What are these parts on a generator? Please help me identify and understand.

[TheBrick]

Hello again. I know this is not an electronics question but I guess most people on here would also have at least a basic idea with regards to power electrics.

I've gotten my hands on a generator and while  I wait for a carburettor seal to arrive I thought I'd take a look at the electrical side as I've never had one apart before. I was expecting just some leads off of the brushes to run up to the the plug but was meet with a bit more than that. I took some photos and would like to see if anyone can give me some incite into what is going on here.

The stator looks like it is made up of two coils fed from the green finned thing (looks like they are a heat sink but for what?) in the bottom right of the picture, I guess the coils are energised once the generator is up and running. But how dose the generator get start producing any electricity to create the magnetic field in the first place. Seem like a chicken and egg situation. Or maybe there are some weak permanent magnets are in the middle to start things off?



I'm also guessing the small black box with 4 terminals is a bridge rectifier used to feed the stator coils? If not what could it be?

The last part I'm unsure of are the two white cylinders with three terminals and what look like fine green thread wrapped around them. A clear picture shown below. What is the function of these? One of the terminals on one of these cylinders is attached to one of the leads coming off of one of the brushes. I can't remember about the others. I try and fins some time tomorrow (in-between the festivities)  to trace and draw up a schematic.



One more pic for completeness.

[david77]

The green finned thing is a selenium plate rectifier (free translation from German name for it).
The rectangular thing is indeed a bridge rectifier.

I don't know why there are two rectifiers in the generator, though. Maybe the original plate rectifier is broken and was replaced with the bridge rectifier at some point - just as as guess?

[PA0PBZ]

The green finned thing looks like a selenium rectifier to me, however I have no idea why it's there, the last one I've seen is like 30 years ago:



The next one looks like a bridge rectifier indeed, although anything could be hiding in there. There should be some markings on, like ~+ -

The white cylinders must be resistors, but again their exact function is hard to tell without a wiring diagram.

[TheBrick]

Interesting about the selenium rectifier. I'll try and sketch out a diagram tomorrow. Thanks

[mc]

Standalone generators work by relying on residual magnetism to start generating. If they're not run for a prolonged length of time, then they can fail to start generating, and the usual fix is a brief application of power to certain terminals.

And going by your description, you don't quite understand alternator basics. The rotor (i.e. the bit the brushes run on) is essentially a spinning electromagnet, and by varying the amount of power applied to it, it controls the voltage produced in the stator coils, which is what you're power leads get attached to.

[SeanB]

The selenium rectifier ( green stack of plates) is there to provide the DC to the rotor via the slip rings, the power resistors ( cylinders) are there for controlling the current through the rotor and thus the output voltage. The second bridge is most likely there for the 12V output to charge a battery.

It produces AC voltage that is fed to the socket from windings wound in the field coils, the rotor is just there to provide a magnetic field from a big coil of wire fed from DC made from a 12V coil wound on the field as well. Generally the coil that provides the excitation voltage is wound so you can get 12V at a few amps more than the fiels uses ( normally around 5A or so depending on output power rating)  to do battery charging if the unit has electric start, so you can charge the starting battery.

[TheBrick]

Thanks some good info there.

And going by your description, you don't quite understand alternator basics.

Quite possibly!

The rotor (i.e. the bit the brushes run on) is essentially a spinning electromagnet, and by varying the amount of power applied to it, it controls the voltage produced in the stator coils, which is what you're power leads get attached to.

I was assuming a stationary magnetic field in the stator coil and a Ac induced in the rotor coil. So effectively the reverse and a frame of reference change.

Ok I've sketched out the connections below. The top b.r seems to only feed the upper coil and the lower coil is off of the selenium rectifier. Also one thing that does not make sense from the above descriptions is that the rotor coil feeds into the b.r labeled AC.  Any further thought on this?

[TheBrick]

Standalone generators work by relying on residual magnetism to start generating. If they're not run for a prolonged length of time, then they can fail to start generating, and the usual fix is a brief application of power to certain terminals.

That's interesting. What do you mean by brief? and I assume you mean something of the order 12v as it is implied somewhere else that this is what is sent to the coil during operation.

[mc]

It varies between alternators, however most modern ones involve 12volts across the capacitor(s) via a fused jump wire (fuse is more to do with avoiding exploding parts if there is a major fault) without it running.

You really need to see what the manufacturer recommends, as applying power in the wrong place can quite easily damage things, especially on ones with electronic voltage control.

However with something like your's without any fancy control electronics, I'd try 12v with a 21W bulb in series across the + and - of the bridge rectifier with it spinning and see if that was enough to kick it into life.

[TheBrick]

Ok makes sense thanks mowicu.

I was just double checking the diagram I copied down and the was the selenium rectifier is wired in does not make sense to me compared to how the silicone  rectifier  is wired in. The selenium rectifier seems to only take the voltage drop over the R2 as its AC input. Also on this generator it does appear as if the stator coils are receiving the DC current and not the rotating coil as suggested earlier by mowicu. Is this just a design choice?

[SeanB]

OK, so the selenium rectifier is actually doing voltage regulation, the current being used to boost the output voltage somewhat via the extra coil saturating a portion of the stator winding as output increases, so that the field gets stronger and increases the output voltage to compensate for load and speed changes. Not terribly good but better than nothing. If you want to repolarise the rotor then simply take a 12V battery and a 50W lamp and connect them in series, with the positive connected to the + on the bridge via the lamp and the negative connected to the - on the bridge, so that about 5A flows through the coil. Then start it, and it should bootstrap itself and give the AC output.  Disconnect the lamp and it should continue to generate AC.

[G7PSK]

As that is a rotating field machine you can flash it by connecting 12 volts across the slip rings the usual thing is positive is the one closest to the end bearing all that is needed is to just touch the wires from a 12 volt source onto the brush gear I use a 12 volt lantern battery, it is just a case of touch the wire and remove it less than a second of contact.  Is that alternator Italian?

[TheBrick]

OK, so the selenium rectifier is actually doing voltage regulation, ....

Interesting. I think I kind of get it but will need to do some more reading to fully understand. Thanks for pointing me in the correct direction.

[TheBrick]

As that is a rotating field machine you can flash it by connecting 12 volts across the slip rings the usual thing is positive is the one closest to the end bearing all that is needed is to just touch the wires from a 12 volt source onto the brush gear I use a 12 volt lantern battery, it is just a case of touch the wire and remove it less than a second of contact.

Thanks more good info.

  Is that alternator Italian?

Well the stationary engine driving it is a Honda G300 but I haven't looked to see wh0 the manufactured of the alternator is.

[TheBrick]

Just an update on all of this. The silicon rectifier was duff. I energised the coil as suggest and it all work lovely. Thanks for the help explaining the way the regulation worked.