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A Bearingless Switched Reluctance Motor (BSRM) has a complicated character of nonlinear coupling; therefore, it is a hard work to operate BSRM stably. In this paper, a new type of BSRMs with novel rotor structure is proposed by analyzing relationships between motor structure and theoretical formulae of levitation force and torque. The stator structure of this new motor is same as that of traditional BSRM and each stator pole can coil one winding or two windings, while the pole arc of rotor is wider. In order to analyze the characteristics of the proposed BSRM, finite-element (FE) models are used and a 12/4 one-set-winding BSRM and a 12/8 two-sets-windings BSRM are taken as examples. The analysis results indicate that the new scheme is effective for a stable levitation. It can realize decoupling control of torque and radial force, thus simplifying its control strategy and improving the use ratio of winding currents. A control system is designed for the 12/8 BSRM based on deducing its mathematical model. Compared with traditional BSRM, the proposed scheme is easier to be implemented.

Because of the similarity between the structure of a magnetic bearing and that of a conventional switched reluctance motor (SRM), a Bearingless Switched Reluctance Motor (BSRM) integrates the magnetic suspension winding into a motor. It combines merits of a magnetic bearing and a conventional SRM such as ruggedness, low cost, fail-safe, no friction, no contact, high efficiency, fault-tolerance, and possible operation at high temperature; therefore, BSRM can operate at a high speed [

According to the number of coil windings embedded in the stator, the BSRMs are divided into one set of windings and two sets of windings [

Scholars at Kyungsung University have proposed one method for two-phase BSRM with 8/10 or 12/14 hybrid pole type, in which each stator has one-set-winding. This proposal takes advantage of the hybrid structure of narrow and wider teeth to separate the radial force stator pole from the torque stator pole [

NASA Glen Research Center proposed a one-set-winding BSRM with hybrid rotor, which consists of two parts: circular and scalloped lamination segments [

The high-speed motor research center of Nanjing University of Aeronautics and Astronautics improves the suspension winding arrangements of traditional 12/8 BSRM with two-sets-windings, proposing a three-phase, two-sets-windings, 12/8 series-excited BSRM [

In this paper, a new type of BSRM with novel rotor structure is proposed based on 12/8 two-sets-winding BSRMs and 12/4 one-set-winding BSRMs. The scheme uses wider pole arcs in the rotor and makes the winding inductance curve a flat area in the inductance maximum position, thus realizing decoupling control of the levitation force and torque. First, the structure characteristics of the proposed BSRM are summed up by analyzing relationships between torque, suspension force, and inductance. Then, operating principle of the new structure BSRM is illustrated. Accordingly, the torque and suspension force performances of the proposed BSRM are analyzed in detail with finite-element (FE) calculation. Mathematical model for suspension force is deduced and a control system is designed for the 12/8 BSRM with two-sets-windings. Compared with traditional BSRM, the proposed scheme has a simpler suspension force model and is easier to be controlled; besides, it can realize decoupling between torque and radial force.

Figure

Configuration and inductance curve of traditional 12/8 BSRM.

Configuration

Inductance curve

The stored magnetic energy

According to the principle of electromechanical energy conversion, the torque

When the shaft has no load or light load in its radial direction, the suspension current value will be very small because a small levitation force is needed to maintain rotor stable suspension in that case. Therefore, the contribution of suspension winding currents on the torque can be ignored. The torque

In the same way, suspension forces in two directions produced by the current in phase-A can be obtained by the virtual displacement method and can be written as

It is known from (

We can also see from (

According to the above analysis of traditional BSRM, the proportional coefficient

The aim form of inductance curve.

On the premise of the fact that the stator structure of the new BSRM is same as the stator structure of traditional BSRM, we increase the width of rotor pole. The rotor pole arc will be greater than the stator pole arc. We name it wider-rotor-teeth BSRM. The reason to do so is that when the stator and rotor pole of the motor are in aligned position, the proportional coefficient

The structure of wider-rotor-teeth BSRM not only can be two-sets-winding BSRM but also can be one-set-winding BSRM.

Taking a 12/4 structure with wider-rotor-teeth and one set of windings BSRM and a 12/8 structure with wider-rotor-teeth and two sets of windings BSRM as examples, this section introduces the rotation operation principle of the novel BSRM.

Figure

Configuration and operating principle of 12/4 BSRM with one set of windings.

Flux lines

Magnetic flux density vector

Obviously, because phase-A is in the flat area of maximum inductance as the rotor position shown in Figure

The number of coil windings embedded in the stator can also be two sets. A three-phase 12/8 BSRM with proposed novel rotor was taken as example to illustrate the suspension and operation principle of the BSRM with two sets of windings. Figure

Configuration and operating principle of 12/8 novel BSRM with two sets of windings.

Flux lines

Magnetic flux density vector

Similar to 12/4 BSRM above, it is necessary to turn-ON two phase windings at the same time. One phase winding is used to generate radial force, while the other phase winding is used to generate torque. In Figure

In order to verify the validity of the suspension operation principles and provide the basis theory for motor control strategy, the above two kinds of wider-rotor-teeth structure BSRM are analyzed through FE method. In order to facilitate the comparison between the two prototypes, the same rated condition is adopted here. The rated power is 2 kW, the rated speed is 20000 r/min, and the maximum radial force is 100 N. The dimensions of the simulation motors are shown in Table

Parameters of 12/4 and 12/8 wider-rotor-teeth BSRMs.

Stator diameter/mm | 95 |

Rotor diameter/mm | 49.8 |

Stator yoke/mm | 6.1 |

Rotor yoke/mm | 7.65 |

Stator pole height/mm | 16.5 |

Rotor pole height/mm | 7 |

Stator pole arc/°M | 15 |

Diameter of axle/mm | 20 |

Gap length/mm | 0.25 |

Length of stator stack/mm | 55 |

Rotor pole arc of 12/4 BSRM/°M | 45 |

Number of windings of 12/4 BSRM | 13 |

Rotor pole arc of 12/8 BSRM/°M | 30 |

Number of main windings of 12/8 BSRM | 9 |

Number of suspension windings of 12/8 BSRM | 13 |

The software of Ansys is used to calculate electromagnetic field. The 2-dimensional (2D) FE models of 12/4 BSRM and 12/8 BSRM are established and their 2D FE mesh models are shown in Figure

2D FE mesh models of 12/4 BSRM and 12/8 BSRM.

12/4 BSRM

12/8 BSRM

For a single loop magnetic system, if the operating current of the loop is

The increment of magnetic field coenergy

Then, combining (

To the 12/8 BSRM shown in Table

Inductances of 12/4 and 12/8 wider-rotor-teeth BSRMs.

Figure

Suspension forces and torques of 12/4 and 12/8 wider-rotor-teeth BSRMs. (a) Suspension forces. (b) Torques.

Suspending forces at

Torques

The results also show that the output torque width of 12/4 BSRM is only 1/2 of the output torque width of 12/8 BSRM, so the torque angle characteristic of 12/8 BSRM is better. The 12/4 BSRM is more suitable for light load applications.

Keep the phase-A currents in [−60°E, 60°E] region the same as that in former simulation

Suspension forces and torques with two phases excited simultaneously. (a) 12/4 BSRM. (b) 12/8 BSRM.

12/4 BSRM

12/8 BSRM

The radial suspension force and torque acting on rotor should be acquired in order to control the BSRM. This paper derives the mathematical model through virtual displacement method as traditional BSRM [

Magnetic circuit diagram.

Accordingly, the magnetic permeance can be expressed as

Here,

The levitation force in two directions and the torque can also be obtained by the virtual displacement method. Equation (

Also according to the principle of virtual displacement, the instantaneous torque of phase A can be expressed as

In inductance rising region, the positive torque coefficient can be expressed as

It can be seen from (_{0} will be revised to

Figure

Control scheme of new 12/8 BSRM.

According to the above analysis, the control system can be made according to Figure

Block diagram of control system.

The 12/8 BSRM dimensions of the simulation motor are the same as shown in Table

Simulation waveforms of the currents, torque, and levitation forces without compulsive excitation unit.

Simulation waveforms of the currents, torque, and levitation forces after compulsive excitation unit is added.

This paper studied a new-structure BSRM, which adopts a wide rotor pole structure. The nonlinear FEM and system simulation analysis based on MATLAB are used to analyze the performance of the novel motor. Analysis results of two prototypes indicate that

the novel BSRM has linear mathematical model of suspension force, since it can produce a flat area at the position of maximum inductance on the winding inductance curve;

the novel BSRM can realize decoupling control of torque and suspension force. Therefore, the algorithm is easier to be implemented and demands lower requirements upon digital controller;

since no negative torques are generated in control, the availability of winding current is higher than traditional case;

to avoid the problem of poor suspension force performance caused by current delay during converting the main winding currents from torque production region to suspension force production region, it is necessary to add a compulsive excitation unit into the converter of the main winding.

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

This study was cosupported by the National Natural Science Foundation of China (no. 51207073) and the scientific research fund of Nanjing University of Posts and Telecommunications (no. NY211021).