Advanced EMI filters keep brush DC motors “low cost”: Page 6 of 6

April 27, 2020 //By Christophe Cambrelin, Johanson Dielectrics
Advanced EMI filters keep brush DC motors “low cost”
Brush DC motors need to meet higher EMC requirements as electronic environments get more crowded and “noisy”. Further, EMI issues are going to become more of a problem with the higher frequencies with Bluetooth, Wi-Fi and now 5G devices. EMI filters will have to handle wider frequency ranges while allowing the appropriate signals to pass through.

Pulse Width Modulated signals

Regardless of the type of filter, an often-overlooked factor is the fact that many brush DC motors are controlled by Pulse Width Modulated (PWM) signal.

With PWM signal, the voltage is switched on and off at a very fast rate between a few kilohertz (kHz) and tens of kHz. The total power supplied is based on the time the switch is on compared to the off periods. The PWM signal is particularly suited for motors because the time constant of a motor is very long compared to the period of a PWM signal. That is why the brush DC motor acts as if the average of the PWM signal was applied on the power leads.

When you first test the motor in the lab the EMI filter may perform well, but everything changes when you apply a PWM signal on the power leads. You want to filter out the noise, but not unintentionally filter out the PWM signal. If you don’t choose the right filter, the motor may not even start.  

The bottom line is that EMI issues are going to become more of a problem with the higher frequencies with Bluetooth, Wi-Fi and now 5G devices. EMI filters will have to handle wider frequency ranges while allowing the appropriate signals to pass through. This will also help OEMs meet regulatory standards that exist in most countries that limit the amount of noise that can be emitted.

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