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Drivelines For High Efficiency Centrifugal Compressors-Direct Drive

Posted January 15, 2014 2:12 PM by larhere

This is the 3rd of a 3-part series on Centrifugal Drivelines by Hans Wallin

who has more than 30 years experience working for SKF with new technologies and applications for the global compressor industry. After retiring in 2012, he continues to work for SKF as an independent consultant and an associate with GEA Consulting

As mentioned in my first blog post, 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.

In a fixed speed driveline with speed increasing gears, an electric motor, typically a 2 or 4 pole induction motor drives the low speed shaft at 1450 or 2950 rpm on a 50 Hz grid, or at 1750 or 3550 rpm on a 60 HZ grid. The high speed (impeller) shaft is driven by the gears and the shaft speed is determined by the motor speed and the gear ratio. Different gear sets are used for different end user applications.

Two technical and economic trends over the last 20 years have enabled the development of high speed direct drive technology. One such technology is Variable Speed Drives (VFDs), the other high speed motors. In high speed direct drives there is no low speed shaft and no gears, the impellers(s) are mounted directly on the extended motor shaft. This means simplified design, elimination of the cost and power losses in gears and simplified maintenance. It also enables oil free operation, since there are no gears that need oil and oil free bearing technologies have emerged and are available today to make oil free operation possible.

With direct drive the absence of the gears also simplifies the mechanical design of a two stage compressor with back to back impellers positioned at opposite ends of the motor. Such a design is advantageous for high speed because of the shorter overhang of the impellers, compared to having both impellers on the same side. A drawback with this design is that it requires a crossover pipe between the two compressor stages.

In a variable speed direct drive, the motor current is produced by a VFD. The impeller speed depends directly on the output frequency of the VFD, which can be either higher or lower than 50 or 60 Hz. With variable speed capability it is possible to adjust the pressure produced by the compressor to the condensing pressure for optimal performance and efficiency. Variable speed can also be used to mitigate surge.

In a VFD, AC at grid frequency is first converted to DC through a rectifier, and then converted to AC at a different and controllable frequency though an inverter. The key components in the inverter are Insulated-Gate Bipolar Transistors (IGBTs) which switch the DC to produce AC of a controllable frequency to drive the motor.

The AC produced by the inverter also contains frequencies of higher order which are not useful to drive the motor, but causes losses and heat generation. To limit the high frequencies a filter can be installed between the VFD and the motor.

The rectifier at the front end of the VFD also produces high frequency AC which goes back and contaminates the grid. Filters between the grid and the VFD can be used to limit the contamination. Alternatively an inverter can also be used at the front end (active front end). With an active front end the contamination of the grid is reduced. With an active front end it is also possible to use grid power of a lower voltage to produce the same motor voltage.

The permanent magnet motor (PMM) is the most common motor type used in high speed direct drive centrifugal compressors. It has half-circle shaped permanent magnets attached to as solid steel rotor. The magnets are held in place by a metal or carbon fiber sleeve. The sleeve is needed to keep the magnets from being flung out by centrifugal force. Carbon fiber sleeves are used for very high speeds. The magnets are made of rare earth materials and very strong. The stator is similar to that of an induction motor. A rotating electromagnetic field is produced in the stator. The rotor magnets are compelled to follow the rotating electromagnetic field, similar to a needle in a compass, producing torque on the rotor. The speed of rotation is synchronous with the output frequency of the VFD. The motor current must be exactly controlled by the VFD to produce an electromagnetic field and torque that is equal to the required torque from the compressor, (including required torque for acceleration). If the electromagnetic field is too weak, the rotor will slip, if it is too strong the motor will overheat. PMMs have high energy efficiency, up to 98%, and high power factor, close to 1.0. Motors with lower power factors use more current and need larger size VFDs.

The speed limiting factors of an electric motor are:

  • Rotor stiffness. Too low stiffness means low natural frequency for bending
  • Strength of the rotor to withstand centrifugal forces
  • Internal motor heat generation and cooling. With high power density heat generation must be low since available cooling is limited
  • Speed capability of bearings

Features that make PMMs suitable for high speed:

  • Solid steel rotor supporting the magnets, meaning high rotor stiffness and high natural frequency
  • High rotor strength with the high strength sleeve holding the magnets in place
  • High energy efficiency and low energy losses in the motor
  • Available high speed, low friction bearings
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