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G8MNY > TECH 30.06.05 19:26l 99 Lines 4481 Bytes #999 (0) @ WW
BID : 52255_GB7CIP
Read: GUEST OE7FMI
Subj: Petrol Generators for /P SSB 4/4
Path: DB0FHN<DB0FOR<DB0SIF<DB0EA<DB0ACC<DB0GOS<ON0AR<ON0AR<GB7CIP
Sent: 050630/1038Z @:GB7CIP.#32.GBR.EU #:52255 [Caterham] $:52255_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (updated Mar 04)
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!
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.
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!
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 facts.
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.
Why Don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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