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Bearingless induction motors combining functions of both torque generation and noncontact magnetic suspension together have attracted more and more attention in the past decades due to their definite advantages of compactness, simple structure, less maintenance, no wear particles, high rotational speed, and so forth. This paper overviews the key technologies of the bearingless induction motors, with emphasis on motor topologies, mathematical models, and control strategies. Particularly, in the control issues, the vector control, independent control, direct torque control, nonlinear decoupling control, sensorless control, and so forth are investigated. In addition, several possible development trends of the bearingless induction motors are also discussed.

The idea of bearingless motors can be traced back to 1973 when Hermann proposed probably the first radial active magnetic bearing topology [

So far, various types of motors have been put forward as bearingless motors, such as synchronous reluctance [

Its rotor topology is relatively simple and robust.

It provides nearly constant rotational speed.

Its torque ripples and cogging torque are less.

The cost is low for open loop operation.

Being continually fueled by new motor topologies and control strategies, bearingless induction motors are becoming more and more attractive and have been identified to be one of the most promising motors for the special industrial applications.

The purpose of this paper is to give an overview of the bearingless induction motors. Thus, the state-of-the-art technology of bearingless induction motors, including operation principles, motor topologies, mathematical models, control strategies, and latest development trends, will be reviewed and discussed.

The basic winding configuration of a bearingless induction motor in the stationary perpendiculars

Principle of radial suspension forces generation.

The main 4-pole flux

At present, the bearingless induction motor topologies are mainly concentrated in the 2-DOF 3-phase squirrel cage type bearingless induction motors which are usually used as principle prototypes in the laboratory. However, the standard squirrel cage rotor will cause problem when it is used in a bearingless induction motor to produce the radial suspension force. As known, a sudden change in the radial suspension force will lead to a sudden change of the stator flux of the suspension force winding. The rotor flux, however, does not change suddenly, because the voltage is induced into the rotor circuit, which generates the rotor current. Therefore, the interaction of stator and rotor circuits will engender a delay in the flux of the suspension force winding which in turn delays the radial suspension force response.

In order to conquer the problem associated with the conventional squirrel cage rotor, Chiba and Fukao [

In the 2-DOF bearingless induction motors, the motors with two sets of windings, one for generating torque and another for producing the radial suspension forces, are primarily researched. Besides, Ribeiro et al. [

Configuration of the split-phase bearingless induction motor drive system [

As it is well known, the multiphase motor, with phases number not less than 5, has the multiple orthogonal

As stated earlier, the 2-DOF bearingless induction motors are primarily used in the laboratory as the principle prototypes. It is worth to mention that evaluating the performance of the rotor levitation as well as electric motor characteristics is necessary for them to apply in the practical application. In [

Structure diagram of 4-DOF bearingless induction motor.

In Figure

Structure diagram of 5-DOF bearingless induction motor (one structure) [

In view of the insufficient of the 5-DOF bearingless induction motor proposed in [

Structure diagram of the 5-DOF bearingless induction motor (another structure) [

The mathematical models, including electromagnetic torque and radial suspension force equations, are the theoretical foundation of the bearingless induction motor. Obtaining the accurate mathematical expressions of the electromagnetism torque and the radial suspension force is critical to realize the stable suspension operation of the bearingless induction motor.

Basically, the method of calculating the radial suspension force can be classified as two kinds of methods virtual displacement method [

The inductance matrix

Finally, the model of the radial suspension force can be obtained by the virtual displacement method, which is expressed as

The airgap magnetic flux density of the bearingless induction motor is collectively produced by the torque and suspension force windings, and it can be written as

By integrating (

The airgap flux linkage of torque and suspension force windings can be expressed as

Based on the vector multiplication operation, (

In the process of practical operation, the eccentricity of the stator and rotor is inescapable, which will lead to complex analysis and computation of the radial suspension force. In order to improve the control precision of the radial suspension force, an accurate radial suspension force expression considering the rotor positioning eccentricity was established [

In comparison to the magnetic field produced by torque windings, the magnetic field produced by suspension force windings is so small that it can be ignored. So the airgap flux linkages equation of the torque windings can be expressed as

The vector control (VC) strategy includes airgap flux oriented control and rotor flux oriented control, and in this section, the rotor flux oriented control for the bearingless induction motor is presented as an example.

When the rotor flux oriented control method is adopted, the rotor flux vector of torque windings is aligned with d-axis, which means that the rotor flux linkages components of torque windings can be given as

From (

Control block diagram of the rotor flux oriented control.

Radial suspension force and torque are the control objectives that determine the machine performance of levitation and rotation in a bearingless induction motor. However, from (

Control block diagram of the IC [

Direct torque control (DTC) is a powerful and attractive control strategy with a simple structure, fast behavior, and tolerance to the variation of motor parameters, which offers direct and effective control of stator flux and torque by optimally selecting the inverter switch states in each sampling period. The DTC method provides a systematic solution for improving the operating characteristics of the motor as well as the voltage source inverter. In order to overcome the limitations such as complexity, unsteady torque, and nonlinearity in the VC algorithm, Wang et al. proposed a space vector pulse width modulation (SVPWM-) based DTC method for the bearingless induction motor drives [

The bearingless induction motor is a multivariable, nonlinear, and strong coupled system, which makes the control of the whole system more complex. To realize the bearingless induction motor suspension operation steadily and reliably, it is necessary to achieve the nonlinear decoupling control (NDC) between torque and radial suspension forces independently. As for the decoupling control of the bearingless induction motor, the traditional approach is the VC method. Its control performance is acceptable when the rotor always suspends at the center of the airgap. Moreover, the VC method is essentially a steady-state decoupling control, which can only achieve the decoupling control between radial suspension forces and electromagnetic torque but cannot realize the dynamic decoupling control of them. Therefore, the dynamic response performance is not satisfactory.

In [

As it is well-known, the static artificial neural network (ANN), such as radial basis function network and multilayer neural network, has the ability to learn and self-adapt, approximate any arbitrary nonlinear mapping, and have the effective control to complex and uncertain system. Taking the advantage of approximating ability of the static ANN, a kind of continuous dynamic ANN structure was formed in [

Control block diagram of the ANNI method [

Similar to the conventional induction motors, the rotor position information, which is traditionally measured by a physical sensor, is mandatory for the proper operation of the bearingless induction motor. In addition, the mechanical displacement sensors are also indispensable for the stable suspension operation. In order to avoid some difficulties resulting from the traditional sensor control method and effectively reduce the cost and short a shaft length, the sensorless control (SC) of bearingless induction motor was implemented [

The main advantages, major disadvantages, and typical techniques of the aforementioned control strategies are compared, and the corresponding conclusions are summed up as shown in Table

Comparison of control strategies.

Advantages | Disadvantages | Techniques | |
---|---|---|---|

VC | Realizing easily; no special requirements for hardware and software | Causing coupling between two sets of the windings | Generating the current control signals using the feedback and coordinate transformation |

IC | Achieving IC of radial suspension force, and hence improving the flexibility of torque control scheme | Requiring additional magnetic flux observer | Obtaining the torque winding flux linage dependently |

DTC | Fast torque response; less parameter dependence; no need for current control | Causing errors due to variation of stator resistance and drift in flux linkage estimation | Generating the voltage vectors using independent torque and flux computations |

NDC | Flexible control algorithms; adapting nonlinear theories and parameter variations | Requiring sophisticated hardware and intensive computation or experiential knowledge | Taking advantage of neural network or other nonlinear theory |

SC | Removing the mechanical sensors, and hence reducing the system cost and size; readily merging into other controls | Requiring sophisticated hardware and intensive computation | Estimating the position and displacement based on the motor mutual inductance |

In this section, several emerging development trends of the bearingless induction motor are identified and discussed.

Generally, to make the bearingless induction motor drives satisfy the requirements of engineering application, it is necessary to achieve the suspension control of the radial 4-DOF and axial single-DOF control of the rotor. The aforementioned two kinds of the 5-DOF bearingless induction motors must be configured with magnetic bearings, which increases the axial length of the rotor and the reactive power losses of the system and restricts the enhancement of the critical speed and the system efficiency. Therefore, an intensive study on designing novel 5-DOF bearingless induction motor prototype structure with a simple motor topology, low power losses, and high efficiency is of great practical significance.

As for the control strategies of the suspension force windings, the most common method is the radial displacement negative feedback modulation-based control strategy, which requires the complicated force/current transformation and coordinate transformations. As a result, the algorithm is more complex, which seriously influence the dynamic response and anti-interference ability of the bearingless induction motors. Therefore, it is necessary to develop the new control strategy for the suspension control of the rotor. Among these control algorithms, the direct radial suspension force control, which can be inspired by the DTC method, will be the next research hot spot of the radial suspension force control for the bearingless induction motors.

Current, speed, displacement, and temperature sensors, which are the very important part of the drive system for bearingless induction motors, play an important role for the table suspension operation of the whole system. However, these sensors are also the potential fault hidden troubles, since after long-time use they cause problems and then lead to the misoperation and malfunction of the whole control system. In this case, the sensorless technologies may be the preferred solution. Although some sensorless technologies have been proposed separately for the position or displacement detection by international and domestic academics and experts, the position and displacement sensorless operation of bearingless induction motor was still investigated separately, which cannot realize the integrated self-sensing of the rotor position and displacement. Therefore, to do some research to implement the integrated sensorless suspension operation, the function integration of the prototype and sensors has important practical significance and theoretical value.

Reliability is one of the most critical requirements in some special applications, such as turbomolecular pumps, canned pumps, and centrifugal machines, where a continuous operation must be ensured. Under the premise of satisfying each performance index of the bearingless induction motors, the study on developing the redundant or conservative design techniques and the fault-tolerant control strategies, so as to reduce or avoid the unnecessary losses caused by the failures, has very important practical meaning and is also an important issue for the researchers in this field.

In this paper, the concepts, key technologies, developments, and potential trends of bearingless induction motors have been reviewed, with particularly emphasis on the operation principles, motor topologies, mathematical models, and control strategies. Some selected control strategies are also compared. In addition, there are many issues which still need to be investigated, such as the novel motor topologies, advanced control algorithms, and sensor integration and sensorless technologies, as well as reliability technology for specific applications.

The authors declare that there is no conflict of interests regarding the publication of this paper.

This work was supported by the National Natural Science Foundation of China under Project nos. 51305170 and 61104016, the National Science Foundation of Jiangsu Province of China under Project no. BK20130515, the China Postdoctoral Science Foundation which funded the project under Project no. 2012M521012, and the Startup Foundation for Advanced Professional Talents of Jiangsu University under Project no. 12JDG057.