[Milsurplus] A generator question...

[Kenneth G. Gordon]

On 24 Oct 2017 at 23:17, Bruce Gentry wrote:

> The only type of generator that can be motored to roll an engine is one 
> with a DC communtator.

Yes. I had thought of that.....but....see below.

>  Applying power to the field alone will not do 
> anything. I had a PE-75 and used batteries to start the engine as you 
> have. HOWEVER, the field is connected to the DC brushes and armature.

Yes. Exactly.
  
> Newer generators do not have DC communtators,, they use small permanent 
> magnets to give enough excitation to produce a small output. This weak 
> AC output is rectified and fed into the rotating field to cause the 
> generator to build up to full voltage.

Hmmm.....OK. I can see that.

This particular generator does have a DC field winding. But the DC to excite that field 
comes from the AVR (Automatic Voltage Regulator) module.

I suspect that whatever takes place in the AVR is somewhat complex, but I don't know for 
sure.

The schematic diagram for the entire generator SEEMS to show brushes at the input of the 
FW (Field Winding) but the schematic representation is not one I've seen before. To me the 
symbol looks like what a commutator would look like to someone who isn't familiar with the 
older symbols.

If you don't mind, I'd like to send you the schematic for you to take a look at. It is a small 
PDF of 103 kB.

IFF the FW does have brushes attached to its input, I suspect I could install a 2PDT relay at 
that point, and connect an external battery to those brushes, motor the generator, then 
when I release the relay, the system would go back to the normal connection....but I don't 
know and won't until I try it. And this will work ONLY if there is sufficient residual magnetism 
left in the core.

To me, the critical component is that AVR, and I have no schematic of that, and since I have 
no idea what is in it, I don't want to apply voltage to it..

>  The only practical way to start 
> these electrially is to add an electric starter if the engine can accept 
> one.

This particular engine has no place to mount a starter motor; BUT, if all else fails, I can use 
part of a recoil-mechanism from a rope starter attached to a battery-powered electric drill to 
remotely start the engine. The recoil mechanism automatically releases itself from the 
engine when the engine starts.

But if it is possible to "motor" the generator, that would be far simpler....and quieter... I like 
"quiet."

> The newest generators use an inverter, the rotating field is 
> permanent magnet. Electric starting is possible, but would require a 
> total re-design of the inverter circuits and probably a 24 or 36 volt 
> battery.

This generator is NOT an inverter type: from the schematic, it looks pretty much to me like 
every other generator I have ever had.

It does have two additional windings.... and two additional connections to one of the main 
windinngs that I have not seen before. 

The two additional windings are 1) an exciter winding labeled EXW (obviously AC) and 2) a 
center-tapped DC winding, labeled DCW which feeds a solid-state full-wave bridge rectifier 
to provide "12 VDC" to charge a battery. 

The two "extra" connections to one of the MW "Main WIndings" are connected to the AVR.

There are six wires going to the AVR. It looks to me as though 4 of those wires are outputs 
and only two, the two which go to the MW, are inputs. The outputs go to the EXW and a DC 
output goes to the FW through what I ***think*** are brushes...

BTW, whoever designed the generator was pretty clever. It is very simple, and therefore 
pretty reliable.

Anyway, to me it is in interesting generator.

BTW, I had a PU-286G once, which was superb. Designed for a radar set.

Thanks for the input, Bruce.

vy 73 for now,

Ken W7EKB

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