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Evnetics Soliton1 - Motor Cable Length

3902 Views 4 Replies 4 Participants Last post by  Tesseract
Hi all,

I'm upgrading the controller in my '69 VW Type-1 to a Soliton1.
I have an ADC 8" motor and 45 cell 90AH Thundersky pack.
I'm considering mounting the controller on the "parcel shelf" (behind the back seat) under a perspex shield. This would mean that the controller->motor cables would be ~600mm (2ft) long. My question is: is this too long?

The cables are 95mm^2 (3/0 AWG) flexible welding cable rated at 794A @ 30%. It can be twisted together easily. Pack is only 144V/90AH so battery power output is limited to about 15kW for normal use and 40kW peak. 90% of driving will be below 60km/hr (40m/hr). So I won't get close to using the 1000A motor current capacity of the soliton1, or if I do it will only be for very short bursts at low rpm.

Given that I^2R losses only increase linearly with cable length
and the motor cables are oversized, I assume that power loss shouldn't be a problem.
(The cable is 0.2mOhm/meter = 0.24mOhm/2x600mm, so even at 1000A the cables will only dissipate 1000*1000*0.00024 = 240W)

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

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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 ;)


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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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)
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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