ABB RELION REX640 Technical Manual page 1596

Section 9
Control functions
1590
are visible in the Monitored data view. If the maximum number of five gap locations
is exceeded, the first pair is overwritten.
The coil position should be known and the controller operation should be guaranteed
even if no new calibration is done. When potentiometer gaps are present in either both
ends or in the middle of the potentiometer range, the potentiometer should be replaced.
These are the three critical points of the calibration where ohmic measurements must
be precise.
The maximum percentage length of the potentiometer gap can be set with setting Pot
Max gap. The controller determines a gap length by measuring the distance between
both ends of a gap in ohms. A gap is indicated when the ohmic position exceeds 2000
Ω.
If the coil potentiometer enters a gap during the normal operation, the controller
estimates the coil position based on its previous known position and coil movement
speed. If the actual gap length exceeds the setting Pot Max gap or a new gap appears
during daily operation, POT_ALARM output is activated. Moreover, TUNING_STS
monitored data indicates the reason of alarm. High values of the Pot Max gap setting
introduce inaccuracy to the coil movement as the coil position is based on a calculated
estimate. The purpose of the gap detection functionality is to enable the controller
operation even if the potentiometer is worn out during the normal operation. The
potentiometer should be replaced during the next service break in order to guarantee
accurate operation of the controller. The gap detection functionality can be disabled
by setting the Pot Max gap value to 0. Potentiometer gaps are illustrated in
826.
Potentiometer
value [Ω]
max
min
0
GUID-11752736-37A4-461B-84AC-920D2B4EAE3D V1 EN
Figure 826: Potentiometer gaps
Gap length
1MRS759142 C
Figure
Potentiometer
relative position [%]
100
REX640
Technical Manual