# Generator - Double Delta?



## LightGuy48 (Oct 14, 2007)

Just curious if anyone in the forum has experience with various generator winding configurations?

I was out at one of the sites I maintain that uses a single phase 120/240v service. The generator is a 12 lead 3 phase unit, when it was installed it was wired as a 4 wire delta with the high leg unused to provide the necessary 120/240v service for the building.

It was suggested to me by our generator contractor that it would be better to convert the 12 lead unit to a double delta configuration.

Is there a compelling advantage to running double-delta over a 4 wire delta? I've also seen some 12 lead diagrams using a zig-zag configuration for single phase use (not to be confused with a zig-zag or dogleg transformer).

Is there any significant difference between these three (DD, Delta, and ZZ) configurations to obtain a 120/240v supply?


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## micromind (Aug 11, 2007)

The way it's connected now, you can use very little of it's capacity without serious long term damage to the stator. Delta connections are designed for 3 phase loads, with a very small allowance for 120V loads. 

A zig-zag connection will allow a 3 phase gen to produce single phase (only) power at 100% of it's capacity. 

A genset like this one is comprised of 2 units bolted together, a generator and a prime mover. If the prime mover is a diesel engine, you've got to be careful sizing and connecting it. The generator end doesn't care what it's connection or load is, as long as it's not overloaded. A diesel, on the other hand, cares alot. It doesn't like to putt around with not much load on it. It likes to work. Somewhere between 50% and 80% of it's capacity will result in longest life and fewest maintenance issues. 

If you're pulling 50% single phase load from a 3 phase delta, you'll burn up the stator in fairly short order. If you're pulling 50% load on a zig-zag, you'll have the happiest genset anywhere.


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## brian john (Mar 11, 2007)

Simple enough to do suprised it was not done from the get go.


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## LightGuy48 (Oct 14, 2007)

Thanks, what about double-delta, I've heard that ZZ has fallen out of preference and DD is preferred because it's symmetrical whereas a ZZ is not.

I checked a newer single phase unit that was wired from the factory and it was in fact a three phase unit wired double delta. I don't have a picture of a DD configuration but I do have a link to a PDF showing a Marthon double-delta wiring diagram: http://www.marathonelectric.com/cgi-bin/nph-drawing.cgi/14000_1125.pdf

It appears from the research I've done that the configuration would be:

L1: 1 & 11
L2: 9 & 10
N: 3, 4, 5, & 7
floating connection: 8 & 6
floating connection: 2 & 12

I just wish I could find some official documentation on what the pros & cons are to each configuration, something with some technical data.


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