Applying Inverters to Centrifugal Compressors in HVAC Chillers

When applying an inverter to centrifugal compressors on HVAC chillers, there are a number of factors to consider. For some compressors, lubricating oil circulation may be driven by the rotation of the compressor shaft itself, via a gear-driven pump arrangement. If you slow the motor speed, the volume of oil provided for bearings, etc. will be reduced. The lube oil pump may have to be replaced or modified. Consult the compressor manufacturer to determine any special requirements. Not all centrifugal compressors can be adapted cost-effectively to variable speed operation. Some chillers have built-in logic which may be tied into the existing starter. A modification may be required from the chiller manufacturer to allow proper operation with a VFD. You may need to bypass the control logic of the existing chiller and use temperature or pressure feedback and PID to optimize speed control while not compromising the safeties of the built-in logic system. Speed control should take precedence over compressor vane control.