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What are the characteristics of permanent magnet materials and what are their effects on the performance of the motor?

One of the structural characteristics of the motor is that the stator magnetic pole is composed of permanent magnet materials. The performance of the magnetic material directly affects the size of the magnetic circuit, the volume of the motor, the functional index and the motion characteristics. Permanent magnet materials, also known as hard magnetic materials, are characterized by high coercivity and high residual flux density. Permanent magnet materials can maintain stable magnetism for a long time after removing external magnetic field after saturation magnetization, which can excite permanent magnet motor and establish a stable magnetic field in the air gap.



Remanence BR, coercivity HCB



After the permanent magnet is magnetized to saturation, the magnetic field intensity H of the external magnetic field is gradually reduced to zero, and the magnetic flux density B of the permanent magnet is reduced from BS to BR, which is called remanence. When the reverse magnetic field is applied, BR is reduced to zero. The absolute value of the reverse magnetic field strength is called the magnetic induction coercivity, or coerce force (HCB), as shown in the figure below. The B-H closed magnetization curve is called magnetic hysteresis loop when the magnetic intensity of external magnetic field is changed continuously and slowly for one cycle. The hysteresis loop in the second quadrant is demagnetization curve, which is the basic characteristic curve of permanent magnetic materials and an important basis for characterizing the quality of permanent magnetic materials.



Intrinsic coercivity Hcj



In vacuum, the relationship between magnetic field strength h and magnetic induction intensity B is: B = 0h; in magnetic materials, it is: B = 0m + 0h, where m is the magnetization intensity (unit: A / M), which is an important physical quantity indicating the magnetization degree of permanent magnetic materials. 0 (= 4 π x10-7 H / M) is the permeability of vacuum.



Since H value of magnetization magnetic field intensity in the second quadrant is negative, for convenience, it is advisable to reverse the H coordinate, so that h is defined as demagnetizing magnetic field strength, which is positive, then the formula should be rewritten as: B = 0m0h, in which, when h = 0, B = br = 0m; when h = HCB, B = 0, M = HCB are positive values, and do not regress to zero. In order to make m return to zero, the demagnetizing magnetic field intensity H should continue to increase until Hcj. As shown in the figure below, the curve BJ = B + 0h is called the intrinsic demagnetization curve. BJ is the intrinsic magnetic induction strength after magnetization of permanent magnetic materials, i.e. the intrinsic magnetic induction strength, and Hcj is called the intrinsic coercivity.



Recovery permeability R



After the permanent magnet is magnetized, the external magnetic field is removed, and the magnetic density is br. Under the strong demagnetization, the magnetic density decreases to a certain point along the demagnetization curve, such as point K in the figure above, and then decreases the demagnetization until the field strength H = 0. However, the magnetic density does not return to br according to the demagnetization curve, but goes to a lower point, such as point M. after adding the magnetic field strength to HK, the magnetic density will follow the new curve to point K to form a new one Local small loops. Because the area of the local loop is very small, it can be approximately represented by the straight line km, which is called the recoil line. The slope of the recovery line is called the recovery permeability (R), which is approximately equal to the slope at BR on the demagnetization curve, that is, the compound line is parallel to the tangent line at BR on the demagnetization curve. R is an important parameter in the dynamic operation of permanent magnet materials. When R is small, the permanent magnetic materials have good dynamic performance.



Maximum magnetic energy product (BH) max



Because of the difference of permanent magnetic circuit, the working point of material is different. The magnetic energy provided by unit volume of material to air gap is directly proportional to the product of magnetic density B and demagnetizing intensity H, i.e. w = BH According to the formula, when B = BR, H = 0, w = 0; when h = HCB, B = 0, w = 0. The product of point d with the largest energy (BDHD) is the largest, which is called the maximum magnetic energy product (BH) max. this point is the best working point of the permanent magnet (see the figure below). For the permanent ferrite, the demagnetization curve of B = f (H) is generally a straight line, and the magnetic induction intensity can be written as: B = br-0rh; when h = HCB, B = 0



Magnetic induction temperature coefficient AB, Curie point TC



Temperature. Coefficient of Br (AB) AB, Curie temp. TC refers to the coefficient of reversible variation of remanence br with temperature in the working temperature range (generally – 40 ° C to + 80 ° C)




Where B1 and B2 are the magnetic induction strength at T1 and T2 respectively.



The magnetic induction intensity of permanent magnetic materials decreases with the increase of temperature, so AB is negative. When the temperature rises to a certain value, the saturation magnetic induction strength BS decreases to zero, and the basic characteristics of permanent magnetic materials are lost. This temperature is called Curie point TC (or Curie temperature). The smaller the temperature coefficient AB is, the better the temperature stability of the permanent magnet material is; the higher the Curie point TC is, the higher the allowable temperature is.

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