Good afternoon, SF. Wanna help me fix an old generator?

It's a Kohler portable, built in the 80's, I believe. On the engine side of things, it's sweet -- purrs like a kitten after getting over its initial cold-naturedness. But on the generator side, I have problems. The voltage output isn't constant, and won't adjust for load, so I'm thinking it's the voltage regulator. And that's about the extent of my knowledge so far.

As for the machine: it's labeled as a Model 1750 MBM25, Spec 142105. "Rated AC: 1500 watts, 12.5 amps, 60 Hz. 12 volts, 10 amps, DC"
The engine is actually a Briggs, I've learned -- Model 113908.
I used to have a manual around here somewhere, but darned if I can find it right now.
Looks like the one pictured here: https://offerup.com/item/detail/534407283

I've been surfing the web a little, looking for information on a replacement voltage regulator. Findings haven't been good, with some listed at over $500. One, that assumes I'm looking up the right part number and two, it's more than the gen sold for to begin with. So I feel like I'm pointed in the wrong direction so far.

Anyone got experience or a good parts list for this little critter?

Thanks, all.

Most of those used capacitors for voltage regulation.
With no load what is the HZ? Or frequency

The generator engine speed determines HZ or frequency. A two pole generator runs at 3600 rpm to make 60hz. A four pole generator runs at 1800 rpm to make 60hz

So at no load it should run at that rpm and when a load is applied the throttle should open to allow more fuel to keep the rpm constant, so make sure the throttle is opening when you apply load
The speed should stay the same just like giving a car more gas to keep the cars speed at 55 when going up a hill.

Need to use a meter and check the HZ and voltage and see which is dropping when load is applied if HZ drops (engine speed) then the voltage will also drop and it is a fuel or governor issue.

If the HZ stays the same and voltage drops or is erratic then its a voltage requlator issue or generator end issue.

Look at the generator end shaft away from the engine if there are two brass slip rings with carbon brushes that ride against the slip rings and they are dull and tarnished that could be your issue and you can use a scotch bright green pad and clean the brass slip rings

Thank you, kg! I don't know the frequency right now -- I have a tach on order, should be here Monday. I'll at least be able to check engine speed at that point. I'll try to rig up a load and a meter over the weekend and check for voltage drop.

I can tear into it this weekend to look for the wear/tarnish on the slip rings.

If you have a basic multi volt meter most will read HZ and voltage.

Most of those small generators and rv generators are set by droop. They set approx no higher than 62 hz at 124 vac with no load applied and when full load is applied they drop down to no lower than 58 HZ at 118 vac

Small space heaters and hair dryers make a good load as they are normally 1500 watts

I can hear my future: The Lovely Girlfriend says, "why do you have my hair dryer in the garage?"

quote:

Originally posted by vthoky:
I can hear my future: The Lovely Girlfriend says, "why do you have my hair dryer in the garage?"

Not as bad as having one scream that there are Garand parts in the dishwasher!

Easy way to make sure all cosmoline is cleaned off

She also didn’t like me using her clothes iron to raise up dents in the stock

You might go to YouTube and type “James Condon” in the search box.

This guy fixes a lot of generators, you might be able to contact him. I’ve spent many hours watching his videos.

Good luck to you.
.

Ugh. Mac (the neighbor who helped get Little Genny's engine running again) told me the other day, "she's as cold-natured as can be." It's true. It took about 5-10 minutes of yank-sputter-stall-repeat to get a smooth idle going. Once going again, quoting Mac, "she purrs like a kitten."

Once Genny was settled and idling, I set to measuring voltage. Yep... two volts AC. Two.

So I'm pulling off covers now, to see if somethings connected poorly (or not at all) behind the panel.

quote:

Originally posted by kg5388:
Not as bad as having one scream that there are Garand parts in the dishwasher!

Haven't gotten caught doing that yet!

- - - - - -

Well. Not much to find behind the panels, really. There's a capacitor (D33C3715E01), a circuit breaker (PR11-62-15.0A-XX), and a rectifier (250JB1L) in there. If I replaced them all (got sources already), it would cost less than 50 bucks. I can deal with that... IF it fixes it.

Five wires come off the winding, three to "the box," and two to the capacitor. I haven't been able to unwrap them all yet to map them out completely. But if I can isolate the winding and measure resistance (to make sure all three combinations are balanced) and find nothing's wrong, then I'm thinking it's in the rectifier.

Gotta tap the electronics gurus at work tomorrow and do some figuring on the winding. If I know the winding resistance and the engine speed, I don't think I yet know enough.

I didn't get it taken apart far enough today to check the slip rings.

Look up field flashing there are videos out there and some do it with an old drill and spin the drill by hand
They use residual magnetism and the capacitor charge to produce voltage and sitting up the capacitor will slowly discharge
I will try to find a good video when I get home tomorrow

I hit up Kohler's web site this morning and found the trusty "contact us" link. Filled in the information, asked for any available manuals, and went on about my day.

About an hour later, I got an email from "Christine" at Kohler, with a copy of the service manual. (Cool!)
There's a portion of the manual that details a setup for flashing the magnets. I stopped by Lowes on the way home and bought the necessary bits and pieces. Made the cable, did the flash job, and performed the old yank-and-crank bit for a few minutes. After a dozen or so attempts, Genny ran smoothly, at about 2800 RPM (the tach arrived today).

1. We're about 20% short of the 3600 RPM target. I didn't get to measure actual frequency.
2. Still only 2 (well, 1.8 - 1.9) volts at the AC outlet.
3. I removed and checked the capacitor -- all good, at the stated 15 microfarads.
4. I talked to one of the design gurus at work today, and showed him the wiring diagram I made yesterday after opening the covers. As soon as I pointed out the rectifier, he said "bring it in tomorrow, let's look at it." I mentioned that I've found a source for a replacement already, and it's $18. He laughed and said, "Pfft! That's a 50-cent part!" I've got more surfing to do, I suppose...

Anyway, I've removed the rectifier and will take it in tomorrow. We'll see what more is learned then.

If that rectifier is one of those ~1.25" square, flat mount bridge rectifiers, he's not wrong, that's absolutely not a $18 part.

I mean you can buy 5 of them on amazon for $9, if it's that part...
Looks like this

You could even build your own using 4 diodes for a couple bucks.

Somebody help me out. My knowledge of AC portable generators appears lacking.

What is the function of a rectifier on an AC generator? Does it have some DC out ports that require the AC to be rectified? Does the generator use DC to create or regulate the magnetic field in the armature or stator? I’ve really never worked on these much, so I don’t understand how they work.

Sigcrazy, this particular set has two screw terminals (DC) on the front panel, stated to be suitable for charging a 12V battery.

As for how the rectifier figures into the AC, I don't think I know yet.

quote:

Originally posted by stoic-one:
If that rectifier is one of those ~1.25" square, flat mount bridge rectifiers, he's not wrong, that's absolutely not a $18 part.

I got a quote by email for the exact replacement for this rectifier -- fifty bucks! That's nuts.

Some good talks at work today led to a verification that the square rectifier is good, and a the next two steps:

1. Do that flash-the-rotor thing again, but for 2-3 seconds (maybe 2-3 times). Oh, and put a current probe on one line to see how much is actually getting there.
2. There are two diodes on the rotor assembly. I need to scrape some varnish and test those. That involves a 40VAC (yes, AC) input on the coil. A great friend is bringing me a variac tomorrow.

- - - -

This generator is way different from motors I worked with in a previous job. I'm used to rotors with permanent magnets, and a wound stator. This thing has a wound stator, and a wound rotor with a pair of diodes attached. As I understand the explanation, the diodes take that sinusoidal AC current and "lop part of it off" to make each wound pole on the rotor something more like a permanent magnet. I'm not sure yet why the rotor just doesn't use permanent magnets, but that's more learning for a later date.

In the big picture, I'm a "digital input, discrete output" kind of guy.
This AC, DC, rectifier, capacitor business is voodoo to me. And when I get a handful of it, it Hz.

I’d be figuring out why it isn’t running 3,600rpm before anything else. At 2,800rpm, you’ll be getting 46.7Hz.

I found a couple interesting bits while learning what “flashing a generator” to "excite the field”really meant:

“ This tip comes from the Briggs & Stratton Customer Education Department. As an alternative to flashing a rotor winding with a battery applied to the brushes, an electric drill may be used. Follow these steps to flash the generator:
• Plug the electric drill into the generator receptacle. (Cordless drills do not work)

• If the drill is reversible, move the direction switch to the forward position.

• Start the generator

• While depressing the trigger on the drill, spin the drill chuck in reverse direction. This will excite the field and the generator will now produce electricity. If spinning the chuck one direction does not work, try spinning the chuck in the other direction as you may have the reverse switch positioned backwards.
Use caution not to get your hand or other materials caught in the chuck. As soon as the field is excited, the generator will produce power and the drill will turn on.
The reason this works is because the electric motor in the drill will act as a small generator when spun backwards. The magnets in the drill's motor induce a voltage into the motor windings, which is fed back through the trigger, cord and into the generators receptacle. From there it goes into the power winding of the stator. The voltage going through the power winding creates a magnetic field, which is intensified due to the iron core of the stator laminations. The rotor intersects this magnetic field as it is spun past the power winding, thus inducing a voltage in the rotor winding. Once current flow is present in the rotor winding the rotor has been flashed.
If flashing the field does not make the generator work, you may have additional problems, besides a lack of magnetism in the rotor. Further testing will be needed. Hopefully, this will give a simple way to field flash your generator if needed”

And:

"Thanks for the reply.
I dug a bit deeper and found that this generator has a separate winding with a rectifier to power a dc circuit. The DC from that circuit is also used to excite the field. So technically it's really an alternator, not a generator.
My ohm meter showed the rectifier to be defective.
Disconnected that winding and hooked a 12vdc battery to the field, started it up, and voila! 130 volts ac came out.

A new rectifier shouldnt be too hard to find or make.”

Link

Here is a great troubleshooting manual for capacitor regulated generator ends.

If you can flash the field and get 130vac then the rectifier can be bad but make sure to test the square rectifier in all directions as one side goes one way for pos and the other side goes the other way for neg
Those diodes down in the generator end are used as surge suppressor.

You may have to do the 120 volt method to flash the field and totally charge the capacitor

Marathon capac regulated light tower generator end

Thank you, kg and trapper. I haven't gotten to work on it this week (and may not this coming weekend), but I'll get back on it next week. Will keep you posted.