pic: A little teaser from 3571

[Alan Anderson]

Originally Posted by Caleb Sykes

...we are not dealing with AC currents here.

Actually, we are. The motor speed controllers work by "chopping" the power at high speed, and there really is significant EM produced. It almost never causes problems for PWM signals, but it definitely affects certain analog sensor measurements.

The pseudonymous poster from team 930 seemed to be warning about high power, not about EM interference, which is why I asked for clarification.

[philso]

Originally Posted by Caleb Sykes

I'm really having trouble understanding how there would be any appreciable electromagnetic interference caused by a single wire with a constant (or at least, low frequency) current running through it. This situation would only cause a slight magnetic field around the wire, and should not be creating any electromagnetic waves. The situation would clearly be different if the current was AC, but we are not dealing with AC currents here.

It is quite possible I am in this way over my head and that I really don't understand what is going on. Could you possibly link to a more detailed description of this interference you are describing? Everything I am finding in searching relates to AC circuits.

We really don't have just DC in the robot power system. The nominal voltage is 12Vdc but there are fluctuations due to a number of factors. The current flowing is mostly AC due to the PWM action of the motor controllers. Every time the output transistors in one of the motor controllers turns on and applies a voltage across the motor, current flows through the motor. This current comes from the battery. When the transistor turns off, the current stops flowing. You now have AC current flowing of a significant magnitude. The current is changing at the PWM frequency which is in the kiloHertz range. When you measure the current flowing from your battery with a meter, you are typically measuring the average (or RMS, depending on your meter) value of the current. This average/RMS current is what we think about when we are considering the trip rating of the breaker. The peak current (on a microsecond time scale) is typically much higher. Please keep in mind that when I am using the term AC, I am referring to a time-varying quantity where the waveshape is arbitrary, not the (nominally) sinusoidal 60 Hertz AC voltage waveform that one finds at a wall socket.


just one article that describes magnetic field coupling
www.learnemc.com/tutorials/Magnetic_Field_Coupling/H-Field_Coupling.html

Got to go to our build site now.

[TheThings]

After a brief discussion with a couple of our mentors, we were informed that if you have to run wires, specifically signal and PWM wires, you should run them 3 in. apart to be safe, according to their calculations.

[Caleb Sykes]

Originally Posted by TheThings

After a brief discussion with a couple of our mentors, we were informed that if you have to run wires, specifically signal and PWM wires, you should run them 3 in. apart to be safe, according to their calculations.

Could you please share those calculations?

[Sqrgz]

They were done awhile ago, so I don't believe we have them anymore.

[philso]

https://plus.google.com/u/0/10096186...7 19407130044

This is one page from the binder we were given at a 3 day course, Introduction to Electromagnetic Interference and Compatibility (EMI/EMC) Best Practices" given at the University of Wisconsin last October.

Assuming a motor controller switches 40A in 1 usec in one wire arranged in a loop with radius of 1cm, a second wire 3 inches away (0.0762m) also arranged in a loop with a radius of 1cm would have a voltage of 4mV induced in it.