SCR rectifier

Question 1

How do you derive the equation for the fundamental component of the phase current:
Îph(fund) =2/Pi root(3) * Id = 1.1Id

Answer 1

You want to find the fundamental component, so use the A1 part of the fourier series formulae. Always integrate over the ‘on’ parts of the wave - don’t worry about phase shift of alpha, just integrate it as if it is the simplest form of that type of wave

Question 2

What do you have to remember when using the

P=3 Vph Iph cos(phi) equation?

Answer 2

The phase current as given by Îph is the PEAK value, have to divide by root(2). Also if given Vll(rms), have to divide by root(3) to get PHASE voltage in rms (same as in Ken’s module)

Question 3

Explain commutation in an SCR rectifier

Answer 3

e.g. going from state [61] to [15]
By the end of the switching process, S6 will have turned off and S5 will have turned on.
S5 turns on when it is forward biased AND gated (according to alpha). Current through rises (over some time) from 0 to Id. At the same time, current through S6 falls from Id to 0.

Question 4

What are the equations related to commutation when

Ls =/= 0

Answer 4

2 VPG equations:
VPG = va - Ldia/dt
and VPG = vc - Ldic/dt (or equivalent for another phase)
Add those 2 equations together.
Set the sum of the di/dt terms to 0 because the sum of the two currents = 0.
Gives:
VPG = (va - vc)/2

Question 5

What happens to the shape of VPG when LS =/= 0

Answer 5

There is that small slanted bit on the rise up to the line voltage (happens on the rise triggered by gating the switch)

Question 6

How do you derive ΔV

Answer 6

ΔV = (area above the slanted bit)/(Pi/3)
ΔV = 3/Pi * integral of Ls*dia/dt with limits from (Pi/6+alpha) to (Pi/6 + alpha + gamma)
ΔV = 3/Pi * integral of w*Ls*dia from 0 to Id
ΔV = 3/Pi *w*Ls*Id

Question 7

Which row of the circuit is the common cathode bit

Answer 7

The top row. Cathode is the line bit of the thyristor

Question 8

Explain the gate sequence for the common cathode devices

Answer 8

Assume S1 is on
Say what voltages S3 and S5 have over them
S3 will turn on once its voltage = 0 AND gated
Sequence is S1, S3, S5

Question 9

How to label the stuff on the wave paper

Answer 9

Vd=VPN, and hops along the LINE voltages
VPG hops along the PHASE voltages.
Work out which line voltage is which (from the normal set of 3) by looking at the cross points of the phase voltages. Work out the other 3 by looking at the wave that is reflected over the x axis

Question 10

How much does one ‘block’ on the prinout paper equal in terms of angle

Answer 10

30 degrees

Pi/6 rad

Question 11

How do you sketch ib

Answer 11

Start by thinking of it as in phase with ib. It will be at the pk values for 4 blocks and 0 for 2 blocks. Then phase shift it by alpha.