foolish question about 3 phase

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Ed Carr

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OK when I see a diagram of three phase waveform it occurs
to me how come they don't start at the same point?
Is the starting point for each phase actually a matter of timing?
If it is time related how is that accomplished?
Also how would this affect a single phase motor that is rated for
208 volts?

Thanks, Ed
 
Here's a site that might explain it.

(And dog-gone it, every time someone starts a thread with "stupid question" or some derivative, I actually would like to see a genuine stupid question. To date, I have yet to see one.)
 
Because the Start of 1st phase gets to go first as "zero" in a 360? rotation.

The 2nd phase starts (at it's zero) after the first rotation is cleared at 120? later as it rotates about an axile.

The third phase will be start it's postive work after 240?.


So if one was to drop the negative values in sine wave, and draw a line
about 3/4 the way up on a sine wave, that where the work of 3 phase happens.
With the 60 Hz pulsing these waves causes a "AC" wave to turn into a "DC" type electrical sine wave or more flat line (like DC) where work can be done in motors, All with the Help of RMS, Root Mean Square, thats another Topic...
 
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Would there be a difference then in the construction of an
electric motor rated single phase 240 volt vs the same motor
rated 208/240 volt?
Also 480 thank you.

Ed
 
Ed Carr said:
OK when I see a diagram of three phase waveform it occurs
to me how come they don't start at the same point?
Is the starting point for each phase actually a matter of timing?
If it is time related how is that accomplished?
Also how would this affect a single phase motor that is rated for
208 volts?

Thanks, Ed
Click to expand...

1. If all the waves started at one point, you would have single phase.
2. Yes. The waves are separated by approximately 5.6 milliseconds.
3. In effect, you have 3 generators coupled to the same shaft with the rotors skewed 120 degrees relative to each other.
4. You would use only one phase of a 208V sysem to drive a single phase motor.
 
480 I was just getting ready to post that same question.
Wouldn't that be 2 phase.
And I have in my possesion as we type a 208 volt single
phase 5hp motor!

Ed
 
480sparky said:
1.Wouldn't that be two phases? One phase would only be 120v.

2. Then again, I have yet to see a 208v single phase motor.
Click to expand...

1. It would be one phase--208V line to line. Neutral does not connect to the motor. Two phases would require three wires.

2. The OP specified a 208V, single phase motor. It doesn't have to exist.
 
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480sparky said:
Wouldn't that be two phases? One phase would only be 120v.

Then again, I have yet to see a 208v single phase motor.
Click to expand...

1. One phase, 208V line to line. Neutral does not connect to the motor.

2. The OP specified the motor; it does not have to exist.

Also, the instantaneous power to the motor is constant with three phase. Should make them run smoother and quieter.
 
480sparky said:
Line to line would be 2 phases. 3 wires would mean three phase.
Click to expand...

A delta config uses 3 wires, but in a wye, Va-N and Vb-N would be 3 wires and two phases.

Drop one wire from any 3 phase system, and you are left with one line to line waveform--one phase.

Two wires, line to line or line to neutral, can only carry one waveform--one phase.
 
Ed Carr said:
Then rattus would not one of those line to line be
separated by the 5.6 milliseconds vs a single phase 240 volt?

Ed
Click to expand...

The 5.6 ms applies to 3-phase, line to line, or line to neutral.

But, there is nothing to separate in a single phase system. There is only one generator, and let's not get into phase inversions through transformers.
 
And, let's clarify our terminology. Let's not use phase and wire to mean the same thing. To eliminate confusion, let us use "phase" to mean the waveform which exists between two wires. That is, the sine waves we see in the diagrams.
 
rattus said:
And, let's clarify our terminology. Let's not use phase and wire to mean the same thing. To eliminate confusion, let us use "phase" to mean the waveform which exists between two wires. That is, the sine waves we see in the diagrams.
Click to expand...

That's where we're not jiving. I was talking about using two 'phases' as in hooking to a 2-pole breaker in a 3-ph panel.
 
rattus..then if I am connecting a single phase motor to
a 3 phase system by just using two hot legs one of the
legs is leading the other? What effect would this have on
a capacitor start motor?

Ed
 
480sparky said:
That's where we're not jiving. I was talking about using two 'phases' as in hooking to a 2-pole breaker in a 3-ph panel.
Click to expand...

Yes, I suddenly realized you were using the trade jargon which is not precise enough for this discussion.

If we have a neutral, then Va would mean the voltage between line A and neutral.

But if there is no neutral, Va is meaningless because there is no reference. It could mean Vab, or it could mean Vac.

What I am saying is that for discussions such as this, a 'phase' is observed between two wires.

Now, in the field, where the real work is done, it will be business as usual.
 
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