Re: Air Compressor Question OT


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Posted by John B [68.69.26.160] on Thursday, February 13, 2014 at 22:23:42 :

In Reply to: Air Compressor Question OT posted by Mike Hernke (MO) [63.151.218.18] on Wednesday, February 12, 2014 at 18:06:28 :

Mike: If you are just using that compressor for light duty work like blowing up foot balls, beach balls, pool toys, the odd car or truck tire, and light duty shop work you will get by with the motor operating through the heavy duty 5hp pressure switch. But for more serious shop work you would be wise to have a contactor or a magnetic motor controller installed.
Trust me they are a little more complicated than wiring a light switch. Your 5 h/p motor probably already has built in motor protection (the reset button.) If so a suitable sized magnetic contactor will do the job. If not, or for double overload protection choose a magnetic motor controller that suits the h/p of the motor. Ensure the magnetic coil is marked as 230 or 240 volts. Check the chart inside the cover and buy two overload heaters to match the amperage rating on the motor nameplate. The overload heaters chosen from that chart will allow the motor to draw 115% of the nameplate amperage before one of them trips out.
The motor controller may come prewired with a set of maintaining contacts. They are used when wiring in stop start stations and more complicated control systems. You won't be using those.
The conductors from the panel terminate on the LINE terminals. The conductors to the motor terminate on the LOAD terminals.
For the control circuit feed from line 1 to a pressure switch terminal, from the mating terminal come back to the controller box, loop through the overload wiring and then to one side of the magnetic coil. The other side of the coil needs to connect to line 2. This may already be prewired.
The power conductors of the circuit should be rated for 125% of the nameplate amperage on the motor. There are some variables there (but that gets more complicated.)
As the motor can draw about 5 times the nameplate amperage on start up you may require fuses or a breaker 2 to 3 times the nameplate amperage for the circuit.(And of coruse there are more variables there as well.) I hope this helps a bit. JJB.




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