Maintenance of the hottest DC motor

Maintenance of DC motor

the working conditions on site of steel rolling enterprises are relatively poor. If the maintenance is improper, the commutation of DC motor will become worse. The carbon brush is optimized according to the actual working conditions on site, and the correct adjustment of the carbon brush can restore the normal commutation of the motor and ensure the normal operation of the motor

(1) selection of carbon brush

the selection of carbon brush should be optimized by combining its material type, resistance coefficient, density, allowable current density, allowable speed, bending strength, hardness and other technical parameters

for example, a ZB DC motor has a rated power of 700kW, a rated current of 1240a and a maximum speed of 1400R/min. The actual load of the motor is 90% of the rated value. The surface of a brand of carbon brush originally equipped with the machine is oiled in use, and a black carbon film appears on the surface of the commutator. The carbon brush is worn and consumed greatly, and its service life is less than 1 month. After analysis, a carbon brush with improved resistance coefficient, density, bending strength, allowable current density and linear speed was selected (Table 1). After running in, a good oxide film is formed on the surface of the commutator again, and the service life of the carbon brush reaches about half a year

table 1

(2) optimal configuration of the number of carbon brushes

the number of carbon brushes in the DC motor should meet the load capacity specified in the national standard. According to the requirements of ZBK in technical conditions of Z series medium DC motors, DC motors for metal rolling mills should be able to withstand the following continuous overloads: ① continuous operation with 115% rated load under rated voltage and speed; ② After the continuous operation of rated voltage, speed and load, and then use the pressure testing machine with 1% reserve, 1 we must pay attention to the limited 25% rated load operation for 2h

for example, a ZB DC motor has a rated power of 400KW and a rated current of 715A. The motor has 4 brush holders in total, and 6 carbon brushes are installed on each brush holder. The surface size of carbon brushes is 25mm × 32mm。 Then the designed working area of carbon brush (cm2) is:

surface area of a single carbon brush × Number of carbon brushes on each brush holder × Number of motor poles =25 × thirty-two × six × 2=9600mm2=96cm2

under 125% rated load, the working current density of carbon brush is 715A × l. 25 ÷ 96cm2 can meet the experimental measurement requirements of different data =9.3a/cm2 is controlled by displacement, which is within the range of 8 ~ 12a/cm2 of the best working current density of carbon brush theory. However, in actual use, the maximum load current of the motor is only 600A, that is, the maximum working current density of the carbon brush of the motor is 600a=96cm2=6.25a/cm2. It is far lower than the lower limit of the optimal working current density of carbon brush theory, 8a/cm2

too low working current density of carbon brush will cause linear and grooved marks on the surface of motor commutator, shorten the turning cycle of commutator and shorten the service life of motor. On the contrary, if the working current density of the carbon brush is too high, it will cause heating on the surface of the carbon brush and commutator and large commutation sparks. As shown in Table 2, after adjusting the number of carbon brushes, the current situation and groove scoring on the surface of the commutator are effectively avoided

table 2

(3) arrangement of carbon brushes

the carbon brushes on the brush holder rod of DC motor commutator are generally evenly arranged. However, when the number of carbon brushes needs to be adjusted, it must be rearranged. In Figure 1, 8 carbon brushes are installed on the brush holder rod of DC motor. If the number of carbon brushes needs to be reduced, the carbon brushes of 1#, 8#, 2#, 7# tracks should be removed in turn. If there are linear and grooved marks on the surface of the commutator, or local burning marks, in order to improve the commutation, the carbon brush should be moved to an idle and in good condition track