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EMI Filter Schematics for Motor Drives

Author Cadence System Analysis

Key Takeaways

  • EMI filters for motor drives can be either passive or active filters.

  • Passive EMI filters are efficient in minimizing EMI problems, leakage current issues, high-frequency common-mode noises, and overvoltage transients. 

  • The type of passive EMI filter is selected according to parameters such as motor drive power rating, operating voltage, full load current rating, and EMC requirements as per EN 61800-3 for the power drive system.

Electric motor drive

EMI filters are a significant part of electric motor drives

Working on a motor drive project was a learning curve for me; my engineering team was tasked with building a PWM inverter-based induction motor drive. As a part of one of my tasks, a literature survey was conducted on EMI filters for induction motor drives. It was clear there was a need for EMI filters between inverter output terminals and induction motor input terminals to limit the effect of conducted EMI, radiated EMI, common-mode noises, differential noises, etc. The literature survey introduced me to a wide array of EMI filter schematics, all suitable for induction motor drive applications. I learned that choosing the right EMI filter schematic can be challenging, and your choice really depends on the type of noise or EMI in the system.

Let’s take a look at EMI filter schematics and what they do. 

The Importance of EMI Filters 

EMI filters are a significant part of any motor drive--power-electronic converters that supply voltages of appropriate magnitude and frequency for speed control of machines. These converters utilize switching devices, such as MOSFETs, IGBT, or TRIACs, to convert the input voltage to the required magnitude and frequency.

High-speed switching is usually used in these power converters to minimize the size of magnetics and to increase the power density of the converter. High-frequency switching leads to the generation of electromagnetic interference, which can be either conducted or radiated. The various current flow paths present in a power electronic system also lead to common-mode and differential mode noises. These unwanted signals can jeopardize the control system, and, in the worst cases, the entire power electronic system interfaced motor.

Filtering out EMI and noises is necessary for the safe and reliable operation of the drives. EMI filters are capable of suppressing unwanted emissions and signals at their generation point. Filters make the coupling paths inactive for conducted EMI and make the device immune to radiated emissions. Regulations set by authorities such as the Institute of Electrical and Electronics Engineers and the International Electrotechnical Commission guide engineers on emissions limits to protect the circuit and its neighboring devices.

Passive EMI Filters for Motor Drives

Passive EMI filters use lumped elements such as resistors, capacitors, and inductors to mitigate EMI. EMI signals are of frequencies greater than the operating frequency. The passive EMI filter prevents the EMI from reaching the circuit downstream of it. Inductors in passive EMI filters allow low-frequency signals and block high-frequency signals. Contrary to inductors, capacitors allow high-frequency signals through them. By arranging passive elements in different fashions, various EMI filter schematics are obtained. The following section introduces some common passive EMI filter schematics used in three-phase motor drives. 

Passive EMI Filter Schematics 

Passive EMI filter schematics are commonly used between the power electronic output terminals and the three-phase motor input terminals. Passive EMI filters are efficient in minimizing:

  • EMI problems and leakage current issues.
  • High-frequency common-mode noises. 
  • Overvoltage transients at the three-phase motor terminals.

There are several passive EMI filter schematics available for use in motor drives. Passive EMI filter schematics are selected according to parameters such as motor drive power rating, operating voltage, full load current rating, and EMC requirements as per EN 61800-3 for a power drive system. Cadence software offers simulation and circuit design tools that can help with the design of passive EMI filters. 

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