VESC v4.10 and battery cable length

Discussions regarding the Field Oriented Control (FOC) implementation on the VESC.
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david
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VESC v4.10 and battery cable length

Postby david » 06 Jan 2016, 21:14

Made some tests with different VESCs on FW 2.8/FOC and realized, that the cable length between capPCB pads and battery matters.

While a VESC with 38cm (no intention!) cables still runs under BLDC, it doesn't under FOC and generates a lot of errors. After cutting the cables to 20cm everything was fine again with FOC, no errors at all.

Could it be that ripple currents have a bigger impact on FOC compared with BLDC?

Anyway it might be a good idea to limit cable length to 20cm though...
VESCs made in Germany

okp
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Re: VESC v4.10 and battery cable length

Postby okp » 06 Jan 2016, 21:29

great testing david !!!!

rfvito
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Re: VESC v4.10 and battery cable length

Postby rfvito » 28 Feb 2016, 17:25

Hi, long cables introduce higher indutance into power line, this delays current, so board is not able to supply very quick high currrent demand.
Capacitors act as a backup energy for quicker response, the more capacitors in paralell the quicker is the current suppply for peak demand, more torque, quicker brushless response accelaratior/desacellaration. Also increases max peak current available to board.
Or can use gold audio capacitors because this ones are for ultra fast response.
Capacitors must be near as possible of power input of board. From capacitors to board very high gauge cable.

Planar cables (flat speaker cable or ground cable minimal tickness, some are cooper adhesive strip)could also replace the round wire, because planar cable minimize inductance. It is used for speakers connection in high quality high power audio systems. To supply the most pure audio signal is necessery that line does not have any inductance because delays current. Same rule in VESC because current demand needs to be very quick, so cable can't delay current. Delayed current means less torque!! less everything!!

Same rule cable from board to brushless. This works at high current high frequencies, so to avoid current delay use minimal lenght cable and/or planar flat cable.

Dont worry about money spent on cables and capacitors! Allways good capacitors and pure cooper cable. Performance and torque matters!

Peace

rew
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Re: VESC v4.10 and battery cable length

Postby rew » 27 Mar 2016, 08:11

Rfvito, that is not true.
I have a 4700 uF capacitor. When I accelerate the current spikes to 20A or something like that. Reworking the capacitor formula, i = C dU / dt into dt = C dU / I, we can calculate that it takes 0.2 miliseconds for 20A to cause a 1V drop across the capacitor. The current spike is more like 0.2 seconds long. It is the battery that provides that extra current.

Sure, the capacitors provide the current peaks when the controller needs a bit of current at short notice, but the time scale is a lot quicker. The battery will provide the required current at the milisecond level, the capacitor helps out at the microsecond level.

WrinklyWink
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Re: VESC v4.10 and battery cable length

Postby WrinklyWink » 28 May 2016, 04:47

Would you know if extraneous cables along the line would effect this 20cm? Such as a charger cable hooked up between the VESCs and the batteries?
Also if there is a relay switch in there as well?

rew
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Re: VESC v4.10 and battery cable length

Postby rew » 28 May 2016, 07:24

If you have a short cable to the VESC, but a longer cable running the other way, I don't think that will influence things a lot.

WrinklyWink
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Re: VESC v4.10 and battery cable length

Postby WrinklyWink » 01 Jun 2016, 02:33

what about a situation where the negative line runs through a BMS. Will a BMS affect FOC?

rew
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Re: VESC v4.10 and battery cable length

Postby rew » 02 Jun 2016, 06:37

It should not. There should be a "big current" mosfet in that line causing a good connection between the VESC and the battery.


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