Start/Runn Circuit Design for Old IR Compressor

I have an old Ingersoll Rand two stage compressor in a sound dampening enclosure. No integral tank, but external storage tanks.



It is 3HP, 3 phase and currently has a simple latching contactor start/stop button setup.





I would like to switch it over to running from a VFD driven from 1ph, soft starting, with a pressure switch controlling start/stop.

Anyone have a simple circuit diagram for this?
Recommended pressure switches?

Thanks!

Why the need for soft starting? Is your branch circuit inadequate for the starting demand?

Why not just use a simple two-wire control circuit? Adding a VFD to a constant speed load when you've already got a single phase motor sure seems like reinventing the wheel.

{he wants to run the existing 3 ph motor from single phase power- hence the vfd)

jim rozen said:

{he wants to run the existing 3 ph motor from single phase power- hence the vfd)

Click to expand...

That is correct!
I figure I’ll just use a small 230V breaker box for the power disconnect and find an appropriate pressure switch for the remote start and include an e-stop or two in series- one remote in the garage and one at the compressor (not sure best circuit practice for those).

VFDs don't "like" reciprocating compressors much, you need to over size them because of the "stroke" loading. I recommend that a VFD be over sized by 50% for a recip compressor, I have never had issues at that level, whereas I have with less. Do not vary the speed though, most of these compressors have mechanical lubrication tied to the crankshaft and slowing them down results in poor lube flow.

Then you need to of course over size the VFD for the single phase input (if the VFD is not rated for single phase input already). That needs to be ON TOP OF the other over sizing, because it's all about the effects on the capacitors.

Other than that, you would just tie the pressure switch to the Run command of the VFD. The VFD will handle the Overload protection of the motor.

"Stroke loading".... a good point. The usual compressor has a speed reduction, meaning that the motor makes more turns, or at least more of a turn, during a compression stroke.

That probably does overheat /overload the VFD even though the actual RMS current is not exceeded. And the peak current limit overload capability response to long term repetitive overload is "not defined" as far as I know, for any VFD.

Having never applied a VFD to a compressor (just never came up), I find that is interesting information which makes sense.

I have found that the speed has not bothered the oiling at reductions to 50 or 60% of normal speed (done with pulleys to use smaller motors for limited power situations). That is hardly a guarantee, however.

Hmmm... I wonder if you could run it in vector and set a limit on the torque ramp rate or something? Let the speed vary a bit between revolutions and try to just put constant torque into the flywheel.

With a pressure transducer, you could do some fun things even with the requirement to stay above 50% RPM. Reduce cycling by using PID between 50-100% load, and run a bit above nameplate speed when starting with an empty tank due to the lowered torque. Also should be substantially quieter if only run up to 50% RPM in low load situations.

Makes no sense, just buy a 3hp 1 phase motor and call it a day...Phil