[RFI] Sine wave correction for small generators

Grant Saviers grants2 at pacbell.net
Sun Nov 1 00:01:27 EDT 2020

Is your gen an inverter type? At 7.5kw a bit unusual for a residence. 
Then the inverter types use a multi KHz inverter frequency and produce a 
reasonable sine wave (some near perfect).  OTOH, cheap DC to AC 
"modified sine wave" inverters do have the stepped wave you have heard 
about.  "Pure sine wave" use a multi KHz frequency so more closely 
approximate sine. Both can have bad RFI.

You might have a standard AC generator, which can have a high distortion 
output when they skimped on the iron & copper.  I've seen awful.  It 
might have a spec.  Worth a look on a scope.

Making a standard isolation transformer a tuned circuit may be possible, 
but it might create a large circulating current. The classic passive 
regulating technology is a ferroresonant transformer.  The primary 
magnetic circuit is modulated by a secondary resonant winding and capacitor.

There are alternative designs depending on the goals for the output.  We 
used 100% passive ferroresonant power supplies for many years at DEC for 
transistor logic computer DC supplies, about +/-10% line+load regulation 
which was designed into the R, S, K & B logic families.  When TTL came 
along then active regulated supplies were necessary.  For an 
industrial/lab/realtime computer, ferroresonant was a great technology 
for noise isolation, increased line dropout resistance, and great 

The Sola "Constavolt" is a classic passive AC line regulator, available 
on ebay to a several hundred VA capacity.

see also https://www.generaltransformer.com/ferroresonant-transformers

Grant KZ1W

On 10/31/2020 20:06, Kenneth G. Gordon wrote:
> I am considering adding a 60 Hz "tuned circuit" in line after my emergency 7500 watt
> generator. Although I have not yet looked at the wave-form of the output, from what I read, it
> is essentially a "stepped" representation of a sine wave, which is, in my opinion, very dirty.
> I have two, very heavy, 115-to-115 VAC, 15 amp transformers (they weigh at least 50 lbs
> each).
> I am considering paralleling enough capacitors on the inputs and outputs of the two
> transformers to result in 60 Hz tuned circuits in an attempt to smooth out the "stepped" 60 Hz
> waveform.
> Has anyone tried this and if so what was your result?
> Ken W7EKB
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