Subordinate Control Loop; Closed Control Loop Without Subordinate Control - JUMO 709061/8-01-032 Operating Manual

6 Special device functions

6.5 Subordinate control loop

6.5.1 Closed control loop without subordinate control

Example
Furnace/kiln
control system
Disadvantage:
84
Subordinate control loops are used to eliminate or compensate for external
disturbances, such as mains voltage fluctuations and changes in load resis-
tance, that would have a negative effect on the control process.
w
Controller
-
x
The electrical voltage supply is connected to the power controller. The control-
ler derives the output level y
the furnace temperature and the actual value (x) which is acquired by a sensor
inside the furnace. The controller output level can range from 0 to100 % and is
output at the output of the controller as a standard signal, for example, 0 to10
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 a thyristor power controller using phase-angle operation, this
means that it alters the firing angle over the range from 180° to 0°, corre-
sponding to a controller output level of 0 to100 %
- If the thyristor power controller is in burst-firing operation mode, it in-
creases the duty cycle T from 0 to100 %, corresponding to a controller out-
put level of 0 to100 %
If the mains voltage drops from AC 230 V to AC 207 V (-10 %) at controller
output level YR, the power fed to the furnace is reduced by 19 %.
ΔP
P – =
230V
P : Power in the load resistance at a supply voltage U of 230 V
230V
ΔP:
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.
A continuing constant temperature is no longer assured.
The power controller recognizes the deviation through the relatively slow re-
V1.04/EN/00561071 [Thyristor Power Switch TYA 201]]
power unit
y
R
from the difference between the set value (w) for
R
2
( , ) ( ,
U 0 – 1U 0 9U
=
------------------------- - ----------------- -
R
Supply
voltage
SCR
y
2
)
•
= 0,81 P
230 V
R
Furnace
Sensor
(2)