A Thought Experiment

[SPECIAL LOCATION]

Ok..

here goes again...

First off. my assumptions are...

Three terminals all with equal resistances between them..

which also supply a balanced load will "look electrically identical"

UNLESS you do something to un-balance the arrangment..

e.g. Stick a known value resistance equal to the reading between the terminals across one pair of terminals... 

Then using Sidesnakes voltage dividing theory..... 

Think it will work...

see 3x pictures to see if Ive missed something?????

First seeing how they all balance out to start off...



Then unbalance it..



Summary...



????? opcorn

 

[Sidewinder]

I think that your theory for the Delta winding is flawed as you have a voltage divider forming part of a current divider.

I stand by my theory, so far, but I do admit my drawing with the delta winding was incorrect due to a diagram error.

The corrected version is below:

 

[Sidewinder]

OK,Specs,

I don't quite follow your theory, please see pic below.

 

[SPECIAL LOCATION]

Full voltage potential is across BOTH a single resistor and a two resistors in series.....

Like loads of parallel light bulbs on a light ciruit...

you stick 2x 100watt in series across the supply..

they will divide the voltage down that leg irrespective of how many other lamps there are in parallel further down the cicuit...

 

[Sidewinder]

No Specs,

The current divides FIRST, then the voltage divides.

You have too many unknown values to make a definitive calculation.

You have a current flowing proportional to a single winding, say R, and a current flowing proportional to two single windings, say 2R.

Now you have NO idea what R is, and in your example you have no idea what the current is, so how do you know what proportion of the current divides down each leg to state that it is the equations you give?

OK, just re-read your post, you cannot use numbers to prove your theory, you have to prove it with algebra else it is not a proof.

Sorry.

Plus you have based your theory on a voltage source, this is flawed, it must IMHO be a constant current source.

 

[SPECIAL LOCATION]

OK,Specs,

I don't quite follow your theory, please see pic below.

Receipt-2014-03-23-18-24-15.jpg

If  want to add  v/3x  +  v/6x

then adding fractions....   (Think of it as  1/3 + 1/6)

got to make the denominators the same...

so multiply top and bottom of v/3x by '2'

i.e

v/3x    must equal  2v/6x

just as 1/3  =  2/6

Plus you have based your theory on a voltage source, this is flawed, it must IMHO be a constant current source.

I disagree.

You are trying to tell me that 2x equal light bulbs connected across a supply will not divide the voltage where the two lamps join?

 

[Sidewinder]

They will, but, you have 2 in parallel with one, see my earlier diagram, so you have a 2 way split of current, but the voltage at the mid point of the two in parallel with the one, will depend on the values of the lamps, which you don't have, and you can't describe in algebra to get a solution with just voltages in the way you describe.

 

[Andy™]

just give me 10 minutes with a grinder and you will have your answer.

 

[Sidewinder]

No Andy you won't because the motor won't run after!  ]

Plus you are missing the point!

 

[NozSpark]

However if this were a real life situation then it would be more than worth while for the company to have this motor destructively investigated so that they can get the info to enable them to get more motors made for the future

However I do get the theoretical quiz

 

[Sidewinder]

OK, Noz, perhaps, but, IF it were essential to know before connecting it up and it was a production line with a cheap downtime cost of say £100k per hour, then there would not be the option of sending it to the re-winder, or, destructive inspection.

You would HAVE to non-destructive test it, and, ensure that you were correct with your answer, because IF you were wrong, it could be in the region of 72 hours before the motor would be back to re-install, that is a downtime cost of nearer £1M than £1!

 

[Andy™]

100k per hour x 72 hours = 72 million. not 1 million. no wonder you cant do the basic maths to figure out a bloody motor!

 

[Sidewinder]

Well, you're not much closer either are you!

Still I was correct, as it is nearer £1M than £1

 

[SPECIAL LOCATION]

They will, but, you have 2 in parallel with one, see my earlier diagram, so you have a 2 way split of current, but the voltage at the mid point of the two in parallel with the one, will depend on the values of the lamps, which you don't have, and you can't describe in algebra to get a solution with just voltages in the way you describe.

two EQUAL lamps...

the value IS irrelevant if they are equal..

Go back and read the described task....

From the motor are 3 wires only.

You can use any test equipment you like.

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

I have assumed that whether Star OR Delta it is balanced...

i.e. ALL three coils are equal...

We can read the resistance between any of the three wires described...

Using an ohm meter (any test equipment)

if it is star the reading will be across 2 coils 

so the coil resistances WILL be known by  r/2  or 0.5r.

if it is delta the reading will be 1 in parallel with 2 in series 

so coil resistance WILL also be know by r/0.6666 or 1.5r.

So we DO know two possible solutions for the coil resistance..

0.5r  or 1.5r....

say r was 200ohms... 

draw the pictures..  star will be 3x 100ohms 

2x 100 in series = 200ohms our initial test reading 

Delta will be 3x 300 ohms 

(2x 300 in series) in parallel with 1x 300..

which  is same as 1 x 600 in parallel with 1 x 300..

which is 200ohms  our initial test reading...

and using a known stable voltage source...

(another bit of any test equipment)

we can have a known voltage across two coils that are either

2 x 0.5r

or

2 x 1.5r

So using any test equipment we do know two possible coil resistances...

and thus voltage division if we stick another known resistor in parallel with one of our two in series.

(another bit of any test equipment)

Unless the rules have changed????

You have not proved that my way will not work!!

Or go back to my previous option calculate physical coil sizes!!!!!

post #100

Thats two solutions I have given!

:innocent

 

[Sidewinder]

No mate, I don't think it will work.

You have now brought in a different point with the lamps.

We'll take things in stages.

Let's take the lamps first.

Two lamps in parallel.

These will current divide equally, the voltage across them will be the applied voltage.

Yes?

 

[SPECIAL LOCATION]

Parallel aspect is irelevant....

Two equal resistances across a supply voltage will divide the voltage equally.....

OR unequally you happen to have unequal resitances.....

Try thinking of a center tapped transformer....

or the old door bell transformer with known coil resistances giving 3 optional outputs...

A-B 4v

B-C 8v

A-C 12v

the voltage is divided proportional to the supply irrespective of how many other appliances are also plugged in

or lights switched on....

other parallel loads have no effect on the proportional dividing properties of the transformer...

Nor the two identical coils across a know controlled supply volatge.... 

Unless you are going to try and prove that traditional wound friedland doorbell transforms have not been working correctly for decades due to your theory that parallel loads stop them dividing properly..

then we will just have to agree to disagree...

Guinness

 

[Sidewinder]

Specs,

Yes you are correct, 2 equal resistances across the supply WILL divide the voltage equally.

However, there is NO way you can configure a Delta winding across a supply to present 2 equal resistances to the supply.

Simply LOOK at how a Delta supply is configured, it is a current divider, i.e. two resistors in parallel, however, one of these resistances is two resistances in series, so you have R1 in series with 2 x R1.

Thus they are NOT equal resistances.

 

[SPECIAL LOCATION]

Consider three lamps..

part of a lighting circuit..

ALL lamps exactly equal rating...

the voltage will Divide across the two in series....

even if one (or more are correctly wired in parallel to the supply)

Swap the lamps for know equal coil resistances.....

and a known test supply voltage 

slightly colour the picture differently to highlight the delta part...



So you are telling me this is now different?????

Or using another picture....



Try using your "current" method...

current divides between the single resistor and the two in series..

the current passing through the two in series will be equal...

potential difference across the resistors = I*R

I is constant down that branch..

the two 'Rs' are equal..

thus TWO halves of the potential difference (voltage) are equal...

How is that NOT a voltage divider????

:coat

 

[Sidewinder]

Because you have another lamp in parallel with the two in series, thus you do not know what current is flowing through the two resistors in series to know what voltage they are dropping across them, as you do not have a know current flowing through the circuit.

You are presuming that your voltage source has an infinite current I am guessing?

You cannot do this experiment, or come up with any conclusions with more than one unknown value, it is not possible to solve an equation for three unknown values, the same as it is for two unknown values, you can only solve an equation for one unknown value, unless you have a series of equations with various unknown values that can be described and substituted into the series to resolve for unknowns, thus reducing the total of unknown values.

Your diagrams have two unknown values in an equation for three unknowns, thus cannot be solved.

I can't see your argument works, I can see your reckoning, but I feel it is flawed in the fundamental theory, as the current will divide and you have no idea what the current is.

 

[SPECIAL LOCATION]

So you are now saying..

because my theory and reckoning are actually correct...

We can now NO LONGER use any test equipment we like...

and we cannot have measured the resistance across the the wires to get two possible values of coil resistance....

As with two possible coil resistances a known test volatge..

and only two possible configurations...

why can we not calculate a current and/or a divided voltage??? 

:C