A Thought Experiment

[Marvo]

Hi Marvo,

Errrr, no.... I went through all that in post 29. Difference is exactly the same for both star and delta. The difference between 15 and 20 is NOT 25% it is 33% . What is 33.333% of 15??? 5.. add 5 to 15... 20!!!!

john...

Yeah, my math was flawed, scratch that idea. Guess it's one of the risks of getting involved with this kinda thing at 1 in the morning.

You could use a thermal camera all you liked too, would not work either, cos the windings are still in the same place. Have to be, otherwise a motor could not be operated in both star and delta. Windings in same place, connections to ends different is all....

john...

I'm not understanding what you mean. This method wasn't my favourite because there's so many reasons it may not work in real life but on paper it's possible. If you apply a battery voltage across any two terminals if the motor is internally star connected two of the windings will heat evenly and the third winding will remain cold. If it's delta then whilst all three windings will heat up one winding will heat twice as fast. There'sa lot of 'ifs' but if your FLIR camera has good accuracy and temperature sensitivity and if the windings heat quick enough so the body of the stator doesn't have a chance to sink the different temperature gradients together and maybe this method would work better on a 2-pole than it would on a 6-pole.But just thinking out loud, maybe a better way of detecting which windings are heating and by how much would be to monitor their resistance. Doing it the resistance way would get rid of a lot of the 'ifs'.

 

[apprentice87]

Hi Marvo, Ah yes!! see what you mean, suppose that might work, no idea if thermal cameras are that good though, who knows!! Try it on a runnning motor with your one, just as an experiment and see what happens...

john..

 

[Marvo]

....no idea if thermal cameras are that good though, who knows!!...

I know my thermal camera isn't that good unfortunately it was a fairly early version and doesn't have anything like the granularity and accuracy of the new models. I might give it a go with the resistance monitoring method though if I can lay my hands on a decent car battery or two.

 

[apprentice87]

Yes, that might work, give it a try, should only draw a few amps.. You will have to calculate how much power each winding will consume and how much it will warm up and the resistance increase.. That will keep you busy for a bit!

john..

 

[SPECIAL LOCATION]

The wires are copper cored.

The wires are 2.5mm sq.

How can you tell if the internal wiring configuration of the motor is star or delta?...

Ok I have been pondering this for a bit...

doing a few spreadsheet calcs ands sketches..

considering several options of strapping and/or connecting other components onto one or more terminals to see if any different values or ratios can be obtained between star & delta...

But as the motors are balanced and all we have electrically to start with is a resistance between any two terminals then whatever we do to a star motor will give the same readings as if it were also a delta motor...

i.e. electrically they are identical... 

Say we have a continuity between any two terminals call that "X"

all we can then deduce is

i)  If it is Star then each coil will be equal to  (0.5*X)

ii) If it is Delta then each coil will be equal to (1.5*X)

iii) If any two terminals are shorted the continuity via the non shorted terminal will be equal to (0.75*X) 

e.g. using easy numbers if x=20,   Star,(0.5*20)=10,  delta(1.5*20)=30,  strapped(0.75*20)=15.

So you just end up going round in circles with electrical calcs...

BUT...

What about physical dimensions....?

We know our 'X' reading and we have two possible solutions for the coil sizes

0.5X   and  1.5X

According to the snake these coils are wound from 2.5mm copper wire....

How about if you calculate the length of copper needed to achieve either of the two possible coil resistances..

Wind them up..

see how big they are and if they would physically fit inside your motor??

By my reckoning you will have two possible coils one of which is three times the size of the other.... 

Then if you have to fit three of the said coils inside you motor housing that you have in front of you..

It just may be physically impossible to fit the one option inside???

Or one may look like its going to rattle around cuz there is just tooo much motor space ...

opcorn

 

[Technician]

I did read something many years ago about drying out motors

using welders or MG sets as welders.  The nature of the note

was more cautionary than procedural because the originator

clearly knew it went on and that damage had been done.

Assuming that the motor coils are intact a small heating current

could be passed between any two terminals, monitored of course.

After removal of the connections the motor coil resistance could

be measured.  If two sets of coils have heated up, it is in star.

If only one, then it is in delta.

Some would say that this flies fully in the face of acceptable

practice but I have heard of this method being used to find the

temperature rise of the winding in the motor stator provided

that the room temperature resistance has been measured.

 

[Sidewinder]

Specs,

I "THINK" I have a solution, as they are NOT electrically identical.

I will post the first bit of my theory later, not because I am trying to drip feed my ideas, but, because I have not had time to do any more.

All, please feel free to comment on, develop, or extrapolate the theories I post.

I am not definitely right on this one, I am trying to develop a theory from first principles.

 

[Sidewinder]

OK, first musings on how a Delta winding behaves from first principles.

Please feel free to develop or comment.

A bit fuzzy, but I hope it is readable.

 

[SPECIAL LOCATION]

Specs,

I "THINK" I have a solution, as they are NOT electrically identical.

I will post the first bit of my theory later, not because I am trying to drip feed my ideas, but, because I have not had time to do any more.

All, please feel free to comment on, develop, or extrapolate the theories I post.

I am not definitely right on this one, I am trying to develop a theory from first principles.

In the real world they are NOT identical...

But in your question as we only have 3 terminals....

They are effectively identical...

i.e.  we DON'T have six conductors.... (or it would be bleeedin obvious if its star or delta)...

So we cannot establish any currents OR voltages down any single coil...

Every single terminal we have access to is connected to more than one branch either directly..

or though the star point (which is also inaccessible) to all three...

So all of the tests and readings must relate to combined values though more than one coil.

The question is not much different to me saying ..

I have two sealed boxes each has a pair of terminals..

one has a 90ohm & 60ohm resistor in parallel with each other.

the other has a 36ohm resistor on its own...

How can we tell the difference????

Electrically they are different BUT testing and overall resistance they are identical...

Back to our three coils in our motor..

basically one's a potential divider with a parallel resistor..

the other is a potential divider with a series resistor...

BUT the external values could be identical...

opcorn

 

[Sidewinder]

No Specs, there will be differences, I am sure, have a look at my last post, and see what you think?

 

[NozSpark]

I understand your calculations,,, however in your "black box" you will not be able to measure I2 or I3

 

[steptoe]

the more I think about this

the less clue I have

:C

 

[Sidewinder]

IF you are doing the black box bit then measurements are irrelevant, the theory will prove the situation.

Forget about taking measurements of any values and express them in terms of Ohms Law, Kirchoff's Laws, & Ampere's Law.

 

[Evans Electric]

Any milage in measuring the R  of any two leads .....the reading is either 1 winding or 2 windings   we don't know which.

We now have a  R    and V  @ 400 V .     Ohms Law should give A  .    Connect & start motor ...read the current  and  whatever it reads tells us ...erm ...that ....er.....we still don't know if its Star or Mesh  :C    So there you have it ....not .

I'd still take it to the rewinders and ask .

 

[Sidewinder]

OK, I have put, some, thought into this, not saying I am right or not.

I started thinking about firstly the Delta connection, following on from my last picture where I started some circuit analysis on a delta winding, I carried on and had a thought, after getting this far:



So I swapped to analysing the Star winding and came up with this:



Open to thoughts and criticism.

I'm not saying it is right, but, it seemed so late at night when I came up with it, so I've just posted it anyway.

It still seems to make sense to me.

 

[Doc Hudson]

I think your comment that V1=2V2 will be true for delta as well. As if you superimpose a delta over your three star resistors you still have a voltage divider. It is either a voltage divider with one additional series resistor or a voltage divider with one additional parallel resistor.

Doc H.

 

[Sidewinder]

I don't think so Doc, see my pic of the Delta winding in the earlier post.

 

[poni]

I think the Doc is right, but in there lies the answer

V1=2V2 Star

V1= 4V2 Delta

 no forget that too many dead spiders on my page, I will have get back to you

 

[Doc Hudson]

I don't think so Doc, see my pic of the Delta winding in the earlier post

Not withstanding the fact that in your sketch your V1 will be zero as you have a short circuit across its terminals. A voltage applied across any pair of terminals in a delta arrangement will put a potential across one single coil in parallel with two equal value series coils. They will become the voltage divider to the third terminal. 

Doc H.

 

[Sidewinder]

In my first sketch for the delta winding you have a voltage divider forming part of the circuit of a current divider.

In the second sketch for the star winding you have a voltage divider, with a branch off this at the half voltage point.

The short is an error in the sketch, that is the short in parallel with the voltmeter V1 is an error and should not be there.

I will post the corrected version later.

I still stand by my circuit analysis to date, not withstanding the error in the diagram pointed out above.

Which I will correct later and re-post the corrected version.

I cannot see how you can make a voltage divider from a Delta winding with sufficient known values to derive known voltages.