ISRO HSFC Technician B Electrician Solved Question paper

Answer: "Exclusive NOR "

• The given truth table represents XNOR gate.
• The output is low when both the inputs are different.
• The output is high when both the inputs are the same.
• XNOR gate produces an output only when the two inputs are same.

Answer: "High starting torque "

A squirrel cage induction motor is a type of electric motor that converts electrical energy into mechanical energy. It’s widely used in various applications due to its simplicity and robustness.

How it Works:

1. Stator: The stationary part of the motor, where electrical windings are placed. When AC current is supplied to these windings, it creates a rotating magnetic field.
2. Rotor: The rotating part, shaped like a squirrel cage, consists of copper or aluminum bars embedded in a cylindrical core.
3. Induction: The rotating magnetic field from the stator induces a current in the rotor bars. This induced current, in turn, creates its own magnetic field.
4. Interaction: The interaction between the stator’s rotating magnetic field and the rotor’s induced magnetic field causes the rotor to start rotating.

Advantages:

• Simple Construction: Its design is relatively straightforward, making it easy to manufacture and maintain.
• Robustness: It can withstand harsh operating conditions, including high temperatures and vibrations.
• Self-Starting: It doesn’t require any external starting mechanism.
• Constant Speed: It maintains a relatively constant speed under varying loads.

Disadvantages:

• Lower Starting Torque: Compared to other types of motors, it has a lower starting torque, limiting its use in applications requiring high starting loads. The fixed rotor bars in a squirrel cage induction motor limit its starting torque. Unlike slip-ring induction motors, where external resistance can be added to the rotor circuit to improve starting torque, squirrel cage motors rely on other techniques to achieve higher starting torque.

Answer: "Increases "

• A lead-acid battery uses sulfuric acid as the electrolyte. During the discharge process, the sulfuric acid reacts with the lead plates to form lead sulfate, which reduces the concentration of sulfuric acid and, consequently, the density of the electrolyte.
• When the battery is charged, the reverse reaction occurs. The lead sulfate is converted back into lead and sulfuric acid. This increases the concentration of sulfuric acid in the electrolyte, thereby increasing its density. With a fully charged battery, its specific gravity is about 1.28.
• Therefore, measuring the specific gravity (density) of the electrolyte is a common method to determine the state of charge of a lead-acid battery.

Answer: "Half stepping "

• Wave excitation of a stepper motor results in half step.
• A stepper motor converts electronic signals into mechanical movement each time an incoming pulse is applied to the motor.
• There are three commonly used excitation modes for stepper motors, full step, half step and microstepping.
• One-phase excitation (wave excitation): While it does result in low torque due to the single coil being energized, it also has the advantage of being the simplest and most energy-efficient mode. It’s suitable for applications where torque requirements are minimal, but precise positioning is crucial.
• Two-phase excitation: It provides the highest torque because both coils are energized. The fixed excitation sequence limits its resolution to full steps, which might not be suitable for high-precision movements.
• Microstepping: This advanced mode uses variable currents in multiple coils simultaneously, achieving very high resolution and incredibly smooth movement. It Requires a more complex driver but provides the best control and flexibility.

Answer: "Induction motor "

Power Factor is a measure that indicates how effectively electrical power is being converted into useful work output. It is defined as the ratio of active power (real power) to apparent power in a circuit. The power factor can range from 0 to 1, where 1 indicates that all the energy supplied is being used effectively.

1. Heater: A heater is a resistive load, and it typically operates at a power factor close to 1. This means it uses almost all the supplied energy as active power, resulting in efficient operation.
2. Induction Motor: Induction motors usually operate at a low power factor, especially under light load conditions. The power factor can be as low as 0.2 when the motor is running without load. This low power factor occurs because, under light loads, the motor requires more reactive power, which reduces the overall power factor.
3. Incandescent Lamp: An incandescent lamp also operates at a power factor close to 1 since it converts nearly all supplied electrical energy into light and heat, using active power effectively.

Conclusion

The correct answer is Induction Motor, as it operates with nearly zero power factor, particularly under light load conditions.

Answer: "0.9 mA "

Balanced Wheatstone Bridge:

• In a balanced Wheatstone bridge, the ratio of resistances in the two arms is equal. This means:

P/Q = R/S

In this specific case, the values are:

10/30 = 20/60

• When the bridge is balanced, no current flows through the galvanometer. This is because the potential difference across the galvanometer is zero.

2. Current Drawn from the Cell:

• To find the current drawn from the cell, we need to calculate the equivalent resistance of the circuit.
• 30 and 10 ohm resistance is connected in series and 20 ohm and 60 ohm are connected are connected in series.
• The 40 kΩ and 80 kΩ resistors are connected in parallel. The equivalent resistance (Req) of two resistors in parallel is given by:

1/Req = 1/R1 + 1/R2

• So, for the given resistors:

1/Req = 1/40kΩ + 1/80kΩ

Req = 80/3 kΩ

• Now, we can use Ohm’s law to find the current (I) drawn from the cell:

I = V/Req

Where:

• V = Voltage of the cell (24 V)
• Req = Equivalent resistance (80/3 kΩ)
• Substituting the values:

I = 24 V / (80/3 kΩ)

I = 0.9 mA

Therefore, the current drawn from the cell is 0.9 mA.

Answer: "180°C "

• The maximum safe temperature of class H insulation180°C.
• Insulation classes are a system for categorizing electrical insulation materials based on their maximum safe operating temperature.
• This helps ensure the safe and reliable operation of electrical equipment by preventing overheating and potential damage.

वर्ग (Class)	Material	सामग्री	अधिकतम संचालन तापमान (Maximum Operating Temperature)
A	Cotton, silk, paper (impregnated or immersed in oil), enamel	कपास, रेशम, कागज (तेल में भिगोया या डुबोया हुआ), तामचीनी	105°C (221°F)
B	Mica, asbestos (rarely used), glass fiber	अभ्रक, एस्बेस्टोस (अब कम इस्तेमाल किया जाता है), कांच फाइबर	130°C (266°F)
E	Intermediate between A and B	A और B के बीच मध्यवर्ती	105°C से 130°C के बीच (221°F से 266°F के बीच)
F	Mica, asbestos (rarely used), glass fiber with high-temperature resistant binding (e.g., silicone)	अभ्रक, एस्बेस्टोस (कम इस्तेमाल किया जाता है), कांच फाइबर उच्च तापमान प्रतिरोधी बंधन (जैसे सिलिकॉन) के साथ	155°C (311°F)
H	Mica, asbestos (rarely used), glass fiber with even higher temperature resistant binding (e.g., silicone)	अभ्रक, एस्बेस्टोस (कम इस्तेमाल किया जाता है), कांच फाइबर और भी उच्च तापमान प्रतिरोधी बंधन (जैसे सिलिकॉन) के साथ	180°C (356°F)

Answer: "Geyser "

L-Series MCB:

• Designed for circuits with resistive loads.
• Ideal for protection of heating equipment like:
  • Geysers
  • Ovens
  • General lighting systems

G-Series MCB:

• Designed for circuits with inductive loads.
• Suitable for protection of:
  • Motors
  • Air conditioners
  • Hand tools
  • Halogen lamps

Answer: "Series with armature "

Interpole windings are connected in series with the armature winding of a DC motor. This specific connection is crucial for improving commutation, a process that ensures smooth transfer of current from one commutator segment to the next.

Why Series Connection?

• Synchronizing Current: By connecting the interpole windings in series with the armature, the current flowing through them directly corresponds to the armature current. This ensures that the magnetic field produced by the interpoles is always synchronized with the armature current, leading to effective commutation.
• Counteracting Armature Reaction: Armature reaction is a phenomenon where the magnetic field produced by the armature current opposes the main field. Interpole windings, being in series, produce a magnetic field that counteracts this effect, improving commutation and reducing sparking at the brushes.
• Opposing polarity: Each interpole has a magnetic polarity opposite to the main pole ahead of it in the direction of rotation. As the armature current increases, the interpole field strength also increases, generating a magnetic field that opposes the distortion caused by the armature reaction.
• Neutralized main field: This opposing field partially cancels out the armature reaction, resulting in a more uniform main magnetic field.

Answer: "6 "

• Armature windings are mainly of two types: lap winding and wave winding.
• Lap Winding: In lap winding, the number of parallel paths is equal to the number of poles. This type of winding is used for high current, low voltage applications.
• Wave Winding: In wave winding, the number of parallel paths is always 2, regardless of the number of poles. This type of winding is used for high voltage, low current applications.

For multiplex wave winding, A = 2m, where m is the multiplicity of the winding.

⚬Simplex Wave Winding: m = 1, so A = 2

⚬Duplex Wave Winding: m = 2, so A = 4

⚬Triplex Wave Winding: m = 3, so A = 6

Therefore, for triplex wave winding, the number of parallel paths is 6.