EEE Interview Questions6 Apr 2018
ANS The Ferranti effect is an increase in voltage occurring at the receiving end of a long transmission line, above the voltage at the sending end. This occurs when the line is energized, but there is a very light load or the load is disconnected
How the corona loss can be reduced?
ANS Corona loss can be reduced by increasing the diameter of conductor and increasing the distance between two conductors
How skin effect can be reduced?
ANS To reduce skin effect the shape of the wire should be less for stranded conductor than that of solid conductor .
What is circuit breaker?
- Circuit Breaker is a device which opens when the fault occurs, the contact of the circuit breaker opens and separate the circuit from overcurrent
- It consists of contact lever with one end connected to electromagnet and other end connected to fixed contact if the current flow exceeds the safe operating range electromagnet gets energized and the contact opens from the fixed end thus separating the circuit
- Again if the current reduces the electromagnet get de-energized so contact will join to include the circuit again
- Relay is a electromagnetic device which opens in the occurrence of fault. It is based on the principle of electromagnetic attraction and electromagnetic induction
- When the fault occurs supply to the relay will be lost hence the contact opens to prevent the line from fault
Relay prevents phase lines from fault where as circuit breaker prevents circuts from fault
Why star delta starter is preferred with induction motor?
• Starting current is reduced 3-4 times of the direct current due to which voltage drops and hence it causes less losses.
• Star delta starter circuit comes in circuit first during starting of motor, which reduces voltage 3 times, that is why current also reduces up to 3 times and hence less motor burning is caused.
• In addition, starting torque is increased and it prevents the damage of motor winding.
State the difference between generator and alternator
Why AC systems are preferred over DC systems?
a. It is easy to maintain and change the voltage of AC electricity for transmission and distribution.
b. Plant cost for AC transmission (circuit breakers, transformers etc) is much lower than the equivalent DC transmission
c. From power stations, AC is produced so it is better to use AC then DC instead of converting it.
d. When a large fault occurs in a network, it is easier to interrupt in an AC system, as the sine wave current will naturally tend to zero at some point making the current easier to interrupt.
How can you relate power engineering with electrical engineering?
What are the various kind of cables used for transmission?
• Low-tension cables, which can transmit voltage upto 1000 volts.
• High-tension cables can transmit voltage upto 23000 volts.
• Super tension cables can transmit voltage 66 kV to 132 kV.
Why back emf used for a dc motor? highlight its significance.
What is slip in an induction motor?
Explain the application of storage batteries.
• For starting, ignition and lighting of automobiles, aircrafts etc.
• For lighting on steam and diesel railways trains.
• As a supply power source in telephone exchange, laboratories and broad casting stations.
• For emergency lighting at hospitals, banks, rural areas where electricity supplies are not possible.
Explain advantages of storage batteries
• Most efficient form of storing energy portably.
• Stored energy is available immediately because there is no lag of time for delivering the stored energy.
• Reliable source for supply of energy.
• The energy can be drawn at a fairly constant rate.
. What are the different methods for the starting of a synchronous motor.
• By means of an auxiliary motor: The rotor of a synchronous motor is rotated by auxiliary motor. Then rotor poles are excited due to which the rotor field is locked with the stator-revolving field and continuous rotation is obtained.
• By providing damper winding: Here, bar conductors are embedded in the outer periphery of the rotor poles and are short-circuited with the short-circuiting rings at both sides. The machine is started as a squirrel cage induction motor first. When it picks up speed, excitation is given to the rotor and the rotor starts rotating continuously as the rotor field is locked with stator revolving field.
Name the types of motors used in vacuum cleaners, phonographic appliances, vending machines, refrigerators, rolling mills, lathes, power factor improvement and cranes.
• Vacuum cleaners- Universal motor.
• Phonographic appliances – Hysteresis motor.
• Vending machines – Shaded pole motor.
• Refrigerators – Capacitor split phase motors.
• Rolling mills – Cumulative motors.
• Lathes – DC shunt motors.
• Power factor improvement – Synchronous motors.
. State Thevenin’s Theorem:
Connected across any two terminals of a linear active bilateral network is the ratio open circuit voltage (i.e. the voltage across the two terminals when RL is removed) and sum of load resistance and internal resistance of the network. It is given by Voc / (Ri + RL).
. State Norton’s Theorem
• One is terminal active network containing voltage sources
• Another is the resistance that is viewed from the output terminals. The output terminals are equivalent to the constant source of current and it allows giving the parallel resistance.
The Norton’s theorem also explains about the constant current that is equal to the current of the short circuit placed across the terminals. The parallel resistance of the network can be viewed from the open circuit terminals when all the voltage and current sources are removed and replaced by the internal resistance.
. State Maximum power transfer theorem
Explain different losses in a transformer.
• Constant losses or Iron losses: The losses that occur in the core are known as core losses or iron losses. Two types of iron losses are:
o eddy current loss
o Hysteresis loss.
These losses depend upon the supply voltage, frequency, core material and its construction. As long as supply voltage and frequency is constant, these losses remain the same whether the transformer is loaded or not. These are also known as constant losses.
• Variable losses or copper losses: when the transformer is loaded, current flows in primary and secondary windings, there is loss of electrical energy due to the resistance of the primary winding, and secondary winding and they are called variable losses. These losses depend upon the loading conditions of the transformers. Therefore, these losses are also called as variable losses.
Explain different types of D.C motors? Give their applications
• Shunt motors: It has a constant speed though its starting torque is not very high. Therefore, it is suitable for constant speed drive, where high starting torque is not required such as pumps, blowers, fan, lathe machines, tools, belt or chain conveyor etc.
• Service motors: It has high starting torque & its speed is inversely proportional to the loading conditions i.e. when lightly loaded, the speed is high and when heavily loaded, it is low. Therefore, motor is used in lifts, cranes, traction work, coal loader and coal cutter in coalmines etc.
• Compound motors: It also has high starting torque and variable speed. Its advantage is, it can run at NIL loads without any danger. This motor will therefore find its application in loads having high inertia load or requiring high intermittent torque such as elevators, conveyor, rolling mill, planes, presses, shears and punches, coal cutter and winding machines etc.
. Explain the process of commutation in a dc machine. Explain what are inter-poles and why they are required in a dc machine.
. Comment on the working principle of operation of a single-phase transformer.
An AC supply passes through the primary winding, a current will start flowing in the primary winding. As a result, the flux is set. This flux is linked with primary and secondary windings. Hence, voltage is induced in both the windings. Now, when the load is connected to the secondary side, the current will start flowing in the load in the secondary winding, resulting in the flow of additional current in the secondary winding. Hence, according to Faraday’s laws of electromagnetic induction, emf will be induced in both the windings. The voltage induced in the primary winding is due to its self inductance and known as self induced emf and according to Lenze’s law it will oppose the cause i.e. supply voltage hence called as back emf. The voltage induced in secondary coil is known as mutually induced voltage. Hence, transformer works on the principle of electromagnetic induction.
. Define the following terms:-
• Maximum demand,
• Reserve-generating capacity,
• Availability (operational).
Maximum Demand: It is maximum load demand required in a power station during a given period.
Reserve generating capacity: Extra generation capacity installed to meet the need of scheduled downtimes for preventive maintenance is called reserve-generating capacity.
Availability: As the percentage of the time a unit is available to produce power whether needed by the system or not.
. Mention the disadvantages of low power factor? How can it be improved?
• Line losses are 1.57 times unity power factor.
• Larger generators and transformers are required.
• Low lagging power factor causes a large voltage drop, hence extra regulation equipment is required to keep voltage drop within prescribed limits.
• Greater conductor size: To transmit or distribute a fixed amount of power at fixed voltage, the conductors will have to carry more current at low power factor. This requires a large conductor size.
State the methods of improving power factor?
• By connecting static capacitors in parallel with the load operating at lagging power factor.
• A synchronous motor takes a leading current when over excited and therefore behaves like a capacitor.
• By using phase advancers to improve the power factor of induction motors. It provides exciting ampere turns to the rotor circuit of the motor. By providing more ampere-turns than required, the induction motor can be made to operate on leading power factor like an overexcited synchronous motor.
. State the factors, for the choice of electrical system for an aero turbine.
• Type of electrical output: dc, variable- frequency ac, and constant- frequency ac.
• Aero turbine rotational speed: constant speed with variable blade pitch, nearly constant speed with simpler pitch- changing mechanism or variable speed with fixed pitch blades.
• Utilization of electrical energy output: in conjunction with battery or other form of storage, or interconnection with power grid.
What are the advantages of VSCF wind electrical system?
• No complex pitch changing mechanism is needed.
• Aero turbine always operates at maximum efficiency point.
• Extra energy in the high wind speed region of the speed – duration curve can be extracted
• Significant reduction in aerodynamic stresses, which are associated with constant – speed operation.
. Explain the terms real power, apparent power and reactive power for ac circuits and also the units used.
• Apparent power: It is the product of voltage and current. Apparent power = V I and basic unit of apparent power is volt- ampere. Expressed as VA or KVA.
• Reactive Power: It is the product of voltage, current and sine of angle between the voltage and current i.e. Reactive power = voltage X current X sinj or Reactive power = V I sin j and has no other unit but expressed in VAR or KVAR.
Define the following: Average demand, Maximum demand, Demand factor, Load factor.
• Maximum Demand: The maximum demand of an installation is defined as the greatest of all the demand, which have occurred during a given period. It is measured accordingly to specifications, over a prescribed time interval during a certain period.
• Demand Factor: It is defined as the ratio of actual maximum demand made by the load to the rating of the connected load.
• Load Factor: It is defined as the ratio of the average power to the maximum demand.
Explain forward resistance, static resistance and dynamic resistance of a pn junction diode.
• DC or Static Resistance: DC resistance can be explained as the ratio of the dc-voltage across the diode to the direct current flowing through it.
• AC or Dynamic Resistance: It can be defined as the reciprocal of the slope of the forward characteristic of the diode. It is the resistance offered by a diode to the changing forward current.
How does Zener phenomenon differ from Avalanche breakdown?
. Compare JFET’s and MOSFET’s.
• JFET’s can only be operated in the depletion mode whereas MOSFET’s can be operated in either depletion or in enhancement mode. In a JFET, if the gate is forward-biased, excess-carrier injunction occurs and the gate-current is substantial.
• MOSFET’s have input impedance much higher than that of JFET’s. Thus is due to negligible small leakage current.
• JFET’s have characteristic curves more flat than that of MOSFET is indicating a higher drain resistance.
• When JFET is operated with a reverse-bias on the junction, the gate-current IG is larger than it would be in a comparable MOSFET.
Explain thin film resistors and wire-wound resistors
b. Wire wound resistors – length of wire wound around an insulating cylindrical core are known as wire wound resistors. These wires are made of materials such as Constantan and Manganin because of their high resistivity, and low temperature coefficients. The complete wire wound resistor is coated with an insulating material such as baked enamel
What is a differential amplifier? Also, explain CMRR.
CMRR: It can be defined as the ratio of differential voltage-gain to common made voltage gain. If a differential amplifier is perfect, CMRR would be infinite because in that case common mode voltage gain would be zero.
Q1) Is two wattmeter method is applicable for balance as well as unbalanced load?
Q2) what are the methods of transformer protection?
Q3) why silicon is used in the core of transformer?
Q4) why secondary of current transformer never open circuited?
Q5) what is phase error in potential transformer?
Q6) what is phantom loading?