Compressor Running on VFD

http://www.powerqualityanddrives.com/variable_constant_torque/

The simplest way to save energy on a compressed gas piping system, is to eliminate leaks from it downstream of the compressor.

A screw compressor is very inefficient when operating unloaded, taking up to 60% of full load hp to not deliver any air. The particular efficency of your compressor running unloaded may vary from the worst case that I suggested, depending if it has any modern devices on it to reduce the unloaded power, such as blowing down the sump pressure.

The best way to reduce power is to shut off the compressor rather than unload it. However, this does not work well unless you have adequate air storage to allow the compressor to be down for around 10 minutes, since motors should not be started more than about 6 times per hour to avoid overheating due to inrush currents.

And, of course, reducing downstream air leaks is a dead-easy way to save energy, as another poster has suggested!!

The VFD will reduce energy use when the compressor needs to have its output reduced because it will still be delivering air at a lower speed rather than be throttled off. (The output air volume VS input horsepower will be closer to a straight line through the origin.) The compressor will not be able to be reduced to as low a speed as commericially available variable speed screw compressors because the screws are designed to be operated at lower speeds without losing as much efficiency.

Your duty cycle, at 60% "on" time, suggests that the compressor might be operated at 60-70% of full speed to meet your compressed air requirements.

Variable torque will be fine, as the horsepower drawn will reduce as the air output reduces.

Since this is a small compressed air system, unless you already have a VFD you might be better off investigating the installation of a storage tank and allow your compressor to stop between loadings.

Jon.

Not all of that is correct. Positive displacement compressors are essentially constant-torque loads, based on ΔP.

With that in mind, however, an advantage of a VFD is that the compressor can run almost all the time, without repeated start/stops and starting current issues.

Why do you have to run with a VFD? Take a look at a Mag-drive System with associated controller if you want to unload the motor or control the output within paramaters..

What is the Compressor running?

If it is a chiller compressor remove the unloaders from the sequence and move the demand percentages to the drive which keeps the compressor running but reducing motor load and saving energy instead of maintaining a full speed motor and by passing refrigerant. This will also reduce start stop times but maintain proper capacity for the chiller to run.

If it is an air system fix the leaks like PW said and reduce its run time, don't add a drive to a pressurized air system.