A lesson in voltage and amperage, please

[dstig1]

Simply put, it boils down to cost. It is considerably more expensive to build and maintain a three phase line than a single phase line and in the vast majority of instances, a single phase service is more than adequate for the customer.

And the main benefit of 3 phase is found when running large electric motors, which residences have few of, if any.

Interesting discussion.

 

[EE_Bota]

They MIGHT have thought that a system of more distributed generation would have saved SO MUCH in distribution costs as to be worthwhile.
How about a much higher frequency to support transformer efficiency ?
Say 1KHz ?

Twenty years ago, I used to design transformers in the 5MVA to 14MVA range, but I am pretty darn rusty now. But if I remember correctly, most were 99% eff. That last % would represent a boat load of savings, but a super-cooled transformer won't do it.

Although I am sure a higher frequency can be smaller and lighter, I am not sure how much the efficiency would improve. But that doesn't mean it wouldn't, just that I don't know.

 

[Matt_Jr]

OK, as I understand Ohm's Law (I = V/R), there's an inverse relationship between voltage and amperage. That is, the more volts, the less amps, and vice versa. Correct?

And a plasma cutter needs more amps to cut through thicker metal - correct? Ditto for a welder, more amps are needed to weld up thicker pieces of metal... correct?

So why is it that the heavy-duty plasma cutters and welders pretty much require a 240V power supply, since those units would then have lower amperage outputs than a light-duty 120V unit?

I must be missing something here Please point out my mistakes!

Interesting question...

(note:the higher the amperage, the higher the heat)

lets say you have a welder that has an input voltage requirement of 120 on a 30 amp circuit. As you increase the heat(turn up the welder to weld thicker materials) your output voltage will go down, but the amperage will go up. So at max settings, lets say it's 140 amps, your voltage will go down to as much as 15 or 20 volts. This is done by the transformer in the welder. What the transformer is doing (through electromagnetic fields in the windings) is converting the high voltage(120) into low voltage(15-20), and the low amperage circuit(30 amps) into a high amperage circuit(140). By using a machine that requires 240 volt input(24amp circuit) you can actually get more amperage because the higher voltage creates a stronger electromagnetic field in the transformers windings to convert to low voltage/higher amperage output. That 240 volt welder is now capable of puting out... say... 250 amps. again, since the amperage went up, what happens to the output voltage is it goes down. down to somewhere between 15 and 30 volts. As amperage goes up, so does heat. that's why welders are rated at amps. The higher the amperage, the thicker the material you can weld on a single pass. So... why do we have duty cycle?... well.. all that current flow(the high amps) generates a lot of heat, and that heat can melt the windings inside the transformer in your welder, so it has to shut off so that it can cool down(to protect itself). then, once it's cool, you can start welding again.

This is the reason power lines carry electricity in higher voltage than what's in our house. It then goes through a transformer so that it can be transformed into 120/240 for home use.

correct me if I'm wrong, but I believe it was Thomas Edison that wanted our powergrid to run off of dc current, and Nicholi Tesla found that a/c current was much more effecient, required less wiring, and was much more reliable than dc current.

Now, on to more confusing stuff
An electric motor that has an input of 120 volts, with the same power as and electric motor that has an input of 240 volts, requires twice the aperage to do the same amount of work.
ex: a 2 horsepower motor=1492 watts
(100%effeciancy for ease of calculations)

a 120 volt 2 hp motor draws 12.4333 amps
1492watts/120v=12.4333(amps)
a 240 volt 2hp motor draws 6.21666 amps
1492/240=6.21666

now, even more confusing... in an electric motor, when mechanical resistance increases, so does amperage. So when these same electric motors are put under a load, the current in the circuit increases, and the voltage goes down. try: connect an inductive amp probe to your cars battery and measure cranking amperage. try it again with the spark plugs out. the amperage will go down, because the mechanical resistance of the compression stroke won't be there.

hope I didn't bore you guys to death.
oh yeah
Hello, it's been a while
-matt

 

[mjncad]

correct me if I'm wrong, but I believe it was Thomas Edison that wanted our powergrid to run off of dc current, and Nicholi Tesla found that a/c current was much more effecient, required less wiring, and was much more reliable than dc current.

-matt

You're correct. War of Currents - Wikipedia, the free encyclopedia

A friend of mine is a big proponent of DC replacing AC, and I saw an article some time ago in the N.Y. Times or Wall Street Journal advocating a DC power grid with all the proliferation of DC powered electronic gizmo's in our homes and businesses.

 

[SPYDERLK]

Interesting question...

(note:the higher the amperage, the higher the heat)

lets say you have a welder that has an input voltage requirement of 120 on a 30 amp circuit. As you increase the heat(turn up the welder to weld thicker materials) your output voltage will go down, but the amperage will go up. So at max settings, lets say it's 140 amps, your voltage will go down to as much as 15 or 20 volts. This is done by the transformer in the welder. What the transformer is doing (through electromagnetic fields in the windings) is converting the high voltage(120) into low voltage(15-20), and the low amperage circuit(30 amps) into a high amperage circuit(140). By using a machine that requires 240 volt input(24amp circuit) you can actually get more amperage because the higher voltage creates a stronger electromagnetic field in the transformers windings to convert to low voltage/higher amperage output. That 240 volt welder is now capable of puting out... say... 250 amps. again, since the amperage went up, what happens to the output voltage is it goes down. down to somewhere between 15 and 30 volts. As amperage goes up, so does heat. that's why welders are rated at amps. The higher the amperage, the thicker the material you can weld on a single pass. So... why do we have duty cycle?... well.. all that current flow(the high amps) generates a lot of heat, and that heat can melt the windings inside the transformer in your welder, so it has to shut off so that it can cool down(to protect itself). then, once it's cool, you can start welding again.
Hello, it's been a while
-matt

Altho the observations you make are real and some of the reasoning is valid, I think you will confuse the issues by giving them a surface treatment. Heat comes from Watts, which are Amps x Volts. In welding, an arc of 100A at 30V has the same heating value as one of 200A at 15V. When you turn up your welding transformer the arc volts only go down a little as the Amps go up. That "lost" voltage is what it has taken to drive that higher current thru the transformer windings. The windings have a resistance [and there are also inductive losses/impedance] -- so it takes a V to push the current thru. The arc [= to winding] amps times that 'lost' voltage is the Watts that cause the greater heating of the tranformer. Compared to the arc it is quite a bit less heat, and thats a real good thing. Getting heat out of a dense "ball" of wires is difficult and the fan cant keep up when using the higher ranges.
larry

 

[Haywire]

Hmmm... OK then, here's data off the UL label on the motor of a table saw:

12.8A @115V or 6.4A at 230V

I had thought that meant the motor produced 12.8 amps at 115 (or 120) volts, and 6.4 amps at 230 (0r 240) volts. An inverse relationship. What am I missing this time?

This has probably already been said.. didn't read the whole thread. You are confusing input voltage and amperage with output power. 12.8*115=1472... 6.4*230=1472. Increase voltage, decrease amperage and you still get the same power output.

Someone said above that 220v wire was bigger than 110v wire. Not true. 12/2 w/ ground and 12/3 (220v cable) are both still just 12 gauge wire.

Ian

 

[dcyrilc]

This has probably already been said.. didn't read the whole thread. You are confusing input voltage and amperage with output power. 12.8*115=1472... 6.4*230=1472. Increase voltage, decrease amperage and you still get the same power output.

Someone said above that 220v wire was bigger than 110v wire. Not true. 12/2 w/ ground and 12/3 (220v cable) are both still just 12 gauge wire.

Ian

Although they said it backwards, I think what they were refering to was that this motor could be run on 14 guage wire on 110V or somewhere between 18 & 20 guage on 220V. (Using an appropriate breaker for the wire size of course.)

This motor isn't a good example, but the advantage of 220V over 110V is the ability to use smaller guage wire for items which have a large load. IE: electric range, dryer, furnace, etc. Without looking, I believer our 220V electric furnace requires a 220V 50A breaker with #6AWG wiring. To run that at 110V would require a 100A breaker and #2AWG wire.:confused2:

 

[crazyal]

I'm surprised that this thread hasn't gotten into why some single phase motors use a start cap or run cap.

 

[955Lincoln]

I'm surprised that this thread hasn't gotten into why some single phase motors use a start cap or run cap.

You really must be crazy to throw out that gauntlet

 

[EE_Bota]

I'm surprised that this thread hasn't gotten into why some single phase motors use a start cap or run cap.

I did, just not explicitly. I mentioned the equal and opposite rotating magnetic fields of single phase requiring special starting means to lock the rotor with one of the fields vs the singular rotating magnetic field of a 3 phase, and all of that was off topic, really, to the OP's question. But the OP seems to be broadly curious about most aspects of electricity, so hopefully he did not mind.

Once, on a job interview at a dairy farm in Florida, they could not wash up their parlor due to a failed single phase motor on the booster pump. The herdsman, a British fellow, was surprised that I started it for him with bailing twine. I wrapped the twine around the shaft such that it would likely release at full pull, and gave it a yank, and it started. Although this was before I went back to school for the EE, I was able to explain why I though that would work.

Interviewing with the owner, I could hear a loud SNAP over and over in the other office. It was that herdsman, he kept plugging that capacitor into the wall and discharging it...SNAP!