According to the different ways of electrical noise, can be divided roughly electromagnetic noise, mechanical noise and air dynamic noise. Judge various kinds of noise from a motor, usually takes on some methods to achieve. Using the method of cut off power supply to analyze mechanical noisenoise,After outage motor continue running through the noise produced for the main part of the mechanical noise；According to the characteristic of electromagnetic noise change with the current，Judging by changing voltage method, the voltage quickly to some extent, as the voltage drop greatly reduced and the main part of noise is electromagnetic noise For air dynamic noise, in the factory the simple method is to compare noise changes before and after taking down the fan. Or to change of different diameter types of fans, with the method to drive two motor each,To distinguish the difference noise in different speeds, this method can accurately identify air dynamic noise.