Best 10 EEE Job Viva Questions & Answer

• Why used stone in electrical substation?

Answer: Stones are used in electrical substations for several important reasons:

1. Safety: Stones help to reduce the risk of electric shock by increasing the resistance between a person’s feet and the ground. This is especially important during a fault, when high voltages can be present.
2. Fire prevention: Stones can help to prevent the spread of fire in the event of an oil spill from a transformer.
3. Weed control: Stones can help to prevent the growth of weeds, which can create a fire hazard and interfere with equipment.
4. Drainage: Stones can help to improve drainage and prevent water from pooling around equipment.
5. Aesthetics: Stones can improve the appearance of the substation.

• Why transformer rated KVA and motor rated in KW ?

Answer:  Transformers are rated in kVA (kilovolt-amperes) while motors are rated in kW (kilowatts) due to fundamental differences functions:

Transformer rating KVA: Transformer losses (heat generated) depend on the voltage and current flowing through the windings, not the power factor of the load. The power transferred in a transformer is the product of the voltage and current, which gives us apparent power, measured in KVA.

Since transformers deal with both active (real) power and reactive (imaginary) power, their rating in KVA covers both. Power factor (which affects real power in kilowatts) varies depending on the load conditions, and transformers do not depend on the power factor to determine their capacity. Hence, KVA is used because it represents total power (without factoring in efficiency or power factor).

Motor Rating in KW: Motor Rated in KW because it consumes real power to produce mechanical output (torque and rotational speed). This real power is measured in kilowatts (KW). KW directly corresponds to the useful energy that a motor is converting into mechanical work.

Motors typically have a specified power factor (which can range from 0.7 to 1.0). The real power, in this case, is more important because it reflects the actual power that is being used to do mechanical work. The apparent power (KVA) would include both real power and reactive power, but for practical applications like motors, the focus is on the real power they consume, so they are rated in KW.

• Why three phase alternator generally star connected?

Answer:

1. In a star connection, the line-to-line voltage is √3 times the line-to-neutral (phase) voltage. For example, if the phase voltage is 400V, the line voltage would be 400V × √3 = 692V. By using star connection, the alternator can produce higher line voltage while keeping the phase voltage lower, this reduces the voltage stress on the insulation of the windings, allowing for:

• Use of less insulation, reducing cost and size.
• Higher voltage ratings for the alternator.

1. Star connection provides a neutral point, which is essential for grounding. Grounding helps to:

• Balance the current flow in each phase.
• Provide a path for fault currents to ground, protecting the equipment and personnel.
• Limit overvoltages during ground faults.

1. For the same line voltage, a star-connected alternator requires fewer turns in its windings compared to a delta-connected alternator. This leads to:

• Reduced cost and weight.
• Increased efficiency due to lower resistance.

1. Star connection helps to produce a purer sinusoidal voltage waveform, reducing harmonics and improving the quality of power output.
2. By using a star connection, it is easier to generate a phase voltage that is suitable for such applications.

• For example, if the alternator produces a 400V line-to-line voltage, the phase voltage will be 400V/√3 ≈ 230V, which is common for many equipment and appliances.

• Why are alternator rated in KVA?

Answer:  Alternators are rated in KVA because they generate both real power (KW) and reactive power (KVAR), and KVA represents the total (apparent) power output.

• Real Power (kW): This is the power that does actual work, like running a motor or lighting a bulb. It’s measured in kilowatts (kW).
• Reactive Power (kVAR): This power doesn’t do work but is necessary for establishing electromagnetic fields in inductive loads like motors and transformers. It’s measured in kilovolt-amperes reactive (kVAR).
• Apparent Power (kVA): This is the vector sum of real and reactive power. It’s the total power flowing in the circuit and is measured in kilovolt-amperes (kVA).

Why kVA for Alternators?

1. Varying Loads: Alternators often supply power to a variety of loads with different power factors (the ratio of real power to apparent power). The power factor can vary significantly depending on the types of loads connected.
2. Unknown Power Factor: When an alternator is manufactured, it’s unknown what type of load will be connected to it. Therefore, it’s impossible to predict the exact power factor.

• Maximum Capacity: The alternator’s physical limitations (like the current-carrying capacity of its windings and the insulation of its components) determine its maximum apparent power (kVA) capacity. This is independent of the power factor of the load.

1. Safety and Efficiency: Rating the alternator in kVA ensures that it’s not overloaded, regardless of the load’s power factor. Overloading can lead to overheating, damage, and reduced efficiency.

So, by rating alternators in KVA, we account for the total power it can deliver to the system, including the effects of reactive power.

• : What is the importance of grounding in a power system?

Answer: Grounding provides a low-impedance path for fault currents, protecting equipment and personnel from dangerous voltages. It also helps to stabilize the voltage and prevent overvoltage’s.

• : What are the different types of power plants?

Answer: Power plants can be classified based on the energy source they use, such as thermal (coal, gas), hydro, nuclear, solar, and wind. (Discuss the advantages and disadvantages of each type).

• : What is the difference between a circuit breaker and a fuse?

Answer: Both are overcurrent protection devices. A fuse melts and breaks the circuit when the current exceeds a certain level, while a circuit breaker can be reset and reused after tripping.

• : Explain the working principle of a transformer.

Answer: A transformer works on the principle of electromagnetic induction. It transfers electrical energy from one circuit to another through a shared magnetic field. A changing magnetic flux produced by the primary winding induces a voltage in the secondary winding.

(Click Here for the types of transformers and their applications).

• : What are the different types of DC motors?

Answer: DC motors can be classified as series, shunt, and compound motors, based on how the field winding is connected to the armature winding. Each type has different speed-torque characteristics. (Explain their applications and advantages/disadvantages).

• : What is the difference between a synchronous motor and an induction motor?

Answer: A synchronous motor rotates at a speed synchronized with the supply frequency, while an induction motor rotates at a slightly lower speed. Synchronous motors are used where constant speed is required, while induction motors are more common due to their simplicity and robustness.