How to flash an AC generator

Do you have the owners manual? It should describe how to do it.

This begs the question as to WHY a previously working genertor would need to be flashed in the first place?

The rotation iron always magnitized in the same direction so what even could cause it to lost its residual magnitism to the point where it can't "bootstrap" up from near zero but NOT zero magnetism?

EMWTK

The residual magnetism in the iron will weaken over time. After a few years it can be too weak to bootstrap the generator.

Anthony

Here's how I have always done it...

Get to the brushes and disconnect them from the circuit board or rectifier. Connect jumper leads to a 12V battery then connect them to the brushes for a few seconds. Polarity doesn't matter. That's all there is to it.

Q

Anth>> This begs the question as to WHY a previously working genertor would >> need to

While the generator is NOT running.

Temporarily (~10secs) connect up a charged 12V battery to one of the

110v outputs. Disconnect battery and start generator..

What works for me is a special connector attached to the brushes, with a 1 amp diode in series. If the alternator doesn't produce power at startup I plug the starting battery (12 volt) in for a few seconds. Free men own guns - www(dot)geocities(dot)com/CapitolHill/5357/

Forgot one step.

After it's started, connect a resistive load, like a pair of 110v incandescence light bulbs, to the unit as you bring the genny up to speed & freq.

This method

is supposed to work. Easy enough to try.

Wayne

Before you flash it try prodding the throttle and see if some increased RPMs will get it going.

I've done it with a 6 volt 5 Ah battery with success. I just connect one side of the battery to the AC output and while the engine is running momentarily connect the other side. Once the AC voltage starts to climb I disconnect the battery. You might want to put a fuse in line with the battery so you don't blow it up. Then run it for a while with a load (like someone else said).

I've not done it sucessfully with the generator not running. It sounds safer but had no effect on my particular genny.

Basically you need to magnetize the field winding with say a 12vdc source. The owners manual typically will describe the procedure. The last gen that I flashed was a industrial 50hz 3kw motorized generator. I disconnected the field and fed 12vdc into the field windings. As long as the field core is slightly magnetized it will be enough to get the stator to produce enough voltage to get the exciter going.

Cheers

Get a 12 volt battery, use the one in your car or use a battery charger if you have one with sufficient current capability. Open up the connection box on the generator and find the leads coming from the stator. These connect to the outlets. Disconnect them from any other wiring and momentarily connect them to the 12 volts. Simply brushing the wire from the battery against the generator wire will be enough. Reconnect the stator wires and crank.

This pulse of current re-establishes the residual magnetism necessary to bring the generator up when started. This magnetism can go away over time.

If the above "static flash" doesn't work then do a "dynamic flash". Connect a "Jesus cord" (plug on one end, alligator clips on the other) to the outlet on the generator. Connect a 100 watt 120 volt bulb in series with the hot leg. Apply the 12 volts between the bulb and the neutral of the line. The instant the DC current flows, the generator will start generating 120 volts AC. The bulb is to prevent much AC current from being applied to the battery. The bulb should light instantly and remain lit as long as the battery is connected.

If this doesn't work and if your generator has slip rings and brushes then the next step is to see if the brushes are stuck in their holders and/or the slip rings are glazed over, preventing the rotor field from being energized. At this point you really do need the manual to assist with locating the brushes and accessing them.

John

-- John De Armond See my website for my current email address

Hi Guys, I've read the article on flashing a generator and have tried all recommendations to no avail. I have a small 3KW generator attached to a Honda G200 petrol engine. I've had several problems with the engine but have resolved these and it is running fine. My problem now is that I have no output from the generator. I'm not sure how long the generator has been stood without any use and suspect it is simply a matter of flashing it again.

I've con checked the stator with a DVM and the windings seem to be intact as are the windings in the rotor.

The unit is a dual 110V / 220V output and the stator has two separate windings which via a switch and are either used in parallel for 110V or in series for the 220V output. There is also a 10uf capacitor in the circuit but I'm not sure where this connects?

The rotor would also appear to have two separate windings with what looks to be a diode in series with each winding. Unfortunately I can't tell from the markings on these to confirm if these are indeed diodes.. I also assume the rotor is self exciting based on residual magnetism?

Would anyone have a circuit diagram of a typical generator set up as this or provide me with a description as to the principles of how self exciting rotors work.

Many thanks

Kindest Regards

Tom

It's a separate winding in the stator. The voltage is generally quite high -

250 or more. It's strictly a parallel LC circuit.

John

-- John De Armond See my website for my current email address

Thanks John, I suspect the capacitor at the moment. I've removed this and checked it with a DVM I get a momentary short circuit reading and then it returns to about 56 Ohms. I would have thought it should have returned to a value much greater than this? I've ordered a replacement capacitor of the same value and voltage to see if this makes any difference.

Many thanks

Tom

That cap sounds maybe OK, if you meant 56k instead of 56 ohms. Motor run caps like that commonly have an internal bleeder resistor. You should see the ohmmeter kick toward zero ohms and then drift back to the value of the bleeder. 56kohms is about right for a bleeder.

Can you run the generator? If you can, measure the voltage across the cap. If the cap is bad or leaky, the voltage will be below about 150 volts, usually way below. If the cap is good then the voltage is typically 250 volts or more. If you can measure frequency, the frequency will be the 3rd harmonic of whatever speed your generator is running at. If the cap is bad the frequency will be primarily the fundamental,

50 or 60 hz, depending on where you live.

The generator may not build any voltage at all if the cap is bad. If it doesn't then hook a 12 volt car battery through a 120 volt, 100 watt lightbulb to the generator's output. That'll supply enough field to get it to come up and if it happens to come up to full voltage, the 100 watt bulb will protect the battery.

BTW, that's a standard motor run cap so you can pick up one at any appliance parts store, HVAC supplier or electric motor repair shop. If they see you coming, it'll be $30-40. If they give you the normal "wholesale" price, figure $5 to 10.

You can substitute a plastic cap, polypropylene or whatever, of the same voltage and capacity. Motor caps are no longer oil and paper. Inside the can IS a plastic cap. They just put 'em in the cans so as not to confuse the HVAC parts changers. The can is mostly hollow or oil filled.

John

-- John De Armond See my website for my current email address

Hi Guys, I've done some testing this morning on the generator with a 240V

60W lamp in series with the stator windings to try and induce an EMF in the windings.

I then took the following measurements:

Exciter windings with the Capacitor removed (Test points A - B) = 2.46V AC Exciter windings with the Capacitor installed (Test points A - B) = 0.93V AC

Rotor Windings with the Capacitor removed = 153mV DC (Test points A1 - B1) and 174mV DC (Test points A2 - B2) Rotor windings with the Capacitor installed = 1.42mV DC (Test points A1 - B1) (1.94V AC) and 1.42mV DC (Test points A2 - B2) (up to 10V AC changing). I suspect one of the Rotor diodes may be faulty as I get the same changing readings on the same winding if I rotate the rotor 180deg.

I've also rechecked the capacitor with the DVM and it is only reading 56 ohms after the initial charge kick.

The following is a link to the test set up and how I believe the windings relate to each other.

I'm not sure about the orientation of the diodes as these will have to be disconnected before I can check them. As soon as I have disconnected and checked the diodes I will update the list.

I can't get your schematic to load any farther than the bottom of the rotor diagram so if there are any notes there I didn't see 'em. Tried Opera and Firefox.

That's odd behavior. Rotor diodes are a good suspect but I might also suspect at this point a shorted turn in the stator. I'd do a couple of things. First, increase the current input. When I recommended a 100 watt light bulb, I was thinking around 1 amp for our normal 120vac. If you had a 500 watt heater that you could put in series, so much the better.

My other thought is, can you remove the stator from the rotor and test it separately?

In the motor shop I'd now move to the Baker Surge Tester

Unfortunately they've password protected their user manuals (!) so I can't send you there to see how this test works. In a nutshell, the instrument "rings" the winding with a high voltage pulse and analyzes its decay behavior. An OK winding will ring for 3 or 4 cycles while a compromised/shorted winding will either not ring at all or will damp down in one cycle.

If you have an O'scope then you can somewhat simulate this test by passing DC through the winding and snapping open the circuit with a cap-shunted switch (same as old points-type car ignition) while monitoring the winding with the scope.

We have another gadget called a core loss tester. This involves passing a couple of loops of heavy cable through the stator and passing high current AC through it. An AC buzz-box welder will do. Then we scan each stator bar looking for differences. The instrument has a probe coil connected to a meter and headphones. The probe is held over the stator and gradually moved around. Any variation in intensity registers. A shorted turn will essentially eliminate the magnetic field. That area of the stator will also get slightly warm from the circulating current.

I'm still not happy with that capacitor reading. 56 ohms is far too low for a bleeder. I also don't think the meter would kick much initially with that little resistance across it. Are you sure your meter is OK?

I normally don't suspect test instruments because they are so reliable but every reading you reported below is highly abnormal. Do you have another meter you can try? I've never seen a shorted rotor diode cause the excitation winding voltage to be that low.

At this point, if your meter is OK then I'd probably pull the rotor and stator apart and test each separately.

John

-- John De Armond See my website for my current email address

Hi John, many thanks for your reply. The schematic is just a jpg image with no additional notes. It should just show the stator windings (two coils in series) the exciter windings with the capacitor in parallel and the two rotor windings which I have now updated to include the capacitor in parallel with these and the diodes. Let me know if you want me to send this to you directly via e-mail.

I've rechecked the capacitor with another DVM and I get the same readings as before, 56 Ohms. I don't suspect my original DVM as I've had this for a few years and it has never let me down yet.

I've removed the diodes from the rotor and they appear to be ok. Forward bias on both diodes gives me about 143 ohms and infinity on the reverse bias. The diodes also have a small ceramic capacitor in parallel with them which I didn't see until I removed them from the rotor.

The capacitors do not register anything when checked with the DVM and I assume these are ok. I can't work out the size of these from the markings (V350K10) but I'm guessing either nF or pF. The diodes are not like any other diodes I have come across and look more like two diodes in series on the same body. There are two sets of numbers on these, one on each body casing I9445-6 & I9433-6. Can you identify these from the numbers?

The rotor winding would also appear to be ok as both of these show about 3 ohms with the diodes removed.

I think I'm back to replacing the main Capacitor when it arrives, it was ordered on-line over the weekend and should be with me later in the week.

Thanks again

No idea what those numbers are. I tried my usual SWAG and stuck the numbers into Google. No hits that made sense.

Yep, that's what I'd do next.

John

-- John De Armond See my website for my current email address