2–14 MM200 MOTOR MANAGEMENT SYSTEM – INSTRUCTION MANUAL
ELECTRICAL INSTALLATION CHAPTER 2: INSTALLATION
Two CT configuration
Figure 2-16: Two CT connection
The proper configuration for the use of two CTs rather than three to detect phase current is
shown. Each of the two CTs acts as a current source. The current that comes out of the CT
on phase A flows into the interposing CT on the relay marked CT1. From there, the current
sums with the current that is flowing from the CT on phase C which has just passed
through the interposing CT on the relay marked CT3. This summed current flows through
the interposing CT marked CT2 and from there, the current splits up to return to its
respective source (CT).
Polarity is very important since the value of phase B must be the negative equivalent of
A + C in order for the sum of all the vectors to equate to zero.
Only one ground connection should be made as shown. If two ground connections are
made, a parallel path for current has been created.
In the two CT configuration, the currents will sum vectorially at the common point of the
two CTs. The diagram illustrates the two possible configurations. If one phase is reading
high by a factor of 1.73 on a system that is known to be balanced, simply reverse the
polarity of the leads at one of the two phase CTs (taking care that the CTs are still tied to
ground at some point). Polarity is important.
NOTE:
Change CT wiring only if the system is de-energized!
A
C
B
Contactor
B3
B4
B6B5
Phase current inputs
MM200
Motor Management System
To switchgear
ground bus
888714A2.CDR
B7
B8
CT1 CT2 CT3
Control power
+
-
A10
A9
A8
Control power
N
L
HI
A10
A9
A8
LO
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