Operating Instructions (Rev. 1/99)
24
2.
DEFROST TIMER. This timer automatically places the
unit in defrost every 4 hours. The defrost timer also ter-
minates defrost 45 minutes after it starts if the defrost
termination switch does not open.
3.
POWER RELAY. This relay supplies power to the mas-
ter On-Off switch when the truck is turned on. In Model
20 units the electric standby relay disables the power
relay when the unit is connected to electric standby
power.
4.
ELECTRIC MOTOR RESET (Model 20 only). Push-
ing this button resets the electric motor overload relay
if the overload relay has opened.
5.
ELECTRIC STANDBY RELAY (Model 20 only). This
relay is energized when the unit is connected to electric
standby power. When energized, this relay disables
engine-driven compressor operation and energizes the
compressor motor contactor.
6.
COMPRESSOR MOTOR CONTACTOR (Model 20
only). This contactor is energized by the cool relay dur-
ing electric standby operation. When energized, this
contactor energizes the electric standby compressor
motor.
Other Controls
1.
DEFROST TERMINATION SWITCH. This tempera-
ture sensitive switch is mounted on the evaporator coil
and is used to control defrost. The switch closes when
the evaporator temperature drops to 42 F (5.6 C). This
enables defrost. The switch opens when the evaporator
temperature rises to 52 F (11.1 C). This terminates
defrost if the unit is in defrost. It disables defrost if the
unit is not in defrost.
2.
SUCTION PRESSURE REGULATOR VALVE. This
valve is located in the suction line and limits the suction
pressure at the compressor. The normal pressure setting
for this valve is 18 psi (124 kPa).
UNIT PROTECTION DEVICES
1.
CONTROL SYSTEM CIRCUIT BREAKERS. The
control system circuit breaker is located in the unit con-
trol box. It trips if the control circuit is overloaded.
Model 20 units have a second control system circuit
breaker that protects the control circuit during electric
standby operation. 40 amp circuit breakers are installed
in 12 Vdc control systems, and 25 amp circuit breakers
are installed in 24 Vdc control systems.
2.
HIGH PRESSURE CUTOUT SWITCH. The high
pressure cutout switch is a pressure sensitive switch
located in the discharge line. If the discharge pressure
rises above 300 psi (206 kPa), the switch opens the cir-
cuit to the compressor clutch and standby compressor
motor contactor to stop unit operation.
3.
CONDENSER FAN PRESSURE SWITCH. The con-
denser fan pressure switch is a pressure sensitive switch
located in the discharge line. When the condenser head
pressure rises above 180 psi (1241 kPa), the condenser
fan pressure switch closes, starting the condenser fan.
When the condenser head pressure falls below 130 psi
(896 kPa), the condenser fan pressure switch opens,
stopping the condenser fan.
4.
LOW PRESSURE CUTOUT SWITCH. The low pres-
sure cutout switch is a pressure sensitive switch located
in the suction line. If the suction pressure falls below 5
to 11 psi (-17 to -37 kPa), the switch opens the circuit to
the compressor clutch or the standby compressor motor
contactor to stop unit operation.
5.
HIGH PRESSURE RELIEF VALVE. The high pres-
sure relief valve is designed to relieve excess pressure
within the refrigeration system. The valve is a spring-
loaded piston that lifts off its seat when refrigerant
pressure exceeds 500 psi (3447 kPa). The valve will
reset when the pressure drops to 400 psi (2758 kPa).
The valve could possibly leak refrigerant after it has
relieved excess pressure. Tapping the valve lightly may
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Page 110: ...100 Model 20 Single Phase with Heat Option Schematic Diagram...