The generation of motor electromagnetic interference

Peak voltage：come from the disconnection of brush and conmutator plate contact。

Mutation current：the sudden change of the current that flowing through the brush when it passes through the gap between commutator.Generally，when di/dt ≠0 or dv/dt ≠0,Electromagnetic disturbance will occur.Di as differential of current，dt as differential of time，Dv as differential of voltage.Di/dt is the derivative of the current which means that the current changes very rapidly in an instant. Dv/dt is the derivative of the voltage which means that the voltage changes very rapidly in an instant.

This interference is transmitted to the grid through the power line or radiates outward through the power line, will influence the quality of the power from the grid, and the surrounding electromagnetic environment.

Restraining interference

Method to restrain interference：

1. Reduce the interference of the source

It is known that the peak voltage when the brush and commutator contact are disconnected and the mutation current current when the brush is closed will cause eletromagnetic interference . We can reduce the peak amplitude by changing the brush material to a softer material or increasing the pressure of the brush on the commutator, but this will shorten the working life of the brush.

2. The filter of the transmission approach

2.1 Interference characteristic

According to chracterization of the common mode and differential mode interference produced by motor,divide the whole EMI test frequency into three parts,as below:

0.15 ~ 0.5MHz differential mostly

0.5 ~ 5MHz both mode

5 ~ 300MHz common mostly

2.2 Introduction and application of filter element

Common filter elements include capacitance, inductance, ferrite magnetic ring and so on.

2.2.1Capacitance

The role of capacitance in the circuit: it has the characteristics of cutting off DC, connecting AC and preventing low frequency. Widely used in coupling, isolating, filtering, tuning and so.

Capacitance indicates the amount of electric energy that can be stored.the unit is Ｆ，also mF、uF 、nF 、pF.

Converting like：

1Ｆ＝1000000uF

１uF=1000nF=1000000pF

The impedance of the capacitance is Zc = 1/2πfC, and the higher the frequency, the smaller the impedance of the capacitance, that is, the capacitance provides a parallel low impedance for the line at high frequency.

Capacitance can be used to filter differential mode signals as well as common mode signals, but in a different connection.