Precision Control Systems 915 - Manual (Page 17)

Page 14 of 21
90 ° PHASE ANGLE
360 ° 270 ° 180 ° 90 ° 360 ° 270 ° 180 ° 90 ° 0 °
MEDIUM POWER
OUTPUT
135
° PHASE ANGLE
360
°
270
°
180
°
90
°
360
°
270
°
180
°
90
°
0
°
LOW POWER
OUTPUT
45
° PHASE ANGLE
360
°
270
°
180
°
90
°
360
°
270
°
180
°
90
°
0
°
HIGH POWER
OUTPUT
AC PHASE ANGLE
AC LINE VOLTAGE
OUTPUT VOLTAGE TO LOAD
Figure 4-5 Phase angle power control
The output voltage of a phase angle controller does not change linearly with respect to the
number of degrees the SCR conduction phase angle changes. Therefore, the SCR controllers
have a built-in load feedback circuit to linearize the relationship between the control signal and
the voltage output. The feedback circuit also adjusts the power controller output to compensate
for changes in line voltage and load impedance (this is called “line and load regulation”).
Phase angle power control offers a number of attractive features. Because the power controller
output level is adjusted by varying the SCR on time within each AC half-cycle, the controller’s