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MEP 003A Blew its Ammeter Circuit

reybo

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Been running fine and still does, only now the ammeter stays unmoved on zero. Happened when I powered the AC while under gen power. The gen's overload switch kicked off while the gen continued to purr. I turned off the AC, flipped the overload switch back on, and the house powered back up. Only the ammeter quit working.

I'll post an ad here looking for a used ammeter for an 003. Correct part number is 6625-01-038-6829. Of the many with that number, the perfect match is 35.7001. Ebay has none tonight.

Since there are shunts in ammeter circuits, this meter may be ok but a shunt has gone bad. That's beyond me. All I can do is put in a known-good meter and see what happens. If it reads zero, a shunt is shot.

But should a 10kw 003A with a maybe 2% daytime load go overload when the AC is connected? I vaguely recall there are different kinds of switch box switches designed to control and balance these things, something neither I nor my licensed electrician knows much about.

If anyone can comment, please do.

Nice to be back here. Been away for 4 years of trouble-free 003 power, aside from needing a fuel tank. Easy replacement. I have more respect for mil specs after the ease of working on this 25-year-old machine.

If you recall, the gen is linked to a 1000 gallon underground fuel tank. My wife needs her oxygen machine and I my CPAP, grid or no grid.

Rey in Charlottesville
 

Elden DuRand

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78
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12/22/2017
It could be something as simple as a bad connection between the shunt and the meter.

Meter movements sometimes go open but it's rare when that happens.
 

Wayne 440

Registered
...But should a 10kw 003A with a maybe 2% daytime load go overload when the AC is connected?...
The answer to that question depends on the size and details of "the AC", and how much load other than said AC you have. If you actually only have 200W of other loads, your application is very unusual.
 

reybo

Registered
It could be something as simple as a bad connection between the shunt and the meter. Meter movements sometimes go open but it's rare when that happens.
Glad to hear that, Elden. The circumstances of this tend toward thinking some part failed. I may have a wiring diagram around and barely enough experience to understand it.

The answer to that question depends on the size and details of "the AC", and how much load other than said AC you have. If you actually only have 200W of other loads, your application is very unusual.
Our AC heat pump manuals cover several sizes of unit. It's been puzzling since the day it was installed to understand the load. I don't need to know the heat pump load, just the AC. The heat pump is back-up for an oil furnace, and would never be used under gen power.
 

Thaumaturge

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68
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07/12/2019
A current shunt is just a length of buss-bar of a known resistance that will produce a known voltage drop with a known current flow. Meter is actually a small value voltmeter. Shunts are often stamped with ratings such as 50mV/100A. Meaning a meter movement that takes 50 mV to read full scale will do so when 100 amps is flowing through shunt. Scale on meter is printed to match. Any 50mV full scale meter will show proper deflection, but you may need to swap scales. If power is flowing shunt cannot be bad.
Doc
 

Vanman

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Last Subscription Date
07/10/2019
Is it really a shunt? Of course it could be. But I am used to seeing current transformers for AC current measurement...
 

reybo

Registered
A current shunt is just a length of buss-bar of a known resistance that will produce a known voltage drop with a known current flow. ... If power is flowing shunt cannot be bad.
Doc
Is the shunt built-in to the meter, or an external part in the circuit?
 

Thaumaturge

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68
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07/12/2019
Is the shunt built-in to the meter, or an external part in the circuit?
Could be either, or even a CT as others have pointed out. A CT is a current transformer. Typically a torroid with line to be sensed passing through it. If you aren't picky about staying original there are now MANY very versitile and ecconomic replacement options. Just check Amazon.
Doc
 

dependable

Registered
The 003 uses a CT. It is in bottom of output box. Check for loose nuts on any of the terminal blocks and the big recconection switch. Also, the cannon plug from control box to out put box could have loosened up. (I'd check that first as it is easiest.)
 

Vanman

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Last Subscription Date
07/10/2019
My vote is for a stuck or otherwise physically damaged meter movement. If the secondary (meter side) of a current transformer opens at any point, high voltage results and I'd expect to see some smoke as the CT is being damaged. But maybe not , if the load was light... :shrug:
 

Thaumaturge

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07/12/2019
Nothing on meter face to reveal actual meter specs, other than it is a REAL D'arsonval movement. (Jeweled pivots with balanced armature.) To test meter movement power with a single battery series connected with a 10k potentiometer, a 1 megohm resistor. Reduce value of potentiometer until meter reaches full scale. If full scale not reached return pot to max and lower resistor by half. Repeat until meter reaches full scale. Then measure ohms of pot and series resistor, then voltage of battery. Ohms law will reveal current and extrapolate full scale voltage. Can measure current in series, but measure meter with resistors for proper total.
Doc
 

reybo

Registered
Interesting. I'll file that.

I looked at the schematic and found two transformers of interest: one labeled current transformer and one called a current voltage transformer.

Locating these on a schematic is a whale of a step away from knowing where a part is physically, and its specs. All my experience has been in front of the dashboard, not behind it.
 

Thaumaturge

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07/12/2019
Uh, in addendum above method works on DC meter movements. If meter contains integral rectification you must either dissasemble to reach zctual movement or use AC in place of battery. As to locating components, CTs are typically toroids, so look for a donut shape.
Doc
 

dependable

Registered
The Ct and CVT is a black bakolight type plastic rectangle with the output wires to lugs looping though it. Sitting on bottom of output box.
 

Radiomike

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The meter, on its face, says FS 1A AC. I looked carefully on the photo for a wound former between the pivots, but didn't see one. That plus the non linear scale may indicate a moving iron meter. However running a CT with an open circuit secondary is inviting trouble, expensive trouble. Check the meter still has continuity, check the CT has secondary winding continuity. Moving Iron meters will respond to DC as well as AC so it is easy to check the movement with a D cell and a swamp resistor.

Mike
 

reybo

Registered
The Ct and CVT is a black bakolight type plastic rectangle with the output wires to lugs looping though it. Sitting on bottom of output box.
Are they equally suspect in causing this? The circumstance -- turning on the AC kicked the overload switch --certainly suggests a part got overloaded, not some other demon like mouse-gnawed wiring.
 

Vanman

Subscriber
Last Subscription Date
07/10/2019
I would be inclined to *guess* that that current meter reads to the 100% mark on the scale when 5 amperes AC are flowing through it... Just since 5 amps is a fairly typical full load CT secondary current. Have no idea if the military stuck with that standard or not! :shrug:

Whoops! Just saw Mike's post. So not 5 amps to 100%, 1 amp all the way to the 133% mark... Interesting.
 
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