HAL 12 November 2008

Question

Answer


Question………………………



In phase controlled rectifiers PF

V(rms)/Vs


An SCR has half cycle surge current rating of 3000A for 50Hz. One cycle surge current will be?

I2t=I2subT
I=Isub*√(T/t)
t=total time period
T=half time period


A step-up chopper is feed from a 220v dc source to deliver a load voltage of 66o v.If the non conducting time is 100micro-sec then required pulse width will be.

Vin/(1-D)=Vout
220/(1-D)=660
D=2/3
Toff=Ton(1-D)
Ton=300 Micro sec


Good transformer oil contains how much amount of water in ppm?

8PPM


Lightning over voltage create how much volt?

Lightning over voltage depends upon the magnitude of lightning (KV)


K/s(s+1)(s+5) then find out the value of K so that system become stable

S3+6S2+5S+k----CE
Apply routh hurwitz criteria
S3 1 5
S2 6 K
S1 (30-k)/6
S0 K

(30-k)/6>0
30-K>0
k<30


If a square waveform is given to the supply side of a 2wdg transformer its
o/p waveform will be

Triangular Wave


SC test done on transformer

To find Out the Copper losses And Series Parameters


If a transformer is operating in .8 PF lag & at efficiency of .9
if now P is .8 lead the efficiency will become

0.9 Only Because Efficiency curve is Single valued cure


Dielectric strength of air is?

30 kv/cm peak or 21.1 kv/cm rms


The voltage of a generator and an infinite bus are given
0.92, 10 deg & 1.0, 0 deg respectively the generator acts as a

Synchronous generator


A wave will not experience any reflection when impedance of line is equal to

Surge impedance
400 ohms for over head lines
40 to 50 ohms for cables


Power transmitted is related to system voltage as

P=EV/Xs Sinδ


Surge wave is attended by

Resistor


In galvanometer damping is provided by

Mechanical and electromagnetic


Megger is used for

Measurement of Very high resistance like insulation resistance of the cable etc
Its working principle is based on moving coil


AC is applied against a moving coil meter it will read

Zero if frequency is high
Oscillate around zero if frequency is low


Rectifier type instrument measures what value

RMS


Advantage of electrostatic instrument is

More accurate
Loading effect is low
Fast response etc.


Creeping occurs because

Over compensation due to frictional forces
Over voltage across pressure coil


Why 2 holes are drilled oppositely in disc of energy meter

To eliminate the creeping


Energy meter is_____ type equipment

Integrating / recording


Inductance is measured by which bridge

Maxwell’s, Anderson, Hay’s, Owen’s bridge


Which type of motor is used in refrigerator?

Capacitor Start and run Single phase induction motor


Corona’s advantage

To increases the virtual diameter of conductor
To protect the conductor from Switching over voltages


Reflection coefficient of current in open ended line is

-1


Reflection coefficient of voltage in open ended line is

1


Reflection coefficient of current in short circuit line is

1


Reflection coefficient of voltage in short circuit line is

-1


Calculation of making current when MVA & KV was given.

Making current=2.5*breaking current
Breaking current=MVA/KV


he o/p voltage e1 & e2 of 2 full-bridge inverters added using o/p transformer. In order to eliminate 5th harmonic from o/p voltage, the phase angle between e1&e2 will be-

36◦


A single phase fully controlled line commutated ac-dc converter operates as an inverter when-


α > 90◦


Why rotor of SCIM is skewed?

To eliminate the slot harmonics in the rotor


Magnetostriction is occurs in


AC circuits’ only


Interlopes are connected for?

To eliminate the Armature reaction


Identify type by pu value Xd”=0.3, Xd’=0.3, X0=0.6?

Transmission line


What to do for converting sine wave to triangular wave?

Integrating


Wave shape of armature mmf?

Triangular wave


Emf in a conductor is directly proportional to?

E α ΦZNP


What’s the order of Jacobin matrix for 20 no. of bus bar?

=2n-2
N=no of buses
=38


How many watt meters required to measure 3phase 4 wire system?

3