Most of the compressors used on board ship use electric motors as prime movers. You need to understand that, at the instant of starting, an electric motor demands maximum power from the electrical distribution system. Therefore, when any motor is to be started, it is necessary to have the motor unloaded as much as possible. Any attempt to start any motor-driven pump, such as an air compressor, when it is loaded will cause the circuit breaker on the power distribution panel (and even the main breaker on a switchboard) to trip the unit off the line. Upon starting, the drive motor for a reciprocating air compressor is lightly loaded (only due to friction in bearings and piston rings against the cylinder walls) because of the unloader system of the air compressor. The unloader may use a combination of pneumatic, electropneumatic (solenoid), or hydraulic-actuated valves to control the unloading function. The unloader for an air compressor has the follow-ing two primary functions:

1. When the unit stops, the unloader releases any air trapped in the cylinders. (You should hear the unit go PSSsshhhhh when it shuts down.)

2. When the drive motor starts, the un-loader prevents air from being compressed in any stage until the motor has reached operating speed.

Units with start-stop control devices will have an unloading system that is separate from the control system. Compressors with constant-speed control devices will have the unloading and control systems as integral parts of each other.

We cannot give you a detailed explanation for every type of unloading device that unloads the cylinders of an air compressor. Still, you should know something about the common methods used to unload an air compressor. These methods include closing or throttling the compressor intake, forcing intake valves off their seats, relieving intercoolers to the atmosphere, relieving the final discharge to the atmosphere (or opening a bypass from the discharge to the intake), or opening cylinder clearance pockets.

We will discuss one example of a typical com-pressor unloading device-the MAGNETIC-TYPE UNLOADER. Refer to figure 14-13 and study the unloader valve arrangement. The magnetic unloader consists of a solenoid-operated valve actuated by the controller for the motor. When the compressor is at rest, the solenoid valve is de-energized, and the two ball valves are in the position shown. Air pressure is routed from the receiver to the unloading mechanism. When the compressor drive motor reaches operating speed, the solenoid valve is energized. The two ball valves will be forced downward. The upper ball will block the inlet from the receiver and the lower ball will block the exhaust port, which allows the compressor to load and build up air pressure.

For details about the unloading devices used with the compressors aboard your ship, you should consult the specific NAVSEA technical manual for each unit.