Featured Case Study

Waste Heat Recovery: 125 kW Expander/Generator on Active Magnetic Bearings

Calnetix Technologies has designed a unique solution for waste heat recovery from a variety of heat sources, such as industrial equipment and processes. The waste heat recovery system is based on the Organic Ranking Cycle (ORC) and utilizes a Calnetix designed 125 kW Integrated Power Module (IPM). The IPM consists of a high efficiency centrifugal expansion turbine integrated with a high-speed permanent magnet generator supported on active magnetic bearings. An efficient variable frequency power electronics package converts the generated power to 50/60 Hz line frequency to deliver the generated power to the grid.

Image of our Integrated Power Module (IPM) consisting of a high efficiency centrifugal expansion turbine partnered with a high-speed permanent magnet generator supported on active magnetic bearings.


Bearings of Choice: Active Magnetic Bearings

  • Oil lubricated ball bearings would necessitate a complicated sealing system.
  • High working temperatures and lack of cooling would reduce grease life of lubricated ball bearings.
  • Poor lubricating characteristics of working fluids would compromise the life of sleeve or tilt-pad bearings.

Given the above limitations, active magnetic bearings (AMB) made great candidates. They require no lubrication, practically operate frictionless and have no wearable parts - thereby helping optimize system efficiency and enhance reliability. They are particularly attractive to this application since they can operate in working fluids, thereby obviating need for expensive and complicated hermetic sealing. They have additional benefits that include reduced transmitted vibration and also offer remote monitoring and diagnostic capabilities.


Active Magnetic Bearing Hardware

Calnetix patented active magnetic bearing actuators use a homopolar, permanent magnet bias topology. Bearing 1 (Brg 1 in figure 1) is a radial bearing with two orthogonal control axes. Bearing 2 (Brg 2 in figure 1) is a combination (combo) bearing with three orthogonal control axes: two in radial direction and one in axial direction. The combination bearing is more compact axially than the comparable arrangement of separate radial and axial bearings; therefore, the overall rotor length is reduced, facilitating the rotor dynamics to be more favorable for simpler control. Figure 2 summarizes design parameters for magnetic bearings.



For shaft position sensing, the IPM uses variable reluctance sensors driven from 15 kHz drive signal that is modulated according to changing reluctance of the airgap.


Rotor Dynamics

Finite element model of the IPM rotor shows the first forward bending mode at 36,600 cpm at 0 rpm. At normal operating speed of 26,500 rpm the first forward bending mode is 38,500 cpm – about 45% above operating speed. The machine operates subcritical; the large margin to the first forward bending mode frequency and weak gyroscopic effect enables a simpler magnetic bearing control design.



Magnetic Bearing Control System

The IPM uses the Calnetix Insight™ 3600 controller (/REF) to control the position of the rotor in the air-gap. The control system hardware uses Calnetix designed control board and power system including amplifiers. The control topology is single-input single-output (SISO) enhanced proportional integral derivative (PID) type.

In addition, synchronous cancellation control is used to reduce unbalance response throughout the operating speed range, minimizing housing vibration. The superior performance of synchronous control is clearly seen in figure 4.


Power consumption of the system: 60 – 90 W.



The measured and analyzed transfer functions, are shown in figure 5. From figures 4 and 5, it is evident that the performance of the IPM active magnetic bearing system meets the ISO-14839-2 zone A classification for mechanical vibration, as well as the ISO-14839-3 zone A classification for stability margins.

After rigorous in-house testing, well over 150 IPM systems have been successfully deployed in the field, starting in April of 2009.


Reference: Hawkins L. A., Zhu L., Blumber E., “Development of a 125 kW AMB Expander/Generator for Waste Heat Recovery”, in the Journal of Engineering for Gas Turbines and Power, July 2011, Vol. 133.

To view a detailed technical paper pertaining to this case study in the Journal of Engineering for Gas Turbines and Power, July 2011, Vol. 133,  see link below