What are the performance requirements for magnets in permanent magnet motors?

An electric motor is a device that converts electrical energy into mechanical energy, which is mediated by a magnetic field. In permanent magnet motors, a constant magnetic field is generated by permanent magnet materials. Therefore, the performance of permanent magnet motors is closely related to the performance parameters of the permanent magnet materials used.

Requirements for residual magnetic Br

When the current remains constant, according to the BLI law, the average electromagnetic torque is T=Bg · I · N · L · Da

In the above equation, Bg is the air gap flux density, N is the number of coil turns, L is the calculated length of one side of the coil, and Da is the armature diameter. The torque T and volume of the motor remain unchanged, i.e. L · Da remains unchanged. Increasing Br increases Bg and reduces the ampere turns I · N, which can save patent wire and reduce copper consumption, thereby improving motor efficiency; If the torque T and ampere turns I · N remain constant, increasing Br can reduce volume, weight, and save materials. Therefore, in general, materials with higher Br should be used to increase the air gap magnetic density Bg. However, increasing Bg will increase the positioning torque, magnetic pulling force, armature chip magnetic saturation, and iron loss. Some motors do not want these situations to occur, so they prefer to use materials with lower Br.

Requirements for coercivity Hcb and Hcj

The actual working point of a permanent magnet motor is not at the Br point where H=0 in the magnetic hysteresis loop, but at a certain point on the demagnetization curve (or recovery line). In order to prevent irreversible demagnetization, the magnet is required to have a sufficiently large coercive force Hcb. The larger the Hcb, the stronger the anti demagnetization ability of the magnet and the better the magnetic stability. The intrinsic coercive force Hcj, which represents the reduction of material magnetization to zero, is also required to be as high as possible in permanent magnet motors. The larger the Hcj, the smaller the leakage magnetic flux. For NdFeB magnets, a larger Hcj can increase the operating temperature, reduce the temperature coefficient, and minimize irreversible thermal magnetic losses.

Requirement for maximum magnetic energy product (BH) max

If the leakage flux of the magnetic circuit is ignored, according to Kirchhoff's first law, the magnetic flux in each part of the magnetic circuit is equal, that is, BmSm=BgSg. Here Bm is the magnetic flux density of the magnet, Sm is the magnetic circuit area of the magnet, Bg is the air gap magnetic flux density, and Sg is the air gap magnetic circuit area. Then Sm=BgSG/Bm. According to Kirchhoff's second law, the magnetic voltage drop is equal at all points in the magnetic circuit, i.e. Hmhm=Hgg. Here Hm is the magnetic field strength of the magnet, hm is the thickness of the magnet, Hg is the air gap magnetic field strength, and g is the air gap length. Then hm=Hg. g/Hm. In the air gap, r=1，Bg = rHg = Hg， If the magnet operates at its maximum magnetic energy point, i.e. BmHm=BdHd=(BH) max, then Vm=Bg ^ 2.Sg.g/(BH) max. If the air gap size is constant and the air gap magnetic density Bg is constant, the higher the magnetic energy product of the material, the smaller the magnet volume. Therefore, permanent magnet motors usually require a large value of the (BH) max of the magnet, and the design should make the magnetic circuit operating point as close as possible to the maximum magnetic energy point (BdHd). Therefore:

For ferrites, the magnetic curve at room temperature is usually a straight line, with a Br/Hcb=Bd/Hd relationship. Therefore, for magnets with large hm/Sm, i.e. thick and small, it is advisable to choose materials with high Br and low Hcb; For magnets with small hm/Sm, i.e. large and thin, it is advisable to choose materials with high Hcb and low Br.

Requirement for magnetic permeability r in response

The magnetic permeability r is high, the slope of the recovery line (or demagnetization curve) is large, the magnetic density at the operating point is low, and the magnetic density at the operating point of the motor varies greatly under different loads, which is something we do not want to happen. Moreover, materials with larger r tend to have larger Br and smaller Hcb, resulting in poor demagnetization resistance. Therefore, for permanent magnet motors working under dynamic conditions, it is required that r be small to achieve good dynamic performance, small changes in magnetic flux density, and strong resistance to demagnetization.

Requirement for linearity of demagnetization curve

In permanent magnet motors, the magnets often operate dynamically, so it is required that the operating point of the motor during maximum demagnetization conditions (such as stalling) be above the inflection point of the demagnetization curve to prevent irreversible demagnetization; At the same time, it is also required that the demagnetization curve above the inflection point of the magnet has good linearity. Under the same Br and r conditions, the working point magnetic density Bd and recovery line of magnets with linearly different demagnetization curves are low. The difference in magnetic density between the recovery line and the demagnetization curve is irreversible demagnetization, and the lower the recovery line, the greater the irreversible demagnetization. Only when the demagnetization curve is a true straight line and the recovery line coincides with the demagnetization curve, will there be no irreversible demagnetization.

Requirements for the magnetic induction temperature coefficient α b

α b is a measure of the degree to which the magnetic density of a magnet can reversibly change with temperature after saturation magnetization. The α b of all magnet systems is negative, meaning that the magnetic density decreases as the temperature increases. The smaller the absolute value of α b, the better the temperature stability of the magnet. In fact, the so-called reversible change is also relative, and it requires several repeated temperature cycles to obtain a completely reversible change in magnetism.

Requirements for Curie point Tc

The Curie point Tc is high, and its allowable operating temperature is also high. In permanent magnet motors, the magnet temperature is usually below 150 ° C. This eliminates the need to consider the Curie point in the design of ferrite, aluminum nickel cobalt, and rare earth cobalt magnets. However, for NdFeB magnets, low Curie point Tc is one of their biggest defects. The Curie points of different grades of NdFeB vary greatly, usually around 300 ° C. Specially manufactured high-temperature NdFeB can reach 450 ° C, with a higher bonding Tc than sintered NdFeB.