Reverse Engineered Generator Controller

Discussion in 'The Projects Forum' started by el bob, Jan 30, 2013.

  1. el bob

    Thread Starter Active Member

    Jan 8, 2009
    Hello, it's been a while since I posted on this forum.

    I have been asked to "take a look at" a circuit board from a backup power generator and to "see if [the circuit board] can be tested." The backup power generator is not currently working correctly and the people responsible for it (referred to as the "operators" from now on) suspect this control board.

    The operators lost the manuals and documentation for the generator. The service company that is the prime point of contact tried sourcing a replacement circuit board (the board's original equipment manufacturer is still in business) but the generator is so old that the guy from the OEM laughed. I have been told the generator may be on the order of 40-50 years old. I have also been told that the operators suspect the board may be responsible for shutting down the generator if certain conditions are met (like a generator protective shutdown circuit).

    Attached are two pictures of the control board (top and bottom) and my first cut at a complete, but messy reverse engineered circuit diagram. I dug up datasheets and traced each circuit to create this Visio diagram. It took a total of 7 hours to get to this version. I slightly cleaned up the diagram last night, but nothing was electrically incorrect so this is still accurate. I only have this version available to upload right now.

    Given that I was not told (and I'm told they don't readily know ... though I have asked them to look into it) what kind of wires/circuits connect to each of the seven spade terminals on the control board, I am left to try and "back solve" what each terminal connects to circuit-wise. Through backsolving, I believe I have identified the following input and outputs.

    Power Input
    1x +28VDC & 0VDC GND Pair (Two Terminals)

    Signal Input
    1x AC Sinusoid (Two Terminals)

    2x +28VDC Discrete Outputs (Two Terminals)
    1x Output Voltage Select (One Terminal)

    What I am looking for is help understanding the signal processing and Op-Amp processing of the input sinusoid signal. I think I have the SCR output circuits pretty well covered, but the chain of events that starts from an input sinusoid, through what looks like antiparallel clipping diodes, into what I think is an inverted negative feedback op-amp loop (possibly with frequency gain constraints), then fed into a second op-amp loop (what I am considering still "input side" signal processing) before feeding an output signal towards two adjustments circuits with trim pots that influence the two output op-amps and their downstream SCR handling circuits.

    Any help in explaining what's going on here in more detail would be greatly appreciated.


    Last edited: Jan 31, 2013
  2. el bob

    Thread Starter Active Member

    Jan 8, 2009
    Controller photos
  3. elec_mech

    Senior Member

    Nov 12, 2008
    I can't speak to the operation of the board, but I may be able to offer some help.

    What, exactly, is the problem? Is the output voltage too high, too low, fluctuates? Or something else?

    What is the output supposed to be, i.e., 240/120VAC 60Hz, 400VAC 50Hz, 200VAC 400Hz, etc.?

    Is this an AVR (Automatic Voltage Regulator)? These are usually found inside the generator saddle box and help the generator maintain voltage.

    If yes, what is the make and model of the generator? Is it a single-phase or three-phase generator?

    Can you post a picture of the wiring inside the saddle box, if possible calling out which wires went to which terminals on the board?

    If this board is not the AVR and is not located inside the generator, can you post a picture of where it was located? This may help give us a better idea of how it is used.
  4. el bob

    Thread Starter Active Member

    Jan 8, 2009
    elech_mech, thank you for your help.

    Although it may sound strange, I was asked to try and test the circuit board independently of the generator. Independently in this case means both physically separate from the generator and also logically separate from the generator, meaning with no knowledge of the specifics of the generator.

    However, like you, I thought it would be helpful to know more about what was actually going wrong with the generator. I asked and was told that the generator would run but would never get past an output frequency of 53Hz. It is a 3Phase 60Hz very old military generator. My understanding is that 53Hz is too low of an acceptable operating frequency and it is on this basis that the operators are saying that the generator doesn't work.

    The operators took a picture of the wires that connect to this control board, but they do not know where these wires connect to on the other end. I personally do not have access to the generator and so for now I have to continue what I have been asked to do with respect to investigating the possibility that this control board has failed in some manner, testing it and seeing if it can be repaired.

    I did ask and was told that the generator has a separate automatic voltage regulator box.

    I have not been told what the voltage output of the generator is, nor the make or model. All I know is that it's a 40-50+ year old military generator.

    I have not been given any pictures of the generator and have not physically seen it myself. I can definitely understand how it would be helpful to have them. For now, they just want me to investigate, test and make sure that the problem is not this control board.
  5. elec_mech

    Senior Member

    Nov 12, 2008
    Hmm. Frequency is controlled directly by the RPM of the motor or apparatus used to spin the generator shaft.

    The AVR only controls the output voltage.

    If the output voltage seems okay, then the problem is likely with whatever is used to spin the generator. If this is a shaft-coupled gas or diesel genset, then they need to look at the engine speed. If it is a belted motor-generator set, then it could be the motor speed or the belts may need replacing as they wear out and loosen over time which will affect the frequency.

    Do you have any idea what the board they gave you controls? If it is not the AVR, does it control the motor side or something else? They have no idea where the wires go that were connected to the board? Do they know if the wires went directly into the generator, the motor, or something else? Any reason they picked that specific board to be examined?

    While I understand they want to give you a board and have you determine if it works or not, this is going to be difficult to do without knowing what is it supposed to do or what is connected to. I'll try to look at it some more and see if something strikes me, but I'm not sure how much you'll be able to help them without more information.
  6. cork_ie

    Active Member

    Oct 8, 2011
    Elec Mech is correct the frequency is directly dependent on RPM and number of poles in the generator.
    How is the rotor field controlled? ,
    There are a number of different types:
    Very old - static field , current is generated in the rotor and fed to load through brushes
    Old Rotating field fed through brushes - very similar to automotive alternator
    Most newer types < 30 years
    Brushless generators commonly have a rotating field fed from a separate auxiliary winding on the same rotor shaft with built in rectifier. Current is generated in the auxiliary winding when it is rotated in a separate static field winding. Voltage is indirectly controlled by varying the current through this static field winding with the AVR.

    You will need to determine what type of generator you have and the current and voltage supply to the field .
    The most common type is the third mentioned above. This type is often controlled at around 12V DC. I think this is the type you have as I see a transformer on your circuit board. If you supply the generators working voltage to the primary of this transformer you should be able to measure the control voltage at the secondary after the rectifier. This will give you a good indication of the field control working voltage.
    It might also be a good idea to connect a non sensitive load to the generator AC output- eg heater rated at about 50% of the generators output.
    You can initially supply the generator field with 6V DC from an external power supply, then gradually increase the voltage until the generator starts to approach normal voltage at the AC output. This will give you a fair indication of the approximate field control voltage.After that it is a simple matter of measuring the field current at No & Full load. It is almost certain that there is a suitable generic AVR available off the shelf for this generator once you know these figures.
    elec_mech likes this.