The traditional driveline for air conditioning centrifugal compressors comprises of an air cooled induction motor, which is connected to the compressor low speed gear shaft through a coupling. A mechanical shaft seal separates the refrigerant and the pressure in the compressor from the outside air. The high speed impeller shaft is driven by the gear shaft through a pinion. Both the impeller and the gear shafts are supported on hydrodynamic bearings, two radial and one axial for each shaft. The motor has separate bearings.

The motor speed is usually fixed and defined by the number of poles on the motor and the grid frequency. The impeller shaft speed is determined by the motor speed and the gear ratio. Different gear sets are used for different end user applications.

An improvement to this design was the introduction of the semi-hermitic design, with the motor rotor mounted directly on the low speed gear shaft and contained within the pressurized shell of the compressor.

This design has several advantages:

• It eliminates the mechanical seal and with that the possibility of refrigerant leakage and need for maintenance of the seal.
• There is no need for separate motor bearings
• The motor is cooled by refrigerant, not air. With refrigerant cooling being much more efficient, the physical size of the motor is reduced.
• Energy efficiency is improved
• Motor cost is reduced
• Overall motor-compressor system footprint is reduced.

The motor is delivered to the compressor OEM in components, rotor and stator, and assembled by the OEM into the compressor.

New challenges on compressors by demands for higher energy efficiency, sustainable operation, new refrigerants, reduced cost and improved reliability are driving improvements of the traditional driveline design. More on these changes in my next blog.