Resistance

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  #1  
Old 10-18-06, 05:38 PM
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Resistance

Can someone point me to some good reading about resistance: what it is, how it affects the current, how to test, etc?

Is that the same as Ohms?

Mostly for my house, but I am also interested in DC current.

Thanks.
 
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  #2  
Old 10-18-06, 06:11 PM
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Resistance is measured in Ohms.

Ohm's Law, written 3 different ways:
Volts = Amps x Ohms
Amps = Volts / Ohms
Ohms = Volts / Amps

To measure resistance, you usually use an "Ohmeter" which is generally built into any cheap multimeter.

http://science.howstuffworks.com/electricity1.htm
 
  #3  
Old 10-18-06, 06:15 PM
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Resistance is sort not relevant for your home. The problems have been accounted for in the table about which wire size you use etc.

Here is some reading

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/resis.html

http://physics.bu.edu/~duffy/PY106/Resistance.html

http://www.answers.com/topic/electrical-resistance-1

http://dictionary.laborlawtalk.com/Electrical_resistance

http://www.google.ca/search?num=20&hl=en&safe=off&q=electrical+resistance&meta=
 
  #4  
Old 10-19-06, 04:54 PM
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well that kinda helped. Maybe I don't know what I am asking for.

As an example, when I use a 110v welder in my garage I am not able to weld the same thickness of material as my friend can with the same welder in his garage. I want to find out why, so I thought I would measure the resistance and ohms and then find out what the problem is. Make sense? Some of the lights in the house will dim because an appliance that uses a large breaker has turned on. I want to find out why that is also.

Anyone know where to start for this?
 

Last edited by JoeVB; 10-19-06 at 05:05 PM.
  #5  
Old 10-19-06, 07:47 PM
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JoeVB,

As far as the welder portion of your question goes, what do you mean by "cannot weld the same thickness of metal"?
Measuring resistance and ohms won't help you here.
If you were to measure voltage at the plug while the welder was operating it would tell you how much voltage drop there is.
Another check is to ensure you have the correct wire size and breaker installed.
 
  #6  
Old 10-19-06, 08:01 PM
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Your friend being able to weld bigger material could be traced back to a few things. He could have higher voltage at his house then you have at yours. He might have 120v at the main panel and you could have 110v. If your 120 volt welder draws 50 amps then a 10 volt difference adds up to a 500 watt difference.

Another thing that could affect the welder is how long the copper path is from the welder to the transformer on the pole. Resistance is based on the size of the wire and how long it is. The longer it is the more voltage drops. If your welder is in a detached garage say 100í from the house which itself is 150í from the transformer then there is at least 250í of wire for the power to go thru. If your friendís house is 50í from the transformer and has an attached garage there is a much shorter path for the power to go thru yielding him more volts to the welder and more total watts of welding power.

As for the lights dimming thatís normal. Your service entry cable coming from the pole is rated at so many amps; probably 200 but could be 100 or less if the house is old. When a device that uses 25%+ of that feedís capacity kicks on things are going to dim. The best way to lessen the dimming lights and out weld your friend is to get a 240v welder. A 240v 30a welder would have 7200watts of power; a 120v 50a welder would only have 6000 watts.
 
  #7  
Old 10-19-06, 08:15 PM
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The welding process is based on the extremely high temperature produced in the arc. Note that if you simply plunk the rod down on the joint, a lot of current flows but it does not get hot much because there is no arc, and also zero resistance at the junction. (amps flowing through resistance = watts = heat)

The temperature that is developed in the arc does vary with the current flowing from the welder. A welding machine is designed to produce very large amounts of current. Ideally, the resistance of the leads, the stinger connector, etc, would be zero. If cables are too small, or connections are loose, then there is resistance and this will affect the heat of the arc.

Ohms law applies. But welding is a little more complicated than that. I would do a google search to determine what is happening in that arc.
 
  #8  
Old 10-19-06, 09:08 PM
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Originally Posted by JoeVB
well that kinda helped. Maybe I don't know what I am asking for.

As an example, when I use a 110v welder in my garage I am not able to weld the same thickness of material as my friend can with the same welder in his garage. I want to find out why, so I thought I would measure the resistance and ohms and then find out what the problem is. Make sense? Some of the lights in the house will dim because an appliance that uses a large breaker has turned on. I want to find out why that is also.

Anyone know where to start for this?

Resistance checks will not tell you much. What you want to see is how much and where the voltage drops when you fire up your welder.
 
  #9  
Old 10-20-06, 08:52 AM
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This is more of what I was looking for...thx.

The main thing I want to make sure of is that I will be able to weld thick metal with a 240v welder. I'd hate to spend the $$ on a 240v welder just to find the same prob. I know that a 240v welder will weld thicker metal than a 120v welder but I want to use the welder to its full capacity, if needed.

My main panel is about 50' away (maybe less) from the garage now. I plan to put in a 60A sub panel and will plug my welder into that.

I'll do more research.
 
  #10  
Old 10-20-06, 09:06 AM
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Ohms law- Volts = Amps x Ohms.

The wires in your house are not perfect conductors- you can think of them like resistors. And in the real world, if you put 120 Volts in one end of a long cable, you might only get 110 Volts out at your lights, 200' down the wires which are snaking from the street and all through your house. (It's not a straight line, and there is a lot of backtracking, especially when 3-way and 4-way switches are used.) Lets assume that's the case in your house- the power company's transformer is putting out 120V, and you're getting 115V at your breaker panel. If your normal house load is 15A on each phase, that means the cables are about 1/3 Ohms [5V = 15A x 1/3O], and voltage drops 5V over those wires. (You have even more voltage drop going from the cables from your breaker panel to the lights and welder, but the current from the welder won't impact the voltage drop in other line here, just the lines that are shared.)

Now, lets say you plug in your welder which draws 45A of power at a low duty cycle. Now, every time you key the arc, the current through the wires to your house spikes from 15A to 60A! The Ohms are the same, but the voltage drop is now 4x more, and instead of getting 115V at your lights, you're getting 100V. [20V = 60A x 1/3O] The same thing happens whenever large motors start up, like in A/C compressors or refrigerator compressors or power tools- your light will dim momentarily as the massive in-rush of current (often 40-60A) causes voltage drop, and then the lights will recover most of their brightness when the in-rush passes. This is most visible when you're using a long extension cord- try plugging in a work light and a skill-saw on to the same cheap 100' extension cord and you'll get a very visible demonstration

Switching to a 240V welder will spread that current out over 2 wires- so instead of one phase taking a 15V drop, you'll have both phases taking 10V drops. Or something along those lines.
 
  #11  
Old 10-21-06, 01:27 PM
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This answered a lot of questions I had, thanks.

Is there a way to avoid the dimming of the lights? Is there something as an electrical conditioner? I have two falsh water heaters that each draw just over 50A measured (60A breaker). The main is 200A. It makes sense that it drops when using 100A for the hot water but is there anything I can do about it?

Originally Posted by Niadh
Your friend being able to weld bigger material could be traced back to a few things. He could have higher voltage at his house then you have at yours. He might have 120v at the main panel and you could have 110v. If your 120 volt welder draws 50 amps then a 10 volt difference adds up to a 500 watt difference.

Another thing that could affect the welder is how long the copper path is from the welder to the transformer on the pole. Resistance is based on the size of the wire and how long it is. The longer it is the more voltage drops. If your welder is in a detached garage say 100í from the house which itself is 150í from the transformer then there is at least 250í of wire for the power to go thru. If your friendís house is 50í from the transformer and has an attached garage there is a much shorter path for the power to go thru yielding him more volts to the welder and more total watts of welding power.

As for the lights dimming thatís normal. Your service entry cable coming from the pole is rated at so many amps; probably 200 but could be 100 or less if the house is old. When a device that uses 25%+ of that feedís capacity kicks on things are going to dim. The best way to lessen the dimming lights and out weld your friend is to get a 240v welder. A 240v 30a welder would have 7200watts of power; a 120v 50a welder would only have 6000 watts.
 
  #12  
Old 10-21-06, 01:30 PM
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The only prob is, I don't buy cheap tools

Originally Posted by grover
This is most visible when you're using a long extension cord- try plugging in a work light and a skill-saw on to the same cheap 100' extension cord and you'll get a very visible demonstration
 
  #13  
Old 10-22-06, 03:23 AM
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JoeVB:

It could be that your welder, although it's the same as your friends, is not putting out the same watts. If practical, I'd ask my friend if we could test my machine in his garage to see how it stacks up there with his.

I'd also try to find out what type of circuit his is on and compare it to mine: what size breaker does his unit operate on versus yours? I'm thinking if his is bigger then his wires are bigger, which would deliver more watts to the unit.

If you have a voltmeter, you could ask your friend to let you check the voltage at the outlet he uses for his unit, and compare it to yours.

Hope you can resolve this without having to buy another welder.
 
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