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G8MNY > TECH 24.08.06 20:12l 486 Lines 24538 Bytes #999 (0) @ WW
BID : 43466_GB7CIP
Read: DL1RX DL1LCA GUEST OE7FMI
Subj: Petrol Generators for /P SSB
Path: DB0FHN<DB0FOR<DB0SIF<DB0EA<DB0RES<ON0AR<GB7CIP
Sent: 060824/0731Z @:GB7CIP.#32.GBR.EU #:43466 [Caterham] $:43466_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (Updated May 06)
(8 Bit ASCII Graphics use code page 437 or 850)
/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.
LOAD SHARING/conditioning
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
O/P soon means lead loss is a problem if the generator is not located near the
battery & rig. (See my bul "Regulating 12V Generator Output")
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.
ÚÄÄÄÄÄÄÄÄÄ¿ mains ÚÄÄÄ¿ 13A FUSE 40A FUSE ÚÄÄÄÄÄÄÄÄÄÄÄÄ¿ \³/
³ SMALL ÃÄÄÄÄÄÄÄÄ´PSUÃÄÄÄo-oÄÄÄÄÄÂÄÄÄÄÄo-oÄÄÄÄ´RIG & QRO PAÃÄÄÄÄÙ³
³GENERATOR³ long ÀÄÄÄÙ ÚÄÄÄÁÄÄÄ¿ ÀÄÄÄÄÄÄÄÂÄÄÄÄÙ ³
ÀÄÄÄÄÂÄÄÄÄÙ cable ³BATTERY³ _³_ _³_
_³_ ÀÄÄÄÄÄÄÄÙ //// ////
////
Also ensuring that no Tx station sharing a generator, has to run full carrier
power for tuning up valve PAs by using "werlos" (not whistles) & PEP METERS,
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´/' ³ / \ _ _ _ mean power
100W´ 100W´. ³ / ³ generator load!
50W´ 50W´ ³/
25W´ 25W´ ³
0 ÁÄÄÄÄÄÄÄÄ 0 ÁÄÄÄÄÄÄÄÄÄÄ
TIME TIME
eg. 400W station needs at least 800W DC I/P (class AB2) for full carrier tuning
up, but only 450W DC for spoken "werlo" 400W PEP tuning up.
MAGNETOs
Small generators having no start battery, they use magneto ignition, which is
housed in the flywheel. Powerful magnets built into the flywheel pass over a
static coil.
The engine axle has a cam on it that
operates a set of shorting contacts SPARK===, __________ IGNITION
called "points". Variable speed PLUG )|( ³ ³ TIMING
engines will have a movable cam, but )|( C === >< POINTS
not constant RPM generators. These MAGNETOÀÄ´ ÀÄÄÄÄÄ´
points short out the primary coil _³_ _³_
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 decaying oscillation with the
condenser. A second high voltage coil (that can be an external Ignition
transformer coil), produces 15-25kV to power the spark plug.
SPARK===, __³\³_______A
PLUG )||( ³/³ +ve _³_ G
)||( \ /ÄÄÄÂÄÄÄÂÄÄÄÂÄÄ2kÄÄÄÄÄ¿ TIMING
MAGNETOÀÄÄ´ SCR ÄÂÄ === _³_ ³ +ve )|| PULSE
³ K³ 1n ³ /_\ 1k )|| PICKUP
³ ÃÄÄÄÄÁÄÄÄÁÄÄÄÙ )|| COIL
_³_ _³_ _³_
//// //// ////
Electronic types replace the points with an SCR that suddenly shorts out the
magneto +ve voltage as the magnets pass. 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 on low oil is also common.
IGNITION QRM
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
very wide.
The ignition suppression described here should reduce QRM by > 40dB. This is
greater than can be achieved with resistive lead or resistive 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 & that it is resistive plug cap type.
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!). It should be
shaped to be a tight fit on the plug hexagonal, & cover the top of the plug
cap. 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 ³ÜÜÜÜÜÜÜ_================================_____ EHT Lead To
Cover ³Ý|n|ÜÜÜ-================================----- Magneto coil
(eg.22mm)³Ý³~³Þ| |
(Cu pipe)³ÚÙ À¿³ | Earth @
_ ³³~~~³³ |_ Cylinder head Ü = Spark plug
CylinerÁÄÁÄÄÅÄÄÄÅÄÄÄÁÄÁ Ý Cap Case
head À~u~ÙSpark 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.
OIL ALARM
Many generators have this feature, 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 lamp may be lit, 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.
ECONOMY
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. eg. changing
a single 100W lamp for a 9W economy type, over a 36 hour period could save as
much œ20/e30/$20 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 œ8/e8/$5 per kW HR. But on a 3kW generator with just the lamp
cost could rise to œ3/e3/$2 an hour as the generator has to be kept spinning.
By comparison a modern 5kW welding generator set, uses a small 50cc petrol
engine & runs (ie. SCREAMS) @ 10,000 RPM producing 8 BHP O/P on full load,
but ticks over at 500 RPM between welds, making the small unit quite
economical. There are now 240V 50Hz generators using this principle, generating
350V DC & into a high frequency switch mode converter to make the accurate 50Hz
240V sine wave. But they are expensive, & the added complexity & power loss &
possible HF QRM may make the advantage less in practice.
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. This may be due to..
a) a blocked carburettor jet,
b) condensed engine oil in jet from engine breather,
c) sticky dried up petrol in the jet, not easily sucked up with low RPM,
d) or a petrol blockage, eg. the filter bowl below the tap.
A quick cure is it strip of the air intake & squirt in a small amount of clean
petrol 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 petrol. If it soon stops you have a petrol blockage, & a clean of
the petrol tap filter & carburettor strip down may be needed.
Prevention is better than cure, always drain the carburettor down with the
drain provided on the carburettor bowl bottom, before storing!
2/ No Ignition. This may be due to..
a) oiled up plug,
b) dirty plug,
c) dampness in EHT wires,
d) dampness in ignition coil/magneto,
e) sticking points,
f) plug spark gap too wide for hand starting!
To test for a spark, remove the plug from cylinder, reconnect the EHT & connect
the plug body with a large earth clip (eg. 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 50-150V 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!
THE ALTERNATOR
These all use a rotating .---~~~~~---.
electromagnet called the /' <EXCITOR> `\
rotor, this is inside a | /\ _-----_ /\ |
fixed outer laminations | |L| /_-"~"-_\ |L| |
called the stator. ³ |O||(_ROTOR_)||O| ³
The stator has the main ³ |A|| ) (+) ( ||A| ³
output load winding & ³ |D||(~ROTOR~)||D| ³
at 90ø around the axis | | | \~-._.-~/ | | |
to this is the self | \/ ~-----~ \/ |
exciting winding. ³\. <EXCITOR> ./³
__³ `---_____---' ³___
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 generators often do not
produce a good waveform.
There are 2 types of rotor excitation used in small generators:-
The first uses a bridge _____________
rectifier on the stators' _³_ _³_ )||exciter ³ load ³
self exciting winding \_/ /_\ )||winding ³winding³
to obtain DC, which is ³+ ÃÄÄÄÄÄ¿ )|| STATORÀCCCCCCCÙ
smoothed with an ÃÄ´ÃÄ´ ³ )|| =======
electrolytic capacitor, ÃÄÄÄÄ)ÄÄÄ¿ ³ ³ =====
& fed through 2 brushes _³_ _³_ ³ ÀÄÄ)ÄÄÄÄÄÄ>(___ccccc ROTOR
& slip rings to the rotor /_\ \_/ ÀÄÄÄÄ)ÄÄÄÄÄÄ>(________³
electromagnet winding. ÀÄÄÄÄÁÄÄÄÄÄÄÄÄÙ SLIP RINGS
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 the excitation, waste engine magneto power can
also be added with another diode.
The second method is brush-
less & more reliable, but __________
more difficult to explain. ³ )exciter ³ load ³
=== C )winding ³winding³
The rotor electromagnet ³ ) STATOR ÀCCCCCCCÙ
winding has just a diode ³__________)
wired across it, & the ROTOR
stator self exciting winding ccccc
has just a large AC capacitor ³ ³
across wired across it. ÀÄ´<ÃÄÙ
On starting the small residual magnetic field in the rotor produces a 90ø
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
rotors' 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 flux repelled from the load winding ends up going
into the exciting winding that is 90ø around the stator. This effect can
produce some 10% increase in rotor excitation & hence 10% increase in voltage
under load that can offset some of the losses. If designed right this increase
balances the extra losses due to the extra load.
VOLTAGE REGULATION
ENGINE RPM
This is normally 3,000 RPM for 50Hz 3600 RPM for 60Hz. 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%
(eg. 50Hz down to 45Hz) for full load, with a resulting frequency & voltage
drop, & also less engine power!
For many items the correct voltage is necessary for the correct & safe
operation. Over voltage is generally damaging, under voltage can cause many
different type of effect, from frequency drift to Tx distortion, to computer
brown-outs that can damage your HDD.
Some generators use overall voltage control, affecting the throttle directly,
and/or feedback that varies the rotor excitation level.
LOAD COMPENSATION
In the simple bob weight RPM control method, better load voltage regulation can
be obtained with some additional load current feedback (feed-forward).
This can easily be applied, by adding a small solenoid (eg. from an old VCR)
rewound with a few turns of suitably insulated & thick enamelled copper wire,
that takes the load current. It is then mounted firmly on the engine/alternator
& linked up to aid the speed setting spring. Even though the DC solenoid is on
50Hz the pull is quite adequate if it has a fully wound bobbin.
__
CRANKCASE BOB ³()³
WEIGHT GOVERNOR³ ³ SPRING ___SPEED
CARBURETTOR__ holes in³ :³-/\/\/\-[___SCREW ÚÄÄÄÄÄÄÄÄ¿
THROTTLE ³()³ adjustment arm³ :³=======================²³SOLENOID³
ARM ³__³======================³__³ BOOST____\ ÀÄÄÄÄÂÄÂÄÙ
GAS---> LINKAGE <--- REVS PULL / Alternator³ ³Load
Neutral Neutral
N.B. The solenoid pull is proportional to:-
1/ the current squared,
2/ the number of turns,
3/ the location of the movable iron slug,
4/ the solenoid size.
Fine load compensation adjustment is best done by varying the length of the
solenoid linkage.
I have done this modification to several generators now, & here is results of a
1.3KW rated (1.5KW peak) generator to make it produce a steady 240V...
LOAD BEFORE MOD AFTER MOD IMPROVEMENT
WATTS VOLTS VOLTS VOLTS %
0 250 250 0 0
100 240 240 0 0
500 230 238 8 3.3
1000 220 242 22 9.2
1500 200 240 40 16.7
With this modification the throttle is opened much earlier when a load is
applied. As soon as the load comes on, you hear the engine rev up under load, &
heavy loads maintaining the voltage much closer to that of real mains! This
means not only is the static regulation much better, but also the dynamic
regulation as well, as there is not the usual hang time while the RPM drops
before the throttle is opened.
LOADS TO WATCH
Switch mode PSUs loads are more immune to voltage variation, but actually
exhibit a negative impedance load to the generator, so they can be the cause of
voltage hunting, as control loop fights the varying current-voltage load. But
normally there are no problems, and they are very efficient for /P use, if
there are no QRM problems. Overvoltage surge protectors are normally part of
SMPSUs & may cause fuses to blow etc if you seriously over voltage them.
Another load hazard worth mentioning are small kettles (350W) that use half
wave rectification (eg. 115V 180W heater misused on 230V with a diode!). On
small magnetic devices like isolation transformers & generators the DC current
will saturate the core (lock up) & reduce the inductance by many times,
resulting in dramatic loss of O/P power or possible damage. Even on a long
resistive lead on real mains with one, can damage other kit (transformers) with
the DC produced!
Constant voltage transformers CVTs that are Tuned to 50Hz generally are not
very useful on generators feeds as the supply frequency is not that accurate.
Inductive loads like rotators & iron ballasted fluorescent lamps, are good if
they are fully Power factor (PF of 0.9 = partly) corrected with a large AC
capacitor.
Typically a 30W rotator needs a
0.47uF @ 300V AC & a 20W fluorescent L ÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄ¿
lamp needs 100VA correction that is test C³ )º INDUCTIVE
5uF @ 300V AC. Having them fully MAINS === )º LOAD
corrected (PF=1) helps with other AC AMP ³ )º
peak pulse loads, as the sine wave N ÄÄ Meter ÄÄÄÁÄÄÄÄÄÙ
shape is maintained!
The exact amount of C needed can be measured if you have a bank of suitable
caps to try out. Use a an AC Ammeter in series with the N wire to the load &
try out various Cs in series/parallel until you find the value needed for
minimum current. Warning mains is dangerous & caps stay charged!
The actual gains for fully corrected loads are small & may not actually be
worth the trouble!
N.B. mains filters & PF correction caps can degrade mains intercoms!
RMS & SINE WAVES?
As mentioned earlier the Alternator design is responsible for making a good
sine wave, but heavy electronic rectified loads also cause problems as the
alternator will only produce an approximation to a sine wave with resistive
loads. A typical linear & electronic equipment usually draws next to no current
over most of the AC cycle except at the crest of the wave when the diodes in
these PSUs charge up large capacitors. The resulting pulse current can be very
high, say 10A peak for only a 100W 400mA RMS computer load!
On real mains, the impedance is normally only an ohm or two, with little
reactance. But a small generator is
quite different a 500W generator ÚÄÄÄÄÄ10êÄÄ¿
may have around 10ê resistive + 15ê º( XL= ³
inductive reactance. º( 15ê LOAD
So a high current pulse on the crest ³ ³
of generated output will just clip ~240V ³
the output to a square wave. ÀÄÄÄÄÄÄÄÄÄÄÙ
With magnetic feedback control on the generator the true RMS power may stay
the same, but real mains is normally 240V (230V EU) & peaks to 340V & your
equipment needs that peak voltage to work properly, & not some clipped 240V
square wave that has the same RMS voltage!
340v PK_³ __ ³ GENERATOR WITH
240v____³ / \ Sine Wave ?? PKÄ´ ,-----, RECTIFIED LOAD
RMS ³ ³ ³ MAINS ³ ³ ³
0vÄ´³- - - ³- - - ³ 0vÄ´³- - - ³- - - ³
³ ³ ³ 254v_³ ³ ³
³ \__/ RMS ³ `-----'
So to partly remedy this, some over voltage is desirable, say 253V the mains
maximum. But at this voltage resistive loads like lamps & valve heaters will
have reduced life, but the HT or PSU headroom will be a little more like
normal!
ACOUSTIC NOISE
There are 4 main sources of noise in any engine...
1/ Engine air intake, a pulsed suck at 50Hz on 2 strokes & 25Hz on 4 strokes.
2/ Engine mechanical noise, bearings, big ends, 4 stroke valve gear, couplings.
3/ Cooling air rush, through Engine & Alternator, higher pitch fan blade noise.
4/ Engine exhaust noise, both from the silencer output & from silencer walls.
1 & 4 change quite a bit with load, with higher frequency components at low
loads to loud predominately lower pitched note under heavy load. Other noises
are often mainly rattles of loose parts.
With their higher compression & very explosive combustion, Diesels are always
noisy, often from the clanking movement in heavily worn highly stressed parts.
The restriction on gasses to & from the engine do degrade its performance,
which is why you do not see mufflers & silencers on racing cars or aircraft.
Commercial industrial generators never used to have much more that small "tin
can" as a silencer, & an oil bath air filter housing designed for cleaning &
no muffing action was normal. But since more awareness of the dangers of noise
at the work place etc many newer generators are quieter.
Slower running 4 stroke generators with 4 poles (1500 RPM) seem much quieter
with their 12Hz exhaust pulse, but they are often VERY heavy for their rating,
as the much larger flywheel has to store energy for 7 load half cycles & the
next compression as well.
108 dBA @ 5M
Noise screens can work _ In___Ex
very well in the open, Four | | ³Genny³ HEAT
here is a good example. Hay |_| ³_____³ SIDE
Bail | |_ _ _ _
Wall |_|_ _|_ _|
58dBA @ 5M
SAFETY
Earthing, is needed for safety to reduce the chance of shock, but floating
generator supplies are generally much safer from this point of view than the
normal N & L mains. This is because it is almost impossible to get a
significant shock current to earth from either power line.
_________ __________
³GENERATOR³=======================³ RADIO TX ³ \³/
³ CHASSIS ³ FLOATING ³ & AERIAL³ÄÄÄÙ
³& SOCKETS³_ 240V AC ~~~~~~~~~_³_
~~~~~~~~~_³_ (typically) ////
//// ( 2x 120V ) EARTH
EARTH STAKE (off earth) STAKE
With actual balanced supplies 120-0-120V where the 0V is hard grounded
(eg. 110V building site transformer) there is still a shock hazard but much
reduced. But the current from either side of feed needs fusing or you could do
welding to earth with it!
I always use an earth stake to provide some static protection at least.
Some generators warn you not to use both DC & AC outputs at the same time. This
may not be due to a loading or regulation problem, but due to safety! This is
because the DC output winding is often part of the 230VAC load winding, & that
can put DC onto the AC output or bypass some of the safety trip features, as
well as MAKING THE 230V NON FLOATING & therefore more hazardous!
If you intend to use both at once do at least use a ELCB/RDC mains trip plug!
Another problem is bad weather (typical contest Wx). Some generators suck in
large amounts of damp cooling air for the alternator. So insulation breakdown
of the windings will eventually happen if high levels of moisture are always
around! So during maintenance the odd spay off light oil/damp start plastic
sprays (not on the slip rings!) may prevent this!
Under rainy field day conditions try a awning or an old gazebo to take the
worst of the Wx off the generator.
The risk of FIRE is always present whether from a damaged carburettor pipe, or
spilling petrol on to a hot exhaust. When generators are ready to run or
running a good fire extinguisher (power type not water!) placed nearby but not
too close is essential.
Some generators have small tanks & others have quite large "safari" tanks.
Obviously the small tanks are safer from the fire point of view, but filling up
more often soon negates this safety advantage.
Spilt Diesel Oil is also a fire hazard if there is any wickering material like
dry grass, straw or even dry soil around & a flame source.
Obviously don't let people smoke near the generators or fuel tanks. If you
allow public on the site, then sign the fact.
One point from Ralph G7IED, was a report of an exploding generator harming
nearby people with shrapnel. This is very rare & might be related to over
reving, bits that could fly out of an engine are the conrod & the flywheel,
most other bits do not have enough energy, so flying bits from a working engine
is very rare.
SECURITY
Generators & full petrol cans, laying about in a field are a magnet to some
types of people. One advantage of using a large earth stake with a welded
eyelet is it can be used to chain & padlock up generators. Even chaining 2
generators together may make then too heavy to move.
A local club has lost a running generator, they went to see why the power had
stopped, only to see a pickup truck driving off with it!
I have never lost a generator to thieves so far, but I have lost full 5 gallon
Gerry cans, so I now lock them up as well!
The end.
See also buls on "Cheaper Generators" & "Regulating 12V Generator Output"
Why Don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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