Micromotor has been widely used in daily life and has penetrated into all aspects of life. Among them, micro permanent magnet DC motor (PMDC) is the most widely used. PMDC has the characteristics of simple structure, small volume, less copper consumption and high efficiency. It is very popular in the automotive industry.

The application environment of automobile motor is generally harsh, and its actual use is not at room temperature, but at high / low temperature; For example, the high temperature rise caused by the frequent locked rotor of the motor, the heat accumulation caused by the motor being wrapped in plastic parts with poor heat conduction, and the use of the car under the conditions of basket temperature or severe cold climate. Therefore, the automobile motor must be able to withstand the test of high and low temperature, generally - 40 ° C. in practical application, customers pay more attention to the high and low temperature performance of the motor, such as low-temperature starting, low-temperature noise, high and low-temperature performance change, heat shock resistance, high and low-temperature storage resistance, temperature rise control protection, etc.

This paper focuses on the increase of starting resistance moment of motor at low temperature. Starting voltage (V) starting current (a) starting voltage (V) starting current (a) forward rotation reverse rotation forward rotation reverse rotation forward rotation reverse rotation forward rotation reverse rotation forward rotation reverse rotation forward rotation reverse rotation 3 the reason for the increase of starting resistance moment at low temperature is analyzed. It can be seen that the no-load static resistance moment t is composed of two parts: mechanical static friction moment TM and cogging positioning torque, Tftvtt

As we know, the no-load static resistance torque changes at low temperature. Test the magnitude of this change on a certain type of motor. The data are shown in Table 2 below The test method of friction torque is to measure the friction torque after the permanent magnet is not magnetized.

Table 2 Comparison of friction torque / total resistance torque at low temperature and normal temperature according to the data in Table 2: 1) the total static resistance torque at low temperature is much larger than that at normal temperature; 2) In the total static resistance torque, the cogging positioning torque is much larger than the friction torque, that is, the total static resistance torque is mainly caused by the cogging positioning torque: 3) the increase at low temperature is mainly the cogging positioning torque.

Cogging torque is an inherent phenomenon of cogging permanent magnet motor and an important parameter of PMDC performance. In this kind of motor, cogging torque is because the rotor has a tendency to align its teeth with the stator (permanent magnet) along a specific direction, which will produce an oscillating torque. The generation of cogging torque comes from the tangential force between the rotor teeth and the stator permanent magnet, which is the result of the interaction between the rotor cogging and the permanent magnet stator.

3.1 influence of temperature on magnetic force the influence of temperature on magnetic force is mainly related to the thermal stability of permanent magnet I ". thermal stability refers to the change degree of magnetic properties of permanent magnet caused by the change of ambient temperature. It is also called temperature stability. When the ambient temperature of permanent magnet rises from t to TL, the magnetic density decreases from B0 to BL: when the temperature returns from to, the magnetic density rises to B ', instead of B0. Later, when the temperature is known and "When the magnetic density changes between 'and Di', the change of 3.2 degrees of magnetic density has an impact on the pole arc coefficient. The pole arc coefficient also has an impact on TJ. Many studies show that 121, the pole arc coefficient has an optimal value for TC. Under this optimal value, TQ is the smallest, and T will become larger if it is greater than or less than this value. According to the data, t of PMDC increases with the increase of pole arc in 130-180 pole arc. The pole arc actually used in our factory is positive Within this range.

For some specific reasons, many magnetizers in our factory sometimes deliberately magnetize the specific area of the permanent magnet (mostly the boundary area in the circumferential direction of the permanent magnet) to be unsaturated during magnetization. For example, the typical magnetic density distribution waveform of S1 magnetization mode, such as, can be seen from the figure: the saturation in the middle of the permanent magnet is the highest, and the saturation is getting smaller and smaller towards the boundary. That is, there is an unsaturated region. At low temperature, the br in the unsaturated region will increase, which relatively increases the magnetic density distribution waveform for calculating the pole arc width eight S1 magnetization. 3.3 the influence of low temperature on the cogging positioning torque is related to the residual magnetic induction intensity or thickness LM of the permanent magnet. IEEE Du Hongjun et al. Cogging torque analysis of permanent magnet motor.