13 years ago
several modes. I will describe what one switch does, and then will
With the continuous/on demand switch, compressor can do the following:
1) Be OFF and UNLOADED.
2) Pump on demand. This is the simplest mode of operation that any
Sears electric compressor would do. When the pressure is below a
certain limit, it would RUN and LOAD, and when it rises above another limit, it
UNLOADS and STOPS.
3) Continuous run. This is what engine driven compressors do, as well
as what is recommended at heavy air use. According to Quincy, this is
the recommended mode when it needs more than six restarts within one
The way it works is that the compressor starts unloaded, then when it
RUNS it is LOADED if the pressure is below a certain limit, and would
become UNLOADED but RUNNING is the pressure rises above a certain
limit. This way, the compressor pumps as necessary, but never stops
I implemented this with a DPDT On-Off-On switch.
I have tested the operation to see that the compressor unloads when
turned OFF, and turns on and off as prescribed in the two modes.
The second switch has a much simpler meaning. If it is in the ON
position, the unloader functions and loads the compressor according to
If it is in the OFF position, the unloader NEVER unloads, and
therefore the compressor would run, or not, but would never pump any
Why this strange mode, you may ask. And the answer is that in this
mode, the 10 HP motor of the compressor is acting as an idler motor,
running with an inertial load, but not really doing mechanical work
(besides 2 HP or whatever it takes to spin the pump that is idle). So
effectively it can make my 17.5 HP phase converter into a 25 HP or so
phase converter, allowing for power dissipation needed to run the pump.
I can post a schematic, if anyone is interested.
In the process, I added a dedicated outlet on my phase converter,
which would be for the compressor only. I used L1520 receptacle and
plug, which is what I am migrating my shop to.
The last thing remaining is to add a START/STOP button to the
compressor, so that it would never restart automatically after three
phase is turned off and then on.