Cable length

General topics and discussions about the VESC and its development.
mrfreedom
Posts: 1
Joined: 29 Aug 2017, 17:02

Cable length

Postby mrfreedom » 29 Aug 2017, 17:12

Hi there
I am building an e-scooter, having started with the L-faster kit here: https://www.l-faster.com/items/1650w-ox ... motor-kit/

With the current design, the motor looses torque and makes a vibrating noise if you put too much throttle; I think this is down to using a "regular" ESC and BLDC motor combo which doesn't understand about high interial loads. It also cuts out when the battery is like 40% used which I don't know the cause of currently.

So I am planning to upgrade to a VESC as a next step.

Question (probably the first of many :) ) : Does it matter how long the wires are from battery > VESC or from VESC > BLDC motor?

Currently the setup is like this:
battery > VESC about 70cm
VESC > motor about 10cm

Would it be ok to change it to this:
battery > VESC about 20 cm
VESC > motor about 80cm

If I can do the second setup it would be better for mounting the VESC.

Thanks all!

Addy
Posts: 7
Joined: 10 Aug 2017, 17:56
Location: BC, Canada

Re: Cable length

Postby Addy » 30 Aug 2017, 18:40

Yes, it matters unfortunately.

Long wires between the battery and VESC are not so bad because the VESC capacitors can provide some current for short bursts.

Long wires between the VESC and the motor are less desirable. The motor phase currents are generally higher than the battery current, so you would be wasting more power this way, and you can't put capacitors on the motor to help supply it. Wasting power in the wires means less power is available for your motor.

If you must have longer wires, try to use thicker gauge wires.

rew
Posts: 905
Joined: 25 Mar 2016, 12:29
Location: Delft, Netherlands.

Re: Cable length

Postby rew » 31 Aug 2017, 05:52

It is not that bad. Because the battery has some internal resistance, the resistance in the wires is insignificant. Similarly some resistance in the wires to the motor will cost you a bit in efficiency, but that's it.

Still: It is better to have short wires from the battery and longer wires to the motor than the other way around. My advice is to braid the wires from the motor to the controller. That is for three wires what "twisting" does for two. This should reduce EMI and inductance in the wires. Thicker wires won't hurt.

Addy
Posts: 7
Joined: 10 Aug 2017, 17:56
Location: BC, Canada

Re: Cable length

Postby Addy » 31 Aug 2017, 05:57

rew wrote:It is better to have short wires from the battery and longer wires to the motor than the other way around.


What makes you think that?

rew
Posts: 905
Joined: 25 Mar 2016, 12:29
Location: Delft, Netherlands.

Re: Cable length

Postby rew » 01 Sep 2017, 12:21

Inductance and some resistance on the motor side of the ESC is expected. You can't avoid it. But inductance in the leads from the battery to the esc might cause voltage spikes that can kill an ESC. So always make sure that you have enough capacitance near the ESC. And calculating what is enough is difficult. The idea is to add more capacitance until your ESC stops dying every time. A costly process.

ThierryGTLTS
Posts: 80
Joined: 09 Aug 2017, 11:10

Re: Cable length

Postby ThierryGTLTS » 01 Sep 2017, 15:30

rew wrote:Inductance and some resistance on the motor side of the ESC is expected. You can't avoid it. But inductance in the leads from the battery to the esc might cause voltage spikes that can kill an ESC. So always make sure that you have enough capacitance near the ESC. And calculating what is enough is difficult. The idea is to add more capacitance until your ESC stops dying every time. A costly process.


A scope can help troubleshooting before frying too much MOSFETs ;)

rew
Posts: 905
Joined: 25 Mar 2016, 12:29
Location: Delft, Netherlands.

Re: Cable length

Postby rew » 02 Sep 2017, 06:15

YEah, and of course you just add "should-be-enough" capacitors and hope for the best. Usually that will work without frying too many ESCs.

Today is "use the scope to figure out what's going on" day. If you think that's easy, you're welcome to come and help. :-)

ThierryGTLTS
Posts: 80
Joined: 09 Aug 2017, 11:10

Re: Cable length

Postby ThierryGTLTS » 02 Sep 2017, 12:16

Analog has some part of "black magic" too.

It's not purely 1 or 0.

After more than 35 years of electronic, I know a good part of the subject, but I continue to learn everyday :!:

More than the value of the capacitor, the placement and the capacitor quality (especially DC Bias for MLCC) are crucial.

That's why for example I've placed my large capacitors just at the input of the board, not after some cm of wires like most do.

Have a Nice W-E.

Thierry


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