MEP-002A and MEP-003A Field Flashing, or Lack Thereof

[Triple Jim]

MEP-003A Field Flashing, or Lack Thereof

I noticed something about the field flashing circuit of my 003A. When cranking, the field is momentarily energized or "flashed" by S1, marked "Master Switch" on the control panel. 24VDC is sent to the positive side of the exciter field, and current is returned from the negative side to ground through an approximately 50 Ohm resistor, located on the diode bridge assembly "A4". This resistor limits the current, but carries enough to get the main alternator producing power to bring up the control circuitry.

Now the interesting part: This resistor is connected to ground through S7, the starter disconnect switch. As soon S7 opens, preventing the starter from running, the field flashing also stops. Holding the master switch in the start position after this does not continue to flash the field. Moving it to the start position after the engine is running similarly does not flash the field. The only built-in way to flash the field again is to stop the engine and restart it.

I also noticed that the polarity of the diode bridge has + and - reversed on all of the schematics I have, including the one on top of my control box.

 

[glassk]

TM5-6115-585-12 has this,

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The circuitbetween 25-27 of the master switch applies batteryvoltage to the field flash resistor through diode (CR2).This action applies field flashing current to theexciter field. It also applies power through diode(CR1), which prevents starting set when the batterypolarity is reversed, and the closed contacts of thestart disconnect switch (S7) to the crank relay coil​

(K3)


Just a thought it may not be reversed if it stops the start function if poles are reversed, page 4-38

 

[Triple Jim]

What you said sounds correct. In the last sentence of my post above, I was talking about the "+" and "-" markings on the schematic, where it shows the 6-diode bridge assembly (A4).

 

[glassk]

Now I know to stay away from Conventional or the other Current flow direction questions,


Just kidding , it was always coming up as a way to get out of classwork,...

 

[Triple Jim]

Ha, yes, I see what you mean. The schematics are marked so if you connected DC to the bridge that way, current would flow. But bridges like that are normally marked as to their output polarity when AC is applied.

 

[Triple Jim]

I had assumed that the -002A was wired the same as the -003A, but it is not. The -002a schematic shows the ground return for field flashing connected directly to ground, not through the starter disconnect switch, so moving the switch to the start position when the engine is running does apply current to the exciter field on that generator.

 

[Speddmon]

Do you have an 003a to verify the wiring, because I believe the diagram for the 003a is wrong. I don't have the time right not to get verify that it is as shown on the 002a schematic or I would. Both generators should be wired exactly the same, that's why I ask.

 

[Triple Jim]

I do have an 003A. I found out about its flashing connection because I measured the voltage on the field during the startup procedure. It was clear that the initial voltage dropped out as soon as the engine was running. Both the field voltage and the generator output voltage stay steady if I turn the switch back to the start position while it's running. When I get a few minutes I will see if I can trace the wires from the resistor to ground see exactly where it gets its ground connection.

 

[Munchies]

You SHOULD be able to flash the field at any time as far as the wiring goes. Some that have been sitting for awhile or have intermittent master switches take some flashing to get energized.

 

[Triple Jim]

I agree that it would be nice to be able to flash the field when needed, but my 003A does not once the engine is up to speed, and the schematic shows that the speed switch disables the flashing function. I have not had time to go trace the wiring in my 003A, but I did confirm with a meter that the flashing does stop once the engine is running before I started this thread.

 

[Keith_J]

Flashing is only needed to restore residual magnetism, the soft iron laminations have more than enough residual magnetism once flashed.

My Lima MAC generator in the M1031 SECM has a flashing procedure, it has never needed it. Not that I would ever want to do it...involves shorting one output when running. MACs are magnetic amplified control, a non electronic voltage regulator.

 

[R Racing]

My mep003a and 002a both flash after start up. The 003a I have to do if the gen sits for more than a month. I think it's a great feature.