Study on conducted EMI from operation of electromagnetic relay contacts

Transient noise that accompanies with electrical arc and contacts bounce is generated when the apparatus contacts operate, which is main source of conducted electromagnetic interference (EMI). The transient noise is non-periodic thus can not be measured by a spectra analyzer. Its prediction and diagnosis are also different from periodic noise. A contact set model that taken into consideration parasitic inductances of the contacts and parasitic capacitances between the contacts and relay shell is employed to obtain equivalent circuit for the conducted EMI. Time domain noises from two linear impedance stable nets (LISNs) are recorded synchronously when resistance load or inductance load are broken and the contacts bounce happens or not. A software based technique is employed to separate the common mode (CM) and differential mode (DM) voltages, and then to analyze the spectra of CM and DM interference voltage. Arc voltage between the contacts is also recorded simultaneously to analyze the EMI conduction path and to propose attenuation methods.