Study and design of dual stator permanent magnet machine with spoke-type configurations using phase-group concentrated-coil windings

Abstract

In this paper, a novel dual-stator permanent magnet machine (DsPmSynM) with low cost and high torque density is designed. The winding part of the DsPmSynM adopts phase-group concentrated-coil windings, and the permanent magnets are arranged by spoke-type. Firstly, the winding structure reduces the amount of copper at the end of the winding. Secondly, the electromagnetic torque ripple of DsPmSynM is suppressed by reducing the cogging torque. Furthermore, the dynamic performance of DsPmSynM is studied. Finally, the experimental results are compared with the simulation results.

Keywords

Dual stator spoke-type cogging torque phase-group concentrated-coil windings

1. Introduction

In recent years, with the widespread use of rare-earth permanent magnet materials in machines, electromagnets in electric excitation synchronous machines have been gradually replaced by permanent magnet materials. Compared with traditional induction machine, permanent magnet synchronous machine (PMSM) has the advantages of small size, light weight, high power density, torque density, high power factor and high efficiency [1]. Rare earth permanent magnet materials such as AlNiCo and NdFeB are widely used due to their high remanence and coercivity. However, they are relatively expensive, resulting in the cost of PMSM greatly increased. Compared with AlNiCo and NdFeB, the remanence and coercive force of ferrite permanent magnet are much smaller, but the price of ferrite permanent magnet is very low. The electromagnetic performance of machines using ferrite permanent magnets is lower than that using NdFeB permanent magnets. How to use ferrite permanent magnet to improve the power density and torque density of the machine has become a problem studied by many scholars [2]. In order to solve this problem, the ferrite magnets are arranged by spoke-type. In this way, it has a higher air gap magnetic flux density than ferrite magnets radially magnetized, which will improve the torque density and power density of PMSM [3].

Traditional winding includes distributed winding and concentrated winding. The copper content of the concentrated winding at the end of the winding is less, which helps to reduce the manufacturing cost of the machine [4].

The cogging torque is the main part of the electromagnetic torque ripple of the machine. The method of reducing cogging torque is mainly from two aspects of machine structure and control algorithm. The common methods to change the structure of the machine mainly include skewing slots and skewing magnet poles, but these two methods reduce the electromagnetic torque while restraining the cogging torque.

Machines using interior permanent magnets typically have better overload capability and wider speed range, which makes the machine easier to adapt to complex conditions.

In this paper, in order to improve the torque/power density, efficiency, cogging torque and torque ripple of PMSM, an advanced design method of PMSM is proposed, including dual stator-PM and rotor-PM machines. For direct-drive applications, the phase-group concentrated-coil windings and an unaligned arrangement of the two rotors/stators are utilized. Phase-group concentrated-coil windings is used to obtain a unity displacement winding factor, and enhance the flux focusing effects together with spoke-type PM configurations. The unaligned arrangement of two rotors/stators can not only achieve further flux magnification by alternating the PM flux from one air gap to the other while keeping the flux relatively constant in the magnet, but also suppress the cogging torque and torque ripple. The machine performance including back electromotive force, cogging torque, and electromagnetic torque are first analyzed by a finite element method under the same operating conditions. Finally, a prototype of the DsPmSynM is manufactured, and some key simulation results are verified.

2. Machine modeling and operating principle

2.1. Machine topologies

The topologies of the DsPmSynM are shown in Fig. 1. To verify the improvement of the proposed design procedure on machine performance, a novel dual airgap stator DsPmSynM with 36 slots/38 poles with phase-group concentrated-coil windings is shown. The two stators in the proposed DsPmSynM are misaligned by one tooth width. Ferrite PMs are arranged as a spoke-type with the magnetization direction 180^° in polarity. Steel sheets of NSSMC 50H470 are used for the ferromagnetic parts of all machines. Figure 2 shows the design concept about DsPmSynM.

Fig. 1.

2-D model of DsPmSynM.

Fig. 2.

Design concepts of the DsPmSynM.

The magnet width, rotor and stator teeth width as well as the stator slot width within one group phase are designed to be the same as θ equaling π/2(elec.) for obtaining the unity displacement winding factor, while the stator slot width between the two different phases will be 5θ/3 equaling 5π/6(elec.) to produce a balanced three-phase back EMF. Thus, a specific combination of stator slots and rotor/magnet poles depending on the stator winding construction can be deduced [4]. Table 1 shows the basic parameters of DsPmSynM.

For the proposed DsPmSynM, a perigon (360 degree = 2π) relationship can be developed as follows: $\begin{eqnarray}\displaystyle 3\left[4{\theta}(n_{1}-1)n_{2}+n_{2}{\displaystyle \frac{14{\theta}}{3}}\right]=2{\rm\pi}. & & \displaystyle\end{eqnarray}$ (1)

The number of slots in one stator is: $\begin{eqnarray}\displaystyle Q=3n_{1}n_{2}. & & \displaystyle\end{eqnarray}$ (2)

Since the rotor pole pitch equals 4θ, the number of rotor poles is calculated as follows: $\begin{eqnarray}\displaystyle P_{r}=3n_{1}n_{2}+{\displaystyle \frac{n_{2}}{2}}. & & \displaystyle\end{eqnarray}$ (3)

For the proposed DsPmSynM, the perigon relationship is: $\begin{eqnarray}\displaystyle 3\left[2{\theta}(n_{1}-1)n_{2}+n_{2}{\displaystyle \frac{8{\theta}}{3}}\right]=2{\rm\pi}. & & \displaystyle\end{eqnarray}$ (4)

And the number of slots in one stator is: $\begin{eqnarray}\displaystyle Q=3n_{1}n_{2}. & & \displaystyle\end{eqnarray}$ (5)

Since the magnet pole pitch is 2θ, the number of magnet poles is: $\begin{eqnarray}\displaystyle P_{m}=3n_{1}n_{2}+{\displaystyle \frac{n_{2}}{2}} & & \displaystyle\end{eqnarray}$ (6) where n₁ is defined as the number of coils for one phase group, n₂ is the number of groups for one phase.

It is also noted here that the electric frequency of the proposed DsPmSynM is calculated as: $\begin{eqnarray}\displaystyle f_{er}={\displaystyle \frac{P_{r}{\omega}_{r}}{60}}. & & \displaystyle\end{eqnarray}$ (7)

And the electric frequency of the proposed DsPmSynM is: $\begin{eqnarray}\displaystyle f_{em}={\displaystyle \frac{P_{m}{\omega}_{r}}{60}} & & \displaystyle\end{eqnarray}$ (8) where P_r is the poles in one rotor of the DsPmSynM, P_m is the magnet poles of the DsPmSynM, and ω_r is the rotor angular speed.

A series of machine models following the above criteria can be designed. In this paper, the DsPmSynM with 36S/38P is selected for investigation.

Table 1

The basic parameters of DsPmSynM

Parameter	Units	Value
Rated power	kW	0.7
Rated speed	rpm	300
Rated current	A	3.53
Rated torque	Nm	22.7
Outer stator diameter	mm	283
Inner stator diameter	mm	222.6
Air gap	mm	0.9
Iron heart	mm	25

2.2. Operating principle

The benefits from the proposed design concept is shown in Fig. 2. Almost all the PM flux corresponding to one phase group (i.e. the area in the yellow dotted line box for phase A) will flow into one airgap when the rotor pole rotates to become aligned with the teeth of the stator, greatly enhanced airgap flux density.

After π/2 (elec.) rotation, when PM flux flows into two air gaps at the same time, the same effect will appear in the other air gap. Therefore, compared with the conventional dual airgap machines which the two air gaps work independently, the air gap flux density of the machine has been greatly improved, resulting in a higher torque. It should be noted here that the phase back EMFs produced by the outer and inner stator windings of the proposed machine models were shifted by π/2 (elec.), resulting from the alternate operating principle for the PM flux concentration and the use of a dual three-phase channel. It helps to obtain a remarkable degree of fault-tolerant capabilities when the proposed machines are fed by the dual three-phase inverter drives.

3. FEM analysis results and discussion

To obtain a reliable evaluation of machine performance, a quantitative comparison among the aforementioned machine models was carried out by utilizing the FEM with the aid of the commercial software, where the simulation techniques and the operating conditions for all machine models were the same. The distribution of DsPmSynM field lines is shown in Fig. 3. The distribution of DsPmSynM flux density is shown in Fig. 4. The waveform of DsPmSynM back EMF is shown in Fig. 5. And the waveform of DsPmSynM cogging torque is shown in Fig. 6.

Fig. 3.

Magnetic field line distribution of DsPmSynM.

Fig. 4.

Magnetic flux density distribution of DsPmSynM.

Fig. 5.

Back EMF of DsPmSynM.

Fig. 6.

Cogging torque of DsPmSynM.

It can be seen from Fig. 6 that the peak value of cogging torque in the positive half cycle is 0.87 Nm, because the outer stator and inner stator of DsPmSynM have mutually shifted by π/2 (elec.).

4. Simulation of vector control system in DsPmSynM

By using the collaborative simulation calculation based on MATLAB/Simulink and Maxwell, the simulation of DsPmSynM speed/current double closed-loop vector control is carried out, and the principle of vector control is shown in Fig. 7.

Fig. 7.

Block diagram of double closed loop vector control system of DsPmSynM.

The simulation settings are as follows: the simulation time is 0.15s, the initial speed of DsPmSynM is set as 400 rpm, the speed is set as 300 rpm at 0.05 s, and the speed is set as 500 rpm at 0.1 s. The load torque applied to DsPmSynM in the whole simulation process is constant as 20 Nm, and the simulation results are shown in Fig. 8 and Fig. 9.

Fig. 8.

Current curve of DsPmSynM.

Fig. 9.

Performance of DsPmSynM under constant torque and variable speed condition.

Figure 8 shows the three-phase current of the outer stator and inner stator of DsPmSynM. Since the load torque of DsPmSynM is constant at 20 Nm, the current of the outer stator and inner stator of DsPmSynM only changes at the moment when the speed changes. At other times, the amplitude of the three-phase current of the outer stator is about 4.2 A and the inner stator is about 4.4 A.

It can be seen from Fig. 9 that it takes about 0.04 s for DsPmSynM to reach the given speed from 0 to 400 rpm, 400 rpm to 300 rpm and 300 rpm to 500 rpm. DsPmSynM outputs stable electromagnetic torque about 0.003 s after a given speed change.

5. Experimental study of DsPmSynM performance

The prototype of DsPmSynM is developed. Figure 10 shows the experimental control system of DsPmSynM. Figure 11 shows the structure of DsPmSynM stator and rotor, and Fig. 12 shows the simulation and experiment comparison of cogging torque and back EMF.

Fig. 10.

Experimental system of DsPmSynM.

Fig. 11.

Prototype of DsPmSynM.

Fig. 12.

Simulation and experimental comparison of DsPmSynM.

Based on the experimental system shown in Fig. 10, the experimental research on the back EMF and cogging torque of DsPmSynM is carried out:

(1) First, keep DsPmSynM static, apply load, DsPmSynM operates in the state of generator, and test its back EMF. The experimental results are shown in Fig. 12(a).

(2) Secondly, keep DsPmSynM static, connect the torque sensor (range: 0–2 Nm) to the rotor, slowly rotate the DsPmSynM rotor, and test its cogging torque. The experimental results are shown in Fig. 12(b).

It can be seen from Fig. 12 that the experimental measurement results of no-load back EMF and cogging torque of internal and external stators are close to those of FEM simulation, which verifies the correctness of the scheme.

6. Conclusion

This paper has proposed a design technique for DsPmSynM, incorporating the advantages of the spoke-type magnets, phase-group concentrated-coil windings, and an unaligned arrangement of two rotors/stators. A quantitative comparison has been carried out among the investigated machines and vector control with the aid of Matlab software and Maxwell software collaborative simulation method. Current, speed, torque, magnetic field distribution and other waveforms are given. The prototype of the proposed DsPmSynM has been manufactured based on the specifications of an electric vehicle standard machine, and some key simulation results are validated.

Footnotes

Acknowledgements

This research was jointly supported by the Project of Liaoning Education Department (No. 201634090), and project support of Liaoning Science and Technology Department (No. 20180550037).

References

Sun

, Research on PMSM harmonic coupling models based on magnetic co-energy, IET Electric Power Applications4 (2019), 571–579.

Wang

W.N.

and Niu

, A novel structure of dual-stator hybrid excitation synchronous motor, IEEE Transactions on Applied Superconductivity26 (2016), 1–5.

Mohammad

M.R.

Kim

and Hur

, Design and analysis of a spoke type motor with segmented pushing permanent magnet for concentrating air-gap flux density, IEEE Transactions on Magnetics49 (2016), 2397–2400.

Zhao

Chen

Lipo

T.A.

and Kwon

, Dual airgap stator- and rotor-permanent magnet machines with spoke-type configurations using phase-group concentrated coil windings, IEEE Transactions on Industry Applications53 (2017), 3327–3335.