voltage drop from long wire run?

rr

Hello,
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I want to power some motors with some parallax motor controllers (HB-25). The motors run off of 12 Volts at about 3-4 Amps (depending on the load). I want the control point to be accessible, which means the motors will be about 100 feet away. I originally thought it wouldn’t be a problem to run a 100’ of 12 volt wire (18 gauge) to the motors, but my friend recently told me that I could loose as much as 6 volts from the long length of wire to the motors. I found a calculator online that seemed to confirm this, but I still have a hard time believing that it will really drop this much voltage.
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Does this sound right???
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I plan on actually testing this; I Just thought it would be easier to ask first before I run a 100’ of wire. If this is the case would you recommend running a 18 volt power supply?
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Thanks for any help.
rr

PJAllen

The resistance of #18 wire is about 6ohms/1,000 feet.· So 100ft would have the effect of a 0.6ohm resistor in series with your etc.· E = I X R: 3 to 4A would result a·1.8 to 2.5V drop across the wire.· The maximum current for #18 wire is about 2.3A

Post Edit -- wire has resistance; in short lengths it's inconsequential, but it adds up.· Larger gauge wire has less resistance than smaller gauge wire.

Post Edited (PJ Allen) : 12/11/2008 12:08:27 PM GMT

Peter Jakacki

100' there and 100' back @0.65ohms per 100' = 1.3ohms where R*I = V = 5.2V. Sounds right to me except of course the motor's current won't remain the same.

There is also the problem with EMI radiating from the long cable run as well especially if you PWM the motors, certainly a problem for commercial equipment.

If the motors are 100' away then that's where I'd put the electronics but then I'd be running a higher voltage like 24 volts for instance. I don't think you are going to get very good regulation just by increasing the voltage as the motors will have a higher voltage when they are not drawing as much current which then makes it draw more current and so on. Then again motors aren't strictly resistive or inductive as they also generate a back EMF depending upon the speed.

But if you aren't worried about speed control then you probably could just increase the voltage, though the motors might get a bit stressed unless they are loaded.

*Peter*

Ugha

The EM problem is a major one... If you absolutely must make a long wire run, try shielded wire or in the very least twist the
wire for its length. That will help reduce the problem a little.

Carl Hayes

Rather than a larger (more expensive) power supply, why not just run bigger wire? #12, for example, will have about 0.3Ω for a 2-conductor run of 100 feet. At 4A you would then lose about 1.2 volts, worst case -- which is acceptable. The #12 wire would also be more robust physically. Copper wire has gone up substantially in price in recent years, but still it's cheaper than power supplies -- and cheaper in the long run than turning energy into heat, too.

I respectfully doubt that EMI would be a problem. In every house, shop, and factory, larger motors than these operate on longer wire runs than the one we're talking about without problems. If the motors are particularly noisy ones electrically -- series-wound DC motors with worn carbon brushes and dirty commutators, for example -- you may need to install some ferrite cores on the wires, at or near the motors -- or simply use some extra length and wind the extra into a common-mode choke. If the wire is, say, 12-gauge zip cord, the conductors will be close enough together (1/8 inch or so) to reduce radiation substantially. But I for one doubt that EMI will be a problem with a 1/15 horsepower DC motor. (Your motor, at 4A and 12V, consumes 48 watts max, which is about 1/15 HP).

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· -- Carl, nn5i@arrl.net

Post Edited (Carl Hayes) : 12/11/2008 3:57:37 PM GMT

Dennis Ferron

If you can, run a 24 volt supply and then PWM it at 50/50 duty cycle. You'll still drop 6 volts, but instead of 12-6 it will be 24-6, so the percentage lost will be less.

As motors speed up, they generate more back EMF (i.e. generate a voltage opposing the input). With all due respect, I think Peter has it completely backwards: he seems to be saying that as the motor speeds up and generates more back EMF, it will oppose the input voltage and draw less current, which makes the wires drop less power, which increases current, and so on. He seems to think this negative feedback loop will lead to oscillations that increase without limit. I'm sorry, this is WRONG. What actually happens (I spent 2 years designing robot motor controllers) is that it leads to equilibrium; the motor will reach a steady speed which balances the resistance of the wire, the voltage, and the back EMF.

Now, I'm not saying that if you suddenly increase the voltage, you won't see some "bell ringing" - you will! But it will die down quickly (seconds or milliseconds).

I'm not trying to jump on Peter Jakacki - his misconception is one that I've caught myself in before too! But it's such a common misunderstanding of how feedback works that it needs to be laid to rest. Here are 4 arguments against the case that runaway oscillations would occur in that system.

1. I've hooked up this exact configuration of resistance and motor described, and run away oscillations do no occur. The motor quickly reaches operating speed and holds it.

2. Oscillations that persist or increase with each cycle require an amplification factor >1 to sustain. There is no amplifying element in the circuit, ergo any oscillations introduced would die down quickly.

3. reductio ad absurdum
i. Motors have internal resistance (your motors probably have about 3 ohms internal resistance).
ii. Series resistors have the same effect no matter what order they are placed in in a series circuit (follows from Kirchhoff's circuit laws)
iii. Therefore the internal resistance of the motor is just as applicable to Peter's argument as a wire resistance
iv. So the stated run away oscillation should be expected to occur in any motor, even if connected without excess wire resistance
v. This does not occur, ergo
vi. It cannot be the case that wire resistance causes runaway motor oscillations

4. i. Peter argues that as the motor draws less current through the resistance, the voltage drop across the resistor decreases (this is true)
ii. Peter argues that as the voltage drop across the resistor decreases, the applied voltage to the motor increases (this is true)
iii. He argues that the suddenly higher input voltage necessarily leads to a sudden increase in motor current draw (this is a fallacy; here's why[noparse]:)[/noparse]
a. A motor, although not only resistive or only inductive, has a highly inductive characteristic over short time scales
b. Inductors resist sudden changes in current; they will generate a momentary voltage to oppose a change in current
c. In this case, the voltage generated by the inductance to resist the decrease in current will be opposite the back emf, aligned with the input voltage
d. So the real equation is (input voltage - back emf + momentary voltage)
e. The more sharp the change in current, the greater the voltage generated by the inductance will be (proportional to the change)
f. So what really happens is:
I. back emf increases
II. current decreases
III. applied voltage increases
IV. inductance resists momentary change in current due to applied voltage increase
V. all elements approach balance

Anyhow, to the original poster, the point is that your only problem from having long wire runs is power waste, not problems with control. Even if you are doing a closed loop control based on the motor shaft speed, the wire resistance will not affect your control loop; refering back to argument #3, if series resistances of wires affected your control loop, so would internal series resistance of any motor you hook up anyway.

Post Edited (Dennis Ferron) : 12/11/2008 4:00:34 PM GMT

Carl Hayes

I agree with Dennis that the stability problem is an imaginary one. A motor that's varying speed because of changes in excitation or changes in load constitutes a velocity servo rather than a position servo. Inertial effects in position servos can cause oscillations, but inertial effects in velocity servos exert a stabilizing influence instead.· But I'd use bigger wire rather than a bigger power supply.

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· -- Carl, nn5i@arrl.net

kelvin james

Any reason you couldn't run your ac source to it instead, and connect the 12v dc supply there for the motors? Losing a few volts on the ac line should not be a problem for the dc power supply.


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Beau Schwabe

kelvin james,

That, or move the batteries physically close to the motor(s) and use the 200 feet of wire for maintaining a charge on the batteries and/or control of the motor(s).

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Peter Jakacki

Hi Dennis,

I haven't read all your post yet but I was presenting the motor as a complex load. In fact the way the motor behaves electrically is heavily dependent upon the mechanical load itself. How do you try to explain something which appears simple but considering the source resistance of the wire and the motor characteristics over varying speed and load? I don't know how best to explain it especially since I know nothing of the motors or load.

Runaway oscillations? No, I don't think I was trying to say that as I do have a little experience with various motors. People with various backgrounds, knowledge, and experience read these posts and I try to remember to speak to the OP on their level. The object of mentioning the voltage/current/speed/load thing is to highlight that there is much more to "calculating the voltage drop" then a simple Ohm's law so that whole thing is not what I would call a "technically correct" statement but an exercise in taking the OP through a quick "in your mind" scenario to show there is much more to it.

But I always appreciate a good discussion.

Beau Schwabe said...

That, or move the batteries physically close to the motor(s) and use the 200 feet of wire for maintaining a charge on the batteries and/or control of the motor(s).

Another one of your redneck ideas mate! Precisely what I have done in some instances or even as a way of using a smaller power-supply and having the battery do the intermittent grunt.

*Peter*

Dennis Ferron

Hi Peter,

Oh, no doubt, motors can exhibit complex characteristics. I found that as the motors I worked with got larger, effects which were small enough that they go unnoticed in small motor projects start to cause problems in large motors, simply because the energy involved is so much greater.