The message <
[email protected]>
from "Robert Green said:
Why does a generator need an inverter at all? Doesn't it put out pure
sinewave A/C to begin with?
====snip====
You'd think so, wouldn't you? However, the reality is that a cheap
standby generator puts out a rather distorted waveform on load (it looks
ok without load) but this isn't the issue. The real problem with a
conventional alternator is its suceptability to overvolting with even a
modest capactive loading which becomes a problem with certain models of
UPSes that switch a large amount of capacitance across the line when
they return to pass through mode.
About two years back, I bought a cheap 2.5KVA petrol(gasoline)
generator from my local Aldi store to supplement the 2KVA APC
SmartUPS2000 that I use to provide a protected supply to my computer
kit. After much testing, it turned out that it was basically useless for
this purpose since even a modest 4.7 microfarad capacitor was sufficient
to make the generator output jump from its regulated 230v to some 275
volts.
The UPS was switching twice that amount of capacitance across the line
every time it saw the generator voltage return to normal and tried to
revert back to "mains" power which caused it to immediately switch back
to battery as the generator voltage shot back up to some 270 volts, a
process that would go on ad infinitum until I reconnected back to the
PSU power.
An inverter type of generator drives a high voltage version of a car
alternator with a permanent magnet rotor which feeds a 3 phase fullwave
bridge rectifier and capacitor smoothing pack and uses throttle control
to maintain its DC output voltage feed to the Sinewave inverter which is
immune to this capacitive loading issue.
It wasn't the 5Khz noise ripple from the cogging effect of the stator
windings, nor the gross amount of harmonic distortion, nor even the
subharmonic content due to the speed modulation effect of using a single
cylinder 4 stroke prime mover nor even the +/-5% speed/frequency
varations between no load and full load on the generator supply as I
initially had suspected, it was simply the total lack of voltage control
in the face of capacitive loading variations.
It took nearly a year before coming to this startling conclusion and
many hours of rigging up test solutions to combat the problem. One
solution I tried was to rig up a parallel inductor to cancel the
capacitive loading but this didn't entirely solve the problem.
Another solution I'm considering is to use a bunch of universal 150W
laptop chargers to create a 54v 2KW battery charger to allow the UPS
inverter to run indefinitely off its battery pack, float charged from
generator power, (in 4 or 5 banks of three wired in series to make up
the 54 volts from 3 times their 18v output setting fed from an
autotransformer set to supply them at a nominal 190vAC from the genny's
230v so that even if the genny voltage jumps up to 280v, they'll not
suffer any overvolt damage).
This, unfortunately depends upon a local fleamarket trader coming good
on his promise to be able to supply me with 15 or so such laptop
chargers at a low enough price to make it a worthwhile proposition. Even
supposing I managed to get hold of the chargers, this still leaves the
issue of uprating the ventillation on the UPS (not a difficult task ;-)
in order to allow it to run in this mode indefinitely.
TBH, I'm now on the lookout for a secondhand 3KW inverter type
generator to replace the cheap 'n' cheerful pile of crap I've currently
got. I couldn't really justify the price of a brand new unit, but it
would neatly solve all the issues at a stroke. If I'd known, when I
bought the generator, what I know today, I would have only considered an
inverter type in the first place.