Figure: Power line EMI/EMC filter concept

A three-phase EMI/EMC filter is an electronic device used to suppress unwanted electrical disturbances, i.e., the conducted electromagnetic interference (EMI) or radio frequency interference (RFI) present on the three-phase AC power supply line. It is a type of power line EMI filter. They come in two variations:   3-line filters (no neutral conductor) and 4-line filters (with neutral conductor). 

Conducted EMI means unwanted high-frequency noise currents emission from a working electrical and electronic device that travels via power lines or other connected wires and can interfere with the operation of other connected devices on the same power line. For example, a drilling machine operating in a neighboring room can disrupt the functioning of a television connected to the same line. Furthermore, the presence of high-frequency noise currents in power lines can transform the power line into an unintended antenna, emitting radiated EMI signals that have the potential to interfere with nearby electronic devices or AM radio signals.  

Common causes of conducted EMI or RFI include SMPS, electric motors, relays, converters, non-linear load switching operations, computers, microwave ovens, fluorescent lights, RF transmitters, microcontrollers, and other electronic devices.

The purpose of the three-phase EMI/EMC filter is to attenuate the high-frequency noise currents (conducted EMI or RFI) present on the three-phase power lines. By attenuating the noise current, the filter prevents conducted EMI from entering or escaping from a filtered device, keeping the device working smoothly without disrupting other connected devices. The EMI/EMC filter is installed at the power entry point (POE) of a device/system to attenuate conducted interference (typically in the frequency range of 150 kHz to 30 MHz).  The filter can be used as a separate device or can be integrated with electrical and electronic devices. 

Figure: Power line filter prevents EMI from entering into equipment and also prevents EMI emission leaking out from the equipment 

Construction and working principle of three-phase EMI/EMC filter:

The three-phase power line EMI/EMC filter is typically constructed by using passive components such as inductors, capacitors, and resistors arranged in a specific configuration to create a low-pass filter effect. The filters are designed to attenuate both common mode and differential mode noises. 

The filter works as a low-pass filter that allows power frequency (50/60Hz) current to pass freely while attenuating the high-frequency noise current on the three-phase power supply lines. The filter operates by redirecting the unwanted noise current away from the device to be protected to the ground. Some filter designs route the high-frequency noise back to the EMI noise source or absorb it. 

Figure: An example circuit of a three-phase EMI/EMC passive filter (filters may use various circuit designs)

In this way, the filter prevents entry or escape of noise current (i.e., conducted EMI) from the filtered device or equipment. Furthermore, by suppressing noise current emission from a device, the filter prevents the noise current flow along the power lines, thereby preventing the power lines from acting as an unintentional antenna, hence reducing the radiated EMI. Thereby, the filter ensures the filtered device/equipment can operate properly in its intended electromagnetic environment without disturbing any other devices connected in the same lines. It ensures electromagnetic compatibility (EMC) of the device that operates on the three-phase AC power supply. 

The filter helps to enhance the EMC performance of a device or product in terms of both the conducted EMI and radiated EMI. Incorporating a three-phase EMI/EMC filter into electrical systems, helps the manufacturers to ensure that their products meet the requirements of electromagnetic compatibility (EMC) standards and regulations, minimize the risk of electromagnetic interference, and enhance the product's reliability and performance.

Applications:

The three-phase EMI/EMC filters are ideal for use in various applications, including power supplies, UPS, motor-drive systems, industrial ovens, laser welders, automated test equipment, robotics, and more.