A lesson in voltage and amperage, please

[grsthegreat]

Spyderlk is correct and grs great is wrong on this. A kilowatt is a kilowatt regardless of the voltage used to get there. Yes, the amps are different. 3-phase allows one to go even smaller with supply wiring than 230 volt does.

Industries use 3 phase for the same reason that a homeowner uses 230 volts over 115 volts. Industries have big machines that require huge amp draws and there is a practical limit to the size wire that you can feed the machine with.

whatever, ive been told this many different times. even our utilities here give a tax refund to companies that change equipment from 120 to 240 or 3 phase. it lightens their loads and reduces consumer costs.

why would they say this if not true? theres no benifit to them. Now mine you, i may be full of cra# about this. i have never run a test to actually see if this is correct. maybe i should do that. I have a tendancy to believe those that are SUPPOSED to know more than i do.

 

[rankrank1]

whatever, ive been told this many different times. even our utilities here give a tax refund to companies that change equipment from 120 to 240 or 3 phase. it lightens their loads and reduces consumer costs.

why would they say this if not true? theres no benifit to them

I guess it could indirectly help the power company's grid system quality in some obscure way (e.g. prevent severe voltage fluctuations) and they might even offer big industry incentives to switch power to lower the amp demands that they would be required to supply - especially in areas where their feeder lines might be borderline taxed as feeders. Regardless, it has no effect on the consumer price paid for electricity since the consumer buys in Watts consumed (or Kilowatts consumed) which is an equal function of voltage and amps consumed. (e.g. Say you and I are on a toll road. You drive 230 mph to the next toll both in 1 hour and your toll fee equal $2.00. On the other hand, I drive 115 mph to the exact same toll booth but it takes me 2 hours to get there. Guess what, I pay the exact same $2.00 that you did even though I spent twice the time on the toll road as you. Why: The consumer was charged on distance traveled (similar to Watts consumed) not charged on time spent on the road (similar to amps consumed)

If your argument held water then I should switch all my 115 volt household power needs to the side of my breaker panel that never registers on the utility's company meter. Each side of a 115 volt feed that when combined produces 230 volts registers equally through the meter so this it will not work.

I think you might be getting confused with effieciency of an appliance. For example lets take a window air conditioner that is 15,000 btu. I have seen 15,000 btu air conditioners that require more watts to to produce the 15,000 btu's on 115 volts as compared to running a similar but more efficient 15,000 btu air conditioner on 230 volt. The 230 volt btu air conditioner is only cheaper from a killowatt perspective because it is a more efficient unit in design.

 

[EE_Bota]

why do yo think large companies use 3 phase..they spread the power over 3 poles. their cost is less.

The power is created with 3 phase machines, so sort of makes sense that a 3 ph machine is the best way to consume the power.

I once knew a professor who said he would die a happy man if all of his students remembered to the day they died:

"When three currents 120 degrees apart in time flow through three windings 120 degrees apart in space, a rotating magnetic field is created which *can* be used for useful work."

I put three in bold, because he didn't say "one", but "three."

Single phase is less desirable for motor loads because two rotating fields are created, and they are equal in magnitude, but opposite in direction, a real drawback requiring clever design to overcome the deficiency.

 

[Qapla]

The way that 3 phase was explained to me was from this aspect.

Since a single phase motor uses a "common" to complete the circuit, then, at 60 Hz (or as they used to say, 60 cycles per second), then the power is only "pushing" the armature half of the time it is running, the other half of the time it is coasting.

3 phase power does not use the common to complete the circuit to the motor (the common is only used if the unit needs a 110 or 220 contactor), then the motor always has at least two hot legs connected at all times. That means it is always "pushing" and never coasts. Therefore, it is more efficient and cheaper to operate.

At least that is how it was explained to me - by an electrician and an electrical engineer)

 

[crazyal]

Wow, kind of got a headache reading all these posts. :laughing: I don't even remember the original question. I think it had something to do with welding.

 

[mjncad]

Not being a sparky; I use the following analogy to keep volts and amps straight. Volts is analogous to pressure in a piping system (e.g. 120PSI, 240PSI, etc), and AMPS is analogous to to flow (e.g. GPM).

Not the best; but it works for me.

 

[Mace Canute]

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?

that means the motor is USING 12.8 amps at 115 and USING 6.4 amps at 230. So the 220 motor is CHEAPER to operate. remember, you pay for current (amps) not volts. This is why large commercial shops run at 480 volts. a similar motor like above at 480 volts would only need 3.2 amps of current to operate. also it would require smaller feed wires and the motor will heat up less and last alot longer.

You pay for Energy. Power [V x A] for a given time .... Kilo-Watt Hours. A 110 motor drawing 12.8A is using the same amount of energy as a 220 drawing 6.4. The electric power [Watts] each uses would be the same -- 220x6.4=110x12.8.
larry

nope nope nope

remember, a 120 volt motor is only pulling on one leg , and this moves meter concurrent with the amperage used. Now if the same machine is pulling 220 and using 1/2 the amperage, only 1/2 the current draw is registered on the meter.

Ive discussed this with the utility company and with my load calculations instructor and they agree. why do yo think large companies use 3 phase..they spread the power over 3 poles. their cost is less.

believe me, Ive talked this over with many different people, as i didn't believe it at first. This is the reason we try to balance a load in a panel. don't want alot of the continuous loads on only one leg. those dang meters move when they sense current thru ANY leg. If current is going thru both legs, it only registers as one leg. Very odd, but it has been confirmed.

Sorry grsthegreat, that's not the way meters work. They are totalizing meters for one thing, meaning they add up the power (watts) used per hot leg. It doesn't matter if you use 10 amps on one 120V hot leg and none in the other or split it up, 5 amps on each hot leg, the end result is the same. Electric meters are also either volt/amp meters or watt meters. Electric utilities charge for either volt/amps or wattage used, not current. Two people could use exactly the same current, but the one with higher secondary voltage uses more power and power is measured in watts.

I have a bit of experience with power and electric meters...35 years worth in fact working as a Lineman and District Operator for the provincial electrical utility.

 

[Mace Canute]

The way that 3 phase was explained to me was from this aspect.

Since a single phase motor uses a "common" to complete the circuit, then, at 60 Hz (or as they used to say, 60 cycles per second), then the power is only "pushing" the armature half of the time it is running, the other half of the time it is coasting.

3 phase power does not use the common to complete the circuit to the motor (the common is only used if the unit needs a 110 or 220 contactor), then the motor always has at least two hot legs connected at all times. That means it is always "pushing" and never coasts. Therefore, it is more efficient and cheaper to operate.

At least that is how it was explained to me - by an electrician and an electrical engineer)

That's not quite correct. In a single phase motor, as the voltage climbs from zero to peak, the torque increases in magnitude, and as the voltage drops from peak value to zero, the torque decreases from a maximum value to zero, but it doesn't stop as soon as the voltage starts to drop, it lessens in value and in proportion to the lessening of the voltage. There is absolutely no coasting for half the time but as the voltage approaches and departs zero, the torque produced is at a very low value.

A three phase motor always has two out of three phases producing torque when the third phase is at zero, hence it has smoother torque output. There is quite a debate about "cogging" of single phase motors and the elimination of it by using a three phase motor instead. The accepted explanation of why three phase motors reduce it greatly is because of this two phases always producing torque when the third is producing none, or next to none.

The reasons for three phase motors being used in industry probably has more to do with them being more compact and less costly than a single phase motor of the same voltage class and rating (no starting windings, etc, eh?) and their inherent higher starting torque, plus VFDs work well with them. They do tend to have higher efficiencies though, an added bonus!

A three phase motor can have a "common"...it's called a star point and is written WYE. It can be a "floating star point" or it can be a "fixed star point" meaning it is connected back to the neutral conductor. 3 phase motors can also be wired with no star point and are said to be DELTA wired. The left one is an example of DELTA winding and the right one is an example of WYE winding. N1, N2 and N3 being of course where the three individual hot legs are connected.

 

[mmurphy]

The reasons for three phase motors being used in industry probably has more to do with them being more compact and less costly than a single phase motor of the same voltage class and rating (no starting windings, etc, eh?) and their inherent higher starting torque, plus VFDs work well with them. They do tend to have higher efficiencies though, an added bonus!


:thumbsup::thumbsup:

Mace, since I believe I seen you were a lineman. On a single phase residential, how does power companies determine Power Factor losses? Is it just a figured in result at best guess?

 

[Mace Canute]

Mace, since I believe I seen you were a lineman. On a single phase residential, how does power companies determine Power Factor losses? Is it just a figured in result at best guess?

To tell the truth, I don't know.
I suspect that one way it could have been done over the years when mechanical meters were the norm was to do random sampling of different residences using a dual meter mount. They look like this:

It plugs into the regular meter socket, then the original meter is plugged into one side and the other meter is plugged into the other side. These mounts used to be used quite a bit to check the accuracy of an installed meter against a new one since they were wired in series. It would be dead simple to use a volt/amp meter in conjunction with a watt meter and record the difference in readings to get the power factor.
They've had mechanical direct reading power factor meters for a long, long time and I would expect in the old days that they would have had a paper graph incorporated into them to record the PF so they could calculate the average PF value over a desired time frame. Of course, with the advent of electronic meters, it's common to have them built to display the power factor along with quite a bit of other information besides just the consumption. FWIW, I have read that the the power factor for the typical home is around 0.97 which is really quite good.