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Tool Talk Discussion Board

Re: Compressor electrical question

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Posted by John T on January 04, 2013 at 12:11:45 from (

In Reply to: Compressor electrical question posted by Stephen Newell on January 04, 2013 at 11:05:33:

If youre constantly burning up pressure switches ITS HP AND/OR CURRENT RATING MY WELL BE LESS THEN THE 5 HP YOU REQUIRE???????? The pressure switch may well be a low current rated device (designed to operate a seperate independant magnetic starter coil) and NOT switch and operate the 5 HP motor itself!!

Of course it takes much more current to START that compressor then to run it, and when those contacts just close they must handle that current ya know.

HERES THE DEAL: You could use a magnetic starter which has sufficient HP and Current rating to start and run the compressor motor, AND JUST USE THE PRESSURE SWITCH TO CONTROL THE STARTERS CONTACTOR which is wayyyyyyyyy less current then the actual motor requires. The pressure switch would ONLY activate the starters low current coil, which when activated and conducting VIA THE PRESSURE SWITCHES CLOSURE then closes the higher HP and higher current rated magnetic motor starter contacts...

A tyical farmer Billy Bob installation could be a wall mounted 2 pole Safety Switch,,,,,,,,,,which feeds the magnetic starter,,,,,,,,,which is controlled on and off via the closed contacts (and low current) inside the pressure switch.

ORRRRRRRRRR when I designed such at our industrial facility I USED A COMBINATION STARTER INSTEAD. It consisted of a combination single enclosure safety switch PLUS a magnetic starter all in a single enclosure and was cheaper then buying BOTH seperate safety switch and seperate magnetic starter

NOTE a magnetic motor starter will have whats called "heaters" or "thermals" sized for the motor to protect against overload and overheating. Size them to match your motor HP, Voltage, Amperage. ALSO the starters coil voltage (which pressure switch activates by its open and closing) can be same as your operating voltage or less (240 or 120 or even lower like 24 etc)

Buttttttttttt if the pressure switch is big enough and the above is NOT the problem, look for excess current especially excess start up current, caused by too long of wire runs or excess voltage drop or too small of wire or a faulty compresor head pressure unloader etc

John T


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