G8MNY > TECH 18.06.18 10:30l 317 Lines 15732 Bytes #10 (0) @ WW
BID : 25696_GB7CIP
Subj: Petrol Generators for /P SSB 1
Sent: 180618/0817Z @:GB7CIP.#32.GBR.EURO #:25696 [Caterham Surrey GBR]
To : TECH@WW
By G8MNY (Updated Jul 16)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
/P POWER NEEDS
A generating set capable of handling several 400W SSB stations at once is very
uneconomical. This is because a typical constant RPM engine & alternator uses
about 30% of it's energy just to overcome the mechanical losses, cooling etc,
as well as the alternator exciting & cooling needs.
Using an under rated generator set, is far more economical, but obviously very
prone to regulation problems on peak load.
By floating a car battery on a small power source, very large peak power can
easily be drained for SSB work. Using a small generator with an unregulated 12V
output soon means lead loss is a problem if the generator is not located near
the battery & rig. (See my bul "Regulating 12V Generator Output")
┌─────────┐ DC 40A FUSE ┌────────────┐ \│/
│ SMALL ├─────────────────┬─────o-o────┤RIG & QRO PA├────┘│
│GENERATOR│ short heavy ┌───┴───┐ └───────┬────┘ │
└────┬────┘ cable │BATTERY│ _│_ _│_
_│_ NOISE ! └───────┘
A 200W SSB Tx needs 35A peak, & regulated 10A PSU will be able to cope with
that provided is well rated. But this is not so good for high power FM, but the
battery will still recover on Rx.
┌─────────┐ mains ┌───┐ 13A FUSE 40A FUSE ┌────────────┐ \│/
│ SMALL ├────────┤PSU├───o-o─────┬─────o-o────┤RIG & QRO PA├────┘│
│GENERATOR│ long └───┘ ┌───┴───┐ └───────┬────┘ │
└────┬────┘ cable │BATTERY│ _│_ _│_
_│_ NOISE └───────┘
Also ensuring that no Tx station sharing a generator, has to run full carrier
power for tuning up valve PAs, so by use "werlos" (not whistles) & PEP METERS.
This can solve excessive mains dips on heavily loaded generators enabling
several 400W SSB stations to be run from a single 1.5kW peak rated generator.
FULL POWER CW/Whistle "WERLO" POWER
400W┤-------- - - - MEAN 400W┤ -, , - - - PEAK READING METER POWER
200W┤ 200W┤/' │ / \ _ _ _ mean power
100W┤ 100W┤. │ / │ generator load!
50W┤ 50W┤ │/
25W┤ 25W┤ │
0 ┴──────── 0 ┴──────────
e.g. a 400W station needs at least 800W DC input (class B or AB2 valves) for
full carrier tuning up, but only 450W DC for spoken "werlo" 400W PEP tuning up.
Small generators having no start battery, & use magneto ignition, which is
housed in the flywheel. Powerful magnets built into the flywheel pass over a
The engine axle has a cam on it that
operates a set of shorting contacts SPARK===, ______________ IGNITION
called "points". Variable speed PLUG )|( │ │ │ TIMING
engines will have a movable cam, but )|( Stop / === >< POINTS
not constant RPM generators. These MAGNETO└─┤ │ C└─────┤
points short out the coil primary COIL _│_ _│_ _│_
with a condenser (0.5uF high voltage
capacitor) across the points as well.
The points are arranged to open a few degrees before "Top Dead Centre", when
the magnets are also across the coil. When the points open the magnetic flux is
allowed to enter the coils, & soon produces a poerfull decaying oscillation
with the condenser.
SPARK===, ______________________________ IGNITION
PLUG )|( │ INTERNAL|( │ │ TIMING
HT )|( Stop/ MAGNETO |( === >< POINTS
TRANSFORMER└─┤ │ PRIMARY │ C└─────┤
COIL _│_ _│_ COIL _│_ _│_
A second high voltage coil (that can be an external ignition transformer coil),
produces 15-25kV to power the spark plug.
PLUG )||( │/│ +ve _│_ G
)||( \ /───┬───┬───┬──2k─────┐ TIMING
MAGNETO└──┤ SCR ─┬─ === _│_ │ +ve )|| PULSE
│ K│ 1n │ /_\ 1k )|| PICKUP
│ ├────┴───┴───┘ )|| COIL
_│_ _│_ _│_
Electronic types replace the points with an SCR that suddenly shorts out the
magneto +ve voltage as the magnets pass. Some use a separate induction timing
pickup coil & small magnet on the crankshaft produce a precise timing pulse,
resulting in the sudden change in the magneto primary winding voltage & the
secondary then produces the high voltage. Ignition kill system on low oil is
On a busy band (contests) it is undesirable to use Rx noise blankers to remove
generator ignition noise, as this normally makes the larger band signals seem
The ignition suppression described here should reduce QRM by more than 40dB.
This is greater than can be achieved with a resistive lead, or resistive plug
cap or resistive plug together!
There are 2 causes of ignition QRM, radiation from the lead (Aerial) &
radiation from the spark plug itself.
Suppress the lead with a large coax braid placed over the ignition lead &
earthed only at the cylinder head! This stops all radiation from the lead, but
not from the plug & cap.
For the plug cap, make a metal cover & use a resistive plug cap type. A thin
Copper (from a pipe) or tin can, is ideal as it is easily soldered (the plug
should not run that hot in service that the solder melts if painted mat black!)
It should be shaped to be a tight fit on the plug hexagonal, & cover the top of
the plug cap with a disk (coin). This must made water tight as any moisture
here with stop the generator starting on a damp morning.
______ Coax Braid as log as possible
Metal │▄▄▄▄▄▄▄_================================_____ EHT Lead To
Cover │Ţ|n|▄▄▄-================================----- Magneto coil
(Cu pipe)│┌┘ └┐│ | Earth at the
_ ││~~~││ |_ Cylinder head ▄ = Spark plug
Cyliner┴─┴──┼───┼───┴─┴ Ţ Cap Case
head └~u~┘Spark Plug
Suppressing plugs & screened caps are also available, but resistive lead as
used on cars is not normally possible due to the magneto lead connection.
Mains lead pick up & re radiation of this QRM might be a route that can be
suppressed with a mains filter at the generator, but in my experience the main
culprit is nearly always the ignition system.
Many generators have this feature nowadays, it saves the embarrassment of a
seized engine because you forget to check the oil level. But it is another
reason why the generator will not stay running!
When the oil is too low a panel lamp may be lit, which happens when a vibrating
oil pressure switch in the sump fails to see sump oil. If left after a few
minutes it will kill the engine ignition or operate the shutoff switch somehow.
This is very dependent on engine SIZE, FUEL, & LOAD, in that order. If the load
can be kept to a minimum by using more efficient loads the better. e.g.
changing a single 100W lamp for a 9W economy type, over a 36 hour period this
could save as much as ť25/e20/$15 worth of fuel! This is because unlike at your
home where the power costs are a few pence/cents per kW HR, from a petrol
generator the cost will be around ť10/e10/$5 per kW HR. But on a 3kW generator
with just the 100W lamp, cost could rise to ť3/e4/$1 an hour as the large
generator has to be kept spinning (about 1/3 - 1/2 the peak output of energy).
By comparison a modern 5kW welding generator set, uses a small 50cc petrol
engine & runs (i.e. SCREAMS) at > 10,000RPM producing 8BHP output on full load,
but ticks over at 500RPM between welds, making the small unit quite economical.
There are electronic inverter 230V 50Hz generator now using this principle,
generating 340V DC from an alternator by a varable RPM engine, into a high
frequency switch mode converter to make the accurate 50Hz 230V sine wave. But
they are expensive, & the added complexity & power loss & possible VERY HF QRM,
may make the advantage less in practice.
Using a diesel generator on farmer's reduced tax "pink" fuel, will certainly
solve much of the running cost & also any electrical QRM. But at the cost of a
heavier & MUCH ACOUSTICALLY NOISIER power source. Note also that not all
engines run/start on the PINK fuel so well, but generators generally do once
warm, & the same goes for ecofuels made from old vegetable oil! N.B. some oils
rot the fuel line gasgets etc.
STARTING AFTER NON USE
Hand pull petrol generators can be really difficult to get going after a long
spell of no use. There can normally be only 2 causes for this..
1/ No Petrol being vapourized, e.g. no strong smell of petrol in exhaust. This
may be due to..
a) Fuel tap off?
b) Blocked carburettor jet?
c) Condensed engine oil in jet from engine breather?
d) Sticky dried up petrol in the jet? Not easily sucked up with low RPM.
e) Petrol blockage? e.g. tap/filter bowl blocked with sludge.
f) Water in petrol.
A quick cure is to strip off the air intake & squirt in a small amount of clean
petrol (or use an "easy start" Ether spay) into the carburettor (choke off). On
turning over expect a few back fires out the carburettor before the engine
eventually runs & sucks through the old sticky low volatile petrol. If it soon
stops you have a petrol blockage, & a good clean of the petrol tap filter,
pipes & carburettor strip down may be needed.
Prevention is better than cure! Always drain the carburettor down with the
drain screw provided on the carburettor bowl bottom, before storing!
2/ No Ignition. e.g. strong smell of petrol in exhaust. This may be due to..
a) Not switched on? (e.g. the points or plug is still shorted)
b) Oiled up plug?
c) Dirty plug?
d) Dampness in EHT wires?
e) Dampness in ignition coil/magneto?
f) Sticking points, or failed points cap?
g) Plug spark gap too wide for hand starting?
h) Low oil level? Engine killer on.
To test for a spark, remove the plug from cylinder, reconnect the EHT & connect
the plug body with a large earth clip (e.g. the 1 used for earthing the genny!)
Pull starter cord & look for a spark. If no spark, then use a meter to
determine which part of the circuit has failed. A pulse of 30-100V should be
seen at the magneto primary C.B. (or electronic version).
Plastic sprays like "Dampstart" can be useful on old engine electrics once they
have been properly cleaned & dried out, so as not to seal dirt/dampness in!
3/ Runs erratically e.g. revs up & down. Look at..
a) Blocked carb main jet? Giving a weak mixture when throttle opens.
b) Contaminated fuel e.g. Water. Drain off some from carb, any H2O droplets?
c) Spark Plug tracking? Clean or replace.
d) Arcing out HT/LT leads?
e) Governor or linkage faulty (loose or rusted up?)
f) Loose wires on Load socket/leads?
g) Failing Alternator Bushes?
h) Burnt out Alternator, coils framing out??
You have to check out all these possible causes just in case.
These all use a rotating electro- .---~~~~~---.
magnet called the rotor, this is /' <EXCITOR> `\
inside a fixed outer laminations | /\ _-----_ /\ |
called the stator. The stator has | |L| /_-"~"-_\ |L| |
the main output load winding & at │ |O||(_ROTOR_)||O| │
90° around the axis to this is │ |A|| ) (+) ( ||A| │
the self exciting winding. This │ |D||(~ROTOR~)||D| │
makes around 10% of the power | | | \~-._.-~/ | | |
available for the spinning rotor's | \/ ~-----~ \/ |
powerful electro magnet. │\. <EXCITOR> ./│
__│ `---_____---' │___
If the rotor is shaped correctly & the stator windings are evenly spread the
rotating magnetic field will produce a sine wave in the load winding. But this
is not the most efficient use of materials, so cheap efficient single phase
generators often do not produce a good sine wave waveform!
There are 2 types of rotor excitation used in small generators:-
The first uses a bridge _____________ small large
rectifier on the stators' _│_ _│_ )||exciter │ load │
self exciting winding \_/ /_\ )||winding │winding│
to obtain DC, which is │+ ├─────┐ )|| STATOR└CCCCCCC┘
smoothed with an electrolytic ├─┤├─┤ │ )|| =======
capacitor, & fed through ├────)───┐ │ │ =====
2 brushes & slip rings _│_ _│_ │ └──)──────>(___ccccc ROTOR
to the rotor's powerful /_\ \_/ └────)──────>(________│
electromagnet winding. └────┴────────┘ SLIP RINGS
Some are more complex with an output voltage regulator placed in series with
the DC rotor feed.
On starting the slight magnetic field left in the rotor is enough to overcome
the bridge rectifier loss to enable the excitation to build up the rotor
magnetic field. Sometimes to aid quicker excitation, waste engine magneto power
can also be added with another diode.
The second method is brush-
less & more reliable, but __________ small large
more difficult to explain. │ )||exciter │ load │
=== C )||winding │winding│
The rotor's electromagnet │ )|| STATOR└CCCCCCC┘
winding has just a diode │__________)|| =======
wired across it, & the =====
stator self exciting winding ccccc ROTOR
has just a large AC capacitor │ │
across wired across it. └─┤<├─┘
On starting the small residual magnetic field in the rotor produces a 90°
leading current in the capacitor & self exciter winding. By transformer action
this produces a voltage pulse across the diode in the rotor & charges up the
rotor's magnetic field. As the rotor spins this occurs twice each revolution.
Some magnetic regulation of output voltage occurs in both types of excitation
because, on high load currents some flux is repelled from the load winding &
ends up going into the exciting winding that is 90° around the stator. This
effect can produce some 10% increase in rotor excitation & hence 10% increase
in voltage under load. If designed just right this increase balances the extra
losses due to the extra load.
On modern engines this is normally 3,000 RPM for 50Hz, 3600 RPM for 60Hz. (even
higher frequency on some cheap generators!) The speed is generally stabilised
by a spinning bob weight governor that moves out weights under centripedal
force to close the carburettors' throttle, against a speed setting spring that
opens it. The basic problem with this feedback arrangement, is that the
throttle cannot be opened, unless the RPM drops, often by as much as 10%
(e.g. 50Hz to 45Hz) for full load, with a resulting frequency & voltage drop, &
also less engine power & torque just when you need the power (watts) & torque
For many mains items the correct voltage is necessary for the correct & safe
operation. Over voltage is generally damaging, under voltage can cause many
different type of effects, from frequency drift to Tx distortion, to computer
brown-outs that can damage your HDD.
Some generators use overall voltage control feedback loop, affecting the
throttle directly, &/or feedback that varies the rotor excitation level.
See part 2
Why Don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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