SESSION:2

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1)    Explain advantages of storage batteries.

Few advantages of storage batteries are mentioned below:

a)    Most efficient form of storing energy portably.

b)    Stored energy is available immediately because there is no lag of time for delivering the stored energy.

c)    Reliable source for supply of energy.

d)    The energy can be drawn at a fairly constant rate.

2)    What are the different methods for the starting of a synchronous motor?

Starting methods: Synchronous motor can be started by the following two methods:

a)    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.

b)    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. 

3)    What's electric traction?

Traction implies with the electric power for traction system i.e. for railways, trams, trolleys etc. electric traction means use of the electricity for all these. Now a day, magnetic traction is also utilised for bullet trains. Essentially dc series motors are utilized for electric traction systems.

4)    What is “pu” in EE?

Pu stands for per unit in power system. (pu = actual value/ base value)

5)    Define stepper motor. What is the use of stepper motor?

The motor which work or act on the applied input pulse in it, is called as stepper motor. This stepper motor is under the category of synchronous motor, which often does not fully depend of complete cycle. It works in either direction but in steps. For this, it mainly used in automation parts.

6)    What is a differential amplifier? Also, explain CMRR.

Differential Amplifier: The amplifier, which is used to amplify the voltage difference between two input points neither of which is grounded, is called differential amplifier. This reduces the amount of noise which is injected into the amplifier, because any noise appearing simultaneously on both the input terminals as the amplifying circuitry rejects it being a common mode signal.

CMRR: It can be defined as the ratio of differential voltage gain to common made voltage gain. If a differential amplifier is perfect, CMRR will be infinite because in that case common mode voltage gain would be zero.

7)    What is use of lockout relay in HT System?

A lockout relay is generally placed in line before or after the stop switch so the power can be shut off from one central location. This relay is powered by the same electrical source as the control power which is operated by a key lock switch. The relay itself may have up to 24 contact points within the unit itself. This allows the control power for multiple machines to be locked out by the turn of a single key switch.

8)    What is inrush current?

Inrush current is the current drawn by a piece of electrically operated equipment when power is first applied. It can occur with AC or DC powered equipment, and can happen even with low supply voltages. Transformer inrush current during very first charging is an example.

9)    In a Tap changing transformer where is the tap connected, is it connected in the primary side or secondary side?

Tapings are connected to high voltage winding side, because of low current. If we connect tapings to low voltage side, sparks will produce while tap changing operation due to high current. Also, HV winding remain over the LV winding. Core<LV Winding>< HV Winding.

10) Why transformer ratings are in KVA?

Since the power factor of transformer is dependent on load. We only define KVA rating and does not include power factor .In case of motors, power factor depend on construction and hence rating of motors is in KW and include power factor. 

11) What is difference between fuse and breaker?

Fuses are burned at the time of over current 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 number of times.

12) What is the difference between delta-delta and delta-star transformer?

Delta-delta transformer is used at generating station or a receiving station for Voltage Change i.e. generally it is used where the Voltage is High and Current is low. Delta-star is a distribution kind of transformer where from secondary star neutral is taken as a return path and this configuration is used for Step down voltage phenomena.

13) Capacitor is load free component but why ampere meter shows current when capacitor bank breaker close?

As we know that there are two types of load, Active and Reactive .Capacitor is a reactive load which is not considering as a load. Meter is design based on Current RMS value. Because of this meter is showing the current RMS value.

14) How to determine alternating current frequency?

Zero crossings of the sine wave to trigger a Monostable (pulse generator) is a way to determine alternating current frequency. A fixed width pulse is generated for each cycle. Thus there are "n" pulses per second, each with a constant energy. The more pulses per second, the more is the energy. The pulses are integrated (filtered or averaged) to get a steady DC voltage which is proportional to frequency. This voltage can then be displayed on an analogue or digital voltmeter, indicating frequency. This method is more suitable than a direct counter, as it can get good accuracy.

15) Why electricity in India is in the multiples of 11 like 11kV, 22kV, 33kV?

           Transformer Induced voltage equation contains 4.44 factor. 

           E   = 4.44*f*T*phi 

           E    - Induced emf per phase 

           T    - Number of turns 

           f     - Frequency 

          phi - maximum flux per pole 

      From the equation we see that E is proportional to 4.4 and it is in turn multiple of 11.So always           transmission voltage is multiple of 11 

16) Why we use ac system in India why not DC?

     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 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.

17) Which type of motor is used in trains, what is the rating of supply used explain Working principal?

Dc series is in the trains to get high starting torque while starting of the trains and operating voltage is 1500 V DC.

18) Battery banks are in connected in series or parallel and why?

Battery banks are always connected in series in order to get a multiplied voltage where the AH or current capacity remaining same. Ex: 24 nos. 2 V, 200 Ah batteries connected in series will give 48V, 200Ah output (AH = Ampere hours).

19) What is the difference between megger test equipment and contact resistance meter test instruments?

Megger test equipment used to measure cable insulation resistance whereas contact resistance meter test used to measure low resistance of clamp contacts between Breaker and Bus, CT and jack bus etc.

20) When we connect the large capacitor bank in series?

We connect large capacitor bank in series to improve the voltage power supply at the load end in balanced transmission line when there is considerable voltage drop along the balanced transmission line due to high inductive loading of the line. So in order to bring the voltage at the load terminals within its limits of the rated high terminal voltage the large capacitor bank is used in series.