Precision Control Systems 930 - Manual (Page 17)

Model 930 ControlIR TM Instruction Manual Operating Instructions
Research, Inc. Page 13 of 25
INPUT LINE VOLTAGE
FIRING
CIRCUITRY CONTROL SIGNAL
GATING SIGNAL OUTPUT LINE VOLTAGE
SCR 1
GATING SIGNAL
SCR 2
SCR 1
SCR 2
Figure 4-3 Conduction of two
SCRs connected in
reverse parallel when
turned on at the
beginning of
half-cycles
Principles of Phase Angle
Control
Figure 4-4 Conduction of two
SCRs connected in
reverse parallel when
turned on after the
beginning of the
half-cycles
Because an SCR is a type of diode, it can conduct only during every other half-cycle of
the applied voltage. Therefore, SCRs used to control AC power are usually installed in
pairs, connected in reverse-parallel, as shown in figure 4-3. One of the SCRs then can
be fired during the positive half-cycle and the other can be fired during the negative
half-cycle.
In phase angle controllers, the SCRs conduct during all or part of every half-cycle.
When delivering full power, the SCRs start conducting at the beginnings of the AC
half-cycles. When delivering less than maximum power, the gating signal is de-layed
so the SCRs start conducting some time later than the beginnings of the half-cycles.
Operation of a phase angle controller at partial output is depicted in figure 4-4.
INPUT LINE VOLTAGE
FIRING
CIRCUITRY CONTROL SIGNAL
GATING SIGNAL OUTPUT LINE VOLTAGE
SCR 1
GATING SIGNAL
SCR 2
SCR 1
SCR 2