Get prepared for your electrical job interviews with our comprehensive list of basic electrical interview questions and answers. Ace your interview with expert insights and tips!
Basic Electrical Interview Question And Answer
Ace your next electrical interview with our comprehensive guide to basic electrical interview questions and answers. We cover everything from the basics of electricity to more advanced topics like electrical circuits and power systems.
Q1: How tube light circuit is connected and how does it work?
Ans: A choke is connected to one end of the tube light and a starter is in series with the circuit. When supply is provided, the starter will interrupt the supply cycle of AC. Due to the sudden change of supply, the chock will generate around 1000 volts. This volt is capable of breaking the electrons inside the tube to make electron flow. Once the current passes through the tube the starter circuit will be out of part.
Q2: Why Human body feel Electric shock? Why we don’t get shocked by an Electric train in motion?
Ans: Our body is a good conductor of electricity. Current always flows through the lowest resistant path. If we have insulation on our feet example rubber shoes, the circuit does not get complete. Hence we don’t get shocked. The electric train is well insulated from its electrical system So we don’t get shocked.
Q3: Why, when birds sit on transmission lines or current wires don’t get shocked?
Ans: It is true that if birds touch a single line (phase or neutral) they don’t get an electrical shock… if birds touch 2 lines then the circuit is closed and they get an electrical shock.. so if a human touches a single one line(phase) then he don’t get shock if he is in the air. If he is standing on the ground and then touching the line (phase) he will get a shock.
Q4: what happens if we give 220 volts DC supply to the bulb or tube light?
Ans: Bulbs [devices] for AC are designed to operate such that they offer high impedance to AC supply. Normally they have low resistance. When DC supply is applied, due to low resistance, the current through the lamp would be so high that it may damage the bulb element. Low DC voltage will not damage the appliances
Q5: 1 ton is equal to how many watts?
Ans: 1 ton = 12000 BTU/hr and to convert BTU/hr to horsepower,12,000 * 0.0003929 = 4.715 hp therefore 1 ton = 4.715*.746 = 3.5 KW.
Q6: Why in a three-pin plug the earth thicker and longer than the other pins?
A: It depends upon R= ρl/a where area(a) is inversely proportional to resistance (R), so if AREA increases, RESISTANCE decreases & if RESISTANCE is less the leakage current will take a low resistance path so the earth pin should be thicker. It is longer because it is the first to make the connection and the last to disconnect. This assures Safety for the person who uses the electrical instrument.
Q7: If one lamp is connected between two phases will it glow or not?
Ans: If the voltage between the two phases is equal to the lamp voltage then the lamp will glow. When the voltage difference is big it will damage the lamp and when the difference is smaller the lamp will glow depending on the type of lamp.
Q8: Two bulbs of 100w and 40w respectively are connected in series across a 230v supply which bulb will glow bright and why?
Ans: Since two bulbs are in series they will get an equal amount of electrical current but as the supply voltage is constant across the bulb(P=V^2/R). So the resistance of the 40W bulb is greater and the voltage across 40W is more (V=IR) so the 40W bulb will glow brighter.
Q9: Why should be the frequency 50 Hz 60Hz only why not others like 45, 95 56, or anything, why should we maintain the frequency constant if so why it is only 50 Hz 60Hz?
Ans: We can have the frequency at any frequency you like, but then you must also make your own motors, high-voltage transformers, or any other equipment you want to use. We maintain the frequency at 50hz or 60hz because the world maintains a standard at 50 /60hz and the equipment is made to operate at this frequency.
Q10: Battery banks are connected in series or parallel and why?
Ans: Battery banks are always connected in series in order to get a multiplied voltage where the AH or current capacity remains the same. Ex: 24 nos. 2V,200Ah batteries connected in series will give 48V,200Ah output (AH = Ampere hours)
SYNCHRONOUS MOTOR Interview QUESTION AND ANSWER
impress your interviewer with your knowledge of synchronous motors by reading our comprehensive guide to synchronous motor interview questions and answers. We cover everything from the basics of synchronous motors to more advanced topics like hunting, dampers, and self-starting.
Q: Why synchronous motor is not self-starting?
Ans: the synchronous motor is a doubly excited machine i.e. two electrical inputs are provided to it. Its stator winding which consists of a 3-phase winding is provided with a 3-phase supply and the rotor is provided with a DC supply. The 3-phase stator winding carrying 3-phase currents produces 3-phase rotating magnetic flux. The rotor carrying the DC supply also produces a constant flux. At a particular instant rotor and stator poles might be of the same polarity (N-N or S-S) causing repulsive force on the rotor and the very next second it will be N-S causing attractive force. However due to the inertia of the rotor, it is unable to rotate in any direction due to attractive or repulsive force and remains in a standstill condition. Hence it is not self-starting.
Q: What are the methods of starting a synchronous motor?
- 1. Pony motor starting( using ac or dc motor)
- 2. Starting as a squirrel cage Induction motor or using damper winding
- 3. Starting as slip ring Induction motor or synchronous motor
- 4. Using a dc machine coupled to it.
Q: What is pony motor starting?
In this method, the rotor is made to rotate by some external device called a pony motor. Once the rotor attains synchronous speed, the DC excitation to the rotor is switched on. Once synchronism is established the pony motor is decoupled. The synchronous motor continuously runs at synchronous speed.
Q: Why damper windings are used in synchronous motors? Or How the synchronous motor started as a squirrel cage motor?
The damper winding consists of short-circuited copper bars embedded in the face of the rotor poles. When an ac supply is provided to the stator of a 3-phase synchronous motor, stator winding produces a rotating magnetic field. Due to the damper winding present in the rotor winding of the synchronous motor, emf is induced in damper winding and hence currents start to flow. Hence torque acts on the rotor.
Damper windings in synchronous motors will carryout the same task as induction motor rotor windings. Therefore due to damper windings synchronous motor starts as an induction motor and continues to accelerate. When the motor attains about 95% of the synchronous speed, the rotor windings are connected to exciter terminals, and the rotor is magnetically locked by the rotating magnetic field of the stator and it runs as a synchronous motor. Now as the rotor rotates at synchronous speed the relative motion between the rotating magnetic field and damper winding is zero. Hence there will be no emf induced in damper winding.
Q: What is the function of damper winding?
- To provide starting torque
- To prevent or minimize hunting
Q: What is hunting?
The sudden changes of load on the synchronous motor set up oscillations in the rotor. Such oscillation of the rotor about its new equilibrium position is called hunting.
Q: What are the effects of hunting?
- Loses synchronism
- Develops mechanical stress on the rotor shaft
- Increase machine losses
- Increase the temperature of the machine
Q: What is pull-in torque?
The torque that is required to pull the motor into synchronism when it is running as an induction motor is called pull-in torque.
Q: What is pull-out torque?
The maximum torque developed by the motor without pulling out of synchronism is called pull-out torque. Its value varies from 1.5 to 3.5 times full load torque.
Q. What is the effect on speed if the load is increased on a 3-phase synchronous motor?
The speed of operation remains constant from no load to maximum load in
the motor operating at constant frequency bus bars.
Q.Why a synchronous motor a constant-speed motor?
When load increase in synchronous motor the load angle
δ also increases but speed remains constant. The further increase in load causes a further increase in load angle. When the load angle reaches 90-degree electrical then the motor comes out of synchronism. Hence the motor rotates at a synchronous speed otherwise it comes out of synchronism.
Q: What is normal excitation?
The excitation for which the power factor of the synchronous motor is lagging and the back emf is equal to the supply voltage.
Q: What is under excitation?
The excitation for which the power factor of the synchronous motor is lagging and the back emf is less than the supply voltage.
Q: What is overexcitation?
The excitation for which the power factor of the synchronous motor is leading and the back emf is greater than the supply voltage.
Q: What is critical excitation?
The excitation for which the power factor of the synchronous motor is unity and the back emf is equal to the supply voltage.
Q. What is a synchronous condenser?
An over-excited synchronous motor under no load, used for the improvement of power factor is called a synchronous condenser because, like a capacitor, it takes a leading current.
Q. Distinguish between synchronous phase modifier and synchronous condenser
A synchronous motor is used to change the power factor or power factor in the supply lines are called synchronous phase modifiers. A synchronous motor operated at no load with over-excitation conditions to draw large leading reactive current and power is called a synchronous condenser.
Q. How the synchronous motor can be used as s synchronous condenser?
A synchronous motor is operated on over-excitation so as to draw leading reactive current and power from the supply lines. This compensates for the lagging current and power requirement of the load making the system power factor to become unity. The motor does the job of capacitors and is hence called a synchronous condenser.
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Q. What are the V and inverted V curves of synchronous motors?
The variation of the magnitude of the line current with respect to the field current is called the V curve. The variation of the power factor with respect to the field current is called the inverted V curve.
Q Write the applications of synchronous motors.
- a. Used for power factor improvement in sub-stations and in industries.
- b. As synchronous condenser
- c. Used for constant-speed applications
Q. State the characteristic features of a synchronous motor.
- a. the motor is not inherently self-starting
- b. The speed of operation is always in synchronous with the supply frequency irrespective of load conditions
- c. The motor is capable of operating at any power factor.
Q. In what way the synchronous motor is different from other motors?
All dc and ac motors work on the same principle. The synchronous motor operates due to magnetic locking taking place between the stator and rotor magnetic fields.
Q A synchronous motor starts as usual but fails to develop its full torque. What could it be due to?
- a. Exciter voltage may be too low.
- b. Field spool may be reversed
- c. There may be either open-circuit or short-circuit in the field.
Q What could be the reasons if a 3-phase synchronous motor fails to start?
It is usually due to the following reasons
- a. Voltage may be too low.
- b. Too much starting load.
- c. Open circuit in one phase or short circuit.
- d. Field excitation may be excessive
