Bryant 548J - Manual (Page 21)

21
Overtemperature
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 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
overtemperature or protection device. The type of device
depends on the motor size. See Table 8.
Table 8 – Overload Device per Motor Size
Motor Size (bhp) Overload Device Reset
1.7 Internal Linebreak Automatic
2.4 Internal Linebreak Automatic
2.9 Thermix Automatic
3.7 Thermix Automatic
4.7
External
(Circuit Breaker)
Manual
The High Static option supply fan motor is equipped with
a pilot--circuit Thermix combination overtemperature/
overcurrent 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 overtemperature
protection device that is imbedded in the motor windings.
It is a pilot--circuit device that is wired into the unit’s 24–v
control circuit. When this switch reaches its trip setpoint,
it opens the 24–v control circuit and causes all unit
operation to cease. 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 overload device is a
specially--calibrated circuit breaker that is UL recognized
as a motor overload controller. It is an overcurrent device.
When the motor current exceeds the circuit breaker
setpoint, 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 548J 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
overtemperature.
Control Circuit, 24--V
The control circuit is protected against overcurrent
conditions by a circuit breaker mounted on control
transformer TRAN. Reset is manual.
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. 26 for board arrangement.
DIP
Switches
Speed-Up
Jumpers
C09275
Fig. 26 -- Defrost Control Board (DFB) Arrangement
The DFB is located in the 548J’s main control box (see
Fig. 27). 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 9 for details of
DFB Inputs and Outputs. Detailed unit operating
sequences are provided in the Operating Sequences
section starting on page 62.
548J