# Is LC filter at ESC input a good idea?

A wall climbing robot's controller board distributes power to a 150W brushless suction motor, 2x 30W brushless wheel motors and precious digital electronics (Intel NUC + STM32), the latter is connected through dc-dc converters.

### Would you recommend placing an LC filter at least on the input of the 150W motor controller and why?

Schematic below shows the precautions of the suction motor's esc power connection.

I have seen people attach LC filters to their drone's ESCs to improve analog video transmission. I have a digital link and I am more worried about motors puking current and voltage onto the robot's brain (aka digital electronics).

• Maybe you should put filters to digital supply lines instead. Jul 18, 2020 at 4:50

## 2 Answers

I've built many of these types of filters for drones, and my observation is that they can easily do more harm than good. I would start with good bypassing (ceramic and electrolytic caps in parallel near the esc), and if you're really worried maybe a ferrite. Don't do more than that unless you have a specific reason.

It depends what your goals are for noise immunity. Find out what your camera needs for say 40 dB SNR for low Q video for supply and return ripple vs f. Otherwise, assume <<1% of 1V video for noise. (usually 10mV for > 60dB SNR)

Sources of noise

Conducted and radiated: Differential and common mode:

• motor variable frequency current noise and rise time= 0.35/f
• acceleration rate of motor and thus surge currents up to 12x rated max
• effects of shared ground noise if any are shared, incl ESR of shared batteries for gnd.

Adding filters are a great idea, when there are problems, but you need to determine the resonant frequency, Q and noise frequency, then if radiated, conducted and then immunity vs f.

For example; a 150 W motor on 24V will draw rated current at ~ 6A or Zrated = 24/6=4 Ohms and surge current 10x means ESR is 0.4 Ohms L of coils ??

So I added ESR for a bunch of Lipo's in series (0.1 Ohm est. )and you can see the voltage drop from the current spectrum below.

Common mode filters are more common and this is all motor wires wrapped around many times on a large ferrite with high mu or as desired with Plastic or low ESR ceramic cap added to e-cap.

Always model with actual ESR for reactive parts and use low ESR caps.

You can simulate it easily in both directions here. http://www.falstad.com/afilter