The Phase Power Question

chawalang

Well-Known Member
Trying to make sure that I am explaining things correctly to people.

The advantage to using a three phase power system is that electricity is spread across three legs as oppose to one so you don't have an issue with increased resistance going through one leg. A more efficient way to transfer electricity.

This is the main reason to my knowledge way AC had a major advantage over DC and why Edison wanted to trash Teslas reputation.

So for example what is the correct way mathematically to figure out how may amps are on a leg.

For example 18, 750w lekos on a dimmer rack that is connected to a 60 amp there phase cam lock disconnect, how many amps would be on each leg?
 
For example 18, 750w lekos on a dimmer rack that is connected to a 60 amp there phase cam lock disconnect, how many amps would be on each leg?
The dimmer rack does not distribute the load over the three phase power, it uses each leg to power some of the dimmers in the rack. To compute the load on a leg, simply add the loads attached to dimmers powered by that leg. Which dimmer is powered by which leg? I imagine that different racks are organized different ways.
 
Trying to make sure that I am explaining things correctly to people.

The advantage to using a three phase power system is that electricity is spread across three legs as oppose to one so you don't have an issue with increased resistance going through one leg. A more efficient way to transfer electricity.

This is the main reason to my knowledge way AC had a major advantage over DC and why Edison wanted to trash Teslas reputation.

So for example what is the correct way mathematically to figure out how may amps are on a leg.

For example 18, 750w lekos on a dimmer rack that is connected to a 60 amp there phase cam lock disconnect, how many amps would be on each leg?

ASSUMING the voltage is US standard 120/208v.

And ASSUMING you are distributing those 18 fixtures on dimmers wired for 3 phase use, so 6 fixtures per phase. Doesn't matter if it's 1 - 6kw dimmer on a phase, or 6 - 1kw dimmers on a phase, the LOAD remains the same.

THEN - 37.5 amps.

18x750 = 13,500 watts, divided by 120 volts = 112.5 amps, distributed across 3 phases = 37.5 amps
 
The advantage to using a three phase power system is that electricity is spread across three legs as oppose to one so you don't have an issue with increased resistance going through one leg. A more efficient way to transfer electricity.
A more significant advantage of multi-phase power is that conversion to and from mechanical energy is more efficent (motors and generators). For power company distribution, the lack of a current carrying neutral reduces the cost of infrastructure (ever notice that high-voltage lines are almost always in sets of 3). For theatrical lighting, use of three phase power is more about what is commonly available (so that's what most distribution hardware is built for) than any inherent advantage of three phase power.
 
"The advantage to using a three phasepowersystem is that electricity is spread across three legs as oppose to one so you don't have an issue with increased resistance going through one leg. A more efficient way to transfer electricity.

This is the main reason to my knowledge way AC had a major advantage over DC and why Edison wanted to trash Teslas reputation."


The advantage of AC is that it is - compared to DC - easy to change the voltage - and more efficient to transmit higher voltage, especially long distances. And Edison was a backer of DC and his adversary promoting AS was George Westinghouse, who had indeed bought some of Tesla's patents. DC's advantage was that there was no AC efficient electric motor initially.

As others have said, just calculate and add up the load per leg, and generally be concerned about the largest one.
 
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This is the main reason to my knowledge way AC had a major advantage over DC and why Edison wanted to trash Teslas reputation.
The advantage of AC over DC for power company distribution is the use of transformers to convert between line level at service locations and high voltage for long distance transmittion. AC is much simpler to convert between voltages than DC is.
 
The advantage of AC is that it is - compared to DC - easy to change the voltage - and more efficient to transmit higher voltage, especially long distances. .

And why is this so you might ask.
Simple.

When you want to send electricity over a wire, the wire has some resistance. You dissipate some energy in the wire ( generating heat ) . The longer the wire the more resistance. The fatter the wire the less resistance.

How much power is lost? the formula P = I * I * R ( Power = Amperage squared times Resistance in the wire). Note that the power lost in transmission does not depend on the voltage, but on the amperage and the resistance.

Remember that 100 volts at 10 amps is 1000 watts. but 1000 volts at 1 amp is also 1000 watts. ( West Virginia formula. or W = V*A). So if I transmit my power at high voltage there is less loss.
 
Note that with the invention of reasonable cost AC/DC power invertors capable of high power ratings, many power generation systems will now transmit long distances in DC. It's very common in Europe with transmission lines being placed under water, due to reduced power losses as compared to A.C.

As well, no need to sychronize different systems cycles-per-second speeds.

Our local power company brought in 600 megawatts from New Jersey a few years back, all DC under NY Harbor and in the ocean. Less having to deal with gaining right-of-way for above ground transmission lines as well.
 
Trying to make sure that I am explaining things correctly to people.

The advantage to using a three phase power system is that electricity is spread across three legs as oppose to one so you don't have an issue with increased resistance going through one leg. A more efficient way to transfer electricity.

Not really. Here is my understanding. Corrections are encouraged. ( wikipedia has an article here http://en.wikipedia.org/wiki/Three-phase )

Remember that in AC current the power ( in the US ) is fluctuation positive to negative 60 times a second. In your house a simple Edison outlet has a hot leg ( which is alternating 60 times a second) and a neutral leg ( which is just the return power ). If you put an oscilloscope on the hot leg you could see it change from about 150 volts positive to 150 volts negative 60 times a second. Think of this a single phase power. One hot, One neutral.

Now consider what would happen if you had three legs, where the current in each wire is offset in time a bit from each other.

threephase.png


If you think of the graph above showing the electron flow in the wire, you can see that the sum of positive and negative flow is 0 at all times. This means that you don't need a neutral wire to let the electrons go back to the generator. So instead of six wires ( Three hot and Three neutral) you can use three ( assuming all loads are the same ).

Now in our world, you don't know that they are the same - so you do need a full neutral wire - so you can use four wires instead of six. This is a big win in terms of cost. It also works out that it is comparatively easy and efficient to generate power ( and to transform between various voltages) using a three phase system.

So its not as much a question of decreased resistance, as one of decreased wire. ( and efficiency in transformation of voltage)
 
Bottom line, there is a mild amount of advantage to using 3 phase power for lighting. Usually, you end up running 6 conductors (HHHNNG). The reason there is not a greater advantage is that dimmers are actually single-phase devices, so they can't take advantage of many of the benefits. In addition, because the phase usage does not always overlap, we end up having to double the neutral line up. The advantage is greater when using 208 volt movers as you do away with the neutral.

Outside of the lighting industry, the advantage of three phase is monstrous! Motors are much easier to run due to the natural phase rotation, so efficiency is increased. (None of that sloppy split-phase motor junk.) Three phase generators are also more efficient at producing power as a greater percentage of the rotation is engaged in actual power generation. The list is very long.
 

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