Diesel Generator Winding Pitch 4

[Rodmcm]

I understand generally the problems with not matching the winding pitch of generators synchronised together; this is the subject of several threads in this forum. What I do not understand is which pitch is better for an isonchronous load. I have a Cummins white paper dated 2013 that says "Cummins.. consider that 2/3 pitch is best for most electrical loads with a 3 phase 4 wire system" ( which we have) and a Caterpillar paper that says " There are very good reasons to avoid 2/3 pitch" and " the 2/3 pitch is primarily for the benefit of manufacturers not users"

Admittedly the Caterpillar paper is 1993 but is still being circulated.

Comments please

 

[FreddyNurk]

I believe that Caterpillar had a new arrangement for supply of alternators, and they now offer them with 2/3 pitch. Regardless of the technical basis on which arrangement is superior for operation, if you mix 2/3 pitch alternators in parallel with others, then you can expect circulating current.

I suspect that Caterpillar may have accepted that they were losing sales as people wanted to integrate with other manufacturers, and the different pitch of the alternator made this more difficult than it needed to be, though this is only speculation.

Cummins own Stamford Newage (AVK now too?), and as far as I'm aware all their alternators are 2/3 pitch, so it'd also make sense for them to state that its better than anything else as well. This may well be a VHS / Betamax argument.

 

[waross]

I understand that at low load levels there may be circulating currents between generators with different pitches.
I believe that when the load current is significantly greater than the no-load circulating current, there is no longer a circulating current.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[FreddyNurk]

waross, to be honest I'd only really checked on one installation that had two Caterpillars and a Stamford unit, the Stamford unit was fitted with a contactor on the neutral connection in the switchboard, but had been bypassed. I did managed to measure a not insignificant amount of current in the neutral conductor compared to the load on the station at the time, the connected loads weren't that unbalanced either (3 phase YNd11 transformers feeding isolated networks for the most part, a small amount of LV connected).

Everything else I've seen indicates that problems are expected (such as fitment of the contactor on the 'odd' alternator feed), but I've not seen much quantification of it.

 

[Rodmcm]

Thank you gentlemen, your answers revolve around matching and circulating currents, all of which I undertstand. Perhaps a better way of asking my question is if I was starting from scratch with multiple machines why would I choose 2/3 over 5/6 or vice versa....

 

[davidbeach]

Yes, you would. Just like you would have to choose between a PC or a Mac.

 

[FreddyNurk]

For what its worth, I don't see much of a reason to not go 2/3 pitch. Caterpillar may have been correct in their assertions that their winding pitch is superior for various reasons (it's been a while since I read their documentation) but since most of the suppliers on the market do 2/3 pitch (including Caterpillar who now use Leroy Somer as I understand it), I'd tend to do the same.

 

[catserveng]

Well, there are still a lot of non 2/3 pitch machines going into service daily, not only with CAT tail ends but generators from Stamford (Cummins), Baylor (now National Oilwell Varco), Kato and ABB and others I'm sure.

2/3 pitch is pretty much the standard for most electric power generation systems, unless you operate equipment like oil drill rigs, mud pumps, paper mills or anything else with mostly large drives as loads. ABB as its standard ships 5/6 pitch machines, with 2/3 the option, this is from ABB's tech paper,

Varying winding pitch to influence harmonics
The winding pitch of generators is shortened from full pitch
– typically reduced to 5/6 or 2/3 – in order to change the
harmonic content of the output waveform. At the same time
the fundamental component of the output voltage is slightly
decreased as well. The reduction in output voltage is then
compensated by an increase in field power. A lower pitch
winding requires more field power compensation than a higher
pitch winding. The result is a decrease in efficiency.
The cross section diagrams show 5/6 and 2/3 winding pitches
in 4-pole, 48-slot generators.
ABB’s HV generators up to 6 MVA are engineered to meet
users’ needs for compact generators in this power range.
The generators are therefore offered with 5/6 winding pitch
as standard, as this generally enables compact overall sizing.
This is because 5/6 winding pitch minimizes the 5th and
7th harmonics, which usually increases overall efficiency.
Higher efficiency enables more compact generators
to be constructed for the same output power.

This comes from some old course content on the subject,

One of the design considerations in selecting an appropriate
pitch factor is the harmonic content of the generated voltage
wave form. Pitch factor can be used to reduce or eliminate
specific harmonic frequencies in the generated voltage wave
form as follows:
A. Full Pitch: A full pitch will have no damping effect on any
harmonic frequency.
B. 2/3 Pitch: A 2/3 pitch will eliminate the third harmonic
and subsequent triplens i.e.: 9th, 15th, 21st, 27th, ……. etc.
C. 4/5 Pitch: A 4/5 pitch will eliminate the 5th harmonic.
D. 6/7 Pitch: A 6/7 pitch will eliminate the 7th harmonic.
E. 5/6 Pitch: A 5/6 pitch will:
1. Minimize the 5th harmonic, but not eliminate it as will a
4/5 pitch.
2. Minimize the 7th harmonic, but not eliminate it as will a
6/7 pitch.

The CAT paper is old and written when the majority of tail ends sold by CAT went into oil field service. 2/3 pitch is now a larger part of the market for them, and pretty much consider the "standard" for most generator companies I now of.

I recently did a startup on a land based drill rig system with CAT driven KATO oil field generators, those were all 3/4 pitch machines. Most of the Baylor tailends I see are also 3/4 pitch (and all of these at 600 VAC rating)

Cummins has a couple of white papers on the subject, and if you actually carefully read thru them all while they lean towards 2/3 pitch as being standard they recognize (and build and sell) machines of other pitch configurations.

Hope that helps, Mike L.

 

[Rodmcm]

Thank you for that