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Process Image
WAGO-I/O-SYSTEM 750
750-494 3-Phase Power Measurement Module
Manual
Version 1.2.0
Current
When calculating current-dependent values, the
current transformer ratio
(CTR)
must be taken into account when using a current transformer.
Current transformers
Current transformers are available on the market in different designs. On the
primary side, currents of 20 A, 25 A, 30 A, 35 A, 40 A, 45 A, 50 A, 75 A, 100 A
… 5000 A are supported, on the secondary side 1 A or 5 A.
For example, transformation ratios of 45:1 A or 200:5 A are possible. Thus, the
corresponding current transformer ratios (CTR) are 1:45 and 1:40.
The I/O module allows you to set
user scaling
using WAGO-I/O-
CHECK
or via
Register 32, bits 12, 13 and 14. If the bit is set to 0, user scaling is disabled and
the transformation ratio of the current is 1:1. If a bit is set to 1, user scaling is
enabled and the transformation ratio is already taken into account in the I/O
module.
Operating current transformers the user scaling should always be on
and the
CTR set in the I/O module. Thus a calculation by the higher control system is not
necessary. As the process values are 32 bits long in the PI even big measured
values can be processed.
Difference to 3-Phase Power Measurement Module 750-493/xxx-xxx
With 750-493/xxx-xxx the process values are only 16 bits long. Therefore
calculations with big CTR values have to be performed by the higher control
system.
The transformation ratio is set using WAGO-I/O-
CHECK
or in Registers 39, 40,
41. The
divisor of the CTR (D-CTR)
is set in the registers, i.e. according to the
examples above:
•
1A-variants:
CTR = 1:45,
D-CTR = 45 => Registers
39, 40, 41
•
5A-variants:
CTR = 1:40,
D-CTR = 40 => Registers
39, 40, 41
Therefore, the following setting options and calculations for the measured values
result in the process image for the current-dependent variables:
