Voltage drop through conductors? - Page 1
Forum section: electronics / projects

author: DrDisco
author_rank: Newbie
timestamp: November 11, 2019, 07:13:23 am
content: Hello, my name is Ariyan. I'm a mechatronics technician studying mechanical engineering in Serbia and shooting for a master's in mechatronics after.I and a few of my friends are trying to develop a modular stepper driver system that's daisy-chainable. Something like the dynamixel smart servos,http://www.robotis.us/dynamixel/.However, we would make them Arduino compatible and completely open-source. A feature of these pcbs would be the ability to daisy chain them and use them on any "nema style" stepper motors.However, some professors at our faculty have raised concerns about the motor voltage drop via the daisy-chaining. For some reason, they can explain it, to our bafflement. I kind of understand what they are talking about, but I thought that is something you would have to calculate for when you talk about bigger distances i.e. power cables or something. Where this system would be as long as to be fitted to a CNC machine for example.Is this something that would have an impact on my implementation, and if so is there a way to stabilize the voltage over 1m of conductor for example? Like a purpose-made boost converter?

author: DrDisco
author_rank: Newbie
timestamp: November 11, 2019, 07:15:07 am
content: Sorry forgot to state, the possible input voltage would be between 8 and 24 volts.

author: magic
author_rank: Super Contributor
timestamp: November 11, 2019, 07:43:54 am
content: I think you are looking for this table and the Ω/m column in particular:https://www.powerstream.com/Wire_Size.htm

author: LateLesley
author_rank: Frequent Contributor
timestamp: November 11, 2019, 08:05:13 am
content: The problem you have, is motors tend to be high current, and when you have high current, that is when you get voltage drop. And as you increase the conductor length, you increase the drop. So motors at the end of the chain would run slower than the ones at the start. I think your professors have valid concerns, because you'd have to account for situations where possibly two or three motors are running at once.One way around this, would be to jack up the voltage, have a high voltage bus, then each module has a DC to DC converter dropping it down. This would mean the supply was high voltage low current, but each module could be low voltage high current. It also means each device is regulating it's own voltage, and less susceptible to drops in the supply line. The down side would be more wasted power, as each DC - DC converter, even though highly efficient, would eat some power.Basically, the higher a voltage you can have, the lower a current you can have, and less loss in the cabling, for a given power output. You should also consider the safety side of things too, because as you increase voltage, you start to stray into the safety areas, where you would have to mitigate any risk of someone touching a high voltage line, and getting a shock.

author: DrDisco
author_rank: Newbie
timestamp: November 11, 2019, 08:29:05 am
content: Hey,So yes, i already have some prototypes on a few nema17 motors. Where both motors in the chain run simutainioulsy at the same speed (measured) with 30cm wire between (so ic2, and motor voltage). So that's around 12v, each module already has buck converters to drop the voltage down to 5 for the logic.The thing with my professor was, that I understand what he meant in theory, but he couldn't explain to me what that meant in practice. (how it would behave)."One way around this, would be to jack up the voltage, have a high voltage bus, then each module has a DC to DC converter dropping it down. This would mean the supply was high voltage low current, but each module could be low voltage high current. It also means each device is regulating it's own voltage, and less susceptible to drops in the supply line. The down side would be more wasted power, as each DC - DC converter, even though highly efficient, would eat some power."Thats a nice idea actually, however, I'm trying to keep it as small as possible. To be able to mount on the backside of a NEMA 17 for example. Somthing like the 1270 from trinamic.TRINAMIC-PD42-2-1270-TMCL-30100204-01.jpg(60.61 kB. 1020x570 - viewed 68 times.)

author: DrDisco
author_rank: Newbie
timestamp: November 11, 2019, 08:33:02 am
content: Yeah, i know. I think my question was actually just, "would the voltage drop be a problem". I know it isnt nice engineering, just to say "itll work". However, for now, that will have to do, I would much more like to test it. But I don't have the variety of motors required and a variable power supply.

author: Dave
author_rank: Super Contributor
timestamp: November 11, 2019, 08:37:08 am
content: You don't need a constant voltage to control an electric motor effectively. As long as the voltage is high enough, it will work.A motor controller will be regulating the current and adjust the PWM duty accordingly. If a motor at the end of the chain will be supplied with a lower voltage, it will simply need to produce a higher duty cycle than the motor at the beginning of the chain (for the same phase current).

author: DrDisco
author_rank: Newbie
timestamp: November 11, 2019, 08:52:32 am
content: Right!, yes I forgot about that, in that case the drop won't be a problem. As the length it will span (i.e. around a printer or router) won't be long enough the make the voltage unusable by the drivers.Thanks!