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Military MEP and Aircraft Gen-Sets

MEP-004a Generator burnt current transformer


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  #1  
Old 06-27-2013, 09:07:47 PM
shackwrrr shackwrrr is offline
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Default MEP-004a Generator burnt current transformer

Had a friend bring me a MEB-004a generator to diag, He would only get voltage when holding start. I combed over the innards and traced it back to the exciter/regulator and I pulled it out to inspect. Upon removal of the lid I found everything covered in soot and the transformer melted down. I am trying to source a new regulator but I wanted to know what caused the meltdown so a replacement doesn't do the same thing.

Thanks
Ian
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  #2  
Old 06-28-2013, 07:38:51 AM
nehog nehog is offline
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Default Re: MEP-004a Generator burnt current transformer

Any chance you could post a few images and/or more information about the failure?
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Old 06-28-2013, 10:07:33 PM
shackwrrr shackwrrr is offline
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Default Re: MEP-004a Generator burnt current transformer

The only story I go was that it only had output when holding the start switch. Engine runs great. I pulled open the voltage regulator/exciter box and found the burnt transformer. I don't know why it fried and if I install another VR I don't want it to happen again. He bought it in the current condition.

here are some pics




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  #4  
Old 06-29-2013, 07:41:37 AM
nehog nehog is offline
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Default Re: MEP-004a Generator burnt current transformer

There is a remote chance I have one (the transformer) from an older 15 KW generator set. The rest of that assembly is recoverable, but the large capacitor must be tested (actually, test them all...) to ensure it is still OK and not shorted.
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Old 06-29-2013, 11:12:17 AM
JerryLee JerryLee is offline
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Default Re: MEP-004a Generator burnt current transformer

Most "clamp-on" type VOMs can test capacitors. Every AC tech uses this type of volt-ohm meter so they can test the large capacitor(s) that are found inside the condenser (the outside unit).

You can buy this type of meter at Harbor Freight Tools very inexpensively but make sure that it will test a capacitor before you buy it. They are available on the internet in many brands and prices.

Maybe you should ask your local AC tech to stop by and test the capacitors for you.
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Old 06-29-2013, 09:29:11 PM
shackwrrr shackwrrr is offline
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Default Re: MEP-004a Generator burnt current transformer

Quote:
Originally Posted by nehog View Post
There is a remote chance I have one (the transformer) from an older 15 KW generator set. The rest of that assembly is recoverable, but the large capacitor must be tested (actually, test them all...) to ensure it is still OK and not shorted.

Thank you and I would be very interested in it if you happen to have it.

I do have a DVOM and will happily test the Cap. Am I just looking to see if its shorted or is there a certain capacitance Im looking for? I did notice that the paint was peeling on it.

I was looking over the gen some more and I noticed that the idle speed screw is missing, could idling have caused the meltdown?

Thanks
Ian
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  #7  
Old 06-30-2013, 09:36:10 AM
nehog nehog is offline
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Default Re: MEP-004a Generator burnt current transformer

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Originally Posted by shackwrrr View Post
...
I was looking over the gen some more and I noticed that the idle speed screw is missing, could idling have caused the meltdown?

Thanks
Ian
First, of course, is never allow a generator set to idle. It is possible that (idling) caused the failure but I can't be sure. Personally I'm thinking it is a 'natural' failure of the transformer.

When windings short out (one turn is sufficient) then all current tries to flow in that shorted winding, as it effectively becomes a secondary with a dead short on it. This causes the temperature of the wire to rise quickly leading to more insulation breakdown and more shorts. Soon, you have cascading failures of windings, insulation and lots of magic smoke lost.

What's the condition of that board that was next to the transformer?
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Old 06-30-2013, 10:10:34 AM
Jack Hottel Jack Hottel is offline
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Default Re: MEP-004a Generator burnt current transformer

Quote:
Originally Posted by JerryLee View Post
Most "clamp-on" type VOMs can test capacitors. Every AC tech uses this type of volt-ohm meter so they can test the large capacitor(s) that are found inside the condenser (the outside unit).

You can buy this type of meter at Harbor Freight Tools very inexpensively but make sure that it will test a capacitor before you buy it. They are available on the internet in many brands and prices.

Maybe you should ask your local AC tech to stop by and test the capacitors for you.
I would hope that most AC techs would know better!
Faulty motor capacitors will often test fine on such multimeters, but will not work in service. The capacitor will not perform under the voltage stresses imposed upon it under operating conditions. With the low voltages imposed by a meter it will test OK. Equipment that will voltage stress a capacitor under test exists, but it is expensive, and not common in field use. Bottom line is, "Always test a capacitor by substituting a known good one"!
Jack Hottel
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Old 06-30-2013, 11:29:32 AM
Radiomike Radiomike is offline
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Default Re: MEP-004a Generator burnt current transformer

I agree with jack.

Whilst multimeter will give an indication of a gross failure, short circuit, the test voltage is very low. The easiest is by substitution with a known good device. failing that there are some simple tests that can help; these are one I use on motor start and run capacitors.

1. DC high voltage insulation resistance at less than working volts this will give a reading in Ωx10^6.

2. With a light bulb in series measure the voltage as detailed here http://toad.net/~jsmeenen/capacitor.html

3. With a resistor in series apply mains voltage and measure all three voltages, across capacitor, across resistor, and total applied voltage. also measure current. The resistor should be good to take the whole current if the capacitor fails. Then application of the cosine rule or a graphical method by drawing vectors, the capacitance can be found as well as an indication of capacitor loss angle. This I know as the three voltmeter method.

For motor start capacitors you need to be quick as their rating at voltage is only measure in a few tens of seconds.

Mike
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  #10  
Old 06-30-2013, 12:56:07 PM
Richard L Richard L is offline
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Default Re: MEP-004a Generator burnt current transformer

Contact Brian Clugson at Flight Systems and put in a modern regulator.
I do it with 002/003 and it makes life really nice, updates the set and gets rid of over engineered items like this.
R
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Old 06-30-2013, 09:17:48 PM
shackwrrr shackwrrr is offline
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Default Re: MEP-004a Generator burnt current transformer

Quote:
Originally Posted by Richard L View Post
Contact Brian Clugson at Flight Systems and put in a modern regulator.
I do it with 002/003 and it makes life really nice, updates the set and gets rid of over engineered items like this.
R
Do you have any pictures or details on your conversions? I think this is the ticket on a replacement AVR. Does it work with the existing components or am I looking at a complete rewire? The owner would like to keep it military looking so something that fits inside the factory avr box would be nice.

Thanks
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Old 07-01-2013, 09:17:22 AM
Richard L Richard L is offline
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Default Re: MEP-004a Generator burnt current transformer

Have just done MEP 2-3,and on those I create a lot of room as I strip out the original Reg components and end up with a pile of excess.
I will find a 04 wiring diagram and have a look at it, and see what it would take to change it.
On the 2-3 you have to change/remove components in both the control box and the output box.
R
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  #13  
Old 07-01-2013, 12:38:05 PM
Richard L Richard L is offline
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Default Re: MEP-004a Generator burnt current transformer

Re generator idling.
I think you need to be specific re WHAT generator you cant idle.
If your talking about some of the Military sets then you have a point, however I have started MEP002-003 at low idle and brought them up to 60 cycle.
Not familiar with the MEP004.
With the newer regulators installed you can start at idle, as there is an under frequency adjustment that cuts out the regulator at a preset point.
I have a 100 KW Isuzu powered unit that you start at an idle let things stabilize and then ramp up to 1800 RPM, and after your through turn the throttle down and let it cool off.
Just my thoughts.
R
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Old 07-01-2013, 03:08:36 PM
nehog nehog is offline
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Default Re: MEP-004a Generator burnt current transformer

Let's see if this posts properly:

Code:
Techincal Specification:
BODY HEIGHT	1.940 INCHES NOMINAL
BODY LENGTH	2.330 INCHES NOMINAL
BODY WIDTH	1.660 INCHES MAXIMUM
DISTANCE BETWEEN MOUNTING FACILITY CENTERS	2.812 INCHES NOMINAL SINGLE MOUNTING FACILITY SINGLE CENTER GROUP
FREQUENCY RATING	50.0 HERTZ MINIMUM SINGLE COMPONENT AND 400.0 HERTZ MAXIMUM SINGLE COMPONENT
INCLOSURE TYPE	FULLY INCLOSED
INPUT-OUTPUT PHASE RELATIONSHIP	SINGLE PHASE TO SINGLE PHASE SINGLE COMPONENT
MOUNTING FACILITY QUANTITY	2 SINGLE GROUP
MOUNTING HOLE DIAMETER	0.187 INCHES NOMINAL SINGLE GROUP
MOUNTING METHOD	FLANGE SINGLE GROUP AND UNTHREADED HOLE SINGLE GROUP
RELIABILITY INDICATOR	NOT ESTABLISHED
TERMINAL TYPE AND QUANTITY	4 TAB, SOLDER LUG
WINDING FUNCTION AND QUANTITY	1 PRIMARY SINGLE COMPONENT AND 1 SECONDARY SINGLE COMPONENT
WINDING OPERATING VOLTAGE	102.00 AC VOLTS NOMINAL SINGLE COMPONENT SINGLE SECONDARY
WINDING OPERATING VOLTAGE	120.00 AC VOLTS NOMINAL SINGLE COMPONENT SINGLE PRIMARY
So basically the transformer in question is an isolation transformer with a slight step down voltage (120V in, 102V out.) I'm still searching for one that may be compatible, but as others suggest unless you strongly desire to keep the machine as originally built, I'd use an aftermarket regulator.

One may think that these voltages are odd, but the fact is that in Japan they use 100 volts as their line voltage, and some (older) equipment is modified for export by using a 120 to 100 volt transformer. That small difference in secondary voltage won't affect anything.

BTW, you could use two transformers with 50 volt secondaries with the secondaries in series, or even three transformers! But I think it may be possible to find the 'correct' voltage transformer if one keeps looking.

If you are not in a hurry, there is a place that I visit every few weeks that has a lot of odd-ball transformers (and variacs, but that's a different story) and they may have one.

Oh, and if I have the right transformer, it is located in the regulator for convenience and not used by the regulator. It appears to be connected to terminals A, B, C and D of the main connector on the regulator.

Can you verify that? If this is the case, that capacitor may have no effect on the transformer's failure, though the failure may have damaged the capacitor however.
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  #15  
Old 07-02-2013, 06:56:45 PM
nehog nehog is offline
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Default Re: MEP-004a Generator burnt current transformer

I just got back from my favorite industrial surplus store. Then had a 240 to 200 volt transformer that would work wonderfully as a 120 to 100 volt unit. Price was $25. If you're interested in it, PM me and I'll either tell you where to order it, or perhaps I can pick it up on my next trip over there.
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  #16  
Old 07-04-2013, 10:38:09 AM
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Jim McIntyre Jim McIntyre is offline
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Default Re: MEP-004a Generator burnt current transformer

Another option here is to abandon the original V/R and stick a SX460 V/R in, as described here:

http://www.smokstak.com/forum/showth...ighlight=sx460
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Old 07-05-2013, 07:19:49 AM
nehog nehog is offline
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Default Re: MEP-004a Generator burnt current transformer

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Originally Posted by Jim McIntyre View Post
Another option here is to abandon the original V/R and stick a SX460 V/R in, as described here:

http://www.smokstak.com/forum/showth...ighlight=sx460
Again, the transformer is not necessarily part of the voltage regulator, but may be part of some other circuit. I've not traced the connections, but my information shows all four wires from the transformer going to the unit's connector with no other internal connections. This points out the possibility that there is something else wrong with the generator set and that would have to be addressed.

The only other alternative would be to use an after market voltage regulator, rip out virtually all the other control circuits (or determine what failed and caused the transformer to fail, if that is the case) and totally rebuild/redesign the system.

Personally I'd suggest tracing out those wires (see my posts above) and seeing where they go. This task should only take perhaps half an hour with the manual(s) and the generator apart. It is quite possible that one would find a shorted wire, welded relay or switch contact, etc., that is the cause of the initial failure.

BTW, were this mine, I'd give some serious consideration to adding a fuse or circuit breaker in the transformer's primary circuit if the failure mode cannot be determined.
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Old 07-05-2013, 04:47:25 PM
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Default Re: MEP-004a Generator burnt current transformer

Quote:
Originally Posted by nehog View Post
...
So basically the transformer in question is an isolation transformer with a slight step down voltage (120V in, 102V out.) ...
Nope. You linked to NSN 5950-00-600-0843, which is NOT the transformer in question.

The transformer in question is NSN 5950-00-600-0840. It is a 120 to 208V with center tap. Shown here:

http://www.nsn-now.com/Indexing/View...=5950006000840

The transformer is T2 in the static exciter (VR) schematic. It is definitely a part of the V/R. All the power for the field goes through this transformer, as it is used to drive the SCR bridge. There's no need for "tracing out wires" as its all shown in the schematic, Figure 5-31 of TM 5 6115-464-34. If you don't have the manuals for this set, and are trying to fix it, God help you...

I'd agree that extended idling could have taken out this transformer. Either that or shorted SCRs (Q1 and Q2).
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Old 07-05-2013, 09:29:01 PM
shackwrrr shackwrrr is offline
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Default Re: MEP-004a Generator burnt current transformer

Been a while since I've been able to log on. Talked to the owner and found out when it died and what was going on. The generator was being used to power a building for a fire cleanup and they were only using 1 leg of the 3 phase output. From what he explained it was heavily loaded on one 120v output and the others were left open.

I am guessing this is what most likely killed it.

I am interested in using a modern VR but its confusing where I would wire it up. The generator has so much wiring and components in it that I really dont know what I would need to keep and where to wire up the VR. It looks like that sx460 reg has ony five or so connections and the current AVR box on the genny has two big round connectors with 15 or more wires. I understand a lot of the stuff works with the instrumentation and overload protection.
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Old 07-05-2013, 10:36:23 PM
shackwrrr shackwrrr is offline
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Default Re: MEP-004a Generator burnt current transformer

Alright, Found the service manual and found out more about the big transformer. That transformer is T1, the heart and soul of the static exciter. I would need to use this component even if I were to go with an aftermarket regulator. The aftermarket unit would only replace that vertical board in the foreground.

Looking at the diagram and from my own interpretation the static exciter interfaces between the actual feild coil and the regulator.

here is the diagram I found
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