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G8MNY > TECH 29.06.05 20:20l 89 Lines 4323 Bytes #999 (0) @ WW
BID : 52079_GB7CIP
Read: GUEST OE7FMI
Subj: Petrol Generators for /P SSB 2/4
Path: DB0FHN<DB0RGB<OK0PPL<DB0RES<DB0GOS<ON0AR<ON0AR<GB7CIP
Sent: 050629/1718Z @:GB7CIP.#32.GBR.EU #:52079 [Caterham] $:52079_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (Corrected Sep 04)
OIL ALARM
Many generators have this feature, 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 lamp may be lit, 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. eg changing
a single 100W lamp for a 9W economy type, over a 36 hour period could save as
much œ20/e30/$20 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 œ8/e8/$5 per kW HR. But on a 3kW generator with just the lamp
cost could rise to œ3/e3/$2 an hour as the generator has to be kept spinning.
By comparison a modern 5kW welding generator set, uses a small 50cc petrol
engine and runs (ie SCREAMS) @ 10,000 RPM producing 8 BHP O/P on full load,
but ticks over at 500 RPM between welds, making the small unit quite
economical. There are now 240V 50Hz generators using this principle, generating
350V DC & into a high frequency switch mode converter to make the accurate 50Hz
240V sine wave. But they are expensive, & the added complexity & power loss may
make the advantage less in practice.
THE ALTERNATOR
These all use a rotating electromagnet called the rotor, this is inside a fixed
outer laminations called the stator. The stator has the main output load
winding & at 90ø around the axis to this is the self exciting winding. 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 generators often do
not produce a good waveform.
There are 2 types of rotor excitation used in small generators:-
The first uses a bridge rectifier on the stators' self exciting winding
to obtain DC which is smoothed with an electrolytic capacitor, & fed through 2
brushes & slip rings to the rotor electromagnet winding.
_____________
_³_ _³_ )exciter ³ load ³
\_/ /_\ )winding ³winding³
³+ ÃÄÄÄÄÄ¿ ) STATOR ÀCCCCCCCÙ
ÃÄ´ÃÄ´ ³ )
ÃÄÄÄÄ)ÄÄÄ¿ ³ ³ ROTOR
_³_ _³_ ³ ÀÄÄ)ÄÄÄÄÄÄÄ>(___ccccc
/_\ \_/ ÀÄÄÄÄ)ÄÄÄÄÄÄÄ>(________³
ÀÄÄÄÄÁÄÄÄÄÄÄÄÄÙ SLIP RINGS
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 the excitation, waste engine magneto power can
also be added with another diode.
The second method is brush-less & more reliable, but more difficult to explain.
The rotor electromagnet winding has just a diode wired across it, & the stator
self exciting winding has just a large AC capacitor across wired across it.
__________
³ )exciter ³ load ³
=== C )winding ³winding³
³ ) STATOR ÀCCCCCCCÙ
³__________)
ROTOR
ccccc
³ ³
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
rotors' 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 flux 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 that can offset some of the losses. If designed right this increase
balances the extra losses due to the extra load.
Part 3 has Voltage Regulation, Load Compensation, & Loads to watch.
Why Don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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