Motor vector shift

[Magoo]

Many years ago, we had a university engineer resolve a problem with large 3 phase motor spiking at start and killing contactor links with a star delta starter.
Does anyone have factual information about vector shift with phase cross-over. All comments appreciated.
magoo:)

[Yuri B.]

Good evening Magoo. What you said sounds for me somewhat obscure, agree, probably because I do not have an engineer education. But anyway, could you please explain "vector shift with phase cross-over". What do you mean to schieve by this calculation?

[Brian_UK]

Well Google gave me some info but I'm none the wiser. ;)

[Magoo]

Hi Brian and Yuri.
It was explained to me at the time, that if motor is wired red phase to blue phase, then the vector change , ie slip between coils and rotor are minimal, cannot remember the dgrees, but if motor wired red phase to yellow phase then the vector shift is 15 degrees, which in turn creates a huge inrush to coils at start and a huge current spike at transition if open transition star delta contactor system. What I'm trying to figure out is if the same applies if using a soft starter or a VSD. I think the vector shift will still apply at the motor irrespective of starting system, but my electric motor theory is not good.
magoo
ps. the high inrush and transition spikes prematurally destroy contactors, and stress motor windings and can create burnouts for no obvious reason, so you replace or rewind without actually fixing the real problem.

[Yuri B.]

Good morning. I know there is no difference between waves apart from that their sines lag apart from each other 90 degrees.

[Yuri B.]

In the line, I mean. The inverter and soft start are already invented and in use, we, probably wont succeed in improving their princip. (in them indeed, sometning may be happening to waves lagging)

[Yuri B.]

Self correction, of course, 120 degree lag between current phases in each line.

[Peter_1]

Hi Brian and Yuri.
It was explained to me at the time, that if motor is wired red phase to blue phase, then the vector change , ie slip between coils and rotor are minimal, cannot remember the dgrees, but if motor wired red phase to yellow phase then the vector shift is 15 degrees, which in turn creates a huge inrush to coils at start and a huge current spike at transition if open transition star delta contactor system. What I'm trying to figure out is if the same applies if using a soft starter or a VSD. I think the vector shift will still apply at the motor irrespective of starting system, but my electric motor theory is not good.
magoo
ps. the high inrush and transition spikes prematurally destroy contactors, and stress motor windings and can create burnouts for no obvious reason, so you replace or rewind without actually fixing the real problem.

I don't understand your explanation Magoo, with a 3 phase motor, if you change leads, only rotation will change and has nothing to see with inrush currents or spikes. If you change from Star to delta, then motor slows down a little bit between switchover and if you power on delta, then you can have a non matching rotating fields for some microseconds. But you have this phenomena far more at start up when rotor is not running at all.

[Magoo]

Hi Peter and Yuri.
Sorry about explanation clarity etc., but if I knew all the answers and theory I would not be asking the question. So I will endevour to track the answers and get back to you all.
All I can say is that the changes made certainly resolved the problem of rapid failure of contactors.
magoo

[oogene]

jus a new guy here but the delta-wye sarter is so that the motor in quetion can be started in delta config to take advantage of the higher torque then switched to wye for the extra voltage-in delta-the current is 1.732 times stronger in delta than in wye and the v oltage is 1.732 times stronger in wye than in delta-not sure if I'e helped or harmed but thats why MHOis only worth $0.02 american dollars

[US Iceman]

I've tried to stay out of this discussion because I have forgotten so much about motors and their theory. However, I'm going to jump in here just a little in case my ramblings jog anyone's memory.



...a huge current spike at transition if open transition star delta contactor system.


I thought this was a natural function of open transition starters and a reason why closed transition was preferred.



...slip between coils and rotor are minimal...


I thought this was a function of the air gap between the rotor and stator. If the air gap is minimized the magnetic field does not have a larger gap to jump and as a result the slip is reduced, which increases the motor efficiency.



...but my electric motor theory is not good.


Don't feel bad. I warned you up front mine was very rusty!;)

It would be interesting to learn more about this. I would not be surprised to learn what you are describing with the term vector shift is something we learned in school under a different name though.