KVA- Apparent power
KVAr- Reactive power
KW- Active power
Every load consumes energy : Two kinds of energies one is active (useful), reactive ( which is generally not useful externally-but required for its internal operation )
Showing posts with label Interview Questions. Show all posts
Showing posts with label Interview Questions. Show all posts
what is the diffrence between dol and star delta strater? why star delta starter used instead of dol
First what is a starter : starter is device through which supply would be given to motor.
Which would limit the starting current, when motor is started it draws 3 to 5 times full load current, there is a need to limit the current.
DOL :direct On line starter
Star delta starter
Current has a relation with torque production.
DOL: same voltage will appears across motor terminals, so high current flows, required torque would be available.
Star-delta: First motor terminals would be connected in star so reduced voltage will appear across terminals of Motor.low current would generate ( limited the starting current )
After a min or so terminals would change to delta. Where normal line voltge will appear across terminals.
So it depends on your application, if you don’t require sufficient torque at starting then you can go for Star delta starter.
How you calculate the repturing capacity of a HRC fuse
Rupturing capacity is measured in amp.
The current at which the lead wire should melt o break the circuit
What is the exact load current needed for a place which has 100nos of bulb of each 100w
One bulb= 100 W, 200V , 0.5 amp
Total current would be 0.5x 100=50 amp
Parallel –current is additive..
Can anyone explain me about the capacitor calculation
Load will have P, Q.
P1=VI cos pi
Q1= VI sin pi
Cos pi1= 0.6 or 0.7
To improve power factor to 0.9 (cos pi2)
Need to place capacitor in parallel with load.
Q2= VIsinpi2
Q2-Q1= Net reactive power to be supplied by capacitor=Q
Capacitor bank will have rating in V and Q.
The appropriate Q will be placed in parallel with Load to achive 0.9 powerfactor.
What is the working principle of ceiling fan
Ceiling fan is a Single phase Induction motor.
Motor principle, whenever current carrying conductor is placed in a magnetic field-it experiences force.
Stator: supplied by 1-phase voltage, current will produce in the stator winding-so magnetic field.
Now we have magnetic field.
We need current carrying conductor:
Because of Induction, voltage will develop in rotor as It is a closed circuit current will produce-so current carrying conductorp laced in magnetic field.
Rotor will rotate.
What is the function of starter in tube light circuit ?
In a tube light, Ionization should take place inside tube to get Ions produces which induces light, To get Ionization needs high voltage at starting , Once it gets ionized, after that no need of High voltage,It mean at starting we require high voltage.
Tube light will have Choke ( which is an Inductor) and starter contains capacitor.
If you look into the circuit :
Starter contains capacitor : starting it is closed, one it get charges it becomes open circuit.
Suddenly when it becomes open, high voltage will develop inside the tube because of inductor ( V=L di/dt).Sudden change in current will induce high voltage across Choke.
After that current will be constant and voltage will become also normal.
How much kv gas generator is needed to run 2 hp and 1 hp airconditioners, plus a refrigerator and 4 Tvs?
2 HP = 2x735.5= 1470 W
1 HP = 735 W
4 TVs= 4x80 W= 320 W
Total= 2525 W
10 % Design margin =252 W
Total= 2777W
2.75 KW generator is enough to drive the load, Its not Kv.
Why birds seating in HT line does not get shock? If person hangs in HT line (single hand or both hands) does he get shock ?
To get shock, current should flow.
Current will take low resistance path, so when bird sat on the HT line, the current flows only through the conductor because of very low resistance comparative to Bird.
When you hang with single Hand: No return path through you and already conductor is low resistance path, so no shock to you.
When you hang with both the hands: No shock you will get, because the same conductor resistance is very low compared to you.
Human body resistance is 1 Kilo Ohm.
What is two wattmeter test ?Explain
Two wattmeter test is not a test, it is one of the ways to measure power consumption by load, measure the power for the given circuit under unbalanced condition, if the system is balanced you can measure the power by using one meter.
What is a Bus PT and what is the function of it Explain?
PT is a potential transformer, which would reduce the voltage to measurement levels i.e. 110V or 220V from line voltage 220Kv, 132 KV, 33KV etc..
When this is PT is connected to bus bar in the switchyard then we call it as a bus PT.
What is the function of surge absorber?Explian with Example
Surge absorber, surge arrestor, Lightning arrestor, surge divertor
What is the dc transformer? How it works?Explain
Chopper you can call it as a DC transformer. Chopper main function is to step up or step down the voltage levels. By using thyristors allowing the supply for on time and block during off time to load, which will give u variable and down voltage.
To get step up voltage, need energy storage devices like inductor or capacitor
what is merz price protection?Explain with Example?
Percentage differential protection we call it as a merz-price protection
What is RTCC panel and why it is required With Example Explain
RTCC means Remote Tap Changing Control will give command to OLTC panel when taps need to change.
how we can calculate cable size for any load?
Cable size calculation for carrying load current has to satisfy 3 conditions:
a) Ampacity
b) Short circuit withstanding capability
c) Voltage drop
- Ampacity: Current carrying capacity after considering conditions like temp, grounding, spacing etc..
- Short circuit with standing capability : When fault is occurred at load, before the Circuit breaker trips the Cable has to withstand the fault current (25 KA,40KA etc..)
- Voltage: You can not lay the line for km as it is satisfied above two conditions, because the cable will have R and X, which will have drop of voltage,so IR drop needs to consider.This drop should be 5% during running of load and 15% during starting of load.
If any condition is not satisfying, go to higher size than you selected.
why transformer ratings are taken in 11kv 22kv 33kv but not in 10,20,30kv
ANS:These are the optimal voltages.
Knee voltage of CT ?Explain With Example
This is the voltage at which a CT becomes saturated. When a CT becomes saturated it can no longer transform current. An example of this rating would be C200, C800 etc... This means that the CT will saturate near 200 Volts or 800 Volts respective.
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