Resurch On Eddy Current Loss And Megnetic Measurement Of Permanent Magnets

Rare Earth Permanent Magnet Motors are widely used in wind power,electric vehicles,medical,other industrial fields and people’s daily lives.Most of the permanent magnets in Permanent Magnet Motors(PMM)use Nd Fe Bs which have high coercivity,high remanence,high electrical conductivity and poor heat resistance.PMMs are basically driven by inverters,which generate a lot of time harmonics.At the same time,the distributed winding of the motor will generate spatial harmonics,and the change of the air gap magnetic permeability caused by the slotting of the stator will generate tooth harmonics in the air gap of the motor.Harmonics in the air gaps of these motors can cause a large amount of eddy current losses in the permanent magnets,causing the permanent magnets to lose magnetism.At present,the methods for studying the eddy current loss of Nd Fe B are mainly Finite Element Method(FEM),Analytical Method and Equivalent Magnetic Circuit method.Each method has its limitations.FEM can accurately calculate the eddy current loss of the permanent magnet,and can establish complex models,but FEM also takes a lot of time.Analytical Method can quickly calculate the eddy current loss of permanent magnets,but it has to ignore various boundary conditions frequently,the calculation accuracy is not high,and it is difficult to develop analytical expressions for complex models.The Equivalent Magnetic Circuit Method can calculate the eddy current loss of permanent magnets quickly and accurately,but there is a problem that it is difficult to establish an equivalent magnetic circuit for complex models.There are currently few methods for directly testing the eddy current losses of the permanent magnets.In order to test the eddy current loss characteristics of the permanent magnets, a eddy current loss test system for permanent magnets is designed.It is mainly based on testing the B-H curve of the permanent magnets under alternating magne tic field,and calculating the loss by calculating the area enclosed by B-H curves. The magnetic permeability of the permanent magnet before the knee point on its demagnetization curve is close to 1,therefor the loss measured by this method is only the eddy current loss and no hysteresis loss.Meanwhile,this paper also pro poses a method to test the excitation magnetic field on the surface of permanent magnets more accurately.The eddy current losses of the permanent magnets are mainly tested in the following aspects:(1)The B-H curve of the copper block under sinusoidal excitation conditions is measured.And it’s compared with the B-H curve of the permanent magnet to verify the correctness of the test principle.(2)The eddy current loss of the Nd Fe B under different frequencies and different amplitude excitation magnetic fields is tested,and the estimation formula of the eddy current loss is put forward.(3)The eddy current losses of Nd Fe Bs coated with copper and zinc respectively are tested and compared with the eddy current loss of uncoated Nd Fe B.(4)The difference of the eddy current losses when Nd Fe B is excited in different directions are analyzed.(5)The loss characteristics of non-magnetic,unsaturated magnetized Nd Fe Bs were tested.The relative magnetic permeability of non-magnetic Nd Fe B and the hysteresis loss characteristics of unsaturated magnetized Nd Fe B were analyzed.(6)The phase current harmonics of the permanent magnet synchronous motor driven by the inverter were tested and harmonic analysis was performed.The extracted harmonic waveform is input into the eddy current loss test system of the permanent magnet,and the characteristics of the eddy current loss of the permanent magnet under harmonic excitation are tested,and a method for calculating the overall eddy current loss by data segmentation is proposed.