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G8MNY > TECH 19.01.04 21:35l 93 Lines 4131 Bytes #999 (0) @ WW
BID : 38146_GB7CIP
Read: DG3NCG DB0FHN GUEST
Subj: Petrol Generators for /P SSB 3/3
Path: DB0FHN<DB0RGB<DB0MRW<OK0PPL<DB0RES<ON0AR<WB0TAX<SP7MGD<ZL2TZE<GB7YFS<
GB7KHW<GB7CIP
Sent: 040119/1445Z @:GB7CIP.#32.GBR.EU #:38146 [Caterham] $:38146_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (New 12/03)
LOADS TO WATCH
Switch mode PSUs loads are more immune to voltage variation, but actually
exhibit a negative impedance load on the generator & can cause voltage hunting
as control loop fights the varying current-voltage load. But are very efficient
if there are no Generator or QRM problems.
Another load hazard worth mentioning are small kettles that use half wave
rectification (eg 120V heater misused on 240V with a diode). On small magnetic
devices like isolation transformers & generators the DC current will DC
saturate the core (lock up) & reduce the inductance by many times, resulting in
loss of power or possible damage.
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.
RMS & SIGN-WAVES?
Heavy rectified loads also cause problems as the alternator will only produce
an approximation to a sign-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 the PSU charge up large
capacitors. The pulse current can be very high, say 10 Amp 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 8ê resistive + 10ê inductive reactance. So a high current pulse on the
crest of generated output will just clip the output to a square wave.
ÚÄÄÄÄÄ8êÄÄ¿
³( 10ê ³
³( LOAD
³ ³
~240V ³
ÀÄÄÄÄÄÄÄÄÄÙ
With magnetic feedback control systems 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 / \ Sign 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 254V the mains
maximum. But at this voltage resistive loads like lamps & valve heaters will
have reduced life, but the HT will be more like normal!
SAFETY
Earthing, is needed for safety to reduce the chance of shock, but floating
supplies are generally much safer from this point of view than the normal N & L
mains, as it is almost impossible to get a significant current to earth from
either power line. With balanced supplies 120-0-120v there is still a shock
hazard but much reduced.
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.
Risk of fire is always present whether from a damaged carburettor pipe, or
spilling petrol on a hot exhaust. When generators are ready to run or running a
good fire extinguisher 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.
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 up generators.
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, but I have lost full 5 gallon Gerry
cans so I now lock them up as well!
The end.
/QSL
73 de John G8MNY @ GB7CIP
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