A lesson in voltage and amperage, please

[Perplexed]

Thanks for all the responses, I've learned quite a bit in this thread!

But all this talk about the benefits of a 3-phase setup begs the question: Why use single-phase at all? Why is it prevalent in American households - is it simply a case of "That's what America started with way back when, and we just never got around to changing the circuitry in tens of millions of homes over to 3-phase"?

 

[Reg]

Thanks for all the responses, I've learned quite a bit in this thread!

But all this talk about the benefits of a 3-phase setup begs the question: Why use single-phase at all? Why is it prevalent in American households - is it simply a case of "That's what America started with way back when, and we just never got around to changing the circuitry in tens of millions of homes over to 3-phase"?

Oh, Ohh.....

There are "historical reasons".
The moving hand of history writes and having writ moves on (or somesuch).

Single phase, 60 Hz, 110, AC Why ?
Thought to be less dangerous than 220 (see Canada, Europe), or DC (which also required more expensive (then) motors) simple, advantageous for speed and time - 1/60 also being the division of hours to minutes, minutes to seconds, etc.
I doubt that during the great electrification era it was anticipated that homeowners would be running 50 amp appliances.... maybe, maybe not.
Lights ? that was probably about it.
Vacuum cleaners, clothes washers, DRYERS(BIG power suckers) all came after.

220 is supplied to most houses in the US and is "center tapped" for 110.
I suspect (GUESS) that 3 phase is "on the street" and different houses get wired across different pairs for load balancing.

As I said, history gone done it to us.

 

[mmurphy]

Thanks, Mace. I guess at 0.97 average it would not be enough to worry about. I hope the power companies never check mine when I am running motors and or welder!

 

[mmurphy]

Thanks for all the responses, I've learned quite a bit in this thread!

But all this talk about the benefits of a 3-phase setup begs the question: Why use single-phase at all? Why is it prevalent in American households - is it simply a case of "That's what America started with way back when, and we just never got around to changing the circuitry in tens of millions of homes over to 3-phase"?

It really has a lot to do with costs. Most residential would not require the potential of watts to do work.
Is it safer to run sinle phase in households? I would say yes, but it still only takes 20 to 30 milliamps going through your heart to stop it.
How many times have you heard the 20 some year old saying "I changed the light switch with the circuit on, it is only 15 amps." To that I tell them you just needed to come in contact with the neutral in 1 hand, and hot in other hand to have a heart stopping moment.

 

[EE_Bota]

Thanks for all the responses, I've learned quite a bit in this thread!

But all this talk about the benefits of a 3-phase setup begs the question: Why use single-phase at all? Why is it prevalent in American households - is it simply a case of "That's what America started with way back when, and we just never got around to changing the circuitry in tens of millions of homes over to 3-phase"?

The power requirements are quite small, so no need to run three phases. Also, built into the setup, like Reg mentioned, is the fact that the transformer is center tapped.

By center tapped, instead of having a winding with the bottom of the winding being 0 volts, the center being 120 volts, and the top being 240, there is a good reason to make it -120 at the bottom, 0 at the center, and 120 at the top. Therefore, you cannot touch a single line with your hand and take a 240V jolt, only a 120 volt jolt.

240 results from the two lines, 120 and -120 being 180 degrees out of phase. So, you can have 240, but only take a single line shock of 120.

Obviously, we have AC because it can be transformed with transformers. DC cannot be transformed, so it is not that good to have as a carrier of large power over long distances.

Single phase is still a good idea, and I doubt very seriously if we started all over from scratch if we would do it any differently.


One of the gentleman used the water analogy. I like that analogy, and use it often, and it can aid in the understanding of hydraulics. But, remember, that analogy does not account for the current flowing backwards and forwards 60 times a second. Usually, for many practical things, this can be forgotten. But in a different post, someone mentioned inferiority of single phase due to the "common." This is a case where the water analogy breaks down. That common might as well be the hot wire, since from a motors perspective, current flows in, reverses, and flows out again. The motor does not know the difference. The real crux of the matter is what I said...rotating magnetic field vs two equal an opposite magnetic fields for single phase. Look at a shaded pole motor, and you will see the simplest way of accounting for that.

 

[Reg]

"Single phase is still a good idea, and I doubt very seriously if we started all over from scratch if we would do it any differently."

Ahhh, to start over ???

I suppose with 20/20 foresight to a 100 year horizon they MIGHT have predicted a lot more power being consumed by a lot more households located a lot farther from each other.
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 ?
I am not CERTAIN about this, but I think 3 phase is common in Europe.
Somewhere I read that code requires outlets on different phases must be more than 2 meters apart, so you don't string yourself across 415 (I think its 415, don't remember).
Not that it saves you from bringing two extension cords together from opposite sides of the room to light yourself up with (-:

 

[Mace Canute]

Thanks, Mace. I guess at 0.97 average it would not be enough to worry about. I hope the power companies never check mine when I am running motors and or welder!

Since residential meters are WATT meters, a bad power factor costs the electrical utility, not the consumer. Utilities don't charge home users for a bad power factor. There is a scam going around where a company/companies are trying to convince homeowners that buying their expensive equipment to correct the power factor in their homes will result in savings of 10 to 20 percent and even up to 25 to 35 percent. This is complete nonsense of course. Industrial meters on the other hand are volt/amp meters where a bad power factor does indeed cost the customer and where power factor correction has been done for decades and is the norm.

A bad power factor is when the current is out of phase from the voltage, it either leads or lags the voltage. An inductive load [coil] causes the current wave to lag or fall behind the voltage wave, so that the peak current flow is some time after the voltage wave is at it's peak level and a capacitive load [capacitor] causes the current wave to lead or advance ahead of the voltage wave, so that the peak current flow is some time in advance of the peak of the voltage wave.

This graph illustrates an instance where the PF is 1, a resistive load.

This graph illustrates an instance where the PF is less than 1, a lagging PF caused by an inductive load.

A meter measures volt and amps instantaneously and in the graphs above, that would mean values on a vertical line either above or below the horizontal zero line. When the PF is 1, the values for both volts and amps are considered to be either positive or negative. When the PF is less than 1, there are certain places when one value is considered to be positive while the other value is considered to be negative. When you multiple two positive numbers, you get a positive number and when you multiply two negative numbers, you get a positive number also but when you multiply a positive number by a negative number , you get a negative number. (This is stuff we took in high school and I remember the end result but not the explanation for it...too many years between then and now, eh?) This "negative" wattage is funny in that it doesn't do any work for you but it does require work to produce it. The generators at the electric plants do indeed see this as a load and it does cost them. Residential meters could of course be either watt meters or volt/amp meters but I would bet that a long time ago they found that the PF for homes was a relatively steady number and (I'm making an educated guess here) using watt meters was probably a political decision so the utilities just adjusted their cost per kilowatt to take into account the PF for residences. Industrial users on the other hand can have wildly fluctuating power factors even in house depending on what the electrical load is and definitely between different industries so charging them for a bad PF makes sense.

That's my story and I'm sticking to it! :laughing:

 

[SweBota]

First of all, voltage is written U measured in Volts (V). Don't mix units and denominators.

Power is P, measured in Watts (W)
Current is I (Ampere)
Resistance R (Ohms)
Energy is E, measured in Joules. 1 Joule is also a Watt-second, Ws. 3600 Ws is a kWh, kilowatthour.

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?

Correct so far.

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?

Not so.

Like others have pointed out, welders and cutters have transformers and regulators so that the output current and voltage is not directly related to what power they run from.

You need to think like this, the output voltage should be kept constant. To increase the current (with constant voltage), the machine needs more power. When the power requirement exceeds what can be supplied by a normal outlet (in your case 110V), it makes sense to use a higher voltage circuit for supply.


As for the discussion about 1-phase or 3-phase, I can only say that it's wonderful to have access to 3-phase in all houses, even in larger/newer apartments. All our larger (professional) power tools like log splitters, pressure washers, drill presses, lathes, mlls etc are 3-phase 400V.

Common residential supply is 3-phase 400V 16A, 20A or 25A. A 25A feed will provide 17kW.

In my small workshop I have 400V 35A, upgradeable to 64A. I can run all the same machines as they do in industrial applications since we have the same power everywhere.


BR,

/Marcus

 

[Mace Canute]

Thanks for all the responses, I've learned quite a bit in this thread!

But all this talk about the benefits of a 3-phase setup begs the question: Why use single-phase at all? Why is it prevalent in American households - is it simply a case of "That's what America started with way back when, and we just never got around to changing the circuitry in tens of millions of homes over to 3-phase"?

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.

 

[SweBota]

I am not CERTAIN about this, but I think 3 phase is common in Europe.

Correct.

Somewhere I read that code requires outlets on different phases must be more than 2 meters apart, so you don't string yourself across 415 (I think its 415, don't remember).
Not that it saves you from bringing two extension cords together from opposite sides of the room to light yourself up with (-:

Not at all, that would make 3-phase outlets kind of difficult...! There are no limitations on where you can put outlets on different phases, but normally you would wire all outlets in a room on one phase.

/Marcus