Star or delta wound motors - different starting characterisitics?
Star or delta wound motors - different starting characterisitics?
(OP)
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?
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?





RE: Star or delta wound motors - different starting characterisitics?
RE: Star or delta wound motors - different starting characterisitics?
RE: Star or delta wound motors - different starting characterisitics?
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: Star or delta wound motors - different starting characterisitics?
Otherwise, I don't think that there should be any difference.
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Star or delta wound motors - different starting characterisitics?
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.
RE: Star or delta wound motors - different starting characterisitics?
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: Star or delta wound motors - different starting characterisitics?
RE: Star or delta wound motors - different starting characterisitics?
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: Star or delta wound motors - different starting characterisitics?
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
RE: Star or delta wound motors - different starting characterisitics?
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 Wye:Delta 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