Active Magnetic Bearings – When Magic Serves Engineers

From the beginning of the turbomachinery era, in the 19th century, engineers have been thinking about ways to reduce losses in rotating machines. Losses connected with fluid motion or producing the useful effects are related to the main purpose of machine operation,while losses in rotor bearings are just annoying and inevitable. Fluid film and rolling element bearings are effective solutions, but their operational principles cause increased friction – the best predictor of losses. But what if we could reduce the losses in rotating machines by avoiding the friction in required supports? What if a rotor could levitate and rotate in the air held by some magic forces? And furthermore, what if this magic could give us even bigger dividends, for example, enabling variable stiffness of rotor supports and safe passing through resonances? Luckily, engineers have already invented how to turn this magic into reality with active magnetic bearings.

The early patents of active magnetic bearings principles were recorded during the World War II, but the decisive breakthrough in production and applications of them were made during the the last three decades when the latest research about the active magnetic bearing operation and control made utilization feasible and economically viable [1].

The early patents of active magnetic bearings principles were recorded during the World War II, but the decisive breakthrough in production and applications of them were made during the the last three decades when the latest research about the active magnetic bearing operation and control made utilization feasible and economically viable [1].

Active Magnetic Bearing and its Components
Active Magnetic Bearing and its Components [2]
The main idea of an active magnetic bearing is based on the electromagnetic processes. Electrical current passing through densely wound copper coils creates magnetic fields which interact with a magnetized sleeve connected to the rotor.

Sounds pretty simple, right? But why on Earth did it take so much time to go from the general ideas to a real industrial application of active magnetic bearings?

The answer is in the rotor dynamics. Stabilizing turbomachine rotors in active magnetic bearings is a challenging task, but also, this is the only way to use all the advantages of bearings which need to be compliant or stiff enough at certain rotational speeds of the rotor. Correct variations of the bearing stiffness are ensured by a control system, and this is where the second complexity of an active magnetic bearing comes into play.

The active magnetic bearing control system measures rotor displacements in or near the bearing using special sensors, which transform these displacements into signals coming to a comparator and a control device (controller). In turn, the controller provides the bearing electromagnets with an adequate response in the form of changing current and, thus, the variable bearing stiffness. This response is enabled by specially developed control laws which differ from one machine design to another. Naturally, these laws are far from obvious and require deep studies. The control system of an active magnetic bearing combines mechanical and electromagnetic processes into one interconnected electro-magneto-mechanical process related to the dynamics of the whole rotating machine.

Structural Diagram of a Control System for Controlling Rotor Motion in a Radial Active Magnetic Bearing with Controlled Stiffness and Damping Parameters
Structural Diagram of a Control System for Controlling Rotor Motion in a Radial Active Magnetic Bearing with Controlled Stiffness and Damping Parameters [3]
This interconnected process cannot be modeled using general approaches of rotor dynamics science. It needs development and application of specially designed approaches and standards that bring even more complexity in the system.

High-voltage Electrical Drive [4] and Turboexpander [5] in Active Magnetic Bearings
Model of a Turboexpander in Active Magnetic Bearings Built in AxSTREAM RotorDynamics
Model of a Turboexpander in Active Magnetic Bearings Built in AxSTREAM RotorDynamics™

Scientists are conducting a huge number of investigations concerning the phenomena related to active magnetic bearings and dynamics of rotors suspended by this type of supports. Some have been less successive, some are breakthroughs. Thanks to these breakthroughs, we are at the stage where active magnetic bearings are a standardized and familiar thing. This is where the magic meets everyday life making sci-fi stories and dreams come true – we produce heavy and big machines levitating in the air powered by forces of the human mind and intelligence.

Although there are many examples of active magnetic bearing applications nowadays, this topic still holds various problems for engineers. Each active magnetic bearing unit requires the development of a unique control system algorithm and ensuring the reliable rotor dynamics of a turbomachine in active magnetic bearings with the non-resonant operation, small vibrations, and stable motion. AxSTREAM RotorDynamics™ provides its users with comprehensive modeling of the control system parameters and the dynamics of rotors in active magnetic bearings examining all the criteria of safe turbomachine operation.

Interested in learning more? Register for our upcoming webinar discussing rotor dynamics analysis for turbomachines with active magnetic bearings

To try AxSTREAM RotorDynamics™ for yourself,  request a trial.

References