Hi All,
Hmmm, Now then!! Compressors are funny things... We really need to know EXACTLY what compressor it is, as this will make a HUGE difference to the starting current.
Most small compessors start as follows; They have a non return valve in between the air receiver and the pump. When the motor stops, a valve opens and lets out the air trapped in the pipe between pump and tank. When the motor [usually DOL, but can be star/delta] starts, it only has a few turns, [enough to "pump up" the pipe] in which to get up to speed, so really a compressor motor can be seen as starting on full load. Soooo, the biggest concern is making sure that the cable is sized large enough not to give an excessive volt drop during starting [more later]
Other rather more expensive compressors have a very complicated starting routine to avoid just this problem. Here is how mine works;
There are inlet valves in the cylinder heads. When the thing is off, these valves are raised off their seats so that the compressor cannot pump air at all. Anyway, you switch it on, and the thing starts using its star/delta starter. Once it is running, [and only then] the controller SEPARATELY closes the valves and pumping starts. Once the thing is up to pressure, the controller opens the valves again [to stop pumping] BUT THE MOTOR AND PUMP CONTINUE TO RUN]
Once the pressure drops, the controller closes the inlet valves and pumping starts again.
If you leave it for a predetermined time, say five minutes AFTER it has reached full pressure and opened its valves, the motor will stop. THEN, once the pressure drops, the entire cycle will start again.
This all means that no matter what, the motor never starts against a load.
I know of a chap that owns a farm [You know what is coming!!!!!] Everything, regardless of cable length is wired in 2.5, as "that is what sockets are wired in, my mate is an electrician and he did it" Hmmm.
"The compressor draws less than 13A it is a 13A plug so what is the problem"
Well, that is why they got through not one, but three motors in quick succession!!! I saw the one!!! The cable length was so long that the motor would not start. The cable length was that long, that even the current drawn by the locked rotor did not operate the OCPD either!!!
When i saw the motor the next day, it had literally melted into a clump of aluminium!!! They would still not be told. The replacement motor lasted about 3 days!!!!!!
I suppose the best way would be to ask the motor manufacturers what the max pemitted volt drop during starting for their motor is...
Having said that, Steps is, i would say, exactly right with his theory of allowing 3X running current for volt drop calcs. Sounds sensible to me!!!!!
john..
