I was merrily testing my converted trommel yesterday, and after a few minutes, the machine (one 60hp-480/60/3 ()on a soft start unit) pump drive motor, and a 3/4 hp cooling fan motor), everything got quiet. I did not have the machine actually in action, just the pump and fan running. After about 10 minutes, the genset just went dark. Cummins power never blinked and starts/runs fine. Just won't make any power. Any phase. Well, I can get 25 volts or so across the 480 buss. Three are three fuses in the load sense circuit that are good, the main breaker did not trip, there is one 8 amp fuse on the AVR board....all good. It had not the time to get hot. I have run this setup for about 40 hours in the last few weeks without issue. The only help the troubleshooting section of the manual offers is to polarize the windings which may have lost the residual magnetism. Can't be that, it quit generating while operating normally. I've seen that on older units after they set unused for a long time. Sooo, looking for ideas on where to start digging. I can't see dropping a main winding taking all three phases out but not checked yet. Can't imagine losing an exciter diode with no load. I do have the original books, with diagrams, they are just lacking any how-to-get-to-it and component locations. I know where the AVR board it, but no testing info. Any volunteers?
I know this sounds simple and crazy but is the selector switch door closed ?? And the door safety switch is working. I've actually had that scenario cause service calls more than once. Door not closed or bad safety switch will kill power generation.
Disconnect all loads. Remove the field wires from the voltage regulator. Fire it up and put independent 12 volts DC or 24 volts DC to the field wires, with the proper polarity. You should see output voltages jump up to something near 50% normal, or more. If output voltage is pretty good, the problem lies in the voltage regulator itself, if it is seeing good AC voltage in from the generator and still not working. Another way to gain some info is to fire it up as-is, make sure the 25 volts is making its way back to the voltage regulator, and see what kind of DC voltage the VR is putting out on its field terminals. It should be putting out some meaningful DC to try and get things going, if not, again it's the culprit.
Do you have the key start model or the fancy pants unit with the digital display ?
Have you checked the phase selector switch? Does it produce in 240?
OK....in order: Tx ....first thing I checked. It's fine. This model throws the main breaker "remotely" when that door is opened in use. It's properly closed and was when the failure occurred. Birkin ...Have not done that test, but can. Book says first test is to apply V to the exciter coils, like we used to polarize generators. That really only should apply to a unit that has sat long enough to lose the residual magnetism in the iron. This quit under load. Had been running fine so not suspecting a mag loss. Have replaced the AVR, since it was cheap enough. No change. Will check for the 25 V on that circuit. Tx ...it's the autostart model. No key. No issues with the engine at all. fwf ...it produces nothing (well, about 16 Vac) in any mode. Single or 3 phase. That's why I chose to replace the AVR module. I suspect the diode bridge(s) in the power end. If I'm not mistaken, that's where the 25 volts comes from. Need to study the schematic a bit. Thanks all!
I recently had a Doosan come through that had one broken eye in the generator junction box, of course it was under everything else. Crimped a new eye and everything was good after that. Had nothing, not even 120
The DC voltage application test is a way to magnetize the coils, but it also tests the functionality of the whole generator. If it works like I said, the generator is good and the AVR is the problem, if you have ruled out dumb wire connection issues and such. It is a real mechanics test and not just a shotgun approach. If not, then you go digging for diodes and such.
Rotating rectifier possibly ? We replace a few of those a year on the multi quip units. I would say wiring a more likely cause but just throwing out ideas
You do the battery excitation test first because it is easy and demonstrates function well, then work your way in.
Birken, I'll try to get that test run tomorrow if nothing urgent pushes it back. I was struggling to get a single genset up out of about a dozen and got a 175 kVa up first. The others are fill in work more than anything. I've already gone over the unit for obvious faults, with the likelihood of finding arc evidence in mind since it quit under about 80 amps load. Found nothing that stood out. I'm not one for shotgun repairs, but it was a time vs money decision, and it seemed to be a reasonable gamble since the output was actually present, but only a few volts. I paid less for the AVR than an hour's work, so I figured I'd have a spare on hand if it was not the culprit, since we have a few of these Doosans (Leroy Somers) generators. Sooner and later I'm sure I'll need it. Tx, that's what I plan on checking pending the results of Birken's tests. I'll post results when I get something to report.
Correction: It quit with ~27 amps load (machine idle, no hydraulics active. Pump draw while holding 1,000 psig hydraulics, after half hour running at 80-85 amps, about what it runs with no material in it being screened. There shouldn't be an overcurrent cause here. Birken , the 25 volt return...how clean does it need to be? I can look at it on a scope. If it's not clean could that indicate possible faulty rotating bridge rectifiers?
I can't speak with authority on this, but I don't think a faulty rectifier is going to show up on the AC output in any way. There is so much inductance in the main field coil, one bad diode is not going to be able to vary the current flow 60 or 180 time a second or more. A lot of exciters the AC they generate is higher than the main generator so it is easier to rectify. All it will do is make it weak and not responsive to input. The 25 volt return I assume you are talking about the residual anyway. I am not sure that is coming from the exciter or main field wires at all. It could just be the magnetism of the main rotor iron doing that. I have drug scopes out into the field and never had one tell me anything useful yet, so I just use my DVM. Just put 12 or 24 volts from a separate battery into the main F+ and F-, and you should get a substantial AC voltage on the output, and also draw a DC arc when you disconnect the battery. If it fails this test, start looking at diodes, wires, and such.
Ok Birken, putting 12 VDC on the exciter I do get ~180 VAC on the mains out, with VSS on 480 still, if that is even part of the circuit now. Measuring the AVR on the exciter terminals on the board itself, I get ~26VDC. What’s that telling me?
You mean that the board is putting out 26VDC with no load/open circuit? Probably meaningless I would say. If was able to supply current and put out 26 VDC into the exciter, it should be doing 360 or better on the main output. I would put it all back together and see what the board gets in AC and puts out DC with residual AC first. If DC is low, put 12VDC back on it from the side with an isolated battery, with the board still hooked up, and see if the 180 volts is making it all the way back to the board. If the board is not putting out DC from the AC it gets, it is no good. Also you need to check if it has an auxiliary winding or PMG for regulator power, if that is all working as well.
Correct, the AVR is putting out 26 VDC on the + & - exciter terminals, no load on gen, exciter leads fed correct polarity 12 VDC when d/c from the AVR. When I connect them to the AVR, where there WAS 26 VDC, there is no generator output. It acts as though the AVR is unable to supply enough current to maintain the exciter voltage. The drawings I have are a little less than clear on where the 180 should show back up on the AVR. There are separate windings for the field and exciter, of course, with the exciter as part of the main field. Here's a snap of the book, if it helps. I did also check the voltage adjust pot, and it's correct and varies smoothly within range. No burned spot(s). I wish I had the full schematic of the AVR. I cannot see the source of the exciter voltage that should be present on the AVR....and is, until the field coil is connected. I suppose it's possible that there is a problem with the field coil drawing too much current, but I have no spec on what it's DC resistance should be. I could measure the current draw to see if it was high if I had the spec and tell if that was contributing to the problem.
Exciter field coil should be 20 or 40 ohms plus or minus a bunch. Don't really know, but that is a rare problem but worth measuring. When I said 26VDC no load, I meant, is that with nothing connected to the AVR output terminals? I try to do all this testing with everything connected most of the time. Your main output voltage should be coming in on the 2 "voltage sensing" wires there. You should also have a good healthy AC voltage showing up on red/black or yellow/green. Those are other windings of power whose purpose is only to supply power to the AVR and make it go. Of course L-S has done it more complicated and one of those may only produce when there is a high current load going.
I don't recall what the actual resistance measurement was on the exciter field coil. There were too many people looking over my shoulder when I measured it. I mostly was concerned with open or short at that point. The 26 volts is measured on the terminals with no connections. (12 volts on the field leads) I'll check it with the field coil plugged in, but is seems that if it was able to support the 26 volts, I'd be making electricity. Ok if I understand correctly, the separate windings called "Aux Windings" on that layout are simply to supply the AVR with information to regulate the main field. Looks like a lot of this is simply inductively coupled. That means to me that if the field is powered (as it is with the battery), it energizes the Exciter which in turn drives the main field, thus causing the stator windings to do their thing. Which they are as long as I supply current to the primary field. Something is preventing the primary field from being energized. If the voltage sensing circuit is open, for instance, it would make sense except for the 26 volts I am seeing on the AVR at the field connections. Monday I'll hook everything back up and get some readings on the sensing wires as well as the Aux winding inputs. At least at this point it appears the stator windings are ok, as I am getting substantial voltage there, all three phases, using 12 volts to power the primary field. Would it be the case that the 26 volts I see are sourced from the Aux windings? I guess measuring them might give a clue. I see no other voltage coming into the AVR to source it from, unless it is the sensing circuit.
I suppose we could say, with reasonable certainty, that the exciter windings are ok, since we do get current to the main field, otherwise the stator would not be making any juice. Since the AVR primary field is not being properly stimulated, that may be the root cause. If that winding functions @ 12VDC, it should work at the proper 26 VDC or so. But why it's there as long as the primary field is not connected is not explained. Obviously, some power is getting back to the AVR, or the 26 volts would not be there. But what kills it when loaded with the primary field windings?
The aux windings (power wires) supply power for the AVR to do its thing. They are rectified and serve as the source of power to drive the DC output. The sensing wires drive SCRs to gate the DC from the power wires down to the proper level on the field coil. If the output (sensed) voltage falls, the SCR gates will turn on more and push more DC to the field coil. If the sensed voltage rises, the AVR will turn down the DC onto the field. You will see the DC voltage on the field vary quite a bit, like maybe 18 volts no load to 70 volts full load, or something like that. If you put your own DC onto the main field, you should see a healthy AC voltage on the main sensing wires, plus at least one of the two aux windings. The sensing wires are connected back to the main output in one way or another, it depends on how the unit is configured.
Thanks for the explanation, Birken. I'll run a few more tests Monday and see what I can turn up. I won't be working on it over the weekend, other than mentally. If another test should cross your mind, please don't hesitate to note it here. So basically if the exciter is working (and it is) to generate some voltage on the final output, the power for the AVR is coming from the Aux windings, and it seems to be or there would not be 26 volts on the exciter terminals (d/c), then something may be related to the sensing circuit. If that was having problems it would kill the exciter voltage....but it doesn't, unless the primary field coil is connected. Seems very odd to me. I'm no stranger to electricity or electronics, but I haven't gained a full grasp of this method of regulation/generation yet. With everything connected, the main output is a measly 16-18 volts (not truly zero).....so maybe there is a false flag sent to the AVR by the sense circuit causing to fall off. IIRC, there are toroids that each of the three legs go through just above the main breaker. Seems they are labeled "load sense". Can hardly see how they could fail. But I'll check them.
Those toroids are just for some meter or controller to display how much current the thing is making and won't be anything to do with this. The 26 volts may be what it is making open circuit, but connected to the actual load of the exciter coil, it might fall to nothing. Operation is really quite simple. When it starts, there is some residual magnetism that makes a weak voltage on the output coils. The 12 main coils, plus the 2 aux coils. The AVR takes the output of the aux coils, rectifies them, and sends them out as DC onto the field coil. When sensed voltage approaches the set point (480) then the AVR will cut back on the DC going to the field and hold it regulated at 480. The sensing wires might be tapped to sense the full 480, or they could be center tapped to sense 240, or maybe 277, who knows.
That's pretty much as I thought. Same as most generators. Since the aux windings are powering the AVR, and the 26 VDC sags (or disappears completely, yet to be measured) when connected to the primary field, that seems like a possible trouble spot. The 26 VDC should vary depending on load, but try to maintain a set voltage that keeps the main output stable. But they should supply close to spec just running with no load just to hold the voltage setpoint. That being the case, I would think that voltage would vary with the voltage adjust pot on the front panel. I think it's a 3W 470Ω. Since this is a brand new AVR, I assumed it came in with the trimmer pots adjusted, but I got no paperwork with it. If that assumption is correct, then either it is bad, which since it behaves exactly as the original doesn't seem too likely, or I have to suspect something external to it. It did arrive with the 50Hz jumper in place, so I had to move it to 60Hz. Makes sense on the toroids. Forgot about driving the panel meter for current. I do have the full schematic for the power side of the machine, so I'll have a look to see where those sensing wires connect. I'll check that circuit if they are not providing a signal. However, if the sensing wires were dead, for example, how would it be making the 26 VDC? Or shorted/open? I don't like the idea, but I have a known good G70 with the same AVR I could swap in if I really think the AVR is bad. This will probably come down to a single wire broken or burned that I just haven't found yet. I sometime fall for those easy assumptions. Bad habit. You've been a great help Birken, and I appreciate it. I've not had to work on too many generators, so this is a little unfamiliar territory. For now at least. But it's getting clearer.
The pots adjustment will only be fine tuning. It will still work no matter how they are adjusted. It is the 26VDC open ckt because the residual voltage is getting rectified and supplied in its attempt to get the thing started. But is the 26VDC collapsing when connected to the actual field? Normal sequence is, 1. residual makes weak AC voltage. 2. Weak AC is rectified making weak DC. 3. Weak DC makes more AC. 4. More AC gets rectified into more DC and it takes off and makes full voltage. If the sequence breaks down, driving 12 volts onto the field momentarily will cause it to gain some AC and then take off. I think this is not happening for you. I would hook everything up, apply 12 volts to the field, correct polarity, along with the VR still connected, then measure AC voltages on the sensing and on both aux windings. If any are missing, there is a problem. The wildcard is one of the aux windings here is apparently a current derived one and not voltage derived. So it might not show much with no load. That is apparently a L-S unique design. Actually, what I personally would do is attach a simple AVR that I carry on the truck for things like this. Sensing is also power on the simple ones. So feed AC in the one end and DC comes out the other end, case closed. That can prove that the generator at least can work. But the battery method kind of proves that too.
snipsnip It would seem so, since it will not produce mains voltage with the primary field connected. I have only checked it while running the 12 VDC to the primary field. Connected to the battery, it is present. I will reconnect the primary field to the AVR, AND apply 12 volts (or 24 possibly, since it is closer to the OCV on the AVR terminals) and confirm. If it were holding, the battery would be unnecessary to power the primary field.
Just beware, if the regulator tries to "take off" and power the field, due to the battery, it will also be trying to charge the battery which of course it does not have enough power to do. So the battery test is only for a short blip, or done through a diode so the AVR cannot charge the battery.
On this last mention of connecting 12 VDC to the primary field, while connected to the AVR, you mean only a momentary contact of the battery, if I'm understanding you correctly. Or through a diode. I have a few 1N5404 diodes on hand. Along with a few others.
Yes that's right. Momentary contact to see if it will light off, if leaving it hooked up and there is a chance it might light off, then use a diode on the battery.
Gotcha....thanks!