Low Excitation Voltage in Self Exciting Generator 2

[wjd7]

I have a 20KW diesel generating set with a problem maintaining constant output voltage. The generator is supposed to put out 120/240 volts, but sometimes puts out 100/200 volts and sometimes does not generate at all. If I boost the excitation circuit with external power it will start generating again, but it is unreliable. Could this be a problem with residual magnetism? Can you recommend a device to boost the excitation voltage so that the proper output voltage is maintained?

 

[waross]

There are some self excited units that use capacitors. You may have a capacitor going bad. Adjust the voltage by changing the value of the capacitors.
There is another type of self excitation that rectifies the output of a constant voltage transformer to supply the basic exitation. There is a resistor to set the no load voltage. There is also a current transformer. The output of the current transformer is scaled with a couple of resistors and rectified. This voltage is added to the basic exitation voltage to provide load compensation. The voltage regulation was 5%. There was provision to add a standard AVR to improve the regulation to 1% as I remember.
This was a new 500 KVA Genset.
I'm sure there are a bunch of types of self exited generators that I am not familiar with but am about to learn. All but 3 of the last 40 sets I was involved with used standard AVRs. 2 of the three were 8 KW sets as I have described.

 

[itsmoked]

Often if the rotor has slip rings with brushes they can wear out then get flaky. I have also seen the rectifiers and power supply that sometimes is on the rotor have circuit board problems. What I'm saying is that you don't need some "booster", you need to figure out what's busted.

 

[wjd7]

Thank you both for your helpful and quick responses. The system is very simple. There are no capacitors. The unit is new so the commutator and brushes for the excitation circuit are good and not dirty. The system has a rectifier and a voltage regulator on the excitation circuit. I changed the rectifier thinking that this was the problem, but it did not change anything. The voltage regulator has a small adjustment screw (I assume for a resistor), but this only varies the output voltage by a few volts. The voltage from the excitation winding is about 20 volts. I believe that 30 volts of excitation will get the needed 120/240 volts of output. I will have to verify this. All of this is with no load. If I introduce a load on the output side, could this possibly inthuse more current into the excitation circuit? Would it help if I posted pictures of the parts? Thanks again for your help.
Bill

 

[waross]

Pictures please, and make, we may be able to find some information on-line. If you have a standard exitation system, I suspect failure of some part. If it's a non standard system, I love a challenge.
yours

 

[itsmoked]

A load could be important.. even a 100W drop light.

 

[waross]

You mentioned a comutator. Is it a comutator or slip rings?
Do the brushes carry the field currrent or the load current.
yours

 

[wjd7]

Thank you. Here is a link to pics of the parts in the excitation circuit.

http://www.members.aol.com/wdamico

I am not sure if this is standard or non-standard, but it seems to be very simple. I believe that this is a comutator not slip rings. The only current that the brushes carry is the excitation current (field current?). The rectifier tests OK and the excitation winding does not test open or burned. This is a Chinese made machine. There is not much info online that I have been able to find. It is made by Jinlong. The part number of the generator head is ST-20.
Thanks again, Bill

 

[itsmoked]

I can confirm "SLIP RINGS".

Looking at the schematic I cannot understand why one of those leads (AI) from the AVR goes to the GEN on the way to the field rectifiers? There are already leads coming from the GEN's output that can feedback the GENs output voltage and supply excitation. ?? A low voltage auxiliary winding?

At any rate I have seen bad AVRs take out generators probably 10 to 1 over any other malfunction doing it. So I always suspect the AVR first. Can you get a replacement? Frankly you should have one always on hand if the generator is important.

 

[waross]

Hi wjd7 and itsmoked
You can try connecting a 12 volt car battery across the brushes to exite the field. If your generator output is less than half rated voltage, you can go to 24 volts.
Many small generators will take double or more voltage on the field between no load and full load so don't be surprised if the voltage drops when you put a load on it.
The car battery (or two) should give you a steady output. Disconnet the existing regulator first. Put a load on the set if you can and watch for signs of trouble. Fluctuating voltage, fluctuating current, over heating, smoke.
Re; the voltage regulator. Most of the sets that I see in this size range have a brushless exciter. The voltage regulator supplies and controls the field of the brushless exciter. The brushless exciter supplies the energy for the main rotating field.
This looks like it is feeding the field directly. That is probably more power than standard AVRs can handle.
I suspect that the winding is a dedicated exitation winding and the regulator is controlling the current to the rectifiers and to the field.
The point of this is that it may not be wise to replace the regulator with anything but an original part.
A number that sticks in my head is 16 Ohms. Many AVRs have a warning notice that the LOAD MUST BE 16 OHMS OR MORE. This is the load on the AVR, or the field resistance.
It's possible that a diode or SCR is blown in the regulator and it's working on half wave.
Like itsmoked said probably the regulator.
A couple of other things you can try;
Measure the voltage across the voltage regulator output. Zero volts or close to it may mean that the regulator is trying it's best but there is a problem with the field or the dedicated winding in the generator.
If there is a fairly high voltage across the regulator and the generator voltage is low, the regulator may be bad. A lot of regulators without Under Frequency Roll Off (UFRO) have a tendency to fail if the're operated at reduced frequency. If you don't have a 50Hz./60Hz. jumper, connection or setting of some sort, you probably don't have UFRO.
Try the battery(ies) and measure the voltage and tell us what you find .
yours

 

[ccjersey]

http://www.smokstak.com/forum/showthread.php?t=23389&highlight=chinese

You aren't the first one to have this problem. Maybe you can find some help in this thread.

 

[wjd7]

Thank you everyone. Previously I tested another regulator and rectifier in attempt to solve the problem, but this did not solve the problem. Maybe the new regulator and or rectifier were faulty also. However I don't think the rectifier is bad because it tests OK. There is a small auxilary winding in the stator that is rectified and regulated. It is my understanding that this current goes into the rotor thru the brushes which excites the rotor and in turn produces the main output voltage in the stator. I will preform the tests that you suggested and post the results. Thanks again for your help.

 

[itsmoked]

You could disconnect the aux winding and check its output.

 

[wjd7]

Everyone: Thank you for the info and links. Now that I have looked at the links I seem to know more about what I am dealing with (an ST type generator). It seems that some people dont use a regulator with this type of generator. Some of the information is a bit technical for me. I understand the basic concept of a resistor. Some of the wiring diagrams that I have been able to find show a resistor in the excitation circuit and some do not. Please confirm that the resistor (assume variable) shown in some of these diagrams would be the avr unit. The pictures that I posted via an earlier link show the wiring diagram sticker on the avr unit that I have. This appears to be different that the diagrams I found online. So I am still a bit confused about weather or not I could safely remove the avr from the circuit to see if it is causing the low voltage.

 

[ccjersey]

The set can have Manual voltage control with a variable resistance or have Automatic control with the AVR. I wouldn't jumper the control out due to the posssibility of overvoltage at rated speed from NO resistance. If you want to try it out of the circuit, you will have to have no load and run the engine slow and slowly increase speed while monitoring voltage. I'm not sure what this will tell you anyway, you know the set will perform, sometimes.

Now you ned to find the source of the intermittent problem. There have to be some diodes somewhere to rectify the excitation winding's AC output to DC before it can be fed through the AVR or resister and into the field. I would bet on the AVR or the diodes being the problem.

The diodes can be checked with an ohmmeter or battery operated test light. Basically they should allow current to flow in only one direction. they are usually arranged in "bridges" of diode pairs so the DC output has less "ripple" so you may have some identical looking diodes with the opposite polarity. What is important is that each one has the "one way" function and isn't either open or shorted so it conducts both ways.

If the set needed "flashing" to restore residual magnetism/voltage, the symptom would be not making voltage at start up, not stopping generating after a successful start and voltage buildup. The flashing process as waross describes can determine if the basic alternator (rotor and stator) is functioning since it supplies excitation current from an external DC source. It would also restore the residual magnetism if that was the problem.

 

[wjd7]

ccjersey -
Thanks for your help. Ignoring everything else, the overall problem is low voltage on the output with no load. I have connected a small load, but this does not make a difference in the voltage. So the unit is working but not at full potential. One part that I do understand is the rectifier and how it works with the arrangement of diodes to convert ac to dc current before going into the rotor to provide the excitation. In the link below are pictures of the parts in the circuit posting.

http://www.members.aol.com/wdamico

The rectifier tests OK (one way current only), but maybe the problem with the rectifier is intermittent or it tests OK with an ohmmeter but is bad under a real load. I thought that a diode is either good or bad but I am no expert. I know that a diode can burn and fuse allowing current to flow thru unrectified, but I am not sure how common this is. What I don't understand is the AVR unit or box that has the wiring diagram on it. It has a small opening with a adjustment screw that can be turned to fine tuning the voltage, but the screw will only vary output by a couple of volts. Since as you suggest, this AVR may be the problem, I am trying to figure out what is in it. I guess I will have to take it apart. What is confusing to me is that you said it could be "Manual voltage control with a variable resistance or have Automatic control with the AVR"
Mine is supposed to be automatic so why would there be an adjustment screw?
If it is a simple resistor, I could replace it with another one. I would think that I could determine its value and use a new one from radio shack or another supplier. From what I recall about resistors, they have color coding on them that can be used to determine their value. This is assuming that it has markings on it. What do you think? Some people are saying that they do not use an voltage regulator with these units, but as you say, they would have to use a resistor or risk burning the circuit. Is a voltage regulator the same as a resistor? Can you see why I am confused?
Bill

 

[itsmoked]

wjd7; Just because it has an adjustment doesn't mean you don't have and automatic voltage regulator. That is what AVR means... Automatic Voltage Regulation. The adjustment is just for compensating for the differences in materials, construction, and parts values. So forget about the "manual" adjustment nonsense. You have an AVR.

Can the AVR be disassembled or is it potted?

 

[edison123]

wjd7

Can you measure the voltage across slipring brushes and check whether it matches with the rated voltage on the nameplate ?

 

[ccjersey]

I would say if you haven't disconnected the parallel bridges, you will not know whether there are some that are open or as you say if some fail under load. It also might reveal a poor connection somewhere. A diode bridge like that is commonly available from places like Mouser or Digikey.

http://www.mouser.com

The manual control which you don't have is where you start up a load and then go look at the voltmeter and adjust as necessary. It works ok in some circumstances (like this type of excitation circuit) and is/was required for fail safe operation on ships etc as a backup for the AVR I believe. I have a 1944 BUDA 3 phase generator with GE controls and it is set up so you can either use the regulator or the manual control.

If the alternator is good, you can purchase an aftermarket exciter regulator from companies like Basler, Delco? or Power-tronics and others which will replace all the excitation winding, rectifiers and regulator. They may even know exactly what you need since there are a lot of these sets coming into the US now. If not you will need a full load field voltage and amperage specification to get the right system.