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Small generator regulator - how does it work?

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I was trying to repair a 550 W Honda petrol generator today. It's about 20 years old, and has 12 V dc and 240 V ac outputs.

I found that both output voltages were really low (about 8 V / 135 V) so I had a look inside the alternator. All the stator windings seem to be about the same thickness, which was a bit of a surprise, as I thought that the 240 V windings would need many turns.

What's really odd was the rotor. It seems to be 2 poles, with a separate winding on each pole. The rotor windings connect to a couple of axial components. There are no brushes or sliprings. Most of the rotor is laminated with a small permanent magnet set in the laminations at one end of the alternator.

The alternator voltage is very dependent on motor speed, and it turned out that the low voltage was due to it running slightly slower than it should have. When I increased the motor speed a little bit, the voltage came up to what it should be.

Does anyone know how this sort of generator / regulator works?
I've never seen anything like it. But since the rotor has windings, and there is no comm/slipring to get external power to the rotor winding the logical conclusion is that the electrical current in the rotor windings is induced by interaction with the stator field while it is rotating. Like the reverse of a squirrel cage motor.
Does the engine run at 1800 rpm's? A two pole would be 1800, single pole is 3600 rpm for 60 Hz. (1500,3000 rpm for 50 hz)

The magnets you see are kick starters to get some generation started to feed the field coils to build up full output.

I had an old generator that the magnets died out which prevents the system from starting up. I rigged a battery flash with a single blocking rectifier diode to kick start the generation. Just a quick momentary external battery connection of about 3 vdc would get the field to build up. You have to get the polarity correct. When running, the field winding voltage would be like 90 vdc so I used the blocking diode to ensure I would not blow up the two ni-cads I used to flash the field coil.

During normal operation the coil field windings produce a much stronger field then the magnet and with parallel polarity. Like in grade school, when they told you not to put the magnets back in the box repelling themselves, the magnets embedded in the face of the generator rotor will eventually weaken due to the strong repelling field of the active field winding.

This all assumes you don't have some other problem like a blown field voltage rectifier. The part that does not make sense is you say there is no slip rings. If there is a rotor coil there needs to be a feed to it. Is there a coil off to the side outside the main stator coils that might act as a transformer coupled to the field winding? I assume the small axial component you mentioned is a rectifier diode for the field winding feed. Perhaps the appearent two pole winding is actually a single pole being fed from inducted input from second field winding. This would make sense if the magnets are embedded in only two of the four rotor coil faces which would be the actual field winding pole. If there is no slip ring to do the battery flash you have to fix the magnets. You might get them recharged somewhere.

Regulation of field winding current is based on generator output load. The more current in the stator winding the more current in the rotor field coil. Not a great regulator but is works. As the generator is loaded more there will be some drop in output voltage. All these type of generator need to have rpm regulated which is a mechanical wind vane feedback system to control throttle. There is usually an adjustment on this mechanical linkage to set the rpm of engine. You can use a freq counter to adjust the engine speed.
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The magnet is just to get it started. RB is right on. Circulating current is induced into the rotor, which in turn creates current in the stators. The low Voltage winding is rectified in the regulator. It is also shunt-regulated to control the output voltage.
Sounds like a brushless alternator design. What it uses is a small secondary alternator and a set of diodes on the rotor to actually power the rotor coils. There will be a small field on the outside of the rotor that induces the power into the rotor alternator assembly that powers the actual main rotor fields which in turn drive the actual output windings.

The 12 volt output is likely just wound with the same gauge wire as the 240 volt output circuit so you dont actually see any different windings for it. They just look the same.

The 2 pole design is a 3600 RPM system at 60 Hz. A 4 pole is a 1800 RPM unit at 60 Hz.

Many of the old low end portable generators used a rather poor regulating system. The voltage is in fact set just by RPM's. Its a relationship between speed and the simple voltage/current feedback loop of sorts. It likely has a current sensing winding on the fields that drive the rotor energizing circuit. As the load increases the added current through a specific set of coils increases the power to the rotor in order to keep the voltage up on the actual output windings.

It was a rugged and rather reliable design with the added benefit that once the load exceeded the engines output the systems power feedback loop collapsed and made it nearly impossible to actually burn up the generator. The down side was it had a limited power output and rather poor voltage regulation in some conditions.
Thanks for that.

There were certainly no brushes or slip rings.

The laminations on the stator and rotor are only about 4 cm (1.5 inches) long. The rotor is about 10 cm (4 inches) diameter and the stator about twice that.

There is no separate exciter part. The only thing that changed along the short length of the rotor was the magnet about 1 cm square embedded in the laminations at one end. I can follow TCMtech's description, but on other alternators where I have seen an exciter generator and rotating rectifiers, the exciter was completely separate. On this machine I can't see any exciter.

(I'll admit, the larger machine was 6000 times the power of this, so efficiency becomes more important, and size, weight and manufacturing cost less so.)

I think TCMtech is right in that the voltage was set by the RPM, and when we tried it on a hairdryer that tried to take too much power, the speed would hunt, slowing and speeding up in about 2 - 3 seconds, but not doing any damage. It was stable at lower loads.

It seems to be 3000 rpm, 50 Hz. It is 2 pole, and it is a tiny 4 stroke single cylinder Honda engine, so 1500 rpm would be too slow.
got any pictures? Its sounds like an odd design. I assumed when you said 550w you had misprinted 5500.
I dont recall having ever seen an old 550 watt generator. I can only guess its some sort of inductive resonance type of control system.

The only systems I know of are these type.
If you take a three phase motor and put the right value of capacitors on one of the three phases and spin the motor at slightly higher than is synchronous RPM it will develop a resonance in that winding that will keep the system energized so it works as a brushless alternator.
The other two phases can have power drawn off of them like a generator up to a certain point. After that the LC tank exciter thats made from the third phase looses it power and the system simply drops out. Once the load is removed it will restart again.
If a set of small permanent magnets are added to the rotor where the poles are it will self excite much faster and stays more stable as well.
I suspect that may be roughly how yours works.
It is certainly 550 W not larger. It has one handle and it can easily be picked up with one hand, including a 2 litre fuel tank that lasts half a day. The 12 V output is rated at 8 A or so. Come to think of it, that is nowhere near 550 W so it would account for the winding being the same diameter as the 240 V one.

I'll try to get some photos. It's not my generator, and it seems to work as good as new now that the the speed has been set right, so I don't know if I can open it up for no good reason.
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