# star / delta motors

• posted

Before I start digging around inside it, is there an easy way to tell if a 3 phase motor is wired star or delta with a meter across the three wires?

Regards

Kevin

• posted

Kevin In short, no. Whichever way they are wired, the resistance between any 2 wires will be the same; also, the resistance of the windings on anything other than a small motor is so low that it's difficult to measure with a standard multimeter.

• posted

Yes, I rather thought that might be the case. Oh well, looks like i'll have to go in -it will probably be in star and need re-wiring anyway.

Regards

Kevin

• posted

I think if you short out any two terminals you will find that measuring between one of those terminals and the third terminal will give you a different reading depending on whether it's star connected or delta connected.

With a delta connection you will have two windings effectively in parallel giving a low resistance. In a star connection you will have two windings in parallel in series with the third winding and thus a higher resistance.

Alan

• posted

I dont know what motor you are investigating,but I have found that i

you examine the windings carefully,and find three wires conecte together,and going nowhere,its "Y" wound.I have found that all Cdro motors(Ex rom that is)are "Y" wound.That is ,just the dozen or so Iv looked at

-- oldphar

----------------------------------------------------------------------- oldphart's Profile:

• posted

In message , Kevin Steele writes

With a bit of luck, your motor windings will be brought out to a terminal block on the side of the casing. Given a following wind, you may even have six ends... If you have six ends, then with even more luck they will be labelled A1, A2, A3 and B1, B2, B3. If they are linked A1-B3, A3-B2, A2-B1 then the motor is Delta-connected. If they are linked B1-B2-B3 then it is Star-connected.

A A B / \ A \ / 3 / \ 1 \3 /1 A / \ B \/ --- | B2A |2 | A

Hope this makes sense!

TTFN

Pete

• posted

Or, alternatively, you could find that it has just 3 wires coming from the motor windings (always a good sign it's star wound!). Then you could spend nearly an hour untangling all the wires on the end of the motor windings to find the star point to disconnect it and re-wire into delta -what is that stuff they coat the wires with, it sets like concrete!. And on top of that it's bloody chilly in the workshop tonight, but at least that's another little job done.

Regards

Kevin

• posted

Don't think so. If you do the sums you get the same ratio of 3/4 between the two measurements you suggest whether it's a star or a delta wind.

• posted

In delta with a winding shorted you would get two windings in parallel, i.e. half thier individual resistance. In star with two terminals shorted you would get two windings in parallel in series with the third winding, i.e. a larger resistance.

• posted

Alan If I've got my sums right, Boo and I are correct: If 1 winding has resistance R, then between any 2 windings on a star (Y) wired motor you will measure 2R (R + R in series). On a delta wired motor you will measure 2/3 R (R in parallel with 2R). If you short 2 windings together, on a star (Y) wired motor you will measure

3/2 R (R in series with R+R in parallel). On a delta wired motor you will measure R/2 (R=R in parallel). The ratio of 2R to 3/2R is 4:3, the ratio of 2/3R to R/2 is also 4:3. Granted the resistances are different between star (Y) and delta wired, but that doesn't help unless you knew the value of R in the first place. Martin
• posted

For motors of the same rated HP, star connected windings are wound with thicker wire. This exactly cancels the suggested resistance difference so this cannot be used to star/delta differentiate.

Looking for the triply connected star point is the only reliable method - don't forget that this star point joint is not necessarily at the same end as the main leadouts. It can be located at either end of the lamination stack.

If it's located at the main leadout end, some minor variation of sods law ensures that this is the joint that is made first. This means that you have to dig out all three leadout joints to be sure of locating a buried star point!

Jim

• posted

I take it that you would like to find the connexion without dismantling the motor. If you want to experiment, then feed the motor across two terminals with DC from a bench PSU. If star, for each triplet of windings I in two coils, zero in the other. If delta , I in one, I/2 in the other two. Opposite pattern if the third terminal is shorted to one of the other two.

If you can get at the end windings through ventilation slots, then thermocouple probes on the end windings can show the current distribution - apply about rated current and measure time for (say)

20C rise on each (energised) phase winding.

If the motor is totally enclosed, then there's a fighting chance that a piece of card and iron filings will show the current pattern in the magnetic field around the end windings.

Otherwise, feed the motor from a 3ph source with lots of third harmonic distortion. If the harmonics circulate in the windings, it's delta connected. Probably easier to strip the motor and inspect.

Regards,

David P.

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.