Never mention 120? on this Forum

chris kennedy

Senior Member
Location
Miami Fla.
Occupation
60 yr old tool twisting electrician
I'm going to break this rule because I can't wrap my head around something I have taken for granted for decades.

Given a wye system where the ungrounded conductors are "supposedly" 120? apart, how does exchanging 2 conductors change rotation? Ungrounded conductors are still 120? apart???

Explination for dummys would be appreciated.

Thanks
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
If I can remember back two decades it has something to do with reversing the magnetic fields within the motor. Let's see what the others have to say. :roll:
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
I'm going to break this rule because I can't wrap my head around something I have taken for granted for decades.

Given a wye system where the ungrounded conductors are "supposedly" 120? apart, how does exchanging 2 conductors change rotation? Ungrounded conductors are still 120? apart???

Explination for dummys would be appreciated.

Thanks
I want to try. I assume you know how motors are wound for three phase. Draw a triangle. Label the angles "A","B", and "C". Now draw an arrow around the triangle following the direction ABC. Now swap any two letters, and redraw the arrow following the ABC format. you will see that the arrow will be drawn in a different direction. Same thing with a motor. Another help. When you draw these out, look at smart$ avatar while you draw it for forward rotation. infinity is right. What you are doing when you swap the wires is change magnetic field.
 
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kwired

Electron manager
Location
NE Nebraska
I like what HV&LV had to say and want to add that each side of the triangle he mentions represents a winding in the motor - if a delta wound motor it represents it even better yet. Each winding has peak voltage and current at 120? difference than the adjacent ones, resulting in intensity of magnetic fields to rotate around the stator which in turn will impose a rotating force within the rotor and push it in same direction the field is rotating. For demonstration purposes name the coils left, middle and right. Connect power and lets say we have peak voltage on left first, then middle, followed by right. Now swap two leads and peak voltage will be on right before it is on middle then will be followed by left. That was just one cycle - now imagine it doing that 60 times a second.

This is just the "simple" explanation that just about anyone could understand - may have some holes in the theory but don't worry the engineers can clear that up in say 300 + posts.
 

mivey

Senior Member
How about using three clock positions, like one o'clock, five o'clock, nine o'clock:

clockwise (foward time):
1-5-9
5-9-1
9-1-5

reverse any two to go counter-clockwise (reverse time):
1-9-5
5-1-9
9-5-1


You get the same thing for a 3-road roundabout: one sequence is clockwise and the other is counter-clockwise.
 

Rick Christopherson

Senior Member
.... but don't worry the engineers can clear that up in say 300 + posts.
Ah ha, but you guys are way over-thinking it. :p

It's not about funky angles or even math. It is simply flipping a circle from one direction to the other.

Draw 3 points on a wheel, A-B-C and then rotate that wheel (clockwise for example). No matter where you start, it will always be the same sequence: A-B-C, or B-C-A, or C-A-B. They are still in the same repeating sequence.

Then turn that wheel counterclockwise. The points are no longer in the same sequence. They become A-C-B, C-B-A, and B-A-C.

When you reverse any two conductors, you flip the direction of the sequence....regardless where your sequence started. Compare the two sequences below, and note that each column has the same starting letter, and you will see the differences.

A.B.C__B.C.A__C.A.B.........Clockwise
A.C.B.__B.A.C__C.B.A........Counterclockwise


Edit: Oh, who would have guessed Mivey was posting at the same time as me. Oops.
 
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kwired

Electron manager
Location
NE Nebraska
Ah ha, but you guys are way over-thinking it. :p

It's not about funky angles or even math. It is simply flipping a circle from one direction to the other.

Draw 3 points on a wheel, A-B-C and then rotate that wheel (clockwise for example). No matter where you start, it will always be the same sequence: A-B-C, or B-C-A, or C-A-B. They are still in the same repeating sequence.

Then turn that wheel counterclockwise. The points are no longer in the same sequence. They become A-C-B, C-B-A, and B-A-C.

When you reverse any two conductors, you flip the direction of the sequence....regardless where your sequence started. Compare the two sequences below, and note that each column has the same starting letter, and you will see the differences.

A.B.C__B.C.A__C.A.B.........Clockwise
A.C.B.__B.A.C__C.B.A........Counterclockwise


Edit: Oh, who would have guessed Mivey was posting at the same time as me. Oops.
You have to understand that something is rotating and (at least partially) why before any of that makes sense in the application though.
 
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