pic: A little teaser from 3571

[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.