JUMO 709062 TYA 202 - Three-Phase Power Controller Operating Manual - Manual (Page 77)

2012-12-31/00561073 [SCR power switch TYA202] 75
6 Special device functions
6.5 Subordinate control
Subordinate control loops are used to eliminate or compensate for external in-
terference, such as fluctuations in the supply voltage and changes in resis-
tance which would have a negative effect on the control loop.
6.5.1 Closed control loop without subordinate control
Example
Furnace/kiln
control system The electrical supply voltage is connected to the power controller. The control-
ler derives the output level y
R
from the difference between the set value (w) for
the furnace temperature and the actual (or
process
) value (x) which is acquired
by a sensor inside the furnace. The controller output level can range from 0 to
100 % and is output at the output of the controller as a standard signal, for ex-
ample, 0 to 10 V. The output level signal is fed to the power controller.
The task of the power controller is to feed energy to the heating elements in
the furnace, proportional to the controller output level:
- For an SCR power controller using phase angle control, this means that
it alters the firing angle over the range from 180° to 0°, corresponding to a
controller output level of 0 to 100 %.
- If the SCR power controller is using the burst firing mode, it increases
the duty cycle T from 0 to 100 %, corresponding to a controller output level
of 0 to 100 %.
If the supply voltage drops from AC 230 V to AC 207 V (-10 %) at a controller
output level YR, the power fed to the furnace is reduced by 19 %.
P
230V
: Power in the load resistance at a supply voltage U of 230 V
Δ
P: Power reduction resulting from reduced supply voltage
R: Resistance of the load
This 19 % reduction in the energy being fed in means that the
furnace temperature falls.
Controller
Furnace
Sensor
SCR
power unit
Supply
voltage
y
R
x
y
-
w
(2) P
230V
Δ P –
U 0 1U , ( ) – ( )
2
R
-------------------------------
0 9U , ( )
2
R
------------------
0.81 P
230 V
• = = =