EP1430587A1

EP1430587A1 - Method and apparatus for torque ripple reduction in sinusoidally excited brushless permanent magnet motors

Info

Publication number: EP1430587A1
Application number: EP01977103A
Authority: EP
Inventors: Tomy Sebastian; Sunil Keshava Murthy; Buyun Liu; Scott R. Berggren
Original assignee: Delphi Technologies Inc
Current assignee: Delphi Technologies Inc
Priority date: 2001-09-14
Filing date: 2001-09-14
Publication date: 2004-06-23
Also published as: WO2003026105A1; EP1430587A4

Abstract

A method for determining a dimension in a motor is described. By applying Fourier analysis, a sequence of terms is obtained. Since the fifth harmonic is the most undesirable term, the minimization of the fifth harmonic term will make resultant waveform closer to sine wave. Based upon the above, a determination of an angle δ (22) is described, wherein the fifth harmonic term of the sequence of terms is minimized. An electronic motor having a rotor (26) and a set of slot on said rotor surface (36) having a set of magnets (46) with a width δ (22) along the circumference (30) of said rotor surface (36) is described., The width δ (22) is determined by a method that includes applying Fourier analysis thereby a sequence of terms is obtained. Since the fifth harmonic is the most undesirable term, the minimization of the fifth harmonic term will make resultant waveform closer to sine wave. Based upon the above, a determination of an angle δ (22) is described, wherein the fifth harmonic tem of the sequence of terms is minimized.

Description

METHOD AND APPARATUS FOR TORQUE RIPPLE REDUCTION IN SINUSOIDALLY EXCITED BRUSHLESS PERMANENT MAGNET

MOTORS

TECHNICAL FIELD

This invention relates to a method and an apparatus for torque ripple reduction in electric motors.

BACKGROUND OF THE INVENTION

Electric power steering (EPS) has been the subject of development by auto manufacturers and suppliers for over a decade because of its fuel economy and ease-of-control advantages compared with traditional hydraulic power steering (HPS). However, commercialization of EPS systems has been slow and is presently limited to small and micro-class cars because of cost and perfonnance challenges. Among the most challenging technical issues is the annoying pulsating feel at the steering wheel and the audible noise associated with the type of high performance electric drives needed to meet the steering requirements.

The choice of motor type for an EPS is a crucial one, because it determines the characteristics of the drive and the requirements on the power switching devices, controls, and cost. Leading contenders are the permanent magnet (PM) brashless motor, the permanent magnet (PM) commutator-type and the switched reluctance (SR) motors, each of the three options has its own inherent advantages and limitations.

For the purposes of this invention, PM brashless motors are preferred over commutator-type motors. The large motor size and rotor inertia of commutator-type motors limit their applicability to very small cars with reduced steering assist requirements. Additionally, the potential for brush breakage that may result in a rotor lock necessitates the use of a clutch to disconnect the motor from the drive shaft in case of brash failure. SR drives offer an attractive, robust and low cost option, but suffer from inherent excessive torque pulsation and audible noise, unless special measures are taken to reduce such effects. For column assist applications, the motor is located within the passenger compartment and therefore must meet stringent packaging and audible noise requirements that the present SR motor technology may not satisfy. Therefore, the PM brashless motor with its superior characteristics of low inertia, high efficiency and torque density, compared to commutator motors, appears to have the potential for not only meeting the present requirements but also of future high performance EPS systems of medium and large vehicles.

Despite the relatively low levels of torque ripple and noise of EPS systems using conventional PM brashless motors, they are no match to the smoothness and quietness of HPS with decades-long history of performance refinement efforts. Consumers are reluctant in compromising such features. Therefore, a new torque ripple free (TRF) system is needed, which as the name indicates would eradicate the sources of torque ripple (under ideal conditions) and reduces the noise level considerably. The near term goal is to enhance the performance of EPS systems with the long term objective of increasing acceptability of EPS systems for broader usage.

Several performance and cost issues have stood in the way of broad-based EPS commercialization regardless of the technology used, but with varying degree of difficulty. This requires that following be addressed: 1. Steering Feel: The key to the wider use of EPS is the ability to reproduce the smoothness feel of hydraulic steering systems at affordable prices. Pulsating torque produced by motors would be felt at the steering wheel, if not reduced to very low levels.

2. Audible Noise: The EPS audible noise is mainly emanating from the motor and gearbox. The gear noise is obviously mechanical due to lash caused by manufacturing tolerances. The motor-caused noise is mainly a result of structural vibration excited by torque pulsation and radial magnetic forces in brashless motors and by the commutator/brush assembly in commutator motors. Typically, to get torque ripple free motor from a sinusoidally excited motor, the induced voltage need to be sinusoidal without any harmonics other than the third harmonics resulting from an analysis such as Fourier analysis. Normally this is achieved by distributing the stator conductors to get a sinusoidal distribution with complementary structures on a stator of the motor.

SUMMARY OF THE INVENTION

The present invention offers advantages and alternatives over the prior art in providing a method and apparatus for torque ripple reduction in sinusoidally excited brashless permanent magnet motors. In practice, a so- called sinusoidal composition of the sinusoidally excited brashless permanent magnet motors is not an ideal or perfect sinusoidal form. Thus, based upon Fourier analysis, it is desirous to minimize or eliminate the unwanted higher order components of the sinusoidal composition.

In an exemplary embodiment of the invention, a method for determining a dimension in a motor is described. By applying Fourier analysis, a sequence of terms is obtained. Since the fifth harmonic is the most undesirable term, the minimization of the fifth harmonic term will make resultant waveform closer to sine wave. Based upon the above, a determination of an angle δ is described, wherein the fifth harmonic term of the sequence of terms is minimized.

In addition, an electric motor having a rotor and a set of slot on said rotor surface having a set of magnets with a width δ along the circumference of said rotor surface is described. The width δ is determined by a method that includes applying Fourier analysis thereby a sequence of terms is obtained. Since the fifth harmonic is the most undesirable term, the minimization of the fifth harmonic term will make resultant waveform closer to sine wave. Based upon the above, a determination of an angle δ is described, wherein the fifth harmonic term of the sequence of terms is minimized

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 depicts a relationship between a flux density in the air gap of a sinusoidal excited brashless permanent magnet motor for one electrical cycle (for 2-poles) and poles on a 6-pole motor rotor.

Figure 2 depicts a rotor for an application of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

It can be appreciated that under ideal condition, a sinusoidal induced signal, be it voltage induced, current induced, magnetically induced, or otherwise induced, is a perfect sine wave. The Fourier analysis of this perfect sine wave would be meaningless in that the sine wave would equal to itself. However, in the real world, under experimental conditions, the so- called sinusoidal composition is not a perfect sine wave. Therefore, a Fourier analysis of the so-called sinusoidal composition will yield more terms than merely itself such as a perfect sine wave. Once it is established that the

Fourier analysis yields more terms, the question turns on which terms of the Fourier analysis is more significant.

The concept underlying the instant invention takes into account the fact that the voltage induced in a sinusoidal application is not only a function of the winding distribution, but also a function of the flux density distribution. Thus the magnet pole arc can be designed to eliminate the most significant harmonics such as utilizing Fourier analysis. The most significant harmonics in a STAR (Y) connected motor is the fifth harmonics. The fifth harmonics in the flux density distribution can be eliminated by making the pole arc to be 144 electrical degrees. Or, for a 6-pole motor it is 48 mechanical degrees. This way we can use a one slot per pole per phase (18 slot for a 3-phase 6-pole) and obtain reduced torque ripple.

Referring to Figure 1, a relationship 10 between a flux density in the air gap of a sinusoidal excited brashless permanent magnet motor, and poles on a 6-pole motor rotor is depicted. In an upper horizontal co-ordinate 12, a pair of poles 14, 16 is shown. It is noted that the pair of poles 14, 16 is only partially representative to the instant invention. In an lower horizontal co-ordinate 18, a corresponding flux density in the air gap is depicted. The flux density in the air gap may be written in the Fourier series as:

4R„

^B = 2)

«Σ Sin{nδl =1,3,5.. nπ

B = ∑4B„X

where B_m is the peak value of the rectangular flux density waveform; and δ is the width of a magnet in electric angle in relation to a motor shape. By reducing the fifth harmonic term to zero, or minimizing the fifth harmonic term, we arrive at:

5δ/2=π, 2π, ..., etc.

Thus, an optimum δ value may be derived. A positive rectangular flux density 20 corresponds the north pole 14 with a width δ 22. A negative rectangular flux density 24 corresponds the south pole 16 with a width δ 22 as well. In a STAR or Y-connected motor, the lowest harmonic which will have influence on the torque ripple is the fifth harmonic. Therefore, eliminating the fifth harmonic is important. For example, 5δ/2=π, 2π, ..., etc. Note that the lower the value of the angle δ, the smaller the dimension of a component incorporating the present invention. Therefore, the value of δ that maximizes the component incorporating the present invention is:

δ=144 degrees in electric angle or δ=4π/5.

Referring now to Figure 2, a rotor 26 depicting an application of the present invention described. The rotor 26 includes a first shaft 28 having a elongated shape with a generally cylindrical circumference 30. A center line 32 wherein the first shaft 28 is substantially centered is described. A rotor cylindrical body 34 having a generally cylindrical shape that includes a cylindrical surface 36, a first disk surface 38 receiving the first shaft 28 is described. The center line 32 passes through the center of the a first disk surface 38. The first disk surface 38 is coupled to the first shaft 28 along the center line 32. Correspondingly, a second shaft 40 having a elongate shape with a generally cylindrical circumference 42 is described. The center line 32 wherein the second shaft 40 is substantially centered is described. The rotor cylindrical body 34 having a generally cylindrical shape that includes the cylindrical surface 36, and a second disk surface (not shown) receiving the second shaft 40 is described. The second disk surface is coupled to the second shaft 40 along the center line 32. The rotor cylindrical body 34 comprises notches or slots that are adapted to receive magnets 46. The magnets 46 will preferably have a generally curved smooth surface that coincides with the cylindrical surface 36, which is also smooth. The generally rectangular smooth surface of the magnets 46 have a width or curvature 22 along the circumference of the rotor cylindrical body 34. Note that a set of segments 48 is equidistantly spaced between the magnets 46. The magnets 46 do not have to be pre magnetized, but rather may be magnetized after assembly onto the rotor. In fact, this latter method is preferred for ease of assembly.

To get good sinusoidal distribution of conductors, normally, it requires the slots per pole per phase to be at least 2. This means, for a 3- phase, 2-pole motor, 12 slots are needed. A 3-phase, 4-pole motor requires 24 slots, and a 3-phase 6-pole motor requires 36 slots.

A higher number of poles is preferred where motor size is an issue, because a larger number of poles means that the stator yoke thickness can be reduced and the motor built smaller. On the other hand if the yoke is made too small, then the number of slots that can be accommodated is limited. For example, with about a 30 mm air gap diameter in a motor, the maximum number of slots that could be accommodated would be around 20 to 25. As a general rale, one is advised to use a 4-pole structure to be able to get a satisfactory sinusoidal distribution.

It can be appreciated that a method for determining a dimension in a motor is described. By applying Fourier analysis, a sequence of terms is obtained. Since the fifth harmonic is the most undesirable term, the minimization of the fifth harmonic term will make resultant waveform closer to a sine wave. Based upon the above, a determination of an angle δ is described, wherein the fifth harmonic term of the sequence of terms is minimized.

It is further noted that an electric motor having a rotor and a set of slots on said rotor surface having a set of magnets with a width δ along the circumference of said rotor surface is described. The width δ is determined by a method that includes applying Fourier analysis so that a sequence of terms is obtained. Since the fifth harmonic is the most undesirable term, the minimization of the fifth harmonic term will make resultant waveform closer to sine wave. Based upon the above, a determination of an angle δ is described, wherein the fifth harmonic term of the sequence of terms is minimized

It will be understood that a person skilled in the art may make modifications to the preferred embodiment shown herein within the scope and intent of the claims. While the present invention has been described as carried out in a specific embodiment thereof, it is not intended to be limited thereby but intended to cover the invention broadly within the scope and spirit of the claims.

Claims

CLAIMSWhat claimed is:

1. A method for determining a dimension in a motor comprising: applying Fourier analysis thereby detennining a sequence of terms; minimizing a fifth harmonic term of said sequence of terms; and determining an angle δ (22), wherein said fifth harmonic term of said sequence of terms is minimized.

2. The method of claim 1 wherein said sequence of terms comprising an angle equal to nδ/2 with n being positive integers.

3. The method of claim 1 wherein said sequence of terms comprises an angle equal to nδ/2 with n equal to 1 or 2.

4. The method of claim 1 wherein said motor comprises a sinusoidally excited brashless permanent magnet motor.

5. The method of claim 1 wherein said angle δ (22) has an optimal value of 4π/5.

6. An electric motor comprising: a rotor (26) having a rotor surface (36); and a set of slots on said rotor surface (36) having a set of magnets (46) with a width δ (22) along the circumference (30) of said rotor surface (36) wherein said width δ (22) is determined by a method including, applying Fourier analysis thereby determining a sequence of terms; minimizing a fifth harmonic term of said sequence of terms; and determining an angle δ (22), wherein said fifth harmonic term of said sequence of terms is minimized.

7. The electric motor of claim 6 wherein said sequence of terms comprising an angle equal to nδ/2 with n being positive integers.

8. The electric motor of claim 6 wherein said sequence of terms comprises an angle equal to nδ/2 with n equal to 1 or 2.

9. The electric motor of claim 6 wherein said motor comprises a sinusoidally excited brashless permanent magnet motor.

10. The electric motor of claim 6 wherein said angle δ (22) has an optimal value of 4π/5.

11. The electric motor of claim 6 wherein said set of magnets (46) are spaced equidistantly.