Forums : Your generator pages are an excellent bit of teaching.

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Your generator pages are an excellent bit of teaching.

lumberjack

L
Sun Oct 14, 2007 1:04 am
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But I do still have a question.
I just bought ($1200) a GMC (Detroit diesel) 2-53 motor hooked to what I am told is a Delco 3-phase generator (alternator) head. It has four wires coming out, plus two wires from a separate exciter going in.
It is similar to this
I understand that they were used as refrigerator power plants for rail boxcars.
The nameplate on my motor and other supporting information indicate that the motor is supposed to spin at 1200 rpm.
When I start the set, the engine spins at 1700 rpm. When I throttle it back >1500rpm, it seems to run much smoother... everything suggests that it was built for slower speed.
My question is; my understanding of three-phase power is that it is generated by six separate coils, in sets of two, thus each revolution produces one full wave on each of three phases.
How can a 1200rpm generator produce 60hz power?
Also, since the generator head only has three hot leads outside the head, can it be modified to use the zig-zag setup? The motor guy I talked to suggested that it is impossible unless the generator has 12 leads.
I have questions about the readings I get from the generator, but first I need to understand whether it's supposed to be running slower.

AnOldMan

A
Wed Oct 17, 2007 7:39 pm
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Most generators run at 1800 or 3600 rmp... 60 revolutions per second times 60 seconds equals 3600 rpm - and with ONE coil pass per revolution you would have 60 cycle; double the number of coils/fields and you get two passes per revolution - so you can run at 1800 rpm; TRIPLE and you get 1200 rpm.
Most gasoline motors have a power curve that needs higher rpm's to reach full horsepower - that's why they usually run at 3600. (this goes for LPG and propane as well) Diesels have a lower rpm power curve, so they usually run at 1800 or even 1200, 1200 requires a more complicated coil arrangement though.
You can re-wire zig-zag, you just have to be willing to open the generator head and determine the configuration. Even with only four wires leaving the head, the coils themselves must exist and must be inter-connected inside the unit.
I helped a gentleman in the past with such an arrangement, his coils were on the rotor with four commutators. The three coil sets commoned out at the commutator, so it was a simple re-wire.

lumberjack

L
Wed Oct 17, 2007 11:54 pm
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anoldman wrote:
Most generators run at 1800 or 3600 rmp... 60 revolutions per second times 60 seconds equals 3600 rpm - and with ONE coil pass per revolution you would have 60 cycle; double the number of coils/fields and you get two passes per revolution - so you can run at 1800 rpm; TRIPLE and you get 1200 rpm.

Given the above, wouldn't all three phase generators require 1200 rpm? In other words, would there be no such thing as a 3600rpm three phase generator?
... or does a three phase 1200 rpm generator have 18 coils inside?

AnOldMan

A
Thu Oct 18, 2007 6:38 pm
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You still have "six" coils (it's really more complicated) but you have more than one EXCITER FIELD interacting with those coils on each rotation.
On a really, really simple generator you can count the field plates on the stator and rotor (just like in a simple DC motor). Your generator could have 54 or more such plates.
Here is a complicated motor/generator with 24 stator coils:

The horseshoe shapes in the violet area represent the field coils of about 1/4 of the stator. The rotor is in light blue, the rectangles facing the stator represent IT'S field coils. All of the hairy lines represent the magnetic field interaction.
Simple example with magnets: If I rotate one magnet past the coils per revolution, my cycles equal my rotation. If I mount two magnets 180 degrees apart, I get two cycles for every rotation, ect...
RPM's are mostly a function of the power curve of your engine. The generator head is designed to produce the desired frequency at the target RPM rate.
If I had to have 3600 rpm for 60 cycle, imagine the RPM required for a 400 cycle generator !