Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here


Generator Pitch

Generator Pitch

Generator Pitch

Checking the archives I see several entries that makes sense - however, does not cleanly map to my circumstance. I am looking to parallel to a old (~1965) GE 480V, 3 phase, 6 pole, 1200 rpm generator. The stator shows 108 slots and a pole covers 14 slots. From what is written, the pitch is 14/(108/6)= 14/18 = .778 pitch or 7/9. I expected to come up with a standard value of either of 2/3, 4/5, 5/6

Nothing in the manual regarding pitch

Could it truly be 7/9?


RE: Generator Pitch

I would use 5/6 to eliminate 5th & 7th harmonics but I guess 7/9 is ok too. Are you sure you counted the pitch correctly? The difference is only one slot.


RE: Generator Pitch

7/9 seems like an odd pitch for that size generator. 1200 rpm 6 pole at 480 with a 1965 vintage, probably less than 1000kW.

What is the application? Ships service, drill/hoist equipment or propulsion? Most shipboard older marine generators I ran across were usually 2/3 or 4/5 pitch, propulsion service is usually 5/6 pitch (also usually 600VAC but did come across some smaller 480 systems that supplied both propulsion drives and house loads)

Many drill ship units were 3/4 pitch.

Finding a spec sheet for that vintage unit will likely be a challenge, but you might try posting your model and serial number, someone may have a sheet that could cover it.


RE: Generator Pitch

Thanks for the responses. Yes, the numbers have been counted correctly (checked twice). It is a shipboard ship-service generator driven by a steam plant (original SSTG plant aboard a resurrected T5 tanker.) It is 1 MW. I have access to the manual and will get that - however, the data appears to be generic, aside from what we know above.

RE: Generator Pitch


Here is a screen capture of the specifications with model number if anyone can derive anything.

RE: Generator Pitch

Quote (BlackJackJacques)

I expected to come up with a standard value of either of 2/3, 4/5, 5/6

As far I know there is nothing such a "standard value of pitch" of double-layer winding for 3 phase electrical machine.
The pitch can be reduced, full or extended in order to meet the manufacturer's need.
BTW, the pitch value should be written as: pitch 1-14 or 1-15 instead of pitch 14, to avoid the misunderstanding.

Tech Support in Motor Design and Repair

RE: Generator Pitch

Here are some winding factors I pasted from a spreadsheet:

Let's see if pasting these tables works:

=== 1-13 coil pitch (2/3) =====
Total Slots Q1_ 108.000
Poles 2*pp p_ 6.000
Pole Pitch yp Tau_ 18.000
Coil Pitch yc W_ 12.000
Coils Per gp q_ 6.000

nu Kp_nu Kd_nu Kdp_nu
1 0.866 0.956 0.828
5 -0.866 0.197 -0.171
7 0.866 -0.145 -0.126
9 0.000 -0.236 0.000
11 -0.866 -0.102 0.088
13 0.866 0.092 0.080
15 0.000 0.173 0.000
17 -0.866 0.084 -0.072
19 0.866 -0.084 -0.072

===== 1-15 coil pitch (7/9) ====
Total Slots Q1_ 108.000
Poles 2*pp p_ 6.000
Pole Pitch yp Tau_ 18.000
Coil Pitch yc W_ 14.000
Coils Per gp q_ 6.000

nu Kp_nu Kd_nu Kdp_nu
1 0.940 0.956 0.898
5 -0.174 0.197 -0.034
7 0.766 -0.145 -0.111
9 -1.000 -0.236 0.236
11 0.766 -0.102 -0.078
13 -0.174 0.092 -0.016
15 -0.500 0.173 -0.086
17 0.940 0.084 0.079
19 -0.940 -0.084 0.079

===== 1-16 coil pitch (5/6) ====
Total Slots Q1_ 108.000
Poles 2*pp p_ 6.000
Pole Pitch yp Tau_ 18.000
Coil Pitch yc W_ 15.000
Coils Per gp q_ 6.000

nu Kp_nu Kd_nu Kdp_nu Kdp_nu/nu
1 0.966 0.956 0.924 0.924
5 0.259 0.197 0.051 0.010
7 0.259 -0.145 -0.038 -0.005
9 -0.707 -0.236 0.167 0.019
11 0.966 -0.102 -0.098 -0.009
13 -0.966 0.092 -0.089 -0.007
15 0.707 0.173 0.122 0.008
17 -0.259 0.084 -0.022 -0.001
19 -0.259 -0.084 0.022 0.001

==== 1-17 coil pitch (8/9) ====
Total Slots Q1_ 108.000
Poles 2*pp p_ 6.000
Pole Pitch yp Tau_ 18.000
Coil Pitch yc W_ 16.000
Coils Per gp q_ 6.000

nu Kp_nu Kd_nu Kdp_nu
1 0.985 0.956 0.942
5 0.643 0.197 0.127
7 -0.342 -0.145 0.050
9 0.000 -0.236 0.000
11 0.342 -0.102 -0.035
13 -0.643 0.092 -0.059
15 0.866 0.173 0.149
17 -0.985 0.084 -0.082
19 0.985 -0.084 -0.082

(2B)+(2B)' ?

RE: Generator Pitch

Thank you everyone for your time and input. Another recount again yielded 14 and the number of slots covered by each pole, so 7/9 appears to be a solid number. Edison123's comment regarding only off by 1 (15 v 14) for 5/6 was a good path to explore, but the recount again resulted in 14. There does not appear to be a big difference between the two. Can a 5/6 parallel with a 7/9 for continuous operation, and are the regulating devices within their regions of operation such that a 5/6 || 7/9 mismatch can work?

RE: Generator Pitch

Lots of installations out there with different pitch machines in parallel, a lot of " it depends" in a correct answer.

Are the units operating with neutrals connected or are the neutrals floating? If connected, how are they connected?

Are the units grounded or ungrounded?

What kind of loads are you supporting?

Rarely see an issue with differing pitch machines having any issues with their voltage regulators, but not unheard of. Ideally you'd want three phase sensing PMG type excitation systems. Also would be good to know how you are going to do VAR sharing, as different pitch machines VAR sharing in voltage droop or in Cross Current Compensation sometimes won't play nice together no matter what you try, but load sharing systems with active VAR share controls seem to do better, at least in the systems I have worked with.


RE: Generator Pitch


Controlling circulating currents when paralleling generators that shares same neutral is important. Paralleling generators with different winding pitch could lead to circulating currents in a common neutral power system which in turn lead to overheat the winding and false operation of over-current protection.
Hence, triple harmonic can be avoided by a 2/3rd pitch winding (may boost the 5th and 7th) and as Muthu say, the 5th and 7th voltage harmonics can be suppressed by 5/6th pitch winding (may boost triple harmonics (third, ninth etc.)) there are other factors to consider. In 2/3rd pitch generators the zero sequence value can increase the single-phase fault current.

For low voltage machines and a 3-phase 4 wires system, at least one manufacturer consider the best option is to use a 2/3 er pitch winding. When considering a winding pitch change consider that pitch winding change will impact the effective turns/coil of the winding which in turn impacts all flux densities, so it is strongly recommendable to verify and compare the new and actual winding data.

Best Regards


RE: Generator Pitch

Petro/CatServ...Thanks. You guys rock - everyone. The generators are ungrounded and the neutrals are floating. The guidance you all provided is on target. Thanks so much again!


Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


eBook - Rethink Your PLM
A lot has changed since the 90s. You don't surf the Web using dial-up anymore, so why are you still using a legacy PLM solution that's blocking your ability to innovate? To develop and launch products today, you need a flexible, cloud-based PLM, not a solution that's stuck in the past. Download Now
White Paper - Using Virtualization for IVI and AUTOSAR Consolidation on an ECU
Current approaches used to tackle the complexities of a vehicle’s electrical and electronics (E/E) architecture are both cost prohibitive and lacking in performance. Utilizing virtualization in automotive software architecture provides a better approach. This can be achieved by encapsulating different heterogeneous automotive platforms inside virtual machines running on the same hardware. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close