One of the structural characteristics of the motor is the stator poles by a permanent magnet material, magnetic properties of the merits, the motor directly affects the size of the magnetic circuit, the motor size and motion characteristics and function indicators. Permanent magnetic material known as hard magnetic material, the main features of coercive force (Coercive Force) large remanence (Residual Flux Density) is high, the saturation magnetization of permanent magnet material after removing the external magnetic field still remains static magnetic give permanent magnet motor field, to establish constant magnetic field in the air gap.
Remanence Br, coercivity Hcb
Saturation magnetization of the permanent magnet to the rear, the magnetic field strength of the external magnetic field (Magnetic Field Intensity) H gradually reduced to zero, the magnetic flux density of the permanent magnets (Magnet Flux Density) B from the retreated Bs Br, Br called remanence. Br enable reverse applied magnetic field drops to zero, the absolute value of a reverse magnetic field strength at this time is referred to as the coercive force of the magnetic induction, referred to as the coercive force (CoerciveForce) Hcb, see below. Continuously slowly varying field strength of the external magnetic field one cycle, BH formed in a closed curve is called a magnetization hysteresis loop (Magnetic hysteresis loop), located within the hysteresis loop is the second quadrant demagnetization curves (DemagnetizationCurve), it is a basic characteristic curve of permanent magnet material, an important basis for characterizing the quality of permanent magnet material.
Intrinsic coercivity Hcj
In vacuo, the relationship between the magnetic field strength H and the magnetic induction B is: B = 0H, in the magnetic material, was: B = 0M + 0H, the formula, M is the magnetization (Magnetization Intensity) units A / m) , is a permanent magnetic material is an important degree of physical quantities. 0 (= 4πx10-7 H / m) is the vacuum permeability (Permeability of Vacuum).
Since the magnetic field strength H in the second quadrant is negative, for convenience, may wish to reverse coordinates H, H so as to define a magnetic field strength, is positive, the formula should be rewritten as: B = 0M0H, wherein showed that: when H = 0, B = Br = 0M; when H = Hcb, B = 0, M = Hcb value is not zero retreated. For M retreated zero, but also to the magnetic field strength continues to increase until Hcj H, shown below, Bj = B + 0H curve called intrinsic demagnetization curves (Intrinsic Demagnetization Curve), Bj is magnetized permanent magnet material the intrinsic magnetic induction i.e. intrinsic magnetic induction, Hcj called intrinsic coercive force.
Recoil permeability r
Removing the magnetized permanent external magnetic field flux density is Br, under the action of the demagnetizing field intensity, flux density along the demagnetization curve drops to a certain point, for example, point K in the figure above, and then decreases until the demagnetization field strength H = 0, but the magnetic flux density does not return to press the demagnetization curve Br, but to a lower point, for example point M, and then add to the strong magnetic field Hk, magnetic flux density along the point K to a new curve to form a small local loop. Since the local loop area is small, can be approximated by a straight line KM, KM called return line (Recoil Line). Reply slope of the line is called recoil permeability (Recoil permeability) r, which is approximately equal to the slope of the demagnetization curve at a Br, i.e. return line parallel to a tangent on the demagnetization curve at the Br. r is an important parameter when the permanent magnet material dynamic operation, when r is small, permanent magnet material has good dynamic performance.
The maximum energy product (BH) max
Depending on the permanent magnetic circuit, and thus the different materials which the operating point is proportional to the product of the flux density B and the magnetic field strength H of the material supplied to the gap per unit volume of this material with magnetic operating point, namely: W = BH ... (J / m3), seen from the formula, when B = Br, H = 0, W = 0; when H = Hcb, B = 0, W = 0. The maximum energy point D (BdHd) product of the maximum, as the maximum energy product (BH) max, and this is the optimum operating point of the permanent magnet (see below), for permanent ferrite, B = f (H) to demagnetization curve is generally linear, magnetic induction can be written as: B = Br-0rH, while when H = Hcb, B = 0, are substituted into the formula:
By B = f (H) is a straight line, the midpoint of (BH) max is on a straight line, so
Magnetic induction temperature coefficient ab, the Curie point Tc
Magnetic induction temperature coefficient (Temp. Coefficient of Br) ab, Curie point (Curie Temp.) Tc refers to the temperature coefficient of the remanence Br with a reversible change in the working temperature range (typically -40 ° C ~ + 80 ° C) :
Wherein B1, B2 were temperatures t1, t2 of the magnetic induction.
Magnetic induction permanent magnetic material decreases with increasing temperature, so ab is a negative value, when the temperature rises to a certain value, the saturation magnetic flux density Bs drops to zero, the loss of the basic characteristics of permanent magnetic material, this temperature is called the Curie point Tc (or Curie temperature). Ab value of the temperature coefficient, the better the temperature stability of permanent magnetic material; Curie point Tc of the high, the allowable temperature is high.
Isotropic and anisotropic
Due to different manufacturing processes, there is an isotropic permanent magnet material (Isotropy) and anisotropy (Anisotropy) points, axes of easy magnetization in the permanent magnet of isotropic different grain orientation is disordered, thus relatively low remanence Br, only about half the saturated magnetic flux density Bs, and accordingly the maximum energy product (BH) max is also small. Anisotropic permanent magnetic field is the use of re-sintered molded (or calendaring), the easy magnetization axis of crystal grains forming a magnetic field are arranged in the same direction, Br proximity of Bs, so about double the ratio of Br isotropic, for ferrite (BH) max. nearly four-fold increase. Thus, the motor is generally anisotropic magnet material.
Editor: lq, Read, original title: Dry | What are the characteristics of permanent magnet materials, which they affect the performance of the motor?
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