MEP018 Field Flash question

[cranetruck]

Hoping that there is an obvious answer to this; why would the engine bog down to a point of stalling when the field flash switch is toggled? Holding the switch for just a second or so is all it takes.

1) I'm not happy with the phase/voltage selector switch action, but it seems to work although with a delayed springy snap. Is it possible that some odd combination of windings results if not working correctly?

2) Rectifiers? Haven't checked them yet.

The unit is not producing electricity after a failure that seem to have occurred when the engine was running poorly at low rpm some time ago. I now have a good running engine and am working on the generator/control box.

TIA

 

[Isaac-1]

It sounds like you have more than one problem. My guess is when you are flashing the field it is trying to produce power, but you have a direct short either in the stator or the output leads (perhaps relating the switch issue you mention). Secondly your Voltage regulator is not working and therefore your not generating any electricity under normal running conditions so the engine is not bogging down.

Ike

 

[liftman]

The reconnection switch in my unit has never had the snap feeling to it. When the switch is moved, it has to almost go past the voltage you want to select and then it kind of springs into place. Never really thought about it, since it works.

Was anything taken apart on the gen head when the engine was repaired? If so, I would double check those items first.

 

[jbk]

did you turn the reconnect switch while it was running?

 

[cranetruck]

The reconnection switch in my unit has never had the snap feeling to it. When the switch is moved, it has to almost go past the voltage you want to select and then it kind of springs into place. Never really thought about it, since it works.

Was anything taken apart on the gen head when the engine was repaired? If so, I would double check those items first.

Nothing was touched on the generator when the engine was worked on. The control box was removed for access at which time I decided to "excercise" the selector switch by turning the knob a number of times in both directions. The switch has never felt "good".
During last year's 25 hours or so of good operation, the switch was always in 120/240 1 phase (split phase) position.

JBK, I have never turned the selector switch while the engine is running.

It's perplexing how the engine almost stalls out when the field flash switch is held on. There is a momentary indication on the frequency meter, but no voltage indication.
CB1 is in "OFF" position, so there is no external load.

It's almost as if windings within the generator are bucking each other to load down the engine...but only when the field flash is activated.

I'll be ringing out the selector switch next.

 

[liftman]

If the control box was removed from the unit, you had to unplug the control cables to the gen head, right? Is it possible that some pins in the plug were either not connected or connected improperly (bent) when the plug was reinserted?

Just an idea before you start getting into rectifiers and other components.

It seems as if you have a short now that wasn't there before which is causing the loading of the generator. If you had to disconnect the voltage selector switch, a wire back in the wrong place could cause this type of issue. Remember when you close the field flash switch, you are essentially telling the gen to produce full power.

 

[cranetruck]

Thanks for taking time to respond!

Alright, so I rang out the selector switch according to fig 7-4 (TM 5-6115-275-14). On the chart for 120/240V 1 Ph position, only 3 out of 11 settings were correct!
Spent all day yesterday removing the switch and proceeded to hardwire the settings using terminal blocks (shown below) for the above "split phase" output.
When the weather clears up, I'll install the control box and see how it runs.
BTW, the load percentage gauge has never worked and sure enough, that position was not correctly selected on the switch either.

The "split phase" position is what I use for the house and don't see any reason to change any time soon.

 

[Speddmon]

How did I miss this one???

Well it looks like you have the situation well in hand now. Ike hit the nail on the head earlier. When you flashed the field and the generator started to produce, something shorting out your windings was causing an overload on the engine. It turns out to have been your switch not connecting them up properly.

Also, as Ike mentioned, you may find you have a VR problem since you stated it ran at low RPM's a while ago. Unless you caught the problem quickly and shut it down, VR's don't tend to like low speed running very much.

 

[cranetruck]

How did I miss this one???....

To recap, the unit was running well racking up some 25 hours since last Summer and after sitting for a couple of weeks in December, it barely started, ran terribly (low rpm) so I shut it down. In a separate thread I show how one of the cylinders was replaced on the 4A084 engine and it now starts and runs beautifully. The fuel pump was iffy and it was replaced also.
I'm using a bank of Nicad cells and have also eliminated the pesky LAs. A relay was added to switch the 4A084 battery charger out, since it can't handle the high charging current required by the Nicads. This relay is visible near the ammeter in the images above.

The posts above show where I am at the moment, just waiting for the rain to end so that I can install the control box again.
I did check the power transistors in the VR and they appear okay.

How can the rectifiers be damaged in the generator head? Hopefully, they are good...don't look like an easy fix to replace...

 

[Speddmon]

I don't think you lost the rectifiers in the head. As you mentioned..the coils may have been bucking each other, or more than likely, the switch was just short circuiting 2 of the coils together....when it tried to produce power, the short circuit (long before any overload protection in the circuit) was just too much for your engine to handle.

 

[liftman]

How can the rectifiers be damaged in the generator head? Hopefully, they are good...don't look like an easy fix to replace...

Shouldn't have to bother with that. With the issue in the switch, you more than likely have found the problem.

 

[cranetruck]

Well, I installed the control box and fired up the engine and with a known good wiring it does the same thing as before, bog down the engine when the field flash is engaged...guess the damage was already done.
Next I'll be checking the generator windings for shorts and continuity...

Taking a closer look at the construction of the switch, I find it hard to believe that it can end up with incorrect settings, however, the contacts are made of unplated copper (see image) and have a rather high contact resistance because of oxidation and a waxy coating (lube?), which may simply mean that my continuity measurements indicated an "open" using my DVM. Oh well, not the first time I jumped to a (an incorrect?) conclusion.

 

[PeterD]

I think were this mine, I'd disconnect the field wires (12 wire, right?) and try again. If it still does it, then the generator head is clearly at fault, and if not then something down stream is likely the problem... As well, when disconnected it will be easy to do continuity and shorts.

 

[cranetruck]

Checked the windings of the generator and it appears that I have a short between two of them, the ones terminated via (J1) pins 2, 5, 8 and 12 are shorted...
Not good, will be difficult to work on, probably best to look for a new generator head, so if anybody is parting out a MEP018, let me know.