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G8MNY  > TECHNI   05.10.21 09:20l 161 Lines 6888 Bytes #999 (0) @ WW
BID : 48699_GB7CIP
Sent: 211005/0712Z @:GB7CIP.#32.GBR.EURO #:48699 [Caterham Surrey GBR]

By G8MNY                                     (Updated Aug 21)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

I was given this 87cc genny, it was very clean & hardly used, but been garage
stored (some corrosion) for a few years. It was supplied by NuTools. In layout
it is very similar to my much bigger well-used MEDUSA SIP2300 2.3kW one.

                   __________________   Tube
/~~~~~~Tank~~~~~~~\               /"        6L Tank       "\ Cage
-------------------      Fuel Tap ==========================
/~~~      & filter\o~~~~~~~~~~~~~~~~~~~~~~~~o
        Tap(0)elec           STOP  [] on_ATrip        DC 1x 2Pin
Choke //       \___         SWITCHo off  VC-| DC      o AC 1x 13A
Ŀ  /~~~~~\\.  Pull           
۳In- ,' /~~~\/\\  Start          ۳F ENGINE           
۳Take  ( )                   ۳A    oil(=) ALT    
 \ \___/  /                 ۳N  ______________
     ' Ŀ `                   ~~~      

It started on "easy start" ether spray OK. But the mixture was very lean & need
the choke partly on, to run for several minutes. (old Petrol?) I striped down
the carburettor & clean the main jet, then it was a little better.

There is next to no tank sump, so an unlevelled ground may result in fuel
starvation, despite there being quite a bit of fuel left. This not helped by
the "in the tank" hidden fuel filter & a fully "screwed in" tap!

   Min Petrol   -Filter
        Level---            So make sure the tap in only "just" (2 turns)
 Ŀ____   screwed into the tank, & then do up sealing Nut.
         Sump/ \O ring
Unused thread _    & Nut
          ____/ __ \
Carb Pipe ____ (_) )

No circuit in the handbook etc. So I have reverse engineered this diagram.

   12V    F R O N T   240V AC            STOP
Charging  P A N E L    Ŀ          o\__
  + -                E    __         bk bk     Earth
 3A Bridge          N   L ///        __          __
 Rectifier                           ///          ///
 4A*                    4A                  
 Trip                  Trip                 
  gn   12u 450V   bk rd           External Pointless
-  -  - - - - - -  - -  - - - - - - - - -  - - - - -
        ĴĿ          RĿ | Magneto Coil
        ye ye   bn bn  Ĵ>Ĵ |    ()))))ͻ
  .4    9.6    4.3        |    0.2 5k6      EHT
  ())   (((()   (((()  (((() |            > <1/4
   ===     =====     =====    =====  |   __      __ Plug
  Load    Exciter     Load    Rotor  |   ///      ///
        A L T E R N A T O R          |   F L Y W H E E L

This is a brush less alternator, with the excitation of the rotor magnetically
charged up each half cycle from circulating 90 out of phase currents from the
exciting 12uF capacitor. The rotor winding has a diode across (& back emf R) it
to maintain the current half a cycle after the kick (100 times a sec).
Excitation starting with the weak residual magnetic rotor field, needs at high
enough RPM (2000RPM) to overcome the rotor diode voltage drop, About 10% of the
power is needed for excitation of the rotor, hence alternator fan must be kept
efficiently working.

This system is more reliable than the DC to brushes & slip rings type, but it
does produce a slight kink in the load winding waveform & there is no
opportunity for voltage control other than engine RPM.

CHARGING (20V off load)
The genny came with the special DC plug & lead (- |), but the lead is short &
the trip was only 4A, & it trips all the time @ it's rated 5-6A charging
current, unless the RPM is well reduced. So I swapped the trip for 10A more
reasonable rated one.

The rectifier is only made from 4x 3A diodes inside the connector socket, with
no heat sink! To improve on this, I doubled up on the diodes, putting 4 more
1N5402 diodes on the copper side of the PCB & flowing lots of solder on the
tracks to improve heat dissipation. Note the current trip does not prevent
blowing the diodes on wrong battery polarity! So a 13A fuse in the charging
lead will help that!

This is a simple bob weight & spring RPM controlled engine governor, with the
alternator's capacitor self-exciting winding at 90 to the load winding which
gives some load to voltage compensation. On this generator these two features
alone seem to give quite good results.

  Load  Volts
    0W   240
  275W   237
  500W   235
  775W   230
 1000W   220
 1100W   215  Max Throttle!

This was quite high, noticeable on HF - UHF, as there are NO suppression parts,
despite the CE compliance markings etc. I did 3 things to get it down 30dB.

1/ Change the spark plug for a resistive type (measures between 5K-15K) it must
   be same reach & ideally same temp type.

2/ I added a non-inductive 5K6 in series with spring inside the spark plug cap.
   ( part unscrews)

                    | _==== < spike
 S=Shortened Spring | S ----
 R=5k6 bare ends    | R |
                    |   |
                    |   |

3/ Then I added a coax braid over the EHT lead & earthed it at the cylinder
   head. The braid is kept insulated other than at the cylinder head end!

           .-----================================----- EHT Lead To
           |n---================================ Magneto coil
           ~  //        Added Coax Braid
          ݱ || Braid earthed
Cylinder  to head bolt
Head      ~u~    with a tag.
       Spark Plug

A/ Reduced the tappet clearance as it was quite noisy (rattly).
B/ As with other Gennys, I added an earth lead to clip on earth stakes etc.
C/ And also the 13A socket rattles any plug out a in a few mins, so a Bungee is
   needed stretched over the plug to keep it in.

I had problem with the excitation capacitor (large AC cap), after an 8 day
summer camp where its was heavily used. The moulded 12uF capacitor bolted in
the alternator has dropped to only 3uF. I replaced it with 6x 2uF 450V AC
fitted into the control panel & wired to the alternator OK.

See my TECH buls "Regulating 12V Generator Output", "MEDUSA SIP2300 Generator
info", "MEDUSA SIP2300 Generator Repairs", "Cheaper Generators" & "Petrol
Generators for /P SSB"

Y Don't U send an interesting bul?

73 de John G8MNY @ GB7CIP

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