skewed slots in AC generator´s stator
skewed slots in AC generator´s stator
(OP)
Dear all,
It is well known the skewed rotor bars in induction motors are used for avoiding CUSP, cogging and noise in three-phase induction motors. However, there is also the possibility of skewed slots in the stator. One of my collegues came to me looking for explantions of skewd slots in a 4160 V 5 MVA 4 Pole Synchronous AC Generator.
Does anyone knows for what were intended such skewed slots (all 60 slots are skewed)in the stator?
Thanks and Happy holidays!!
Petronila
It is well known the skewed rotor bars in induction motors are used for avoiding CUSP, cogging and noise in three-phase induction motors. However, there is also the possibility of skewed slots in the stator. One of my collegues came to me looking for explantions of skewd slots in a 4160 V 5 MVA 4 Pole Synchronous AC Generator.
Does anyone knows for what were intended such skewed slots (all 60 slots are skewed)in the stator?
Thanks and Happy holidays!!
Petronila
RE: skewed slots in AC generator´s stator
RE: skewed slots in AC generator´s stator
RE: skewed slots in AC generator´s stator
skewed coils, or rotor bars, provide a means for gaining more circuit resistance.
John
RE: skewed slots in AC generator´s stator
I respect John's opinions on things like this, but I have a hard time imagining why increased resistance would be required... and if required wouldn't there be better alternate ways to achieve such a modest increase in resistance without the complexity of skewed stator slots / bars.
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(2B)+(2B)' ?
RE: skewed slots in AC generator´s stator
I believe too the skewed slots in the stator are associated with harmonic prevention.
The coil pitch for a synchronous alternator should be a chord factor of 0.866 so the coil pitch needs to be 1-11. (4 Pole, 60 Slots).
If the alternator operates solo then the pitch is less critical, but if it is connected in parallel to the line or with other alternators, a 1-11 pitch will avoid the third harmonic.
This alternator had a 1-12 pitch. Then, I suspect the manufacturer skewed the coils for avoiding any kind of harmonic if the machine is paralleled.
Happy New Year!!
Petronila
RE: skewed slots in AC generator´s stator
The skew does not have to be all one way, either. It could look like a herringbone pattern.
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RE: skewed slots in AC generator´s stator
The aim is to reduce step harmonics (order: k * slots +/- pole pairs; k is 1,2,3...) and thus prevent or reduce some side effects such as cogging, cusp and electrical noise, as these phenomena cannot be sufficiently reduced by any winding intervention (the pitch, connection, grouping, parallel paths, etc.).
Changing the pitch from 1-11 to 1-12 alone, can reduce the overall content of harmful harmonics but cannot reduce step-harmonics.
The harmonic content of equivalent winding (30 slots, 2 poles) for pitch 1-11 and 1-12 is shown below.
More at: https://winding.wixsite.com/design/winding-analysis