C.1.1 Checklist - GE MM3 Instruction Manual

APPENDIX C
APPENDIX C DO'S AND DONT'SC.1 DO'S AND DONT'S
For proper and reliable operation of the GE Power Management MM3 Motor Manager 3, it is imperative that
the steps, recommendations and practices listed below be adhered to at all times. This DO's and DON'Ts
checklist has been compiled as a result of years of trouble free operation by a variety of GE Power Manage-
ment products.
a) MM3 GROUNDING
Users are requested to ground terminals 21, 48 (safety ground) and 22 (Filter ground) directly to the GROUND
BUS using a heavy gauge wire or braided cable. Terminals 21, 22, and 48 will accept up to #12 AWG wire.
These terminals must be grounded for proper filtering of noise, and protection against transient conditions.
b) GROUNDING OF PHASE AND GROUND CTS
• All external phase CT and ground CT secondary windings must be grounded to the GROUND BUS to keep
the potential difference to a minimum. If the CT secondary windings are not grounded, capacitive coupling
could allow the CT secondary voltage to float up to the voltage of the mains. This is a serious safety haz-
ard. Note: Terminal 20 of the External Ground CT is internally grounded therefore do not ground terminal
19 since the Ground CT signal would then be shunted.
• It is also recommended that, in addition to the solid grounding of the ground CT described above, a
shielded twisted pair cable be employed when using the GE Power Management 50:0.025 ground CT. The
reasoning behind this recommendation is that the 50:0.025 ground CTs are typically used on high resis-
tance grounded systems where the fault currents are limited to less than 200 A. The alarm and trip levels
on these systems are usually between 0.5 A and 15.0 A. This equates to a secondary current of 0.25 mA
to 7.5 mA. Due to the very low levels that must be monitored by the MM3, any noise picked up by these
secondary wires must be kept to a minimum.
c) RS485 COMMUNICATIONS PORT
The MM3 interfaces with PCs, PLCs, and DCSs using the Modicon Modbus RTU protocol. The MM3 supports
Modbus function codes 01, 03, 04, 05, 06, 07, 08, and 16. The communications port is a very important part of
the MM3's process and control applications. The port allows reading and writing of data as well as full control
to start and stop the motor from a remote location. For these reasons, proper wiring practices are critical.
A shielded, twisted pair cable, such as 24 gauge Belden 9841 (120 Ω characteristic impedance) or equiva-
•
lent, MUST be used for the communications link. The cable should be routed away from all power carrying
cables, such as the motor mains, power supply wiring, CT wiring and noisy contactors or breakers.
• When using the GE Power Management 232/485 converter box at the MASTER, GE Power Management
recommends placing no more than 32 GE Power Management devices on the same data link which
should be of no greater length than 4000 ft. The devices on the data link should be daisy chained for reli-
able operation. Star or stub connections are not recommended. If more than 32 devices are required to
go onto the data link, or the distance must be greater than 4000 feet, consult the EIA 485 standard for
more details on specific calculations. Another way to increase the number of units on the data link or the
transmission length is to utilize a RS485 Repeater.
• The shields of the cable should be daisy chained to all of the MM3 serial commons (Terminal 38) and
grounded at the MASTER only. This provides a common reference for all of the devices on the data link,
as well as, grounding the data link without creating the potential for ground loops. The potential difference
between the MM3 safety ground (terminal 14) and the MM3 serial common (Terminal 38) should not
exceed 36 V.
A terminating network consisting of a 120 Ω / 0.25 W resistor in series with a 1 nF / 50 V general purpose
•
mono ceramic or equivalent capacitor MUST be placed across the positive and negative terminals at both
ends of the data link (terminals 39 and 40 on the MM3). This is to provide the 200 mV separation between
the +ve and –ve terminals of the device, as well as to eliminate any reflected signals and ringing.
GE Power Management MM3 Motor Manager 3
C.1 DO'S AND DONT'S

C.1.1 CHECKLIST

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