MEP-002A and -003A main breaker

ashwood486

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I just got my MEP-002A running (nice clean unit that didn't need anything besides filters, oil and a couple of gauges, but I did tear all the sheet metal off and go over everything with a fine tooth comb) and decided to try it on a 3HP air compressor. Same thing as a lot of the previous posts on this thread!

The air compressor is a very compact but heavy duty industrial 3HP, 3phase 208Vac unit from the early 80's (Toshiba Toscon). With the MEP-002A in 208V 3-phase mode, putting out exactly 208Vac at 60 HZ flipping the breaker with the compressor connected pegs the loadmeter and trips the breaker instantly, without the engine bogging down.

By dropping the frequency to 50HZ and the voltage to 190V it will start, and then increasing voltage and frequency to normal the loadmeter sits around 30% with the compressor chugging away. Engine doesn't sound labored at all.

That tells me the breaker in this unit, just like a lot of people have complained, is overly sensitive to motor starting inrush currents. OK, I might just hold out for a good deal on a MEP-803A, but I'm pretty sure I can make this MEP-002A do a little better. Here's some ideas:

R-3 adjustment, which I have not tried, but it seems no one as had much luck with that route.

Add shunt resistance to the low-current side of the breaker (3 resistors in between each pair or six small terminals on the breaker) was thinking of using resistors equal to the breaker's internal resistance across those terminals. That would divert half the current away from the breaker doubling the trip point. Just an idea, not sure if it would work.

Bypass the breaker and run an external breaker (probably the easiest).

Part if the problem is that box with the breaker is very crammed with stuff and not easy to work inside of, and besides I don't want to butcher it. Part of the beauty (at least to me) of these units is the insane complexity and high quality components inside. I could gut that box with the crazy drum switch and the current transformer, wire the 12-lead alternator for 3-phase and use a modern voltage regulator and circuit breaker but it just wouldn't be the same....

I spent about half an hour trying to understand the electrical schematic of this unit (and I'm used to dealing with complicated electrical schematics) and gave up trying to understand it!

What's the opinions on an MEP-803A's ability to deal with 3-phase motor starting? I might just try to get one of those and sell the -002A.

Another (possibly unrelated) question.... What's the black plastic box behind the current transformer to the left in the high voltage box? The field rectifier board is on the right of it. On mine the plastic is cracked in a few places, but it's hard to get a good look at it without pulling stuff apart.
I had the most luck by adding a hard start capacitor to the compressor motor and also delay timers on my other fan motors when my central a/c system is switched on it reduced the start up amps and helped my situation. I fully intend on playing with different breakers in the future on this machine. good luck.
 

dependable

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Am pretty sure it is not simple to bypass main breaker (to use a conventional one) if you want to keep original gauge & control circuitry. The 6 18g wires that go in & out of breaker are involved in a few circuits.
 

peapvp

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This sure is an interesting discussion.

I would like to through in my 25 cents.

The great majority of these Generators were used primarily for two applications:

1. Powering the electronics for a mobile command center on the front line. Primary generator: MEP 002A / 003A / 004A / 103A / 104A

2. Powering moveable missiles launch vehicles / transport erector launcher for Pershing / Scud etc. Missiles. Primary Generator: MEP 004A / 005A / 006A / 104A / 105A

These generators were never intended to power high inductive loads, but electronics like radar systems, embedded systems, radio equipment and control gear.

The hydraulic pumps on a TEL are powered by the onboard 24VDC Supply.

This is the reason why this circuit breakers trip so fast as they were intended to protect valuable electronic equipment.

Look at the CB as a ultra fast 3AG Type fuse. I guess you get the picture.

BTW. Over the years these generators were used in other applications as well, but powering electronics was the primary design purpose.
 

steelypip

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If a hard start capacitor on the compressor motor solves the problem that's definitely the cheap/easy/simple way to go. I suspect it will if the running load is that small.
 

peapvp

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I am surprised that no one said anything about the "Scud Missile" in my last post.
 

1000eemonarch

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OK, if these things are only designed to power electronics not high inrush-current loads, one would wonder why they are set up for 3 phase.

Here's the common uses for three phase that I can think of:

Balancing load between a large number of 110V loads (like lights) put some on one leg, put others on second, etc. (doesn't really make sense on a small gen like the -002 or -003)

Low ripple DC after being rectified. Three phase make smooth DC with a 6-way diode bridge. (that's the likeliest if they really were meant to power electronics).

And of course running motors which we all seem to agree they do a poor job of doing.
 

Munchies

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The military practically runs on 3-phase. Anything 'militarized'. Any civilian equipment is typically 120v (laptops, battery chargers, smart boards, etc). All your portable field equipment is 3-phase.
 

steelypip

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The military practically runs on 3-phase. Anything 'militarized'. Any civilian equipment is typically 120v (laptops, battery chargers, smart boards, etc). All your portable field equipment is 3-phase.
^^What he said. A minor source of heartburn for people camperizing comm shelters and shop trucks is that their bulkhead mounted AC or HVAC systems (even the little 6K BTU ones) all run on 208/3 phase, which isn't commonly available at your friendly local campground.
 

jamawieb

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Where in the manual is the adjusting the R3 screw. I could not find it. I think my dryer is tripping my genset.
It's in -34 on page 82.
6-8. AC OUTPUT CONTROL BOX ASSEMBLY.
a. Test, on Equipment.
(1) Resistor R3 (20, figure 6-10) must be adjusted
as follows before performing any tests.
(2) Remove wire D26F16 from TB3-1 and connect
0 to 1 amp ac ammeter. Connect test ammeter as shown
in figure 6-7.
(3) Place reconnection switch in
120 position.
(4) With the generator operating at rated frequency,
apply rated load by closing circuit breaker (CB1) (9,
figure 6-10).
(5) Adjust R3 until ammeter previously installed
reads 0.6 amps, tighten locknut on R3. R3 is now adjusted
properly.
 

pclausen

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I have a question about pure resistive loads. This weekend I load tested my 003 using 2 4500W and 2 3500W water heater elements. I started out with 12,500W and ran like that for maybe 5 minutes with no issues. I then added the 2nd 3500W element and the engine barely noticed and I ran the 16,000W resistive load for maybe 1 minute. I then went back to 12,500W and went about my business. About 10 minutes later when I checked on it, I noticed the breaker had tripped. Reset it, and it tripped again right away. Went to 9,000W and it held fine, went to 11,500W and it tripped right away. Went back to 9,000W for another 5 minutes and then shut down.

So is there a 3rd component to these breakers that would cause them to get trip on continuous resistive loads above the 10kw rating (in the case of the 003), like maybe an engine over temp sensor or something?
 

steelypip

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If the output breaker tripped, it's because you were above the current-time line for the breaker. They don't have a single tripping point, but will trip at progressively lower amperages as time increases. That's to allow the circuit to run in a temporary overload state to start a big motor, for instance.

If it's a thermal-magnetic breaker, they are definitely prone to trip sooner once they've gotten hot. You will want to retry this load cycle starting from cold and see if it will let you put as much load on it cold as it did before.

If not, it's possible that your breaker is getting old and flaky and may be tripping prematurely. I find that the breaker on my itty-bitty (450W Kawasaki) generator, the breaker trips well before the set is into the bogging down region. I have yet to get the breaker on the MEP-002A to trip even when it was running at 120% of rated load for 30 minutes.
 

pclausen

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Thanks, that makes sense. I'll retest, this time timing things more accurately. So even if the breaker is working as designed, I should not expect to be able to run a load above 10kw for more than say 10-15 minutes?

I wish this thing had an oil temp sensor. I feel you're kinda flying blind without one.
 

cuad4u

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I have "restored" and sold 22 MEP 002A and 003A generators. I load test EVERY 003A I offer for sale at 13,500 watts for at least 2 hours. I have never had a breaker trip.
 

pclausen

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Ok, in light of that, I'll definitely re-test at 12,500 watts and time how long before the breaker trips. If I can run for 1+ hour at 12,500 watts without tripping, I guess that short burst I did at 16,000 watts got the breaker hot enough to where it would no longer handle 12,500 watts.
 

cuad4u

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We all know the military rates the 003A at 10,000 watts. However the data plate on the side of a 003A plainly states that the ratings are 52 amps at 240 volts. Doing the simple math (52A X 240V) = 12,480 watts. According to the data plate the generator's continuous duty rating is 12,480 watts or for all practical purposes 12,500 watts. When I load test any 003A at 10,000 watts the % current meter indicates around 75%. The % current meter on a typical 003A does not get up to 100% until I increase the load to around 12,500 watts. I understand the exact same Onan generator sold on the consumer market was rated at 12,500 watts continuous duty. I cannot comment whether or not the ratings or trip points on the main breaker used in the military version are the same as the main breaker used in the consumer version. Again I load test every 003A I refurbish at 13,500 watts or 110% on the % current meter for at least two hours before offering it for sale. I never had a main breaker open.
 
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Triple Jim

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The catch is that the generator is rated at 52 amps at 0.8 power factor. The manual states that if it's running a resistive load, full power is achieved when the meter reads 80% rated current. Running the generator with a purely resistive load of 52 amps at 240 volts is overloading it, according to the specification.

If you want to load test a generator at 110% of its rating, and have a resistive load bank, a properly calibrated meter should read 88%.

(page 2-4 and 2-5 of TM5-6115-585-12.pdf)
 
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pclausen

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When I load tested mine using 12,500 watts, my meter indicated 130%. If I multiply 12,500 by 0.8, I get 100%, so should I adjust the meter down to 100% with a 12,500 watt resistive load?

Should I be seeing 41.6 amps with a 12,500 watt resistive load @ 240 volts? 52 x 0.8

I'll be sure to throw a current clamp on when I retest to see what I get.

Here's the 130% reading I'm currently getting @ 12,500 watts.
 

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Triple Jim

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When I load tested mine using 12,500 watts, my meter indicated 130%. If I multiply 12,500 by 0.8, I get 100%, so should I adjust the meter down to 100% with a 12,500 watt resistive load?
That sounds correct according to the manual. Then if you have a typical real-world load of 10,000 VAR, PF=0.8, the meter will read 100%.

Should I be seeing 41.6 amps with a 12,500 watt resistive load @ 240 volts?
No, the oddity is just in how the manual says to calibrate the meter so it reflects the capability of the generator with real-world loads. 12,500 watts (resistive) ÷ 240 volts = 52 amps, and that's a 25% overload, even though the meter reads 100% This means that with a purely resistive load and a meter calibrated according to the manual, the meter is misleading, and reads too low.

Also note that the 10kw rating of the generator is at an ambient temperature of 125°F, so one would assume that at lower temperatures, the generator is capable of higher output without damage.
 
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