Page 339 - Solutions Manual to accompany Electric Machinery Fundamentals

SOLUTION Assume that the three voltages applied to this circuit are:
 V
vt M sin t
A
 
 V
vt M sin  t 2 /  3
B
 V
 
vt M sin  t 2 /  3
C

The period of the input waveforms is T, where T  2 / . For the purpose of the calculations in this
problem, we will assume that  is 377 rad/s (60 Hz).
(a) The when the SCRs start to conduct as soon as they are forward biased, this circuit is just a three-
phase full-wave bridge, and the output voltage is identical to that in Problem 3-2. The sketch of output
voltage is reproduced below, and the ripple is 4.2%. The following table shows which SCRs must
conduct in what order to create the output voltage shown below. The times are expressed as multiples of
the period T of the input waveforms, and the firing angle is in degrees relative to time zero.

Start Time Stop Time Positive Negativ Conducting Conducting Triggered Firing
( t) ( t) Phase e Phase SCR SCR SCR Angle
(Positive) (Negative)
 T / 12 T / 12 c b SCR 3 SCR 5 SCR 5 -30
T / 12 3T / 12 a b SCR 1 SCR 5 SCR 1 30
3T / 12 5T / 12 a c SCR 1 SCR 6 SCR 6 90
5T / 12 7T / 12 b c SCR 2 SCR 6 SCR 2 150
7T / 12 9T / 12 b a SCR 2 SCR 4 SCR 4 210
9T / 12 11T / 12 c a SCR 3 SCR 4 SCR 3 270
11T / 12 T / 12 c b SCR 3 SCR 5 SCR 5 330

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