GE MM300 - Phase Current Inputs (IO_A Module)

CHAPTER 2: INSTALLATION ELECTRICAL INSTALLATION
MM300 MOTOR MANAGEMENT SYSTEM – INSTRUCTION MANUAL 2–15
Phase current inputs
(IO_A module)
Figure 2-17: Typical phase current input connections
The MM300 has three channels for phase current inputs, each with an isolating
transformer. The phase CTs should be chosen so the FLA is not less than 50% of the rated
phase CT primary. Ideally, the phase CT primary should be chosen such that the FLA is
100% of the phase CT primary or slightly less, never more. This will ensure maximum
accuracy for the current measurements. The maximum phase CT primary current is 1000
A.
The ground CT connection can either be a zero sequence (core balance) installation or a
residual connection. Note that only 1 A and 5 A secondary CTs may be used for the
residual connection. The zero sequence connection is shown in the diagram above, and
this is recommended. Unequal saturation of CTs, CT mismatch, size and location of motor,
resistance of the power system, motor core saturation density, etc. may cause false
readings in the residually connected ground fault circuit. Refer to section 2.2.2.1 for CBCT
connection.
Note that if residual ground connection is selected, the CT primary setpoint used by the
Phase CT's will also be used by the residual ground calculations.
Power flow
D1
D2
D4D3
Phase current inputs
D5
D6 D7
CT1 CT2 CT3
D8
G/F
D1
D2
D4D3
Phase current inputs
D5
D6 D7
CT1 CT2 CT3
D8
G/F
D1
D2 D4D3
Phase current inputs
D5
D6 D7
CT1 CT2 CT3
D8
G/F
Two CT Connection
Direct Connection
Residual Ground Connection
Power flowPower flow
Power flowPower flow
853753A1.cdr
A
C
B
Power flow
–
+
D1
D2
D4D3
Phase current inputs
MM300
Motor Management System
To switchgear
ground bus
853744A2.CDR
D5
D6 D7
CT1 CT2 CT3
Control power
LN
D8
G/F
Contactor
R
I
CBCT /
50 : 0.025
Core Balance (Zero Sequence)