25
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 line. The switch is
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 coil. The switch is
stem−mounted. Loss of Charge Switch trip setting is
27 psig ± 3 psig (186 ± 21 kPa). Reset is automatic at 44 ±
3 psig (303 ± 21 kPa).
Freeze Protection Thermostat trip setting is 30F ± 5F
(−1C ± 3C). Reset is automatic at 45F ± 5 F (7C
±3C).
Supply (Indoor) Fan Motor Protection:
Disconnect all electrical power and apply appropriate
Lock−out/Tagout procedures when servicing the fan
motor.
Motors are equipped with an over−temperature device
(Thermik), internal line break, external circuit breaker or
electronic controlled circuits for overload protection. All
protection schemes are automatically reset except for
units having the 2−speed indoor fan option (VFD) or
external circuit breakers. These two protection schemes
are classified as manual reset. The type of device depends
on several factors including motor size, voltage and other
options in the unit (i.e. VFD).
The Thermik device is a snap−action over−temperature
protection device that is imbedded in the motor windings.
It is also a pilot−circuit device that is wired into the unit’s
24V control circuit. When this device 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 device to
correct trouble. Determine the cause of the problem and
correct it.
The External motor overload device is a
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 has 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. The Control Circuit is reset manually.
COMMERCIAL DEFROST CONTROL
The Commercial Defrost Control Board (DFB)
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. 33 for board arrangement.
DIP
Switches
Speed-Up
Jumpers
C09275
Fig. 33 − Defrost Control Board Arrangement
The DFB is located in the 50TCQ’s main control box (see
Fig. 34). All connections are factory−made through
harnesses to the unit’s CTB, to IFC (belt−drive motor) or
to ECM (direct−drive motor), reversing valve solenoids
and to defrost thermostats. Refer to Table 8 for details of
DFB Inputs and Outputs. Detailed unit operating
sequences are provided in the Start−Up section starting on
page 41.
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