# Evnetics Soliton1 - Motor Cable Length



## major (Apr 4, 2008)

samoc said:


> Are there problems with inductance? radiation? that I haven't considered?


Hi Sam,

Inductance should not be an issue. You actually want inductance on the load side of the converter. And radiation is kept low by placing the cables adjacent to each other, or better yet twisted.

Maybe our expert can confirm. Tesseract 

Regards,

major


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## Tesseract (Sep 27, 2008)

samoc said:


> Are there problems with inductance? radiation? that I haven't considered?


Good choice of controller, Sam 

The short answer: Ideally, if not practically, the battery cables need to be as short as possible, with the positive and negative adjacent, or even twisted together, over the entire run. The motor cables only need to be kept short to reduce I²R losses, not because the inductance is harmful (indeed, as major notes, it's helpful) or, even, because the noise emissions are particularly harmful. If you want more of an explanation, keep reading...

I seem to write a post on this every few months, but I'll grant that the topic it isn't exactly easy to search for. Anyway, there are two types of electrical noise - that from a changing voltage, called "E-field", and that from a changing current, called "H-field". Radio transmitters like cell phones, FM stations, etc., primarily emit E-field radiation while induction heaters primarily emit H-field. Both types of radiation can cause noise problems in electronics, but it just so happens that it is nearly impossible to adequately shield against the H-field (magnetic) but almost trivial to shield against the E-field.

A motor controller draws current from the battery in rectangular pulses, but the input capacitor (and batteries) integrate the voltage waveform into a triangular ripple. This still means that there are rapidly changing currents on the battery cables so they will primarily radiate H-field noise. Shielding against this requires lots of magnetic material to concentrate and divert the field lines - ie, lots of iron, mu metal, etc. This is clearly not practical, but you can stop much of the fields from being emitted in the first place by twisting the source and return cables together. One of our early beta testers that is active on this forum, dimitri, had problems with his PakTrakR which were mostly solved by simply twisting the battery cables in the engine compartment (not over the entire run, which would be ideal, but hey - a lot easier to just do so in the last few feet, eh?).

Conversely, a motor controller delivers pulses of voltage to the motor (except when at 100% duty cycle, that is). If the motor were a pure resistance then the current flowing through it would be rectangular, too, and those cables would emit both H-field and E-field noise. However, motor's have significant inductance which integrates the current into a triangular ripple. Thus, the motor cables only have rapidly changing voltages on them and primarily emit E-field noise as a result. This is where a metal shield is effective, as in coaxial cable, though automotive electronics are usually well-protected against E-field noise because they were designed to coexist with spark ignitions systems.


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## DIYguy (Sep 18, 2008)

Thanks Jeff! That was a great explanation.

Tell me, would some capacitance added prior to the controller help to smooth out the ripple and reduce the electrical noise emission? If so, how much would be suitable? (capacitance that is... in farads)


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## Tesseract (Sep 27, 2008)

DIYguy said:


> Tell me, would some capacitance added prior to the controller help to smooth out the ripple and reduce the electrical noise emission?


Adding capacitance to the battery side of the controller will reduce the peak to peak voltage ripple, but it won't reduce the current ripple, and it is this which is the main source of noise from the battery cables. To reduce current ripple you need to add more inductance (which, if you think about it, is what occurs on the motor side of the controller).

Unfortunately, adding too much inductance to the battery side of a controller can result in destructive ringing because a high-Q resonant circuit is formed. If you need to reduce the ripple voltage seen by other devices, such as dc/dc converters and chargers, it's much simpler - and less dangerous - to put the inductor in series with them, not the controller. The inductor is also much smaller and less expensive. I may write a separate post on that since I am alarmed at how benign many manufacturers of chargers and dc/dc converters assume the battery circuit power quality to be. It's anything but benign when a high power PWM controller is operating!


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