The air brake governor is nothing more than a switch, and like any switch it turns the compressor off and on.
Like A Furnace Thermostat
The best analogy for the governor is that it works like a thermostat on your house’s furnace. The thermostat monitors the air temperature at its location in the house. When the temperature drops to a set threshold (minimum temperature), it turns the furnace on. When the furnace heats up the air around the thermostat, the furnace is turned off. The thermostat turns the furnace off and on within a temperature range.
Another analogy is the float switch on a sump pump. When the water level reaches a maximum level the float switch turns on the pump. The water level is reduced to an acceptable level and at this point the float switch turns off the pump.
Unlike the thermostat and the sump pump float switch however, the governor does not turn the compressor on and off. Owing to the fact that modern air brake compressors are bolted to the side of the engine block and gear driven, these operate the whole time the engine is running.
Works Like a Railway Switch
Therefore, the governor is comparable to a railway switch – it either moves the train off the main line or allows it to remain on the main line. And like a train that never stops when it’s on the main line, the compressor pumps air the entire time the engine is running.
When the system reaches maximum pressure, the governor directs the compressor to pump the air into the atmosphere. Conversely, when the air brake system reaches its minimum operating pressure, the governor directs the compressor to pump air into the system’s reservoirs.
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Minimum & Maximum Settings
When the system air pressure reaches a minimum setting—often 20psi (130kPa) below the maximum—the governor puts the compressor into the load (cut-in) phase. When the system pressure reaches maximum pressure, the governor puts the compressor into the unload phase (cut-out).
Minimum standard for air brake systems is to operate between 80psi (550kPa) and 135psi (930kPa). Most modern air brake systems will operate with a maximum air pressure of 125psi (862kPa) and a minimum pressure of 105psi (724kPa).
Bossy Wife - Lazy Husband
Another way to think of the relationship between the governor and the compressor is that the governor is the bossy wife and the compressor is the lazy husband. When the system reaches minimum pressure the wife tells the husband to go to work and put air in the system. When the system attains maximum pressure, the bossy wife (governor) tells the husband (compressor) that he can go in the living room, sit on the couch, watch sports, drink beer, and fart.
The compressor is in the unload (cut-out) phase. Like the husband in his “man-cave”, the compressor is not working at this point and is “cooling off.”