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G8MNY > TECH 20.03.04 12:12l 131 Lines 6501 Bytes #999 (0) @ WW
BID : 49071_GB7CIP
Read: F6GIA DB0FHN GUEST OE7FMI
Subj: Petrol Generators for /P SSB 2/3
Path: DB0FHN<DB0FOR<DB0SIF<DB0EA<DB0NOS<DB0BI<DB0FBB<DB0GOS<DB0EEO<DB0RES<
ON0AR<GB7CIP
Sent: 040320/0959Z @:GB7CIP.#32.GBR.EU #:49071 [Caterham] $:49071_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (New 12/03)
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 10kW welding generator set, uses a small 50cc petrol
engine and runs (ie SCREAMS) @ 10,000 RPM producing 15 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.
VOLTAGE REGULATION
ENGINE RPM
This is normally 3,000 RPM for 50Hz 3600 for 60Hz. The speed is generally
stabilised by spinning bob weight governor that moves out weights under
centrifugal force to close the carburettors' throttle, against a speed setting
spring that opens it. The basic problem with this simple feedback arrangement,
is that the throttle cannot open to full, unless the RPM drops, often by as
much as 10%, with a resulting frequency & voltage drop.
For many items the correct voltage is necessary for the correct & safe
operation. Over voltage is generally damaging, under voltage can cause many
different type of effect, from frequency drift to Tx distortion, to computer
brown-outs that can damage your HDD.
Some generators use overall voltage control, affecting the throttle directly,
or feedback that varies the rotor excitation level.
LOAD COMPENSATION
In the simple bob weight RPM control method, better regulation can be obtained
with some additional load current feedback. This can easily be applied, by
adding a small solenoid (from an old VCR) with a few turns of suitable wire
taking the load current mounted firmly on the engine/alternator, & linked up to
aid the RPM setting spring.
__
CRANKCASE BOB ³()³
WEIGHT GOVERNOR³ ³ SPRING ___SPEED
CARBURETTOR__ holes in³ :³-/\/\/\-[___SCREW ÚÄÄÄÄÄÄÄÄ¿
THROTTLE ³()³ adjustment arm³ :³=======================²³SOLENOID³
ARM ³__³======================³__³ ÀÄÄÄÂÄÂÄÄÙ
LINKAGE Neutral ³ ³ Neutral
N.B. The solenoid pull is proportional to:-
1/ the current squared,
2/ to the number of turns,
3/ to the location of the movable iron slug.
4/ the solenoid size.
I have done this modification to several generators now:-
eg. a 1.3KW rated (1.5KW peak) generator to make it produce a steady 240V...
LOAD BEFORE AFTER MOD IMPROVEMENT
WATTS VOLTS VOLTS VOLTS %
0 250 250 0 0
100 240 240 0 0
500 230 238 8 3.3
1000 220 242 22 9.2
1500 200 240 40 16.7
With this modification the throttle is open much earlier, as soon as the load
comes on, you hear the engine rev up under load heavy loads maintaining the
voltage much closer to real mains! This means not only is the static regulation
much better, but the dynamic regulation as well, as there is not the usual hang
time while the RPM drops before the throttle is opened.
Part 3 has Loads, Sine waves, Safety, & Security
/QSL
73 de John G8MNY @ GB7CIP
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