Generator Windings 3

[rockman7892]

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?

 

[itsmoked]

The multiple windings allow the the generator to be "reconnected" for different voltages.

Yes, varying the 7.2A will vary the generator's output. That's the voltage regulators job.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[rockman7892]

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?

 

[itsmoked]

I believe that was a permanent magnet generator. That means the field coils are more like trimming control. I presume you could oppose the fixed field as well as enhance it with the field coil.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[rockman7892]

If I understand correctly you are saying that the magnetic field is created by a permenant magnet somewhere in the rotor? How can you tell this from the drawing? By trimming you mean that you can add to or subract from the permenant magnet field with the field coils? Meaning the excitation can increase or decrease the permenant magnet field?

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?

 

[davidbeach]

Two winds in series for 480V, two windings in parallel for 208/240V.

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.

 

[niallnz]

The permanent magnet generator provides the power for the voltage regulator. The regulator converts the PMG output to a variable DC voltage which it uses to excit the main field generator. The main field generator is separate three phase generator on the shaft, and its output goes directly to retifier dioeds that are also mounted on the main shaft (rotating diodes), and it is the output of this rectifier that provides the main generator excitation. The excitation system and diodes are not shown on you drawing. The current of the main field will be much higher than the output of the AVR, I guess it will be around 150Amp at full load.

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

 

[rbulsara]

For starters, the drawing you are looking at is not intended to tell you how a generator is designed internally, rather it indicates how to connect it to rest of the system. The drawing indicates only externally accessible terminals necessary to complete the power, sensing and control wiring.

The actual machine is even more complicated than the drawing shows, as many before me tried to explain to you.

 

[waross]

Actually the PMG is an add on on the outside of the case at the tail end of the shaft. Most PMG machines may be operated with a conventional AVR if need be. I have done this a couple of times while wating for replacement parts. Without the PMG there will be more voltage dip when energizing transformers and starting motors. The fault current may be much lower but that can usually be accepted on a small generator. The same machine is probably available without the PMG option and will not have problems. Voltage collapse and low fault current become important issues when you work on the really big stuff like DavidBeach and ScottyUK.

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

 

[davidbeach]

Bill, what you say about PMG placement is probably true for larger units, but for gen sets with brushless excitation (three machines on one shaft), I've never seen it outboard. Diodes are always the last thing on the shaft as they are what is most likely to fail and need the most cooling.

 

[waross]

Hi David. I have seen similar PMGs from about 30kW to 1200kW.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[rbulsara]

Visit Kato's website and there is some useful information on the basic block diagram and construction of PMG exciter and alternators.

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.

 

[waross]

Thanks Rafiq. I'll check out that site this evening.

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

 

[rbulsara]

Bill:

If you can get out of your local generator rep, CAT's service manual has good diagrams.

 

[rockman7892]

So without a PM on this generator we would need some other way of supplying power to the VDC and therfore excitation generator. How is this typically done without a PM? Batteries maybe? With an external source I would guess that you would then need brushes.

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?

 

[waross]

Generators "bootstrap" or build up the field from the residual magnetism in the field. The AVR takes available voltage from the generator output terminals and feeds it to the field of the brushless exciter. At start-up this may be only 4 or 5 volts. However, this slightly increases the field and as a result the output voltage rises. This lets the AVR send a little more voltage to the field and the voltage rises some more. As the voltage rises, it starts to respond faster and faster. It only takes a few seconds for the voltage to build up to full voltage. If you are watching the voltmeter, you can see the voltage buildup on the meter.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[rockman7892]

Waross

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.

 

[waross]

You have it on voltage buildup and are pretty close on loading.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[rockman7892]

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?

 

[rbulsara]

rockman:

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.