Cap 6 To 10

Question 1

Types of generators?

Answer 1

AC GEN > AC CURRENT

DC GEN> DC CURRENT

DC ALTERNATOR > produces AC but then converted to DC with rectifier

Question 2

How to distinguish ac from dc gen and dc alternator?

Answer 2

Ac gen > rotating magnet and stationery armature

Dc gen> rotating armature stationery magnet

Dc alternator> same as AC gen but with rectifier to get to DC

Question 3

Explain the process of generating current from engines in put to the generator out put ( idg / csd)

Answer 3

Variable torque power from the engine is connected to the CSD to keep constant the rotation of the axel of the generator 1 which comes out from CSD by hydro mechanical transmission .This small generator 1 has a permanent magnet which creates a 3 phase current .this current then goes to GCU (generator control unit) which controls the the strenght of the exciter field (electromagnet) of the gen 2 which rotates as the same speed of gen1 /csd. This new current 3 phase then is rectified to make DC current and transferred to the gen3 rotor (electromagnet) which creates another 3 phase current that goes to the load of the a/c and the neutral point of the star connection is connected to the GCU to monitor the voltage variation (if a/c requires more current the gcu detects this and increase the exciter field strenght).
Inside the gcu there is a voltage regulator.

Question 4

Where is ECR (excitation control realy) is placed and what it does?

Answer 4

Is placed between GCU and GEN .

And ECR is open when the GEN is unavailable/faulty

Question 5

Frequency formula?

Answer 5

Freq = (Pole pairs x RPM)/ 60

OR

Freq= (number of poles x RPM)/120

Question 6

Csd ?

Answer 6

Constant speed drive 
Its a hydromechanical transmission  which keeps constant  the axel rotation of the GEN.
Important parametres are :
OIL TEMP
OIL PRESSURE

Question 7

How does GCU keeps the frequency undercoltrol?

Answer 7

It uses the CSD to control the frequency by controlling the out put of an hydraulic swashplate pump

Question 8

What are variable frequency gens?

Answer 8

These are generators which don’t have csd and so they are connected directly to the engines which save weight and can also be used to start the engines .
The out put is a variable frequency (380 hz -800hz)which is then been locally rectified for electronic loads

Question 9

RMS?

Answer 9

Root mean square value
RMS= 0.7071 x Peak
And is the comparison between ac -dc of peak values.
Because AC have high peaks and low peaks so the rms give the average value .

Question 10

Star and delta connection?

Answer 10

In delta each line wires is directly connected across a single winding.Therefore the line voltage is equal to phase voltage (think about like been put ion parallel).
And the line current is equal to 1.73 x phase current.

Start connection opposite ( think about like in series)
Line voltage is 1.73x phase voltage.and the line current is the same as phase current

Question 11

Phase voltage and line voltage differences?

Answer 11

Phase voltage is taken from one line and the ground wire
The line voltage is taken from two lines

Same applies for current

Question 12

Reactive power ?

Answer 12

Is the power lost due to voltage and current been out of pahse

Question 13

Reactance ?

Answer 13

When the voltage leads the current .

The out of sync of electrons and magnetic field

Question 14

Capacitance?

Answer 14

When the current leads the voltage.

More power

Question 15

Zener diodes?

Answer 15

Are diodes used to block reverse current flow

Question 16

GEARB?

Answer 16

GENERATOR > 90 KVA
EXTERNAL POWER > UNLIMITED POWER
APU > 90 KVA
RAT> 5 KVA
BATTERY > 1KVA