230 volt convertor

[Jraef]

So for a small compressor, even if you use a VFD, would you recommend using 'bang-bang' control.

With the lubrication issues that you describe, it seems to me that a VFD would still have substantial value, but it might be limited to 'full torque soft starting'. Though I doubt that this would be worth the cost of the VFD unless you had severe voltage drop issues (eg. large compressor in a home shop )

-Jon

Kind of a waste to use a VFD as a soft starter, but yes, sometimes that's the only option when the source supply is severely limited.

As to recommending bang-bang? I don't know, there are some very successful VFD applications on compressors, it just depends a lot on the compressor mechanics; not all are built the same. A lot of older recips use a mechanical lube pump that has a critical speed necessary to make them work. People tend to discover that critical speed by frying the main compressor. Screw compressors and scroll compressors tend to lend themselves to VFD application a lot more readily.

 

[Besoeker]

Kind of a waste to use a VFD as a soft starter, but yes, sometimes that's the only option when the source supply is severely limited.

As it happens, we shipped one (VFD) out earlier this year for exactly that purpose. It was a fairly big beast - 550kW at 690V.
For reasons of efficiency, it had a bypass contactor so that the VFD was used for run up only.
This was a little more complex that it might at first appear.
There is the obvious need to get both frequency and phasing matched before changeover.
Once in bypass, you no longer have the protection features of the VFD so you need additional hardware for overload, unbalance and earth fault.
Then the supply would see motor power factor so PFC is required with its contactor and fuses.
In addition, the signals normally available from the drive aren't any more. Status, hours run, energy consumed, current etc. to name but a few.

All this to save about 2% losses.