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G8MNY > TECH 24.10.09 23:57l 568 Lines 28521 Bytes #999 (0) @ WW
BID : 40840_GB7CIP
Read: GUEST
Subj: Petrol Generators for /P SSB
Path: IZ3LSV<IK2XDE<DB0RES<ON0AR<GB7CIP
Sent: 091024/2232Z @:GB7CIP.#32.GBR.EU #:40840 [Caterham] $:40840_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (Updated Jan 09)
(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
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")
ÚÄÄÄÄÄÄÄÄÄ¿ DC 40A FUSE ÚÄÄÄÄÄÄÄÄÄÄÄÄ¿ \³/
³ SMALL ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄo-oÄÄÄÄ´RIG & QRO PAÃÄÄÄÄÙ³
³GENERATOR³ short heavy ÚÄÄÄÁÄÄÄ¿ ÀÄÄÄÄÄÄÄÂÄÄÄÄÙ ³
ÀÄÄÄÄÂÄÄÄÄÙ cable ³BATTERY³ _³_ _³_
_³_ NOISE ! ÀÄÄÄÄÄÄÄÙ
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, 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´/' ³ / \ _ _ _ mean power
100W´ 100W´. ³ / ³ generator load!
50W´ 50W´ ³/
25W´ 25W´ ³
0 ÁÄÄÄÄÄÄÄÄ 0 ÁÄÄÄÄÄÄÄÄÄÄ
TIME TIME
e.g. a 400W station needs at least 800W DC input (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, & 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 ³ TIMING
engines will have a movable cam, but )|( Stop / === >< POINTS
not constant RPM generators. These MAGNETOÀÄ´ ³ CÀÄÄÄÄÄ´
points short out the coil primary COIL _³_ _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 poerfull decaying oscillation
with the condenser.
EXTRERNAL COIL
SPARK===, ______________________________ IGNITION
PLUG )|( ³ INTERNAL|( ³ ³ TIMING
HT )|( Stop/ MAGNETO |( === >< POINTS
TRANSFORMERÀÄ´ ³ PRIMARY ³ CÀÄÄÄÄÄ´
COIL _³_ _³_ COIL _³_ _³_
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 system 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 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 ³ÜÜÜÜÜÜÜ_================================_____ EHT Lead To
Cover ³Ý|n|ÜÜÜ-================================----- Magneto coil
(e.g.22mm)³Ý³~³Þ| |
(Cu pipe)³ÚÙ À¿³ | Earth at the
_ 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.
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.
OIL ALARM
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.
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. e.g.
changing a single 100W lamp for a 9W economy type, over a 36 hour period this
could save as much as œ25/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 œ10/e10/$5 per kW HR. But on a 3kW generator
with just the 100W lamp, cost could rise to œ3/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 now 230V 50Hz generators 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 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.
THE ALTERNATOR
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 ³ |O||(_ROTOR_)||O| ³
90ø around the axis to this is ³ |A|| ) (+) ( ||A| ³
the self exciting winding. This ³ |D||(~ROTOR~)||D| ³
makes around 10% of the power | | | \~-._.-~/ | | |
available for the spinning rotor's | \/ ~-----~ \/ |
powerful electro magnet. 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 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' _³_ _³_ )||exciter ³ load ³
self exciting winding \_/ /_\ )||winding ³winding³
to obtain DC, which is ³+ ÃÄÄÄÄÄ¿ )|| STATORÀCCCCCCCÙ
smoothed with an electrolytic ÃÄ´ÃÄ´ ³ )|| =======
capacitor, & fed through ÃÄÄÄÄ)ÄÄÄ¿ ³ ³ =====
2 brushes & slip rings _³_ _³_ ³ ÀÄÄ)ÄÄÄÄÄÄ>(___ccccc ROTOR
to the rotor's powerful /_\ \_/ ÀÄÄÄÄ)ÄÄÄÄÄÄ>(________³
electromagnet winding. ÀÄÄÄÄÁÄÄÄÄÄÄÄÄÙ 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. ³ )||exciter ³ load ³
=== C )||winding ³winding³
The rotor's electromagnet ³ )|| STATORÀCCCCCCCÙ
winding has just a diode ³__________)|| =======
wired across it, & the =====
stator self exciting winding ccccc ROTOR
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
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 90ø 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.
VOLTAGE REGULATION
ENGINE RPM
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
(amps)!
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.
LOAD COMPENSATION
In the simple bob weight RPM control engine system, better load voltage
regulation can be obtained with some additional load current feedback (feed
forward) control.
This can easily be applied, by adding a small solenoid (e.g. from an old VCR)
rewound with a few turns of suitably insulated & thick enamelled copper wire,
that takes the 230V full load current. It is then mounted firmly on the engine
or alternator & linked up to aid the speed setting spring. Even though the DC
solenoid is now operating on 50Hz the pull is quite adequate if it has a fully
wound bobbin. __
CRANKCASE BOB ³()³AXLE
WEIGHT GOVERNOR³ ³
__ AXLE ³ ³ SPRING ___ SPEED
CARBURETTOR³()³ holes in³ :³-/\/\/\-[___[SCREW ÚÄÄÄÄÄÄÄÄ¿
THROTTLE ³ ³ adjustment arm³ :³======================²²³SOLENOID³
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 load 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 (piece of coat hanger wire?)
I have done this modification to several generators now, & here is results of a
1.3kW rated (1.5kW peak) Honda generator to make it produce a steady 240V (UK).
LOAD BEFORE MOD AFTER MOD IMPROVEMENT
WATTS VOLTS VOLTS VOLTS %
0 250 250 0 0
100 240 240 0 0
500 230 238 8 3.5
1000 220 242 22 10
1500 200 240 40 20
With this modification the throttle is opened much earlier than before "when a
load is applied", rather than when the engine has slowed down. So as soon as
the load comes on, you hear the engine immediately "throttle 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.
Although this modification improves the peak load ability, the mean engine
rating should be kept the same, as the cooling, bearing ware etc. are
unaltered. e.g. don't plug in that 2kW heater!
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 the generator control loop fights the varying current to
voltage load. But normally there are no problems, & they are very efficient for
/P use, if there are no QRM problems! Over voltage 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 (e.g. 115V 180W heater misused on 230V with a diode by the
makers!)
e.g.
ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄLÄÄÄÄÄÄÄo/oÄÄÄ´>ÃÄÄ¿
( 5R lead or 240V AC on off 80R
( winding +15V DC ³ 3A 50% of time
ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄNÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ = 360W
On small magnetic devices like an isolation transformers or a generator the DC
current will saturate the core (lock up) & reduce the inductance by many times,
resulting in dramatic loss of output power or possible damage. Even on a long
resistive lead on real mains, the resultant Mains + DC can damage other kit
(transformers fuses especially torroidal).
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,
but they will protect against overvoltage & poor sine wave at a 10% power cost.
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 transformer
needs a 0.47uF @ 300V AC & a 20W L ÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄ¿
fluorescent lamp needs 100VA test C³ )º INDUCTIVE
correction that is 5uF @ 300V AC. MAINS === )º LOAD
Having them fully corrected (PF=1) AC AMP ³ )º
helps with other peak pulse loads, as N ÄÄ Meter ÄÄÄÁÄÄÄÄÄÙ
the sine wave shape is maintained!
The exact amount of C needed to tune the load 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 gain for a fully corrected load is small & may not actually be worth
the trouble. Using 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. ÚÄÄÄÄÄÄ10êÄÄ¿
But a small generator is quite different º( XL= ³
a 500W generator may have around 10ê º( 15ê LOAD
resistive + 15ê inductive reactance. ³ ³
So a high current pulse on the crest of ~240V ³
generated output will just clip the ÀÄÄÄÄÄÄÄÄÄÄÄÙ
output to a square wave.
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
electronic 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´ ,--ÄÄ, 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 `ÄÄ--'
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 (Typically 20dB worse than Petrol), often from the clanking movement in
heavily worn highly stressed parts.
The restriction on gasses to & from the engine degrade 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 an exhaust silencer, & an oil bath air filter housing designed for
cleaning, but no muffing action was normal. But since more awareness of the
dangers of noise at the work place etc. modern generators are much quieter.
Old 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 a much larger flywheel has to store energy for 7 load half cycles
before the next compression & then explosion.
108 dBA @ 5M
Noise screens can _ In___Ex
work very well in Four | | ³Genny³ HEAT
the open, here is Hay |_| ³_____³ SIDE
a good example. Bail | |_ _ _ _
Wall |_|_ _|_ _|
78dBA @ 5M
SAFETY
Exhaust gases, these are dangerous & engines should never be run for any time
in an enclosed area! E.g. no more than 1 min in a garage !
Earthing, 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) EARTH
EARTH STAKE ( 2x 120V ) STAKE
(off earth)
With actual balanced supplies 120-0-120V where the 0V is hard grounded
(e.g. 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. I use a
large galvanised T section one with a eyelet for the security chain too.
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!
Generators are also HEAVY, especially if fully fuelled. Accidentally dropping
one on your foot is unlikely to brake the generator, but your foot is something
else? Backs to are a human weak spot, so get help/lift correctly!
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 an 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/gas type, NOT WATER!) placed nearby &
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 with less fuel
to "go up", 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. In still air Petrol vapour hangs
around & a dropped hot fag end can start a fire at least 20m away!
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
revving, 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
with it covers on 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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