All questions of Complete Technical Mock Tests for Electrical Engineering (EE) Exam

Explanation:
When peak voltage is fed to the primary of a transformer, the following effects can be observed:

1. Flux density: The flux density in the core of the transformer will be maximum when peak voltage is applied to the primary. This is because the voltage applied to the primary is proportional to the flux produced in the core. Therefore, the higher the voltage applied, the higher the flux density.

2. Iron loss: Iron loss is the power dissipated in the core due to hysteresis and eddy currents. When peak voltage is applied to the primary, the iron losses will be less. This is because the magnetic field in the core will be established quickly due to the high flux density, and the hysteresis and eddy current losses will be reduced.

3. Copper loss: Copper loss is the power dissipated in the windings due to the resistance of the wire. When peak voltage is applied to the primary, the copper losses will be higher. This is because the current in the windings will be higher due to the higher voltage applied.

4. Windage loss: Windage loss is the power dissipated in the transformer due to the friction and turbulence of the cooling air. When peak voltage is applied to the primary, the windage losses will be more or less constant. This is because the windage losses are not affected by the voltage applied to the primary.

Conclusion:
Therefore, the correct option is 'A' - the iron losses will be less when peak voltage is fed to the primary of a transformer.

An ohmic conductor is a material that obeys Ohm's Law. The resistance of an ohmic conductor is defined as the potential difference across the conductor divided by the current flowing through it.

Explanation:

The transformation ratio of a transformer is given by the ratio of the number of turns in the secondary winding to the number of turns in the primary winding. So, for a 1:10 step up transformer, the secondary voltage will be 10 times the primary voltage. Similarly, for a 1:5 step up transformer, the secondary voltage will be 5 times the primary voltage.

When the secondary of a 1:10 step up transformer is connected to the primary of a 1:5 step up transformer, the total transformation ratio can be calculated as follows:

- The secondary voltage of the 1:10 transformer is connected to the primary of the 1:5 transformer, which will step up the voltage by a factor of 5. So, the total transformation ratio is:

1:10 x 1:5 = 1:50

- This means that the secondary voltage of the 1:10 transformer will be stepped up by a factor of 5 to give a total voltage increase of 50.

- For example, if the primary voltage of the 1:10 transformer is 120V, the secondary voltage will be 120 x 10 = 1200V. When this voltage is connected to the primary of the 1:5 transformer, the secondary voltage will be stepped up to 1200 x 5 = 6000V.

- Therefore, the correct answer is option C: 50.

In summary, connecting two transformers in series multiplies their transformation ratios, resulting in a higher overall voltage increase.

Solution:

Given data:

Frequency of rotor emf = 24 Hz

Supply frequency = 50 Hz

Number of poles = 8

Formula Used:

Synchronous speed of induction motor = (Supply frequency * 60) / Number of poles

Rotor speed of induction motor = (Synchronous speed of induction motor) * (1 - slip)

where slip = (Synchronous speed of induction motor - Rotor speed of induction motor) / Synchronous speed of induction motor

Calculation:

Synchronous speed of induction motor = (50 * 60) / 8 = 375 rpm

As we know that frequency of rotor emf = slip * supply frequency

Therefore, slip = frequency of rotor emf / supply frequency = 24 / 50 = 0.48

Rotor speed of induction motor = (375) * (1 - 0.48) = 195 rpm

Therefore, the motor speed is 520 rpm.

Hence, option (d) is the correct answer.

Note: The frequency of rotor emf is always less than the supply frequency in an induction motor. The rotor speed is always less than the synchronous speed of the motor. The slip is the difference between the synchronous speed and the rotor speed of the motor.

The diffusion current density is directly proportional to the concentration gradient. Concentration gradient is the difference in concentration of electrons or holes in a given area. If the concentration gradient is high, then the diffusion current density is also high.

Working Principle of Photodiodes. When photons of energy greater than 1.1 eV hit the diode, electron-hole pairs are created. The intensity of photon absorption depends on the energy of photons – the lower the energy of photons, the deeper the absorption is. This process is known as the inner photoelectric effect.

Explanation:
Capacitance is defined as the ability of a system to store an electric charge. It depends on the geometric properties of the system, such as the distance between the two conductors and the area of the conductors.

When the diameter of the core and cable is doubled, the following changes occur:

1. Distance between the conductors: The distance between the two conductors is increased by a factor of 2. This is because the diameter of the core and cable has been doubled, and the conductors are located at the center of the core and cable.

2. Area of the conductors: The area of the conductors is increased by a factor of 4. This is because the area of a circle is proportional to the square of its diameter.

As a result of the above changes, the capacitance of the system is reduced to one-fourth of its original value. This is because capacitance is inversely proportional to the distance between the conductors and directly proportional to the area of the conductors.

Mathematically, the capacitance of a system is given by the formula:

C = εA/d

where C is the capacitance, ε is the permittivity of the medium between the conductors, A is the area of the conductors, and d is the distance between the conductors.

When the diameter of the core and cable is doubled, the value of A is increased by a factor of 4, and the value of d is increased by a factor of 2. Therefore, the capacitance is reduced by a factor of (1/4) x (1/2) = 1/8 or one-fourth.

Accuracy of Different Meters
Accuracy is a crucial factor when it comes to measuring instruments, especially in electrical engineering. Let's explore the accuracy of different types of meters listed in the question.

Moving Iron Meter
Moving iron meters are commonly used for measuring AC currents. They have a moderate level of accuracy but are not as accurate as some other types of meters. They are suitable for general measurement purposes but may not provide the highest level of precision.

Rectifier Meter
Rectifier meters are often used for measuring DC currents. They offer good accuracy but may not be as precise as other types of meters available. They are reliable for many applications but may not be the best choice when high accuracy is required.

PMMC Meter
Permanent Magnet Moving Coil (PMMC) meters are known for their high accuracy. They are often used in precision measurement applications where accuracy is paramount. PMMC meters are sensitive and provide reliable measurements, making them suitable for tasks that require precise readings.

Moving-coil Meter
Moving-coil meters are commonly used for measuring both AC and DC currents. They offer good accuracy and are suitable for a wide range of applications. While they may not provide the highest level of precision compared to PMMC meters, they are still a reliable choice for many measurement tasks.
In conclusion, among the meters listed, the Moving-coil meter has the highest accuracy. However, the choice of meter ultimately depends on the specific requirements of the measurement task at hand.

Speed is inversely proportional to resistance

This would imply that if the secondary is open circuit, the voltage differences between primary and secondary are 90 degrees out of phase. If the secondary is fully loaded according to the KVA rating then the voltages between the two are 180 out of phase.

For maximum power transfer theory in case of transformer we know Zout=Zload
from this equation we can find out the turns ration
Zout=Zload×(N1/N2)^2

Interphase Fault Detection in Three Line System

Interphase fault is a type of fault that occurs when a fault develops between two or more phases of a three-phase system. This type of fault can cause significant damage to the electrical equipment and can also result in a power outage. Therefore, it is important to detect interphase faults quickly to minimize damage.

Relay is an important device used in electrical systems for detecting faults. In a three-phase system, the number of relays required for detecting interphase faults depends on the type of protection scheme used. Here, we will discuss one of the protection schemes used for detecting interphase faults in a three-line system.

Three-Line System

A three-line system consists of three phases (A, B, C) and three lines (AB, BC, CA) connecting them. The voltage between any two phases is called the line voltage, and the voltage between any phase and neutral is called the phase voltage. In a balanced three-line system, the three phases are equal in magnitude and are 120 degrees apart.

Interphase Fault Detection Scheme

The interphase fault detection scheme uses three relays, one for each phase, to detect faults between two phases. The relays are connected in such a way that if a fault develops between two phases, the corresponding relay will operate and trip the circuit breaker.

The operation of the relays is based on the principle of phase angle difference. When the system is balanced, the phase angles between the three phases are equal. However, when a fault develops between two phases, the phase angle difference between them changes.

The relays are designed to detect this change in phase angle difference and operate if it exceeds a certain threshold. The threshold value is set based on the maximum allowable phase angle difference for the system.

Advantages and Disadvantages

The interphase fault detection scheme has the following advantages:

1. It is simple and easy to implement.
2. It provides a fast response to interphase faults.
3. It can be used for both high and low voltage systems.

However, it has the following disadvantages:

1. It cannot detect faults between all possible combinations of phases.
2. It requires a separate relay for each phase, which can be expensive.

Conclusion

In conclusion, the interphase fault detection scheme uses three relays to detect faults between two phases in a three-line system. The relays operate based on the change in phase angle difference between the two phases. This scheme is simple and fast but requires a separate relay for each phase.

Advantage of Salient Poles in an Alternator

Reduced Windage Loss
Salient poles in an alternator have a larger diameter as compared to cylindrical rotors. This results in a larger air gap between the rotor and stator, reducing the windage loss. Windage loss refers to the energy lost due to the friction between the rotor surface and surrounding air. The reduction in windage loss improves the overall efficiency of the alternator.

Other Advantages of Salient Poles
Apart from the reduced windage loss, salient poles also offer the following advantages:

- Adoptability to Low and Medium Speed Operation: Salient pole alternators can operate at low and medium speeds without any significant drop in efficiency. This makes them suitable for various applications such as hydropower plants, wind turbines, and diesel generators.
- Reduced Bearing Loads and Noise: Salient pole alternators have a lower rotational speed, which reduces the bearing loads and noise levels. This makes them ideal for applications where low noise levels are required, such as hospitals and residential areas.

Conclusion
Salient poles in an alternator offer several advantages, including reduced windage loss, adaptability to low and medium speed operations, and reduced bearing loads and noise levels. These advantages make salient pole alternators suitable for a wide range of applications.