Exciter voltage and adjustment on AEAD-200LE serial kb048669

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What is the correct exciter voltage for a 1991 Miller Legend?

I've got a machine that runs, welds and generates, but all of the open circuit voltages are a little low, for example the 240 plug
shows about 219. A 10 amp 120 volt load knocks it down to about 209 VAC. RPM is correct. In welding mode the max OCV
is about 10 volts below spec, but the machine welds anyway. The 100 HZ plug is likewise low, say 105 unloaded, but it too works
with modest (few amp) loads.

A few measurements, using diagram SC-137 004-A
85 VAC from wire 28 to wire 29 (exciter AC output)
75 VDC from wire 21 to wire 26 (exciter DC output)
0.7-0.8 VDC from wire 26 to wire 25 (voltage regulator drop) Seems too small, as if the regulator is wide open

I've checked the brushes, R1, R4 and R2. Nothing is open, all the grounds are tight.

It looks to me as if there's not enough excitation voltage to give the regulator any headroom to control with.

One possibility is to reduce the value of R4 slightly to increase excitation. The wiper on R4 is close to center, so
there's some room for adjustment. What's a prudent maximum excitation voltage?

One other oddity is that S1B1 is open, so CR3 never energizes. It's hard to believe that could be the problem,
since the fine current control only drops a few tenths of a volt at the maximum position. It just dawned on me
that I never measured the voltage across the brush holders. From everything seen so far there's no indication
of significant wiring problems, what voltage should be expected at the brush holders at 1860 RPM and no load?

Thanks for reading, any guidance much appreciated!

bob prohaska

[Aeronca41]

How are you measuring RPM? Per many of Cruizer's posts, you should be seeing about 62.5 HZ at the 120v outlet with no load (power mode). RPM is very critical. Would also be interesting to measure the freq at the 100hz outlet and see if it's low. Also, be sure the brushes move freely in their holders and have good spring tension.

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RPM was measured using a tachometer, unfortunately I don't have a frequency counter. I did check the brushes,
they move freely and are plenty long. Brush resistance appears to be one or two ohms. That's higher than I expected,
but small compared to the resistance of the exciter field winding.

Thanks for reading!

bob prohaska

[Aeronca41]

Check out this thread.

http://www.millerwelds.com/resources...ltage-too-high

while his problem was voltage too high vs your case of too low, I think checking frequency and adjusting speed accordingly may address your problem. Could be wrong but I think worth a check.

Note how surprisingly small adjustments affected output voltage in this thread. The Kill-A-Watt meter is not expensive and readily available.

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Using a Kill-A-Watt makes things much easier, thank you!

It turns out the tach was reading low, so the machine was running around 67 Hz.

I re-did the measurements with the RPM set to 62-63 Hz to start with. It drifted up to 63-64 Hz
by the time the measurements were complete.

Exciter was 75-78 VAC, 67-69 VDC after SR2
Exciter brush to center brush 57-59 VDC
Generator brush to center brush 61-62 VDC
R2 14.1 VDC
R4 8.9 VDC
Regulator drop measured at the wipers on R2 and R4 was 6.9-7.2VDC
This is much higher than the value observed at 67 Hz.
By this time the output voltage was down to 102-104 volts.

Repeating the tests with a ~600 watt load pulled the frequency down to 60
Hz and the voltage to 88 VAC. The regulator drop was 6.9-7.1 VDC.
The exciter AC was about 70 VAC, DC side was 61 VDC, both presumably
because of the speed loss.

At this point I'm quite puzzled, especially at the sudden appearance of a voltage
drop at the regulator. Had the idle module opened CR4 the regulator _would_ go
to weld mode, turning full on. However, there was voltage at the duplex receptacles,
implying CR4 was energized and the regulator was seeing the output. In the load
tests the regulator drop lowered briefly when the 600 watt load was connected,
but then resumed its previous value, suggesting the regulator is at least partly
responsive.

How much speed variance should the governor allow? It seems to shift around
6 Hz during warmup and at least 2 Hz under a 600 W load. That seems a lot.

Thanks for reading, and any ideas! If a service manual can be had please tell
me where to look.

bob prohaska

[cruizer]

Soooo, locked at idle 1860 rpm what is the output power at the 120 receptacle, unlocked, at 3000 rpm, what is the power at the 100 hz receptacle.

What is the frequency of both in both modes

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With no load applied after about ten minutes of warmup:

At the duplex outlet I see 103-104 VAC at 62 Hz. The frequency
wanders 1 or 2 Hz over the span of several seconds

At the 100 Hz outlet I see 119 VAC at 94.5 Hz, quite steady.
The Kill-A-Watt is only rated for 43-67 Hz but it reads smoothly
to 99.9 at least, making it tempting to believe the 94.5 Hz figure.

Thanks for reading and any ideas,

bob prohaska

[duaneb55]

Proper exciter output AC voltage to bridge rectifier SR2 is 80VAC at Idle/Power speed and 145V at High/Weld RPM.

Adjust the tap strap (engine off) on R4 to provide 128-132V at the 100Hz outlet RC4 at proper weld speed. This should provide 70-72VDC at the DC weld output studs with R1 set to maximum.

If the above 70-72VDC is not present at the DC weld output studs after adjusting R4, adjust the tap strap (again, engine off) on R2 to result in spec voltage.

[duaneb55]

How much speed variance should the governor allow? It seems to shift around
6 Hz during warmup and at least 2 Hz under a 600 W load. That seems a lot.

The governors on these units typically have a droop of 100RPM +/- and being a 4-pole generator that would equate to just over 3Hz. This droop is why the no-load power speed spec is 1860RPM to allow for this droop and still stay within the normal recommended operating (loaded) frequency range of 58-62Hz (1740-1860RPM).

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Proper exciter output AC voltage to bridge rectifier SR2 is 80VAC at Idle/Power speed and 145V at High/Weld RPM.

Adjust the tap strap (engine off) on R4 to provide 128-132V at the 100Hz outlet RC4 at proper weld speed. This should provide 70-72VDC at the DC weld output studs with R1 set to maximum.

If the above 70-72VDC is not present at the DC weld output studs after adjusting R4, adjust the tap strap (again, engine off) on R2 to result in spec voltage.

It does not seem possible to obtain rated AC exciter voltage by adjusting R4. With R4 jumpered 100 Hz exciter voltage is only
about 140 VAC at the stator leads on the fuseholder and SR2. RC4 reads 130.5, frequency is 100.1-100.4 Hz.

With R4 in its original setting the exciter shows 138 VAC, RC4 reads 127 VAC.

The DC resistance of the exciter stator winding is 1.2 ohms. Exciter field resistance is 32 ohms, generator field resistance is
38 ohms, both warm.

I've looked for signs of local overheating in the exciter current loop and not seen anything obvious. Something other than
R4 seems to be dominating the current in the excitation loop, but I haven't found it.

I'll keep looking, but any suggestions would be much appreciated. SR4 would be a welcome suspect, but it's not too hot and
checks ok using an ohmmeter on diode test.

Thanks for reading and any ideas!

bob prohaska

[cruizer]

Going to suggest that your Freq meter is out to lunch, and best try a good known meter, perhaps test it in a household plug ,

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Going to suggest that your Freq meter is out to lunch, and best try a good known meter, perhaps test it in a household plug ,

Seems unlikely. It's a brand new Kill-A-Watt which reads 60 Hz when plugged into the wall.

I just re-checked the brushes, the center, shortest one is 14 mm long. I've been seeing an
ohm or so between it and the rectifier positive, but the connection seemed noisy. The holder
screw has been tightened, the slip ring and brush running faces have been cleaned with
220 3M wet-or-dry. The resistance reading is now about 0.4 ohm and steady between rectifier
positive and slip ring.

I'll test again tomorrow.

Thanks for reading,

bob prohaska

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With R4 and R2 bypassed completely the
AC weld voltage is 79 VAC
100 Hz outlet is 131 VAC no load at 99.5 Hz
Exciter voltage is about 140 VAC

At 60 Hz the duplex outlets are at 120 VAC but drop to about 99-101 VAC with an 800 watt load at around 59 Hz.
The exciter is at around 60-65 VAC and seems to wander a bit.

It looks as if the generator works but the regulator is insensitive. The regulator is alive, the voltage across wires
26 and 25 increases from around 5 volts to around 17 as the 800 watt load is applied. It's just not enough.

Removing R2 and R4 seems like a rather drastic adjustment. Both were close to midrange and don't look like
they've been disturbed from factory settings.

Thanks for reading, any counsel appreciated

bob prohaska

[[email protected]]

It looks as if the generator works but the regulator is insensitive. The regulator is alive, the voltage across wires
26 and 25 increases from around 5 volts to around 17 as the 800 watt load is applied. It's just not enough.

It looks as if this is badly wrong. On close inspection the voltage regulator board seems to have lost its
15 volt power supply. The apparent cause is the overheated R70, which lowers the input voltage to
the regulator to 10 volts. Obviously, a 7815 can't make 15 volts out of 10. No other components
looks like they got hot and the soldering looks good, with no obvious cracking or dewets.

I'm very tempted to just replace R70 and see what happens. I admit that this is treating the
symptom, rather than the disease. Without a better understanding of the SCR side of the
circuit the disease is apt to evade me.

It seems fairly clear that the two op amps deliver an error signal to R52, but how that interacts
the the SCR section isn't at all clear to me. Can anybody point me to a description?

Thanks for reading and any comments,

bob prohaska

[Aeronca41]

Since the machine is so old, I wonder if Miller would give you a tech manual with troubleshooting info? I think I'd try asking them. tel:920-735-4356


Actually, with the board diagrams already in the manual, there may not be much additional info in the Tech Manual, but I'd ask anyway.

Agree that a toasted R70 is most likely a symptom. I would check the input diodes-if one shorted, it would have allowed AC onto those old electrolytic caps, which would cook R70-and perhaps then opened a diode as the failure unfolded. Or it could be that the old caps just started to leak a lot and cooked it. Check the diodes and caps.