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DC Motor Cable Sizing

:: EPE Chat Zone ­:: ­Radio Bygones Message Board :: » EPE Forum Archives 2010 - » Archive through 08 November, 2012 » DC Motor Cable Sizing « Previous Next »

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muskrat
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Post Number: 430
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Posted on Tuesday, 23 October, 2012 - 06:42 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I have a tail-ift mechanism for a trailer. It is hydraulically operated, with the hydraulics being driven by a 24V 2000W motor.
The spec plate has: "S2-2min" and "S3=5%ED" neither of which I understand.
Applying the bog-standard P:V:I formula, I come up with 80A running current!
That implies a rather large supply cable to handle the current. Given that the truck battery is close on 17 metres from the motor, volt-drop will also be an issue.
How do I calculate an acceptable cable size?
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alexr
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Post Number: 228
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Posted on Tuesday, 23 October, 2012 - 07:48 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Assume an acceptable voltage drop of say 10% across the cable then plug the figures into a cable sizing calculator such as http://myelectrical.com/tools/cable-sizing-calculator or http://www.energymatters.com.au/climate-data/cable-sizing-calculator.php

With a 17meter run at 80Amp both calculators come up with you needing a 50mm2 cable, i.e. a copper cable with a 8mm diameter core, something like a heavy duty welding cable!
If you could move the battery nearer to the motor you could get away with a much lighter cable.
Alex
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muskrat
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Posted on Tuesday, 23 October, 2012 - 08:40 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Thanks Alex.
Unfortunately the motor assy is at the rear of the trailer, and the battery, obviously, up front on the horse. I also need to be able to disconnect the electrics when the trailer is removed. I'll use an Anderson connector there.
Due to the massive cost of the cable - could the trailer chassis be used as earth?
Bearing in mind that the trailer-horse connection is through a king-pin in a 'fifth-wheel' with a lot of grease around.
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muskrat
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Posted on Tuesday, 23 October, 2012 - 09:24 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

OK, I'm officially an idiot. I can bond to chassis at the Anderson connector on both sides. Problem solved.
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dave_squibb
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Posted on Tuesday, 23 October, 2012 - 10:24 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Might be much cheaper to buy a second battery just for the motor?

(Message edited by dave squibb on 23 October, 2012)
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muskrat
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Post Number: 433
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Posted on Tuesday, 23 October, 2012 - 10:59 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

That was an option - but would involve even more hassles trying to charge it.
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basementboy
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Post Number: 35
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Posted on Tuesday, 23 October, 2012 - 11:56 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

It may be stating the obvious, but each 50mm cable can be made up of eg 2 x 25 in parallel. This may open up other cheaper avenues for the cable purchase. Scrap yards often have 3 phase 16mm SWA, which in parallel would give 48mm2.
I'd hope the trailer chassis would be good enough for the return current, just check which side of the battery is currently connected there first!
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petelobus
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Posted on Tuesday, 23 October, 2012 - 05:58 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Welcome to the joys of low voltage power transmission! The reference book I usually use (Bosch 'Automotive Handbook') suggests a maximum voltage drop in 'other circuits' of 2V at 24V. Quick sum suggests that 34m of 50sqmm CSA cable will drop 1V at 80A... Practically problems are likely to arise if the motor has to start under load with low(ish) batteries (the starting current of the motor will be much higher than the rated current). I'd guess the trailer chassis would provide a good return path, but its as well to ensure that the system isn't 'isolated return' (i.e. neither side of the battery connected to chassis - common on buses in the past, but not used so much now). Obviously returning current through the chassis would have some implications for this type of system...

On a practical note, check out the local welding supplies merchant for large cross section cables. Welding cable is nice and flexible and (at least round here) seems to be much cheaper than automotive starter cable. Insulation rated to at least 100V too. I checked for another project, and my supplier can do 50sqmm cable. For some reason cash sometimes gets a good discount...!

Ian
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ant
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Posted on Tuesday, 23 October, 2012 - 09:07 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hello all,

Depending on how much the winch is used it may work to have a battery alongside the winch, floated off the trailer supply cable. It would take a little while to get topped-up after use but that may be a useable compromise.

Regards Ant
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boris
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Posted on Wednesday, 24 October, 2012 - 03:04 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

"hydraulics"

Maybe you can extend the pipework and shorten the cable run?
Thank you for not breeding.
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boris
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Posted on Wednesday, 24 October, 2012 - 03:06 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Maybe you can move the 'horse' around to the back and plug in a shorter cable? Jumper cable?
Thank you for not breeding.
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muskrat
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Post Number: 434
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Posted on Wednesday, 24 October, 2012 - 02:05 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Thanks everyone.
Fired it up this morning for 'smoke and fire' test. Motor runs fine, cable is cool, chassis giving a good earth return, volt-drop of just under 1V.
I'm quite a happy little puppy!
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mikeb
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Posted on Wednesday, 24 October, 2012 - 02:16 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

@musk

Glad you got it working ... it got me thinking outside the padded cell ...

I wonder if you could have used domestic copper water pipe? It is reasonably cheap, comes with a range of soldered (low resistance) fittings and plenty of attachment options. Its all low voltage, so you can even make use of their range of insulation products.

When in transit, pump the horses water through it for cool refreshment .... time for my medication again ....

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