 51st term $ a_{51} $:

$a_{51} = 2^{51}$

Conclusion

Thus, the 51st term of the progression is  $ 2^{51} $.

Answer: "Boiling water reactor "

The Tarapur Atomic Power Plant (TAPS) uses boiling water reactors (BWR). Specifically, it has two units of 120 MW capacity each that were commissioned in October 1969, which are BWR-1 type reactors.

Summary of the Reactor Type:

• Type: Boiling Water Reactor (BWR)
• Capacity: Two units of 120 MW each
• Commissioning Date: October 1969

Answer: "Both (b) & (c) "

The retardation test, also known as the running down test, is a significant method used to evaluate the performance of DC shunt motors. This test is primarily aimed at determining stray losses, which include both friction losses and iron losses.

Overview of Stray Losses

Stray losses in a DC motor arise from various sources, including:

• Friction Losses: These occur due to mechanical friction in bearings and other moving parts.
• Windage Losses: These are losses due to air resistance as the motor components move.
• Iron Losses: These include hysteresis and eddy current losses within the motor’s core.

Test Procedure

1. The motor is initially run at a speed slightly above its normal operating speed.
2. The supply to the armature is cut off while keeping the field winding energized.
3. As the motor decelerates, the time taken for this deceleration is recorded.
4. The data collected allows for the calculation of total stray losses.

Advantages of the Retardation Test

• Efficiency Evaluation: This test provides an accurate assessment of a DC machine’s efficiency by identifying both mechanical and iron losses, which are crucial for understanding overall performance.
• Comprehensive Loss Measurement: It effectively measures various types of losses, giving a complete picture of operational efficiency.
• Less Power Requirement: Conducting a retardation test requires significantly less power compared to running the motor at full load, making it a cost-effective testing option.

Answer: "Becomes four times "

Step 1: Capacitance Formula

$C = frac{kappa epsilon_0 A}{d}$

For air as the dielectric:

$C = frac{epsilon_0 A}{d}$

Step 2: Initial Conditions

– Initial area: $ A_1 = A $
– Initial distance: $ d_1 = d $
– Initial capacitance:

$C_1 = frac{epsilon_0 A}{d}$

Step 3: New Conditions

– New area: $ A_2 = frac{A}{2} $
– New distance: $ d_2 = 2d $
– New capacitance:
$C_2 = frac{epsilon_0 A_2}{d_2} = frac{epsilon_0 left(frac{A}{2}right)}{2d}$

Step 4: Simplifying New Capacitance

$C_2 = frac{epsilon_0 cdot frac{A}{2}}{2d} = frac{epsilon_0 A}{4d}$

Step 5: Relationship Between Voltage and Capacitance

Voltage across a capacitor:

$V = frac{Q}{C}$

Initial voltage:

$
V_1 = frac{Q}{C_1} = frac{Q}{frac{epsilon_0 A}{d}} = frac{Q d}{epsilon_0 A}$

New voltage:
$V_2 = frac{Q}{C_2} = frac{Q}{frac{epsilon_0 A}{4d}} = frac{4Q d}{epsilon_0 A}$

Step 6: Comparing Voltages

$V_2 = 4V_1$

Conclusion
When the distance between the plates is doubled and the effective area of the plates is made half, the voltmeter reading becomes four times greater than before.

Answer: "Blue to Pink "

When the moisture content of silica gel in a breather becomes saturated, the color change from blue to pink indicates that the silica gel has absorbed moisture and is no longer effective at removing additional moisture from the air. Here’s a more detailed explanation:

Silica Gel as a Desiccant

• Function: Silica gel is a desiccant, which means it is a substance that absorbs moisture from its surroundings. It is commonly used to control humidity and prevent moisture-related damage in various applications.
• Color Change Mechanism:
  • Blue Silica Gel: This type of silica gel contains cobalt chloride, which is hygroscopic and changes color based on its moisture content. When dry, it appears blue. Upon absorbing moisture, it transitions to pink, indicating saturation.
  • Indicating Functionality: The color change serves as a visual indicator for users to know when the silica gel needs to be replaced or regenerated.