How to calculate 3 phase voltage

[goofeedad]

I am lost when it comes to figuring out 3 phase ANYTHING ( just never really understood the concept or the need ).

Anyway, if some wall unit ( it's a steam humidifier ) states that it uses a 3 phase, 60 HZ, 208VAC, 21.2Amp supply. Does that mean that each phase delivers 69.33 VAC (I know that's not right but I don't understand)?

In a previous post "Diver300" was helpful he stated that to find the KWh I could take the 3 * Amp * Vac = KWh. That came pretty close, but I guess my question is why the 3phases and why the wierd 208VAC?

Thanks.

 

[tcmtech]

For a Wye connection this is basically what you have.
Think of a 120/208 three phase system as the capital letter 'Y'. From the center to any of the three end points you have 120 units of measure. From any point on the end to another end point you will have 208 units of measure.

Another way to think of it as three triangles with two legs 120 units long and a base 208 units long. The one corner with the two 120 sides is the center of the 'Y'. the other two corners with a 120 and a 208 represent two of the ends of the letter 'Y'.
If you place the three triangles together with all the 120 sides against each other you end up with three equilateral points 120 degrees apart in reference to a single rotation or circle.
The more common high power connection in North America is the 277/480. Its the same ratios just at a higher voltage.
The more common European standard is the 240/416 I think.

Simple geometry using nothing more than triangles and a circle.

For a delta connection (Δ) you just have one equilateral triangle with no center point. They are most often 240 volt or 480 volt. In some applications they add the center tap to one side of the triangle so it has one leg split into a 120/240 or a 240/480 connection. Then you get the 120/208/240 or the 240/416/480 three phase connection system.
The 208 or 416 volts come from that one center tapped side going strait across to the opposite corner.

On a three phase name plate the amps rating is the amps per each leg. So a 240 volt, 20 amp three phase circuit carries the same power as three single phase circuits of equal voltage or 240 volts at 60 amps single phase while only adding one more wire.

 

[crutschow]

I am lost when it comes to figuring out 3 phase ANYTHING ( just never really understood the concept or the need ).

Here are three basic advantages I can think of for three phase power.

1) It requires the least amount of copper to transmit a given amount of AC power. Only a DC circuit is better which is why some long distance power lines are DC.

2) It naturally generates a rotating field in a motor, thus a motor start circuit is not required, as with a single-phase motor. A 3θ motor also operates more quietly and efficiently than a 1θ motor since the power is continuous, not fluctuating. (Even though the power of each phase fluctuates, the power sum from all three phases is continuous with no variation).

3) If you rectify a 3θ signal to get DC power, the ripple is much lower than from a 1θ signal, thus requiring much less capacitance to minimize the ripple, and lower peak currents in the transformer.

 

[goofeedad]

OK, never had it explained so well.

But unless your powering an electric motor why have it? Or more to my point why would a steam generator need to have such an elaborate electrical hookup? Why not just run it off of 220VAC for example?

I realize no one on the forum knows exactly because no one can see it (I can't either anymore) but why a 3 phase supply? Seems like over kill to me.

 

[tcmtech]

At some point going to a three phase supply makes more sense simply because the load requirements and the related larger wiring makes it a more cost effective way to get that power to the device.

Plus its typical application may be that its manufacturers target market is commercial and industrial settings where three phase power is standard.

I have a number of older commercial three phase machines and devices I run on single phase but they needed wiring modifications in order to handle the higher amperages that are then drawn from them being on a single phase source.

As I mentioned earlier changing a 20 amp rated three phase sourced load over to a single phase source means it now draws 60 amps. The 12 gauge wire and contactors now have to be upgraded to much larger stuff to safely handle the increases in the amperage.

Three runs of 100 feet of 12 gauge for a three phase system cost considerably less than the two 100 foot runs of 4 gauge needed for the same load on a single phase system.

 

[goofeedad]

At some point going to a three phase supply makes more sense simply because the load requirements and the related larger wiring makes it a more cost effective way to get that power to the device.

Plus its typical application may be that its manufacturers target market is commercial and industrial settings where three phase power is standard.

I have a number of older commercial three phase machines and devices I run on single phase but they needed wiring modifications in order to handle the higher amperages that are then drawn from them being on a single phase source.

As I mentioned earlier changing a 20 amp rated three phase sourced load over to a single phase source means it now draws 60 amps. The 12 gauge wire and contactors now have to be upgraded to much larger stuff to safely handle the increases in the amperage.

Three runs of 100 feet of 12 gauge for a three phase system cost considerably less than the two 100 foot runs of 4 gauge needed for the same load on a single phase system.

Again, that makes alot of sense. Thanks for explaining it so well.

 

[crutschow]

But unless your powering an electric motor why have it? Or more to my point why would a steam generator need to have such an elaborate electrical hookup? Why not just run it off of 220VAC for example?

Most industrial motors over a few HP (including all the large air conditioners in commercial buildings) are 3-phase because of their better efficiency. So you need a 3-phase generator to supply power to all those motors.

A large generator is 3-phase for the same reason a motor is. It's smoother, more efficient, and makes better use of the copper wire. Adding a third wire is a small price to pay for all the other advantages of 3-phase power.