Q. What is the meaning of power factor?
Ans– Power factor is the cosine of the angle between voltage and current waveform.
Q. How can it be improved?
Ans– Power factor improvement capacitor banks are used if the loads are mainly resistive.
Q. What is the benefit of improving power factor?
Ans– Power factor close to 1 means for the same real power in kW, the load current is less and hence the I2R losses are less. The system efficiency is higher as the losses are lower. Improved voltage regulation is achieved.
Q. For a given line voltage, four heating coils will produce maximum heat when connected in
Q. If a motor is wound star and you want to change to delta what should you do?
Ans– Open the terminal connection box. Remove the shorting link connecting the common connection (star) point of A2, B2 and C2. With the 3 phase windings marked A1A2, B1B2, C1C2, connect A2 to B1, B2 to C1 and C2 to A1.
Q. What is the voltage used for ignition in boilers/incinerators etc.?
Ans– Normally 10,000V (10kV) – incinerator and around 19k in boilers
Q. What can be possible reasons for a motor failing to start?
Ans– No power , Fuse blown , Overload trip, contactor coil burnt , Contactors contacts bad, control circuit relay faulty, stop switch open circuit and start switch does not operate, hold on contac does not make and motor burnt.
Q. Explain what happens if a DC motor is fed with AC supply?
Ans– Motor will run at low speed, sparking at brushes, heat due to eddy current will finally burn the motor.
Q. Why is 440 V used for motors and 110/220 V used for lighting?
Ans– Motor load currents are large. Motors are 3-phase loads. A higher 440V voltage means lesser current for the same power and hence losses and size of cable wires is lower. Lighting is single phase loads and its load currents are small, hence lower voltage means less insulation in the cable wires.
Q. How to test emergency generator widout switching off main supply
Ans– Many methods
1. Simulation switch
2. Tie breaker on esb to be switched off
3. Tie breaker on msb switched off.
Any one can be used
Q. Is the alternator connected star or delta
Ans– Normally star.
Q. Compare between lead acid and nickel-cadmium batteries?
Ans- Lead acid : Dilute H2SO4, spongy and PbSO2, low efficiency, 2 volt per cell, less strong, requires more maintenance, less efficient wrt temperature, discharges fast, low cost, problem of sulfation.
NiCd : KOH, Ni & Cd, high efficiency, 1.2V per cell, robust, less maintenance, more efficient wrt temp difference, retains charge for longer periods, expensive, no sulfation.
Q. Why are motor ratings given in KW and that of alternator and transformer given in KVA?
Ans– kW is the output mechanical power of a motor and is expressed in kW.
kVA is the net (apparent) power input to the transformer. This input
power is the output + losses.
kW = kVA x system power factor
Q. What is a self-monitoring alarm circuit?
Ans– It self monitors the health of the alarm circuit. That is, it senses whether the power supply to the alarm circuit is healthy and all the relays and contacts are functioning normally. Check with E/O for more details.
Q. What is Under-voltage protection?
Ans– It prevents closure of the breaker by mistake, or the generator that is coming on load during parallel operation. It also provides protection against loss of voltage while machinery is connected to the switchboard.
Q. How does the emergency generator start automatically?
Ans– It is activated by a under voltage relay. When there is blackout, the under voltage relay senses loss of voltage and starts up the emergency generator. Similarly when the power is restored, the relay stops the emergency generator
Q. What is the purpose of the earth fault indication on the switch board?
Ans– It detects and indicates phase to earth fault on a circuit.
Q. There is something done to bearing pedestal of an alternator, what is it?
Ans– Insulation pads made from nylon are fitted under each pedestal bearing and the bearing holding down bolts are similarly insulated. Magnetic interpoles producing the opposite ampere turns fitted which minimizes effect of the stray magnetic fields causing reduced emf in the shaft.
Q. What is a diode?
Ans– In electronics, a diode is a two-terminal electronic component with asymmetric transfer characteristic, with low (ideally zero) resistance to current flow in one direction, and high (ideally infinite) resistance in the other. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material with a pn junction connected to two electrical terminals
Q. Sometimes, when you change the running direction of a E/R blower (from supply to exhaust) it trips. Why?
Ans– If the flaps of the blower are not set or opened properly or the filters are clogged, the air supply is affected and the blower may beoverloaded causing it to trip.
Q. If all your air bottles are at low pressure and there is a blackout how do you start the generators?
Ans– If main air bottles are empty, the emergency air bottle can be used to start the main generator. If even the emergency air bottle is empty, then it can be filled by running the emergency air compressor (powered by the emergency generator). Once the emergency air bottle is filled, this air can be used to start the main generator.
Q. What is the function of a fuse?
Ans– Fuse places a limit on the amount of the current that can be drawn by an electric circuit by opening (blowing or melting) when the current exceeds a preset limit. This protects the circuit & the surroundings from fire or damage in the case of an overload or short circuit.