3 phase question

[Anonymous]

In a shool that we are working in we were changing out some outlets in the shop the panel is a 120/208v panel. i was wondering how the phase to phase voltage can be 208v when each phase to ground reading is 120v and can 208 volts run a 240v motor for like a table saw or any wood shop equipment. We pluged one in the other day and it seemed to run fine.
thanks
BW

[Wgoodrich]

Three phase sine wave is not a complete out of phase design like a single phase sine wave would show. In a single phase you would have one peaking minus while the other is peaking plus on the sine wave because there is only two hot lines. Divide by two evenly

In three phase no two hot lines will be peaking plus and minus opposite or each other because there is three hot lines. divide by 3 unevenly. You would be considere 1.732 voltage output or the square root of three hot lines instead of 2 hot lines.

Single phase is approximate 180 degrees out of a 360 degree circle.

Three phase is only approximate 120 degrees out of a 360 degree circle.

Many motors are dual voltage rated allowing 208 or 240 volt to run those motors. Amperage would increase on the 208 volt about 10% more than the 240 volt supply.

Try visualizing a circle. Put two hot lines opposite of each other on an equal basis. Take the same circle beside it and put three hot lines opposite of each other on an equal basis. You may be able to see what is being said then.


You should be fine.

Wg

[imported_Dznewz]

I was taught that the two hot leads of a single phase 120/240 supply are 180 degrees out of phase and that the three hot leads of a three phase supply are 120 degrees out of phase.

[Wgoodrich]

You are right but I was not talking in the terms you were talking. I will correct my last post to proper numbers. My brain went faster than my fingers did. Sorry.

I was trying to get you to see on a view of a complete circle being 360 degree. It is often necessary to give up accuracy to draw an understandable picture in a person's mind then get them to the correct terms.

Half of a circle is 180 degree of a 360 degree circle if the circle were divided by the two hot wires.

A third of the circle is 120 degrees of a 360 degree circle if the circle were divided by three hot wires or the square root of three this is where teh 1.732 comes into the equation when calculating load and voltage drop.

Just trying to create a visual that can be understood by a laymen beginner that has no idea what a sine wave looks like.

You are learning at the start of a very big subject.

Good Luck and congrats on what you have learned so far.

Wg