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G8MNY > TECH 26.12.05 17:58l 149 Lines 6774 Bytes #999 (0) @ WW
BID : 10487_GB7CIP
Read: DL1LCA GUEST OE7FMI
Subj: Petrol Generators for /P SSB 4/4
Path: DB0FHN<DB0MRW<DB0WUE<DK0WUE<7M3TJZ<ON0AR<GB7CIP
Sent: 051226/1457Z @:GB7CIP.#32.GBR.EU #:10487 [Caterham] $:10487_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (updated Jan 06)
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 may have
around 10ê resistive + 15ê inductive reactance. So a high current pulse on the
crest of generated output will just clip the output to a square wave.
ÚÄÄÄÄÄ10êÄÄ¿
º( XL= ³
º( 15ê LOAD
³ ³
~240V ³
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 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.
Noise screens can work very well in the open, here is a good example.
98 dBA @ 5M
_ In___Ex
Four | | ³Genny³ HEAT
Hay | | ³_____³ SIDE
Bail | |_ _ _ _
Wall |_ _ _ _ _|
50dBA @ 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. 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!
GENERATOR ======================= RADIO TX
CHASSIS FLOATING & AERIAL
& SOCKETS 240V AC _³_
_³_ (typically) ////
//// ( 2x 120v ) EARTH
EARTH STAKE (off earth) STAKE
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 saver 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.
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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