The beginnings and ends of phase windings 2

[zlatkodo]

Distance between the beginnings of three-phase windings is , generally, two thirds of the full pitch.
But this may not always be so, it depends on which shortened pitch is used.
For example: where are the beginnings for three phase winding, 33 slots, 8 poles, double-layer:

- the beginnings of the first, third and fifth pole-phase group or
- beginnings as shown in the attachment?
Which option is correct? Which is better?
Is somewhere I can find a detailed analysis of this topic?
Zlatkodo

 

[rhatcher]

zlatkodo,

For the sake of discussion, assume that the stator has 54 slots. Since I do not understand your point, please perform the slot number analysis and let us know what the results are.

You are right that the 1-7 jumper diagrams have beginnings in the same place. Only the ends are different. Since you are focused on the beginnings, I made another connection for you to consider. It is symmetrically asymmetrical.

Pete: I don't know what LPS is but if it is a good thing then thank you. If it is a bad thing then I do not want to know.

 

[davidbeach]

LPS = Little Purple Star. Not a bad thing all-in-all.

 

[zlatkodo]

Hi, Rhatcher,
First about your earlier example.
For 54 slots and 6 poles , el. angle between two slots is 20 el. degrees.
The full pitch is 9.Distance between beginnings is 6 slots or angle 6*20=120 degrees ( this is 1*120).This is exactly two-thirds of full pitch.
Please see Koizumi post from 10.Sep. and this picture1:

http://img219.imageshack.us/img219/3554/pic154slots6pol.png

Now about your last example.
There is a similar case. The distance between the phase-beginnings is 12, 18 and 24 slots and that is:
- 12 * 20 = 240 = 2 * 120 el. degrees
- 18 * 20 = 360 = 3 * 120,
- 24 * 20 = 480 = 4 * 120.
It is a multiple of two-thirds of the full pitch. See:

http://img140.imageshack.us/img140/3129/pic254slots6pol.png

I mean, there's no reason for such connections in the application with integral-slot windings, when we have a better solution.
BTW, what is the benefit of this arrangement?
Personally, I avoid such a winding arrangement with k = 3, (360/120).
Something more: for me is a bit strange this practice of drawing a diagram specifically for "star" and "delta". In this way, each scheme-collection is unnecessary twice larger and the scheme for themselves seems more confusing.
Zlatkodo

 

[electricpete]

Please see Koizumi post from 10.Sep.

In your given winding diagrams (dated Sep 10), the upper diagram might not applicable. There are two reasons:
1) The electrical angle between the U1-V1 and V1-W1 have to be n*120 degrees (n is a natural number).
The slot-to-slot electrical angle alfa=p*360/Z=4*360/33=43.64 deg.
In the upper diagram, from U1 to V1 you have 3 slots, so the electrical angle here is 43.64*3=130.92 deg., very much difference to 120 deg. The same with V1 to W1.
....*Conclude: the lower diagram is correct.

I'm not trying to pick on koizumi (he has since indicated some uncertainty about this statement), but in the interest of clear communication, I'd like to paraphrase what I think he's saying and then explain why it's not correct

Here's what I think he's saying:
1 - The power system has 120 degrees difference between phase leads.
2 - The point where we connect the leads needs to be 120 degrees apart to avoid mismatch in the phase of the voltages applied by the system and phase voltages induced by the motor.
3 – The phase difference between phase leads can be computed simply by looking at their separation in slots times slot angle.
That's the basic logic, right?

It sounds right on the surface, but it's not (the problem is #3). Here's why it is incorrect: Your are just using a number to represent phase of a voltage, but voltages and their associated phases only have significance when we define the associated loop. So we need to draw a mental picture of the machine and the power supply and do a more methodical comparison.

Let's say the machine is connected in wye: U1/V2/W1 are phase leads and U2/V2/W2 are the neutral point.

Let's say the power supply is balanced and also connected in wye, or at least hast theoretical neutral voltage which we can visualize. For the power supply, the voltage from U to neutral is 120 degrees apart from the voltage from V to neutral, which is 120 degrees different than the voltage from W to neutral.

Does the motor represented by the top diagram of zlatkodo posted 10 Sep 10 12:45 satisfy the same 120 degree phase relationship among the line to neutral voltages? In particular, is the voltage from U1 to neutral 20 degrees apart from the voltage from V1 to neutral?. We certainly can NOT figure that out by looking at how many slots apart are U1 and V1 as koizumi did 10 Sep 10 22:1 because U1 and V1 are just connection points.... we cannot possibly know the voltage to neutral from looking at a connection point without considering it's relationship to the neutral! To find a voltage to neutral of phase U we would have to look at the full path between U1 and neutral. To find the voltage to neutral of phase V we would have to look at the full path between V1 and neutral. The voltage from any line to neutral is the vector sum of voltages of the associated coils connected from line to neutral. Each coil has a phase difference of 43.64 degrees as koizumi noted. For each coils in U phase, there is a corresponding coil in V phase located exactly 11 slots away. 11 slots * 43.64 = 480 degrees = 360+120 degrees = 120 degrees. So for each coil in U phase there is a corresponding coil in V phase exactly 120 degrees away. When we add up the vector sum of all the coils in U phase and the vector sum of all the coils in V phase, the results of course remain 120 degrees apart. So there is no difference in the phase relationship of voltages imposed by the power supply and voltages induced in the motor.

It was a long way around to get to this conclusion this way (I thought it was much mcuh simpler just looking at the known characteristics of series circuit!). But the key was to be very methodical about visualing the whole circuit and clearly specifying what voltages you are looking at (every voltage involves two points). Maybe you can piece it together a different way that makes more sense to you (the same logic can be done with delta connection but it is trickier).. If you still don't agree, you can either 1- ask questions about my analysis above or 2 - try to define more precisely what are the relevant voltages that are being compared similar to what I did above and post them here for discussion.

For those that don't agree, I appreciate your patience. I am optimistic that eventually we will come to an agreement. Again, I am 100% certain of the conclusion.


=====================================
(2B)+(2B)' ?

 

[electricpete]

Correction in [red]bold red[/red]
U1/V2/W1 are phase leads
should have obvsiouly been
U1/V[red]1[/red]/W1 are phase leads


=====================================
(2B)+(2B)' ?

 

[electricpete]

Or if I can propose an alternate "lesson" from my discussion above, it would be that voltage is induced in coils...voltages is not induced in "locations". The voltage induced in a coil does depend on it's location. But we cannot pick a location (such as the location where we connect the T-lead) and assume it represents a voltage. Instead we need to look at associated voltages induced in coils and add them up

=====================================
(2B)+(2B)' ?

 

[electricpete]

Actually the detla version of the explanation is not so much different. U1-U2 represents a phase-to-phase voltage. V1-V2 represents another phase to phase voltage. Since every coil between U1 and U2 has a corresponding coil in V1 and V2 which is 120 electrical degrees away, we can conclude the U1-U2 voltage is 120 degrees from the V1-V2 voltage. Which means the phase to phase voltages are 120 degrees apart.

=====================================
(2B)+(2B)' ?

 

[rhatcher]

zlatkodo,

You should go back and look at the other connections that I provided. Your analysis only works if the connection has beginning lead connections that are all on the outside of the winding or all on the inside of the winding.

This is because each pole in the motor is 120 degrees out of phase from all of the poles of the other phases. If you place the all of the beginning leads on the outside of the winding, the slot # will add to 120 degrees or a multiple. If you place all of the beginning leads on the inside of the winding, the slot # will add up to 120 degrees or a multiple. If you place some beginnings on the inside and some on the outside, your analysis does not work.

Try your analysis on the other connections that I provided. They are attached again for your convenience.

 

[rhatcher]

zlatkodo,

There are many ways to connect a winding. I have seen many variations of the same winding and asked the same questions that you asked me. What is the difference? Is one better than the other? The answer is that in most cases there is little or no difference.

The different variations can be explained by saying that they were created by different people with different ideas of what might be best.

As an example, if you look back at my post on 19 Oct 10 13:39, I stated that two of the connections that I posted were "my version designed to place all of the leads on the outside and in repeating order for ease of winding."

My preference is for a connection that is easy to construct with a minimum chance for error. This means placing beginning leads at the first slot in the group, grouping the leads together for ease in paralleling, and ordering the leads in a some repeating pattern that is easy to follow.

This preference results in connections that match your rule of 120 degree lead separation. However, for me this is simply a preference. It is not a requirement for the connection to work.

That being said, the connection I posted on 22 Oct 10 8:06 was drawn specifically for you. Did you see your name in the 'customer name' place? Although this meets the 120 degree rule, this is not a design I would use. I simply drew it to see what you reaction would be to the different lead locations. Your response was very useful to understand the point you are trying to make.

Finally, to answer your question about the practice of drawing a diagram specifically for "star" and "delta." The number of leads that are used (3,6,9, or 12) is based on the type of controller and the variety of voltages that the motor can be used with. For motors that use a full voltage starter, a soft start, or a VFD drive there is no need to use more than three line leads in the connection. The remaining connections are all made internally. If you did not do this the delta or wye connection would have to be made externally and would require a much larger junction box, would be prone to error from people making the wrong connection, and would be prone to fault from failure of the extenal connection in the junction box.

Again, the simpest solution is the best so internal star or wye connections are often used where ony three line leads are required.