JUMO 709062 TYA 202 - Three-Phase Power Controller Operating Manual - Manual (Page 79)

2012-12-31/00561073 [SCR power switch TYA202] 77
6 Special device functions
tance is proportional to the input signal to the power controller.
Heating elements that have a positive temperature coefficient (TC)(i.e. where
the electrical resistance increases with increasing temperature) are usual-
ly driven by a power controller that incorporates a subordinate control (U
2
control) (Figure 1).
These are resistive materials such as
- Kanthal-Super
- Tungsten
- Molybdenum
- Platinum
- Quartz radiators
Their cold resistance is substantially lower than their resistance when hot (by a
factor of 6 to 16). These heating elements are usually run at temperatures
above 1000 °C.
Figure 1: Heating element with a positive TC
Power controllers need current limiting for the starting phase. The constant
current and the increasing resistance mean that, initially, the power in the heat-
ing element increases in proportion to R, since the power P = I
2
· R.
When the current falls below the preset limit value, the automatic current limit-
ing is no longer effective, and the power controller operates with the subordi-
nate U
2
control (i.e. if the resistance continues to increase at a constant volt-
age level) the power fed to the heating elements is automatically reduced
P
load
= .
This effect supports the complete control loop. As the furnace temperature ris-
(6) P
load
input signal of the power controller (0 to 20 mA) ∼
U
load
2
R
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