Generator Windings
Generator Windings
(OP)
I was involved in the startup of a 480V generator over the last few days and used it as an opportunity to learn more about how generators work.
The one question I cant seem to figure out about the generator that I have is why there are two sets of windings on each of the ouput phases as shown no the attached drawing. Why are there two sets of windings connected in series for each phase as opposed to one winding?
Also the generator exication was rated at 32V and 7.2A. Does this mean that the voltage regulator outputs 7.2A DC to the field coils in the rotor? Will varying this 7.2A output change the output voltage of the generator?
The one question I cant seem to figure out about the generator that I have is why there are two sets of windings on each of the ouput phases as shown no the attached drawing. Why are there two sets of windings connected in series for each phase as opposed to one winding?
Also the generator exication was rated at 32V and 7.2A. Does this mean that the voltage regulator outputs 7.2A DC to the field coils in the rotor? Will varying this 7.2A output change the output voltage of the generator?






RE: Generator Windings
Yes, varying the 7.2A will vary the generator's output. That's the voltage regulators job.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Generator Windings
So I'm assuming that increasing the excitation current will produce more of a magnetic field in the rotor coils and will thus induce a larger voltage on the output of the generator?
RE: Generator Windings
Keith Cress
kcress - http://www.flaminsystems.com
RE: Generator Windings
Back to your first response, would the being able to configure the coils for different voltages be similar to the various windings on a 6 or 9 lead motor?
RE: Generator Windings
The PM generator is separate machine on the same shaft and is the source of the field of the main machine. PM generator feeds the AVR and the AVR feeds the main field. Often there are actually three machines on the same shaft so that there is no need for brushes.
In the three machine case, the PM generator (Machine 1)feeds the AVR. The AVR then controls the field of machine 2, which is built inside out, with the field on the frame and the "stator" on the shaft. The output of machine 2 is rectified and fed into the field of the main generator (machine 3). That way there is no need for brushes. The PM generator is usually in the middle with machine 2 at the bearing end so that the diodes can get the most cooling air.
RE: Generator Windings
Generators like motors are sold all around the world, so they are made to be configurable for all sorts of voltages, hense the number of windings.
Cheers
RE: Generator Windings
The actual machine is even more complicated than the drawing shows, as many before me tried to explain to you.
RE: Generator Windings
Bill
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"Why not the best?"
Jimmy Carter
RE: Generator Windings
RE: Generator Windings
I agree with you about the diode plate being the last thing on the shaft INSIDE the case. The PMGs that I see are two or three inches thick and about 8 inches to about 14 inches in diameter. They mount on the back end of the shaft, outside the case. In the case of the suppliers that I am familiar with, the PMG option is readily recognizable by the "bump" on the back end of the machine. The PMG must be removed first.
But, with all the different strategies that I have seen for generator construction, I probably just haven't yet seen the arrangement you describe.
We may be describing different things.
When I look at the specs of a standard generator it will have an AVR that is powered from the output of the generator. If I order the same machine with the PMG option, the PMG is added to the end of the shaft outside the case and the AVR is replaced with an AVR that has a front end matched to the output of the PMG. Often around 220 V, three phase. I still surprises when I see a machine from a manufacturer that I am unfamiliar with.
Bill
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"Why not the best?"
Jimmy Carter
RE: Generator Windings
Alternator upto 3MW that I have used have PMG exciter and diodes/varisotr assembly on the same shaft, but "inside" the case, but still out side the main stator, but technical it is still "outside" of the main alternator body but under the same enclosure.
RE: Generator Windings
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Generator Windings
If you can get out of your local generator rep, CAT's service manual has good diagrams.
RE: Generator Windings
I guess the advantage of having a PM generator is that you can always have a "Black Start" because you are not relying on any outside source of power to the generator.
We load tested the generator today and the load test called for 100% kW loading for 4hrs. There was no reactive load test done on the unit therefore we only put about 75%-80% load on the unit. (unit has .8pf)
Is there an advantage for using reactive load testing as well other than just putting more of a load up to 100% on the generator?
RE: Generator Windings
A PMG supplies the voltage to the AVR so it doesn't need to bootstrap.
Testing; The KVA rating is usually determined by the thermal capacity of the generator. Rather than looking for a reactive load, you can test at full current by lowering the voltage. If you drop the voltage to 80% of rated voltage, the prime mover will have enough power to drive full rated current through the generator. (Asuming a PF rating of 0.8)
Bill
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"Why not the best?"
Jimmy Carter
RE: Generator Windings
If I understand your "bootstrap" explanation properly, after a genertor is shut down there is some residual magnetism remaining in the generators rotor. When the generator is started again this residual magnetism creates a small voltage at the output of the generator which then in turn feeds the AVR, then to the exciter gen, then back to the rotor field. This process continues until the generator ramps up to full voltage.
So with this being said, any generator even without a PM can be started without any external excitation or brushes due to the fact that there is always some small residual magnetism remaining in the field.
As far as lowering the voltage for testing, I guess your saying that lowering the volage by 80% will increase the current by 25% in order to maintain full output kW of the generator. Although we will not be at rated KVA we will be at the thermal limits of the generator due to the increased current.
This particular unit does not have any external controls for the voltage regulation, so I guess you'd have to somehow gen inside the AVR in order to adjust the voltage.
RE: Generator Windings
If you set your load bank above the rated kW, you can expect to overload the engine and slow it down.
Lowering the voltage does not in itself raise the current, it drops the current. But, by dropping the voltage you may then increase the loading with the load bank proportionally without overloading the engine.
There is usually a voltage adjustment on the AVR board.
Bill
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"Why not the best?"
Jimmy Carter
RE: Generator Windings
With a fixed kW load, wouldn't lowering the voltage cause the current to increase in order to supply the necessary kW to the load?
If you only have a resistive kw load cannont you not increase the load above the rated kW of the generator since no reactive protion is present. For example, for a 750kW generator and a pf of .8 the max current will be 1128A at 480V.
A 750kW load will only only consume about 900A. So with a pf of 1 could you not increase the kW loading to 936kW which would corrospond to this current of 1128A at a power factor of 1?
RE: Generator Windings
You cannot exceed either of kW or KVA ratings. kW is related to the prime mover rating which only provides kW (real power). kVA is the limit of the alternator, which also provides reactive component necessary for magnetic loads.
So a 750kW generator set is a 750kW set, you can not expect it to provide 936kW. Otherwise it would be rated as such.
What it does say is that you can have a power factor as poor as 0.8 or better at the rated kW load. Or any other combination of kW and kVAR (which relates to power facotor) as long as you do not exceed either the rated kW or rated KVA.
RE: Generator Windings
RE: Generator Windings
Dropping the voltage does not increase the current of a resistive load.
You can test the prime mover with a resistive load of 750 kW.
The KVA rating is a reflection of the heating of the set caused by the load current. It is rated Amps times Rated Voltage (Of the generator end only, not the complete set.)
So, at 480 Volts, the rated current will be:
750 kW / 0.8 = 937.5 KVA
937.5 KVA at 480 Volts three phase will be 1128 amps. So far so good.
Now, if you drop the voltage to 480 V x 0.8 = 384 Volts and load the set up to 1128 Amps, you will have a load of 750 Kw.
Your prime mover will not be overloaded and you will be able to verify that the set can deliver 1128 amps without overheating or tripping the protection.
This will cover most of the issues that you want to test on a set in that size range.
Bill
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"Why not the best?"
Jimmy Carter
RE: Generator Windings
Would dropping the voltage increase the current if there were reactive loads involved?