TM 5-4310-377-13
1-13. PRINCIPLES OF OPERATION - Continued.
b. Pressure Switch. The pressure switch is connected directly to the air pressure in the tank. When the air
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pressure in the tank drops to 175 10 psi (12.3 0.70 kgs/cm ) the switch is actuated and causes the motor starter
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to start the motor. When the air pressure in the tank is raised to 200 +0, -10 psi (14.1 +0, -0.70 kgs/cm ) the switch
opens and causes the motor starter to shutoff the motor.
c. Unloader. The unloader is a valve that opens the compressor pump output pressure line to free air and allow
the motor to start running. Once the motor has started, the valve is closed and compressed air is directed into the
tank. The unloader is part of the pressure switch.
d. Motor Starter. The motor starter receives the signal to start and shutoff the motor from the pressure switch.
The voltage from the pressure switch is applied to the starter coil and close the starter contacts. The motor current
will then pass to the motor and start the motor running. When the pressure switch reaches the shutoff (cut-out)
pressure, the voltage is removed from the starter coil and causes the contacts to open. The motor current is thus
removed and the motor will stop.
1-14. WIRING DIAGRAM. (Refer to Figure 1-3).
Figure 1-3. Compressor Unit Wiring Diagram.
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