tailieunhanh - Industrial Brushless Servomotors Episode 4

Tham khảo tài liệu 'industrial brushless servomotors episode 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | The brushless machine 55 Iron core Figure Mmf and flux in air- and iron-cored coils Magnetic field intensity H The shorter the path of the flux in whatever medium the greater the amount of flux which can be established in that medium by a given mmf. The mmf per metre of the flux path is called the magnetic field intensity expressed as A m. Permeability p As its name suggests permeability tells how easy it is for the mmf to establish flux in a particular medium. The permeability of the medium is p PoPĩ where Po is the permeability of a vacuum expressed in the unit of henry per metre and Pt is the permeability of the medium relative to that of a vacuum. The density of the flux in a particular case can be seen to be dependent on two factors. One is the intensity of the mmf around the flux path and the other the permeability of the medium. The flux density is given by 5 PoPrH 56 Industrial Brushless Servomotors In Figure H will have about the same value in the two cases. However Ặír for iron is of the order of several thousand and so the flux density will be much greater here than in the air-cored coil where zr is close to unity. The hysteresis B-H loops The normal characteristic which describes the properties of a permanent magnet is the B-H loop shown in Figure . The dotted line is the so-called intrinsic loop. The normal curve shows the full cycle of magnetic states which can be induced in the magnet starting at the origin with a previously unmagnetized sample. Figure a shows such a sample clamped between the ends of an iron core so that the external magnetizing force NI A-turn can be applied. Note that the x-axis in Figure is not scaled as H but as HoH which is the flux density which would exist in the air between the ends of the iron core without the magnet in place. The y-axis gives the density which actually appears in the magnet when in place. Let US now go round the normal B-H loop of Figure starting at the origin. Assume that