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G8MNY > TECH 21.03.04 10:02l 106 Lines 4961 Bytes #999 (0) @ WW
BID : 49182_GB7CIP
Read: GUEST DB0FHN OE7FMI
Subj: Petrol Generators for /P SSB 3/3
Path: DB0FHN<DB0RGB<DB0MRW<DB0ERF<DB0FBB<DB0IUZ<DB0GOS<ON0AR<ON0AR<GB7CIP
Sent: 040321/0118Z @:GB7CIP.#32.GBR.EU #:49182 [Caterham] $:49182_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (updated Feb 04)
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 are very efficient for /P use, if there are no QRM problems.
Another load hazard worth mentioning are small kettles (500W) that use half
wave rectification (eg 120V heater misused on 240V with a diode!). On small
magnetic devices like isolation transformers & generators this 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.
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 iron ballasted fluorescent lamps are good if they are
fully Power factor (PF of 0.9 = partly) corrected with large capacitor.
Typically a 20W fluorecent lamp needs 100VA correction that is 5uF @ 300v AC.
Having them fully corrected (PF=1) helps with other peak pulse loads, as the
sine wave shape is maintained!
RMS & SINE WAVES?
As mentioned earlier in 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
the PSU charge up large capacitors. The pulse current can be very high, say
10Amp 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¿
³( 15ê ³
³( LOAD
³ ³
~240V ³
ÀÄÄÄÄÄÄÄÄÄÄÙ
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 / \ 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
supplies are generally much safer from this point of view than the normal N & L
mains 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 grounded (eg 110V building site transformer) there is still a shock
hazard but much reduced. But the current form either side of feed needs fusing!
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 (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.
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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