its been a while since i did power, so i;m a little rusty:
the coils are a transformer - provides isolation and step up or step down depending on turns ratios
as in single phase
the source on the left is a delta - that is, the three phases are 120 degrees part and the ends
of the three coils (1 per phase) are connected to look like an equilateral triangle
here is is drawn as a series connection that ends at the start
the voltages are line to line, or just line as they appear in the equations
the secondary is a y - usually drawn as an equilateral y
as in the drawing, the end of the coils tied together are called the neutral
the other ends are the phase voltages, a, b and c as labeled
VL(m) would be the maximum line voltage -
Vphase the phase voltage
Vm the maximum (phase) voltage
Im is the maximum current
Vo(avg) is the average output voltage -
the cos(20) is there because the conduction angle, where you turn the thyristers on, is 20 degrees
(or is it the angle the thyrister is on?)
Io(avg) is the average output current
Ith(m) is the maximum current in a thyrister
Ith(aveg) s the average currnt in a thyrister - they each conduct 1/3 of the time
part of this however seems off, as one needs a + thyrister (1, 3, 5) and a - thyrister (4, 6, 2) on at the same time to conduct
and they have to turn on in the correct sequence
1 and 4 (for example) cannot be on at the same time
1 and 6 are on for 1/2 the conduction time of 1 and 1 and 2 are on for the other 1/2 of the conduction time of 1