If you have a variable speed drive (also called variable frequency drive) compressor installed in your system and want it to work correctly with your fixed speed compressors, then things need to work a little bit differently that normal. A VSD compressor should be taking the partial load, with the fixed speed units either running in full load or turned off in automatic mode.
The traditional way to set up multiple compressors with no VSD is in a cascade arrangement, where each compressor has its pressure band set slightly lower than the next, forming a stairstep arrangement. This loads system compressors in a specific order one at a time, with the one set at highest pressure loading first. But in a system of multiple compressors, with one being a VSD, you typically desire the VSD to be the first compressor on and the last off, the one taking the part loads all the time (called the trim compressor). This requires a different arrangement.
First of all, you should have sized your VSD compressor correctly—the best results are obtained if the VSD controlled unit is equal to or larger than the largest base compressor with which it must work. The general rule is that the variable range of the VSD should be equal to or slightly larger than the base compressor (variable range = full load capacity minus minimum speed); this means that optimum control is gained if the nameplate rating of the VSD is slightly larger than the base units. If this sizing rule has been followed, you should set up your VSD target pressure to be nested within the pressure band of the fixed speed units as per the graphic below.
The graphic shows the general arrangement with three compressors. There are two options; the one on the left should be used if the VSD is equal size to the base units, and the one on the right used if the VSD is larger than the base units. In both cases, the VSD target pressure is nested within the bands of the two other compressors. The diagram shown assumes the load or start setting for the VSD is the same as the target pressure, some brands of compressors allow the load or start point to be adjusted separately from the VSD target pressure. The settings sometimes need to be adjusted by trial and error to ensure proper handoff between VSD and base unit when the load changes. In all cases, you should consult with your service provider, it is often more complicated that it looks.
Do you have more than one fixed speed compressor? Then things are getting more complicated—it may be time to start looking at a system controller that will properly control all your compressors, including one or more VSD units. Most compressor manufacturers have controllers that can be used successfully. Call your service provider.
If your system is set up correctly, and you monitor your operating hours, you should find the ratio of loaded to running hours of the fixed speed compressors is very high, above 85%, meaning the VSD is taking part loads, and the fixed speed units are running mostly fully loaded, the desired condition. If your fixed speed compressors are loading and unloading frequently, this is a sign of a problem.
Note these pressure bands are given for example purposes only, the required bands depend on the characteristics of the compressors and the system. Your requirements may vary.