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1.9 MOTOR PROTECTION MODULES
A Motor Protection Module for each compressor is lo-
cated in the Control Panel. These modules supply motor
over-temperature protection, 3-phase current protection,
phase imbalance, phase rotation, and a programming and
troubleshooting 7 segment display.
The motor over-temperature protection is supplied by
3 temperature sensors imbedded in the motor windings
120 degrees apart. The module monitors these sensors,
allowing it to sense a hot winding and shut down the
compressor if motor cooling is inadequate.
The on-board C.T.s provide 3-phase current protec-
tion which look at 3 of the 6 motor legs and send
an analog signal proportional to average motor current
to I/O Expansion board and on to the microprocessor
board for microprocessor low/high current protection
and current display. This allows the micro to monitor
current and shut a system down if low or high motor cur-
rent is sensed. This is a non-adjustable protection circuit
electronically sized to a system's motor specifications.
Internally, the on-board 3 C.T.s and internal circuitry
allow the Motor Protection Module to protect against
high motor current as programmed on the Motor Pro-
tector dip switches. These switches are set at the factory
according to motor specifications.
126
The module also provides phase rotation protection to
assure the screw compressor does not rotate backwards.
A single phase protection circuit located in the module
also monitors for a phase imbalance. If current imbal-
ance exceeds 17% of the average motor current in one
of the phases, the Motor Protector will recognize it and
shut the system down.
Whenever the Motor Protection Module senses a fault,
internal contacts will open, and shut the system down.
These contacts are wired in series with the compressor
motor contactor. When the contact opens, the micro
will attempt to start the system 2 more times. Since
the motor contactor signal path from the Relay Output
Board to the motor contactor is broken by the Motor
Protection Module contacts, it will lock the system out
after 3 faults. The Motor Protection Module must then
be reset by removing 115VAC power from the Control
Panel. After the Motor Protector is reset, the individual
system SYS switch must be switched OFF and then ON
to reset the microprocessor to allow restart of the system.
Anytime the module faults, a thorough
investigation of the problem should be
performed before attempting to return
the system to operation. Failure to
perform this investigation could lead
to motor or compressor failure. Addi‑
tional details on the Motor Protection
Module can be found on page 16.
JOHNSON CONTROLS
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