A Thought Experiment

[Sidewinder]

I have shamelessly stolen this question from somewhere else, but I have put a little spin on the way it is worded.

I have not thought about it for more than a few seconds and I don't have an answer.

I just felt that it was a very good question to get the mind working, hence the thread title.

You are presented with a sealed electric motor, it is "standard" 3ph, this much we know.

The ends are welded up, so you can't remove them without destroying the motor, which you can't do.

There is no rating plate.

The motor is blue.

There is no terminal box.

From the motor are 3 wires only.

You cannot see into the motor through the cable entry.

You can use any test equipment you like.

There is an external earth terminal on the motor which is present but is not connected.

All 3 of the wires are black.

The wires are copper cored.

The wires are unmarked.

The wires are 2.5mm sq.

The motor is foot mounted.

The motor shaft has a keyway.

There is no manufacturers information on the motor.

The motor does have a fan and cover over the fan.

If you have a definitive solution immediately, I can think of one or two members that may, please wait for some other suggestions/ideas please, before posting your answer.

OK to the question then.

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

 

[steptoe]

its for powering a mangle, 

 

[m4tty]

:slap ^^^

 

[Bruspark]

Ask the person who 'presented' it to you for details ???

 

[Sidewinder]

OK Bru,

You did ask them, and they can't tell you because they are just the store man.

They did tell you it has been re-wound, and there is no record of who did the rewind now in the company, it has been lost in the system somewhere.

 

[kerching]

I cannot see anywhere where it states that it 'cannot be examained with an angle grinder'.....or  'that it has to work afterwards' :innocent

just not wanting to put a serious answer

 

[Sidewinder]

OK,

I knew I would forget a few conditions!

Kerch, it cannot be cut open as it is the only spare, and it has to work, so it can't be destructively inspected!!!

 

[NozSpark]

Surely you can't tell unless its neutral earth tapped

 

[kerching]

OK Bru,

You did ask them, and they can't tell you because they are just the store man.

They did tell you it has been re-wound, and there is no record of who did the rewind now in the company, it has been lost in the system somewhere.

curses  upon theses storemen who cannot maintain the records system....look at the trouble they have caused

 

[Technician]

I have had an idea.

Arrange to pass a small direct current through

the motor windings using a small power source,

like a battery.

Measure said current.  Do this three ways.

Record results.  They should be the same.

Now, let one terminal be A the next B and the next C.

The battery and its ammeter are connected to A & B.

Measure the voltage between terminal A and C and

then B & C.

If this measured voltage is half the applied voltage,

i.e. V (A/C) and V (B/C)  is half voltage A/B then

chances are this is a delta connected machine.

Now, connect another ammeter so that it is teminated

at winding C at one side and to A on the other.

This puts the windings at A and C in parallel.  This

ammeter should show half the reading of the other one

which is measuring total current.

If you do this on a delta connected motor you will short

the energy source and this ammeter will go off scale.

Hope this is clear but I am open to suggestions & questions.

I have sketched this out on a cigar box and it looks OK.

 

[apprentice87]

Yes, i was going to say something similar, Connect up a voltage between two of the terminals, then measure the voltage between one of your connections and the third one. With your knowledge of ohms law, and how voltage dividers work and the rules to do with the connecting up of resistors, you should be able to figure out the internal connections.

If you have the base dimensions and the shaft and keyway size, just look for similar specced motors in a makers catalogue, that should give you a bit of a clue what it might be, unless it has been would as a special, in which case never mind the internal configuration, it might have been wired for the mains voltage in timbuctoo..... Anyway, find similar motors and compare "your" winding resistances with the known values from the "standard" motors. This will give you even more of a clue..

Just connect it as star and see if it goes and how it goes!!!

john...

 

[Technician]

If you do this on a delta connected motor you will short

the energy source and this ammeter will go off scale.

My correction;  the second ammeter should read double

the read on the other one because it will be reading the

current flowing in one winding.  The first ammeter will be

reading the current in two windings in series.

I just went back to the cigar packet.

 

[Evans Electric]

R between a & b    a & c   b & c   would be greater  for the  star   that the delta , I'm thinking  .  

Method 2  .  Ask Bob the Breakdown,

Method 3 .  Ask at the rewinders .

 

[Sidewinder]

Deke,

I don't think resistance will tell us, each of the windings are equal resistance.

Letting the ends be as you have labelled them a, b, & c.

a to b will be two windings, b to c will be two windings, c to a will be two windings, if star.

a to b will be one winding, b to c will be one winding, and c to a will be one winding, if delta.

Trouble is we don't know what the resistance of one winding is, so we also don't know what the resistance of two windings is either.

Not blindingly obvious really.

Hence why I posted.

I'll have a look at what technician has posted later if I get time.

I think we need to stick known currents & voltages in and see what we get as an initial guess.

 

[Evans Electric]

You're right .....I hadn't thought of that .

 

[Sidewinder]

Perhaps I should have put this in the student section?

I have a theory, and I'm going to go through what technician has posted next.

At the moment, I think Kirchoff will give us our theory.

 

[Sidewinder]

Technician,

I am not sure, but, I think you are part way there, I feel some sketches.

Please guys keep the ideas coming, as I am finding this interesting, I have not worked out the answer yet.

Though I am leaning toward a constant current d.c. source, some ammeters & some voltmeters, with a high input impedance, at the moment.

I thought you would have an idea Canoe, it is not something I have ever had to do, I saw it elsewhere and thought it a good conceptual question.

So I have gone back to fundamental principles to try to identify a solution.

The only thing that I am pretty set on, is using a d.c. source so we are only looking at resistance rather than impedance.

 

[SLIPSHOD & SLAPDASH]

Is the answer not based on electrical theory and more facts such as the keyway, its blue, shrouded fan etc that points to only one motor ;

(probably not but maybe)

 

[Andy™]

Kerch, it cannot be cut open as it is the only spare, and it has to work, so it can't be destructively inspected!!!

erm... look at the one that its a spare replacement to?

 

[Sidewinder]

Sorry Andy the spare that it is supposed to replace is in the same boat, and worse it is totally burned out, and seized, plus it is no longer on site and has been sent to the rewinder, where they say it is un-repairable, as it is not possible to trace the internal windings as it is too badly damaged.