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Jumpers JMP5, JMP6 and JMP7 are located in notches
across the top of the CTB. See Fig. 29. These jumpers are
factory cut on all heat pump units. Visually check these
jumpers to confirm that they have been cut.
PROTECTIVE CONTROLS
Compressor Protection
Over- - current
The compressor has internal line- - break motor protection.
Over- - temperature:
The compressor has an internal protector to protect it
against excessively high discharge gas temperatures.
High Pressure Switch:
The system is provided with a high pressure switch
mounted
on
the
discharge
stem- - mounted and brazed into the discharge tube. Trip
setting is 630 psig 10 psig (4344 69 kPa) when hot.
Reset is automatic at 505 psig (3482 kPa).
Loss of Charge Switch:
The system is protected against a loss of charge and low
evaporator coil loading condition by a loss of charge
switch located on the liquid line and a freeze protection
thermostat
on
the
indoor
stem- - mounted. The Loss of Charge Switch trip setting is
27 psig 3 psig (186 21 kPa). Reset is automatic at 44
3 psig (303 21 kPa).
The Freeze Protection Thermostat trip setting is 30_F
5_F
(- - 1_C 3_C). Reset is automatic at 45_F 5_F
(7_C 3_C).
Supply (Indoor) Fan Motor Protection:
Disconnect and lockout power when servicing fan motor.
2.9 and 3.7 bhp motors are equipped with an
over- - temperature or protection device. The type of device
depends on the motor size. See Table 6.
Table 6. Overload Device per Motor Size
Motor Size (bhp)
Overload Device
1.7
Internal Linebreak
2.4
Internal Linebreak
2.9
Thermix
3.7
Thermix
External
4.7
(Circuit Breaker)
line.
The
switch
is
coil.
The
switch
is
Reset
Automatic
Automatic
Automatic
Automatic
Manual
The High Static option supply fan motor is equipped with
a pilot- - circuit Thermix combination over- - temperature/
over- - current protection device. This device resets
automatically. Do not bypass this switch to correct
trouble. Determine the cause and correct it.
The Thermix device is a snap- - action over- - temperature
protection device that is imbedded in the motor windings.
It is a pilot- - circuit device that is wired into the unit's 24V
control circuit. When this switch reaches its trip set point,
it opens the 24V control circuit and causes all unit
operation to stop. This device resets automatically when
the motor windings cool. Do not bypass this switch to
correct trouble. Determine the cause and correct it.
The
External
motor
specially- - calibrated circuit breaker that is UL recognized
as a motor overload controller. It is an over- - current
device. When the motor current exceeds the circuit
breaker set point, the device opens all motor power leads
and the motor shuts down. Reset requires a manual reset
at the overload switch. This device (designated IFCB) is
located on the side of the supply fan housing, behind the
fan access panel.
Troubleshooting supply fan motor overload trips —
The supply fan used in the 50TCQ units is a
forward- - curved centrifugal wheel. At a constant wheel
speed, this wheel had a characteristic that causes the fan
shaft load to DECREASE when the static pressure in the
unit- - duct system increases and to INCREASE when the
static pressure in the unit- - duct system decreases (and fan
airflow rate increases). Motor overload conditions
typically develop when the unit is operated with an access
panel removed, with unfinished duct work, in an
economizer- - open mode, or a leak develops in the duct
system that allows a bypass back to unit return opening.
Outdoor Fan Motor Protection:
The outdoor fan motor is internally protected against
over- - temperature.
Control Circuit, 24V
The control circuit is protected against over- - current
conditions by a circuit breaker mounted on control
transformer TRAN. Reset is manual.
COMMERCIAL DEFROST CONTROL
The
Commercial
Defrost
coordinates thermostat demands for supply fan control, 1
or 2 stage cooling, 2 stage heating, emergency heating and
defrost control with unit operating sequences. The DFB
also provides an indoor fan off delay feature (user
selectable). See Fig. 30 for board arrangement.
20
overload
device
is
Control
Board
(DFB)
a
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