Electrical Engineering Important Viva Questions and Answers - Part 05

Question No. 01: What is the difference between Isolator and Circuit Breaker? What is bus-bar?
Answer: Isolator is an off load device which is used for isolating the downstream circuits from upstream circuits for the reason of any maintenance on downstream circuits. It is manually operated and does not contain any solenoid unlike circuit breaker. It should not be operated while it is having load. First the load on it must be made zero and then it can safely operated. Its specification only rated current is given. But circuit breaker is on load automatic device used for breaking the circuit in case of abnormal conditions like short-circuit, overload etc., it is having three specifications:
  1. 1st is rated current and
  2. 2nd is short circuit breaking capacity and
  3. 3rd is instantaneous tripping current.
Bus bar is nothing but a junction where the power is getting distributed for independent loads.

Question No. 02What is stepper motor. What are its uses?
Answer: Stepper motor is the electrical machine which acts upon input pulse applied to it. It is one type of synchronous motor which runs in steps in either direction instead of running in complete cycle. So, in automation parts it is used.

Question No. 03: What is the difference between synchronous generator & asynchronous generator?
Answer: In simple, synchronous generator supply’s both active and reactive power but asynchronous generator (induction generator) supply’s only active power and observes reactive power for magnetizing. This type of generators is used in windmills.

Question No. 04: Which motor has high Starting Torque and Staring current DC motor, Induction motor or Synchronous motor?
Answer: DC Series motor has high starting torque. We cannot start the Induction motor and Synchronous motors on load, but cannot start the DC series motor without load.

Question No. 05: Why the capacitors work on ac only?
Answer: Generally capacitor gives infinite resistance to dc components (i.e., block the dc components). It allows the ac components to pass through.

Question No. 06: Which power plant has high load factor?
Answer: All base load power plants have a high load factor. If we use high efficiency power plants to supply the base load, we can reduce the cost of generation. Hydel power plants (hydroelectric power plant) have a higher efficiency than thermal & nuclear power plants.

Question No. 07: What is meant by knee point voltage?
Answer: Knee point voltage is calculated for electrical Current transformers and is very important factor to choose a CT. It is the voltage at which a CT gets saturated.(CT-current transformer).

Question No. 08: Use of switch mode power converter in real-time basis?
Answer: Switch mode power converter can be used in the following 5 different ways
  1. Step down an unregulated dc input voltage to produce a regulated dc output voltage using a circuit known as Buck Converter or Step-Down SMPS,
  2. Step up an unregulated dc input voltage to produce a regulated dc output voltage using a circuit known as Boost Converter or Step-Up SMPS,
  3. Step up or step down an unregulated dc input voltage to produce a regulated dc output voltage,
  4. Invert the input dc voltage using usually a circuit such as the Cuk converter, and
  5. Produce multiple dc outputs using a circuit such as the fly-back converter. 

Question No. 09: Why we use ac system in India why not dc? 
Answer: Firstly, the output of power stations comes from a rotary turbine, which by its nature is AC and therefore requires no power electronics to convert to DC. Secondly it is much easier to change the voltage of AC electricity for transmission and distribution. Thirdly the cost of plant associated with AC transmission (Circuit breakers, transformers etc) is much lower than the equivalent of DC transmission AC transmission provides a number of technical advantages. When a fault on the network occurs, a large fault current occurs. In an AC system this becomes much easier to interrupt, as the sine wave current will naturally tend to zero at some point making the current easier to interrupt.

Question No. 10: What is difference between fuse and breaker?
Answer:
Fuses are burned at the time of over current flows in the circuit but breakers are just open (not burn) at the time of over current flow. Fuses are used in only one time but breakers are used by multiple numbers of times.

Question No. 11: State the difference between generator and alternator
Answer: Generator and alternator are two devices, which converts mechanical energy into electrical energy. Both have the same principle of electromagnetic induction, the only difference is that their construction. Generator persists stationary magnetic field and rotating conductor which rolls on the armature with slip rings and brushes riding against each other, hence it converts the induced emf into dc current for external load whereas an alternator has a stationary armature and rotating magnetic field for high voltages but for low voltage output rotating armature and stationary magnetic field is used.

Question No. 12: What are the different methods for the starting of a synchronous motor?
Answer: Starting methods: Synchronous motor can be started by the following two methods:
  1. 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.
  2. 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.

Question No. 13: Mention the disadvantages of low power factor? How can it be improved?
Answer: Disadvantages of low power factor:
  • 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.

Question No. 14: Which type of A.C motor is used in the fan (ceiling fan, exhaust fan, pedestal fan, bracket fan etc) which are find in the houses?
Answer: Its Single Phase induction motor which mostly squirrel cage rotor and are capacitor start capacitor run.

Question No. 15: Why Human body feel Electric shock? And in an Electric train during running, we didn't feel any Shock? Why?
Answer: Unfortunately our body is a pretty good conductor of electricity, The golden rule is Current takes the lowest resistant path if you have insulation to our feet as the circuit is not complete (wearing rubber footwear which doing some repairs is advisable as our footwear is a high resistance path not much current flows through our body).The electric train is well insulated from its electrical system.

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