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Fractional Slot Stator Windings ?
4

Fractional Slot Stator Windings ?

Fractional Slot Stator Windings ?

(OP)
We have a 3.3 KV, 1490 RPM (4 pole), 1000 HP Motor with 84 stator slots, which we want to convert to 990 RPM (6 pole) motor. For 6 pole, we get a fractional slot winding (84/3x6=4 2/3 slots per pole per phase). Can any one suggest the actual arrangement of coil groups around the stator periphery that will be electrically and magnetically balanced for 6 poles? For example, I claim the arrangement  555 455 544   555 455 544 is the best. But, our consultant claims the arrangement 545 545 464   545 545 464 is electrically better ? Views from any experienced ac machine designer will be appreciated.

RE: Fractional Slot Stator Windings ?

I am not a designer. But hey it's a free board so I'll throw in my unifnormed 2 cents for what it's worth.

I do have a reference which indicates that a balanced winding cannot be produced for this configuration.

Using terminology of EPRI5036V17
q = Q/(m*p) = N / B = a + b/B
Q=84
m = 3 phases
p = 6 poles
N and B are smallest integers
q = 84/14 = 42/9=14/3
B = 3
a - largest integer
b = remainder

Condition for balance is
d = (mNP+1)/B
where P is the smallest integer that will make d an integer.

If B/m is an integer, there will be no value of P that makes d an integer. In this case, the fractional slot winding will be unbalanced, and neither the phase voltage nor phase angles will be equal.

Sorry if I am telling you what you already know.

RE: Fractional Slot Stator Windings ?

typo correction:
Should have been q = 84/18 = 42/9=14/3
results are the same

RE: Fractional Slot Stator Windings ?

I  should clarify it is a direct quote from the EPRI reference: "If B/m is an integer, there will be no value of P that makes d an integer. In this case, the fractional slot winding will be unbalanced, and neither the phase voltage nor phase angles will be equal."

NOT my own words.

RE: Fractional Slot Stator Windings ?

(OP)
electricpete,

interesting equation.

I agree that the winding we propose is not ideally balanced. My question was which of the two arrangements proposed in my posting would produce least dyssymmetry.

RE: Fractional Slot Stator Windings ?

edison123:- coil grouping
  a b c a b c a b c
  4 5 5 5 5 4 5 4 5 :-repeat  wind 12 groups 5 + 6 groups4.
 note:- unbalanced elect deg, connect series star only.
also check rotor slotts for cusp or coging.
cannot get a balanced winding for 6 pole (12.87 elect deg between slotts)config.have you looked at 8 polethis you have a balanced winding(17.14 elect deg )

 kind regds

RE: Fractional Slot Stator Windings ?

Suggestion to edison123 (Electrical) Jun 12, 2003 ///\\\
We have a 3.3 KV, 1490 RPM (4 pole), 1000 HP Motor with 84 stator slots, which we want to convert to 990 RPM (6 pole) motor. For 6 pole, we get a fractional slot winding (84/3x6=4 2/3 slots per pole per phase). Can any one suggest the actual arrangement of coil groups around the stator periphery that will be electrically and magnetically balanced for 6 poles? For example, I claim the arrangement  555 455 544   555 455 544
///5+5+5+4+5+5+5+4+4+5+5+5+4+5+5+5+4+4=84
or
15+14+13+15+14+13=84
Please, what is your design basis for this configuration?\\\
 is the best. But, our consultant claims the arrangement 545 545 464   545 545 464
///14+14+14+14+14+14=84
This partitioning is more symmetrical.
Also, considered could be either:
545 545 545 545 545 545=84
or
464 464 464 464 464 464=84
or
545 545 545 545 545 545=84
Nowadays, the magnetic circuits are often designed by software. Has the consultant used any software for the group it is claimed better one?\\\
 is electrically better ? Views from any experienced ac machine designer will be appreciated.
///This may be proprietary.\\\

RE: Fractional Slot Stator Windings ?

Hi edison 123

I am curious of why would you hire a Consultant if you think you have a better answer to your problem?All of the arrangments mentioned so far would probably get your motor running.The problem starts after it runs.Can it run efficiently and up to the rated Horsepower.
I would suggest,if possible,consult the motor manufacturer. They have stator core magnectics information that are quite important for motor performance in addition to the Physical arrangement of coil groups.
If you expect to have equal motor efficiency, you have to have all of the Electrical/Magnectic information related to this motor.
We,working for a motor factory subsidiary,were always very reluctant to change motor's design whenever that change impacted on the Motor Magnectics/Harmonics.Sometimes,what appears to be a straight forward motor change,turns out pretty ugly.The Customer is very seldom aware that his
previous 1000 hp have now shrank to 950Hp.   

GusD

RE: Fractional Slot Stator Windings ?

(OP)
teco
Thanks for your input. I have decided on 455 545 554 ..repeat, which is closer to what you have suggested.

gusD
don't we all live and perish by consultants? it was management (?) decision to counter check an engineer's decision. As usual, the consultant was proved wrong. In our case, we are changing the motor design to suit the new speed requirement on the load side. naturally, the new output at 1000 RPM will be 700 HP.

electricpete
thanks again for your memorable equation

RE: Fractional Slot Stator Windings ?

Suggestion: Is your design based on prerogatives or on any engineering design basis or empiricism?

RE: Fractional Slot Stator Windings ?

(OP)
My design is based on 1. Pole balance (All pole-phase bands have same no. of coils). 2. Magnetic balance (Opposite pole bands have same no. of coils). These two conditions are required to get electrical as well as magnetic balance.

RE: Fractional Slot Stator Windings ?

Suggestion: Please, would you numerically elaborate on how those pole-phase bands have the same no. of coils.

RE: Fractional Slot Stator Windings ?

(OP)
If you add up the no. of coils in each pole-phase band in my above posting (4+5+5) (5+4+5) (5+5+4)... repeat, you get the same no. of coils(14)in each pole-phase band. This satisfies the electrical balance.

RE: Fractional Slot Stator Windings ?

Suggestion to previous posting: Thank you for the clarification that appears to be somewhat different from your original posting.

RE: Fractional Slot Stator Windings ?

Well, since the subject is opened up....
I have to say I am absolutely positive that I don't understand the requirements for a balanced fractional slot winding.  But I'm interested to learn from anyone out there who knows more about it.

Here is one thing that strikes me about
(4+5+5) (5+4+5) (5+5+4) (4+5+5) (5+4+5) (5+5+4)

The grouping of three together is somewhat articial.  It does highlight that there are same number of coils in phase A, B, C.  However 60 degrees later in time I think the same winding could be described as :

..4) (+5+5+5) (+4+5 +5) (+5+4 +4) (+5+5+5) (4+5+5) (+5+4..

The groups range from 13 to 15.  I think it is a reflection as discussed above there is no perfectly symmetrical solution. Maybe there is more to it.

RE: Fractional Slot Stator Windings ?

Appendix 1 of Electric Machinery, Volume 2, by Liwschitz/Garak has a detailed discussion of this subject.  Far above my head. If it can be described in 3 or 4 simple rules I'd be interested to understand, but it looks more complicated to me.

RE: Fractional Slot Stator Windings ?

I have already qualified I don't know the subject. I read a little bit and trying to understand.  

I think the 3 induced phase voltages could be calculated for each of the proposed configurations.  The goal is to generate induced phase voltages for three phases which are  as close as possible to equal magnitude with phase angle 0, 120 degrees, 240 degrees.

If all coils were connected in series, the voltage for a given phase is the sum of each of the included induced coil voltages.

For 84 slots, 6 poles, we could represent the induced voltage for the coil whose top leg is in kth slot as a phasor:
Vk = exp(6*Pi/84)*(-1)^t for k=1, 2, 3, ...84.

Va = Sum(Vk) for all k that belong to phase A.
In the case of proposed solution:
(4+5+5) (5+4+5) (5+5+4) (4+5+5) (5+4+5) (5+5+4)
for phase A we would use k in the set of 1,2,3,4 (first 4), 15, 16, 17, 18,19, 20 (batch of 5), etc.

for phase B we use 5,6,7,8,9 (batch of 5), etc

t is an odd or even number depending on the two possible coil polarities.

RE: Fractional Slot Stator Windings ?

typo correction... should add I (sqrt(-1) as follows:

Vk = exp(I*6*Pi/84)*(-1)^t for k=1, 2, 3, ...84.

RE: Fractional Slot Stator Windings ?

Well, I put that formula into excel.  edison's most recent configuration gives
Magnitude    26.72    26.64    26.72
Phase    36.71    -83.57    156.15
which seems pretty darned close.

The consultant's recommendation gives
Magnitude    23.25    23.35    26.72
Phase    32.14    -78.34    156.15
which is quite a bit off.

I wonder if I'm doing that right. I'll try to post the spreadsheets.

RE: Fractional Slot Stator Windings ?

OK - it is uploaded.

http://www.geocities.com/pschimpf/fractionalslot.htm

Do you guys agree this is a reasonable way to evaluate the balance?  edison - why did you mention a requirement for "magnetic balance" of 180 degrees opposite pole bands?  

RE: Fractional Slot Stator Windings ?

I keep on talking about "induced" voltages. It comes from the terminology of Liwschitz/Garak.  It is as if we treat it as a 6-pole syncronous generator... then look to ensure that the induced voltages add up to create a balanced three-phase set.

RE: Fractional Slot Stator Windings ?

(OP)
pete,

You have to have magnetic pole balance (same no. of amp turns in opposite poles) to prevent uneven magnetic forces. i am sure you'll agree that if one pole has say 5 coils and the exactly opposite (180 deg mechanical) has only 4 coils, then you will have uneven rotating magnetic forces, which will definitely produce vibrations and other undesirable harmonics.

i am yet to go thru' your upload. will get back once i have a look at it.

RE: Fractional Slot Stator Windings ?

thanks edison. That makes sense.  

I made an error on the consultant file.  I changed the A,B,C column, but forgot to change the polarity column. When I change the polarity column it gives me identical results to your's (edison's).
Magnitude    26.72    26.64    26.72
Phase    36.71    -83.57    156.15

Spooky similarity. They look close enough to balanced that the error might be round-off error. Still not sure if I'm doing it right.

RE: Fractional Slot Stator Windings ?

(OP)
pete,

does your xls files have some formula links since i have got only blanks in your angle calculation columns ? may be you can mail me yr results directly.

RE: Fractional Slot Stator Windings ?

Hello edison123

Edison,you have a lot of good information regarding your future motor modification .It would be interesting to do some load testing on this drive after the mods, and compare new motor efficiency/performance versus old one.
If you can provide that information it would be great.

Thanks a lot.

GusD

RE: Fractional Slot Stator Windings ?

(OP)
GusD,

We are right now into the project. Expect to commission the motor in another 6 weeks. Will surely post the results in  this thread.

RE: Fractional Slot Stator Windings ?

Hi edison.  I don't know if you are still interested in looking at those files. I would be intersted to hear your opinion.

I looked at those files and they still read ok on my computer. I think there are differences in how excel is set up. It needs to have the functions for handling complex functions.  Maybe you can try reading on another computer.

One other alternative is that I can convert all the results to values (vs functions) and e-mail it to you. Unfortunately I lost your address. e-mail me if you want me to send it like that.

RE: Fractional Slot Stator Windings ?

Suggestion: Although the machine with 84 slot 6 pole 4 2/3 slots per pole per phase is not included in Referece:
1. M. G. Say "Alternating Current Machines," John Wiley & Sons, 1978, page 88 "Fractional - Slot Winding" section, the 16 pole 3 phase machine with 108 slots and 60deg phase spread with 3 3/5 slots per pole per phase is included.

RE: Fractional Slot Stator Windings ?

(OP)
epete,

my e-mail : eewemf@myrealbox.com

Just now, I rechecked your XLS file. Still not working. Would appreciate if you can mail your program.


jbartos,

Thanks for your reference which I have already checked.


To all interested in this post:

The conversion to 6 poles is under progress. In all likelihood, the converted motor will be ready for testing by first week of august.

RE: Fractional Slot Stator Windings ?

(OP)
An Update to all interested in this post.

Finally, the motor, redesigned and rewound for 6 pole operation, was successfully test run today on no-load with low voltage supply. (Actual load test in its own bed will be done in about 3 weeks). Here are the low voltage test results:

Original parameters for 4 pole : Stator volts 3.3 KV, stator current 162 A speed 1490 RPM (4 pole) with rotor volts of 910 V and rotor current of 500 Amps.

Redesigned parameters for 6 pole : Stator volts 3.3 KV, stator current 115 A  speed 990 RPM (6 pole) with rotor volts 660 V and rotor current of 475 Amps.

Rotor was injected with 450 V and induced stator volts was 2150 volts. The motor ran smoothly at 1020 RPM (the supply frequency was 51.3 Hz from a separate DG). The no-load rotor current at 450 V was 166 amps. (I know, the no-load current is a bit high but then the requirement was for a high pull-out torque necessiating higher magnetic loading).

No electrical vibration (due to fractional slot stator winding in the new 6 pole design) and low mechanical vibration. Max velocity was only 1.8 mm/sec pk.

Thanks electricpete, GusD, teco and jbartos for sharing your valuable opinions with me. Your ideas meant a lot to me.

RE: Fractional Slot Stator Windings ?

Thanks for sharing that.

RE: Fractional Slot Stator Windings ?

edison123

Thanks for sharing the good news about your motor mod.
"All is well that ends well"

Regards
GusD'

GusD

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