Star or delta wound motors - different starting characterisitics? 1

[ScottyUK]

I'm involved in an ongoing discussion at work relating to the starting behaviour of two very similar motors, one wound for star connection at 400V, the other wound for delta connection at 400V. They are otherwise identical - same power output (30kW), same speed (4-pole), frame size (D200L), same manufacturer (Brook), and so on.

One group says that the motors should behave almost identically because the machine characteristics measured from the terminals are almost identical: line current, voltage, power factor. The other group says that the star motor will have a heavier inrush than the delta motor, and that this is why star-wound 400V machines are rare in larger sizes.

I'm currently siding with the first group, but I'm sufficiently curious to see if there's any support for the other school of thought before I totally dismiss it. The transient conditions as the rotating magnetic field establishes itself and any slight time delay between contactor poles seem to be the most likely candidates for any difference in behaviour. The DC resistance measured between any pair of line terminals is also slightly different for the two motors, as might be expected because of the different conductor size used for the windings.

Has anyone either done some analysis or has anyone got a reference on the subject?

 

[davidbeach]

If it matters, it would seem that the only way it can matter is that the winding impedances would differ. The motor performance equations I've seen do not have a delta or wye factor.

 

[ScottyUK]

I agree, I'm fairly sure any significant difference has to be a transient effect. Steady-state motor theory doesn't account for any differences.

 

[jraef]

I believe I have read a paper saying that the magnetization inrush is slightly different, but by definition that takes place before induction, so would have no effect on torque. If I recall though, the inrush was lower on Star (Wye) wound motors. I can't remember though to be honest. I'll see if I can find it, I save lots of stuff like that.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[Skogsgurra]

I can only think of triplenes having an effect on starting caracteristics. Is there a measurable diference between the motors? Is that difference consistent start after start?

Otherwise, I don't think that there should be any difference.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[ScottyUK]

Is there a difference? Not practically - both are quite happy running on a fan drive with a Class 10 overload and no changes to the starter gear. We don't have sufficient test gear to do a full set of controlled tests, and as it isn't causing us a problem I'm not likely to justify any deeper investigation.

The question arose because four motors which had been wound in delta were scrapped and replaced with equivalents which were wound in star. Someone mis-connected the new motor in delta without reading the nameplate: you can imagine the consequences and it has gotten a lot of attention as a result. More than a few have commented to the effect that "why would you choose a star-wound motor instead of a delta, they have worse inrush / starting characteristics?" and I'd replied that there was no difference worth measuring in the behaviour of the motors and it didn't matter which winding configuration was used in a 1-speed DOL application. I heard this comment enough times to make it worth asking if anyone here could substantiate this idea, even though I was somewhat sceptical.

 

[jraef]

I found the paper I was thinking of. The question on inrush was raised at the end, but refuted. This was actually in the Q&A of an article I tore out of a magazine somewhere, and unfortunately because this was the only part that interested me, I didn't get the part that had the name of the author or even the magazine. Most likely is was the EASA magazine, I used to subscribe to that and saved a lot of articles. I searched for the text and didn't find it in their available archives, however they didn't digitize and save their archived articles until a few years ago. So I don't think it's on line, but here is a relevant excerpt:

Why Wye windings?
To achieve the same electrical characteristics, a Wye connection will require fewer turns than a Delta connection; 1.732 in a Wye connection for every 2 in Delta. When winding smaller motors that have narrower slots and high volume manufacturing, having even slightly fewer turns reduces the handlng and manipulation of the coils in the tight spaces involved and allows a greater use of machine winding techniques. When wanting dual voltage motors in larger frames to work with, using a Delta configuration can allow the use of smaller gauge magnet wire, reducing the conductor cost. When you get to very large motors where dual voltage configurations are no longer needed, the fewer turns of a Wye configuration becomes advantageous again. The decision by the motor designer comes down to a compromise between conductor cost, number of coils, turns ratios and assembly labor. These factors result in different manufacturers having different criteria for when to use Wye or Delta connections in their motors, leading to no industry standard as to which type of winding is used.
...

Questions have been raised as to differences between Wye or Delta connections for inrush current or starting characteristics. For the motor to work correctly, series resistances, whether configured in Wye or Delta, must be the same. So inrush current for magnetic field establishment is the same. Starting characteristics are related to flux penetration and back emf from the rotor, neither of which is changed significantly by the way the stator windings are configured for a given voltage, so the starting characteristics can be considered to be identical.

"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[ScottyUK]

Thanks jraef, that's as good an explanation as I have seen. Appreciate the effort taken in typing it out for me.

 

[jraef]

I can't take full credit! I used a scanner and OCR, although some of the OCR interpretations were humorous because the paper was faded and smeared. I must have been eating potato chips when I tore it out...


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[zlatkodo]

Wye or Delta?
Without knowing all the details of motor winding, my first choice would be "Wye".
Why?
- The motor may have a non symmetrical winding , for example, all 6 or 12 pole motors with 24, 48, 60 .... slots. It is known that Delta connection should be avoided in that case in order to eliminate circulating currents.
For some reasons, sometimes the winding is non symmetrical even in case when should not be , regarding the "q" (number of slots per pole and phase).
- another reason could be a harmonic content, ie. whether the impact of harmonics is the same in Wye and Delta connection?
There is few articles written about it and there are some discrepancies between them . I was not reading any serious , documented analysis on that topic, yet.
So, my choice is "Wye" whenever it is feasible.

Winding Design and Repair

 

[waross]

I have wondered about the difference. This thread has given me new thoughts to pursue.
1. When the motors are at rest, there is no measurable difference at the motor terminals.
2. When the motors are running on a balanced supply there should be no measurable difference.
3. When running on an unbalanced supply there may be unwanted circulating currents in the Delta winding. Comments?
BUT
4. Starting is a transient state. There may be a parallel to a transformer connection that I love to hate, The Wye;Delta connection.
This connection may be used with either a three wire primary connection or a four wire primary connection. Both connections may be problematic.
When the three wire primary connection is energized, the wrong combination of point on wave energization and residual magnetism may result in one winding saturating and high over-voltages on one or both of the other windings.
I wonder if there may be a parallel effect with a Wye connected motor winding. A stationary Wye motor may be thought of as a Wyeelta transformer with a three wire primary connection and the rotor forming the delta winding.
The effect may be occasional higher first quarter-cycle transients with the Wye connected motor with possible tripping of instantaneous protection devices. The transient would probably not be seen by thermal devices.
I don't know the answer, but it is a good question to ponder.

Bill
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"Why not the best?"
Jimmy Carter