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amperage draw [message #160606] Thu, 16 February 2012 22:52 Go to next message
RadioActiveGMC is currently offline  RadioActiveGMC   United States
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Someone said once that if you add resistance to a circuit amp draw stays the same. Example if you have a 12v motor and it draws 10amp and you'd like to reduce it to 7 amps, adding a resistor (using ohms law not debating that at all) to the circuit would still have the same amp draw. I've scratched my head since I've read it. If you reduce the speed of a motor and it turns slower, amp draw goes down. The resistor would add "resistance" but not draw as many amps as the load would of originally...

***"Gettin There"-1973 23' Sequoia- Michael, Onans smell, "Go solar/wind power!"

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Re: [GMCnet] amperage draw [message #160624 is a reply to message #160606] Fri, 17 February 2012 06:37 Go to previous messageGo to next message
jhbridges is currently offline  jhbridges   United States
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'someone' was mistaken.  Take your trusty meter and give it a try.
 
--johjnny
 
'76 23' transmode norris
'76 palm beach

From: Michael <radioactive626@msn.com>
To: gmclist@temp.gmcnet.org
Sent: Thursday, February 16, 2012 11:52 PM
Subject: [GMCnet] amperage draw



Someone said once that if you add resistance to a circuit amp draw stays the same. Example if you have a 12v motor and it draws 10amp and you'd like to reduce it to 7 amps, adding a resistor (using ohms law not debating that at all) to the circuit would still have the same amp draw.  I've scratched my head since I've read it. If you reduce the speed of a motor and it turns slower, amp draw goes down. The resistor would add "resistance" but not draw as many amps as the load would of originally...
--
***"Gettin There"-1973 23' Sequoia-
Michael, Casa Grande, AZ
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Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons. Braselton, Ga. I forgive them all, save those who hurt the dogs. They must answer to me in hell

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Re: [GMCnet] amperage draw [message #160625 is a reply to message #160606] Fri, 17 February 2012 06:40 Go to previous messageGo to next message
Steven Ferguson is currently offline  Steven Ferguson   United States
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ELI the ICE man.

On Thu, Feb 16, 2012 at 9:52 PM, Michael <radioactive626@msn.com> wrote:

>
>
> Someone said once that if you add resistance to a circuit amp draw stays
> the same. Example if you have a 12v motor and it draws 10amp and you'd like
> to reduce it to 7 amps, adding a resistor (using ohms law not debating that
> at all) to the circuit would still have the same amp draw. I've scratched
> my head since I've read it. If you reduce the speed of a motor and it turns
> slower, amp draw goes down. The resistor would add "resistance" but not
> draw as many amps as the load would of originally...
> --
> ***"Gettin There"-1973 23' Sequoia-
> Michael, Casa Grande, AZ
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
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>



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Re: amperage draw [message #160635 is a reply to message #160606] Fri, 17 February 2012 07:05 Go to previous messageGo to next message
RF_Burns is currently offline  RF_Burns   Canada
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I think the "someone" was talking about the fact that using a resistor to reduce speed and to save power is not the best idea. The resistor will consume lots of power as well.

What you want to do is use an electronic DC motor controller. That will allow you to reduce the speed (and power requirement) of the motor, without consuming a lot of power doing it.

I don't have time to go back looking for the thread, but I believe that was the jist of it.

Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.
1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that

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Re: amperage draw [message #160644 is a reply to message #160606] Fri, 17 February 2012 09:12 Go to previous messageGo to next message
RadioActiveGMC is currently offline  RadioActiveGMC   United States
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Ok, I honestly wasn't even trying to open a can of worms. I just really scratched my head maybe I misread it as well. I appreciate the replies. RFburn for a long time I've wanted to say that is the best of the best "handles" for a forum.

***"Gettin There"-1973 23' Sequoia- Michael, Onans smell, "Go solar/wind power!"

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Re: [GMCnet] amperage draw [message #160656 is a reply to message #160625] Fri, 17 February 2012 10:04 Go to previous messageGo to next message
n6mon is currently offline  n6mon   United States
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Only applies to AC circuits and has to do with the phase
relationship between Voltage (E) and Current (I) in a
reactive (L or C) circuit (high school and college
electronics 40 or so years ago!)

tt

On 2/17/2012 4:40 AM, Steven Ferguson wrote:
> ELI the ICE man.
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Re: amperage draw [message #160669 is a reply to message #160606] Fri, 17 February 2012 11:19 Go to previous messageGo to next message
RF_Burns is currently offline  RF_Burns   Canada
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To illustrate, say your load is 12 ohms and 12 volts power for easy figuring. Through ohms law you find your load consumes 12 watts.

So you figure you'd like to cut this in half to 6 watts. Through ohms law you find you need to cut your current from 1 amp to 0.707Amps.

12v / 0.0707Amps = ~17 ohms
17 ohms - 12 ohms = 5 ohms needs to be added in series.

That 5 ohm resistor will dissipate 0.707 X 0.707 x 5 = ~2.5 watts.

So instead of cutting your power from 12w to 6w, you are now consuming 6w + 2.5w = 8.5w. Using an electronic controller can reduce that 2.5w to a fraction of a watt.

But since we are talking about motor driving a fan, you can throw all those calculations out the window because the load will change with the motor speed and other variables making it near impossible to accurately calculate.

So just get an electronic controller and don't worry, be happy.

Oh.. I'm in the mobile radio communications business so I know.. RF burns! Rolling Eyes

Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.
1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that

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Re: amperage draw [message #160670 is a reply to message #160669] Fri, 17 February 2012 11:28 Go to previous messageGo to next message
Ken Burton is currently offline  Ken Burton   United States
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I have grabbed a few low power (under 100 watts) active antennas in my life. I choose not to think about it, but I sure remember. I'll take 120 VAC over RF any time.

It is kind of like grabbing a spark plug wire. You remember it forever.

Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana

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Re: amperage draw [message #160677 is a reply to message #160606] Fri, 17 February 2012 11:54 Go to previous messageGo to next message
wally is currently offline  wally   United States
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Mr.RadioActive wrote on Thu, 16 February 2012 22:52

Someone said once that if you add resistance to a circuit amp draw stays the same. Example if you have a 12v motor and it draws 10amp and you'd like to reduce it to 7 amps, adding a resistor (using ohms law not debating that at all) to the circuit would still have the same amp draw. I've scratched my head since I've read it. If you reduce the speed of a motor and it turns slower, amp draw goes down. The resistor would add "resistance" but not draw as many amps as the load would of originally...

Ebay is loaded with DC controllers..low cost too.
http://www.ebay.com/itm/New-12V-40V-10A-13khz-Pulse-Width-Modulation-PWM-DC-Motor-Speed-Control-Switch-/180817335072?pt=LH_DefaultDomain_0&hash=it em2a198d9320
HTH

Wally Anderson
Omaha NE
75 Glenbrook

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Re: [GMCnet] amperage draw [message #160679 is a reply to message #160670] Fri, 17 February 2012 12:20 Go to previous messageGo to next message
jhbridges is currently offline  jhbridges   United States
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If you grab one of mine, you get fuittd for a box.  For eternity.  They ain't low power.
 
--johnny
'76 23' transmode norris
'76 palm beach
 

From: Ken Burton <n9cv@comcast.net>
To: gmclist@temp.gmcnet.org
Sent: Friday, February 17, 2012 12:28 PM
Subject: Re: [GMCnet] amperage draw



I have grabbed a few low power (under 100 watts) active antennas in my life.  I choose not to think about it, but I sure remember.    I'll take 120 VAC over RF any time.

It is kind of like grabbing a spark plug wire.  You remember it forever. 
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons. Braselton, Ga. I forgive them all, save those who hurt the dogs. They must answer to me in hell

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Re: amperage draw [message #160703 is a reply to message #160606] Fri, 17 February 2012 16:38 Go to previous messageGo to next message
g.winger is currently offline  g.winger   United States
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Okay,,,,heres how this works. Motors have maximum torque and maximum current at 0 (zero) rpm,s. Thats why the lights dim when you start a motor,,,, no RPM's. Now if yoy put a meter on a motor when its not running its almost no ohms. Its just copper wire ,right??? So how come its not alot of amps when running???? When a motor spins, it has copper wire passing thru a magnetic field. Remember,,,,,,this makes a voltage. Like a generator!!!! This voltage "bucks" the voltage that drives the motor. Oposite polarity!!! CEMF ,(counter electromotive force.) So the running current is limited by CEMF. When you cut the voltage with a resister that slows the motor and the "resistace" of the motor changes because of less "CEMF",,,,,, bad juju. Or overheating. The resistance of the motor goes down and changes the resistance of the series circut. Fan motors on GMCs are built for this. The resisters are HD and cooled by blowing air. Use pulse width modulation instead. (on/off quickly,,,like 50 times per second). Hpe this helps,,,,,,PL
78 RB Royale
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Caddy 500
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Re: [GMCnet] amperage draw [message #160753 is a reply to message #160679] Sat, 18 February 2012 02:20 Go to previous message
Ken Burton is currently offline  Ken Burton   United States
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Johnny Bridges wrote on Fri, 17 February 2012 12:20

If you grab one of mine, you get fuittd for a box.  For eternity.  They ain't low power.
 
--johnny
'76 23' transmode norris
'76 palm beach
 



I realize that. That is why I'm extra careful working around or on Commercial towers.


Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana

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