Low Energy

Examples of Low Energy Conducted Noise:

Common Mode Noise

Common Mode Noise (CMN) is the voltage between Neutral and Earth.

The two basic reasons for such a voltage is:

• Current flowing down the Neutral conductor creating a voltage drop
• Current flowing down the Earth conductor creating a voltage drop

Impact to equipment by CMN typically occurs in switchmode power supplies. This is due to the output filter, that normally dirvert noise to earth, allowing a back feed of CMN voltages directly to the output supply. Exposure then occurs to every device connected to the DC outputs of that power supply.

Harmonic Noise

Harmonic Noise is created when load current is not linear. Linear loads typically include incandescent lamps and resistive heaters. Non linear or switched loads occur in almost all controlled power equipment. Harmonic noise generating loads include:

• Fluorescent lights
• Battery Chargers and Rectifiers
• Motors and especially when speed controlled
• Switch mode Power Supplies

Solutions:

• Passive harmonic filters
• Active harmonic filters

Precautions:

• Passive filters are best for specific loads. If the load varies the filter needs to be more complex.
• Active filters can have problems when connected to a generator supply.

Spread Bandwidth Noise

Most noise below 2000Hz is Harmonic noise. Harmonic filters solve this type of noise and are basically passive or active in design.

Noise above 2kHz is generated mainly by high current switching noise. If this level of noise presents a problem then solutions include modifying the site impedance by use of inductors and capacitors. The Capacitors attenuate the noise while the Inductors prevent the capacitors drawing high harmonic currents. This is opposite to what is implemented in Power Factor Correction equipment where inductors are added to protect the main capacitors.

Noise above 1MHz can still impact on sensitive equipment. Solutions to this noise problem generally require in series RFI filters.

Magnetic fields are a problem anywhere a current flows. This includes all power cables especially as the currents become large. Mostly this is an issue near transformers. Transformer design should contain most of the flux within the steel, however they are not perfect and leakage occurs. Shielding is difficult and utilises ferrite or bulk steel attenuators.