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From: wtm@neoucom.UUCP (Bill Mayhew)
Newsgroups: sci.electronics
Subject: Re: Ferro-resonant Transformers
Message-ID: <672@neoucom.UUCP>
Date: Fri, 21-Aug-87 09:43:37 EDT
Article-I.D.: neoucom.672
Posted: Fri Aug 21 09:43:37 1987
Date-Received: Sun, 23-Aug-87 10:38:53 EDT
References: <3761@watdcsu.waterloo.edu>
Distribution: sci
Organization: Northeastern Ohio Universities College of Medicine
Lines: 42
Summary: Square wave due to 3rd harmonic content

bmaraldo@watdcsu.waterloo.edu (Commander Brett Maraldo) writes:
>    Are ferro-resonant transformers supposed to output a pseudo-square wave?


Ferro resonant transformers put out a waveform that resembles a
square wave due to the high 3rd harmonic content that their
reguating action produces.

Some sensitive equipment can be bothered by the presence of the
harmonic content.  Electronics that has power transformers that are
designed strictly for 60 Hz can have problems.  Sometimes the
transformers get hot or the core laminations vibrate annoyingly.
The vibration is particularly irritating, since it is 360 Hz due to
the fact the laminations vibrate once per half cycle of the 180 Hz
hamonic wave.

Harmonic neutralized regulators are available but tend to be quite
a bit more expensive than the standard models.

Where the power problems are slow drifts rather than variations
that occur over one or two cycles, I favor a tap switcher
transformer.  Basically, a tap switcher is usually an
autotransformer that is equipped with a small voltage sensor
circuit board.  The output of the sensor goes to SCRs that select a
tap on the autotransformer that will produce the desired output
voltage.  The advantage of such a transformer is that the output is
relatively harmonic free, and the transformer itself is small,
quiet and runs cool.

I did have one nasty experience with a Topaz brand tap switcher at
the last place I worked.  The line input fuse on the sensor card
blew, and somehow caused a commutation effect on the SCRs.  The
rapid switching of the SCRs turned the transformer into a big
ringing choke and it fed 1800 volts into the output side!!
Fortunately, nothing was plugged in when it blew.  It was used in a
test facility, and part of the morning ritual was to test the line
voltage to the bench before plugging in the D.U.T.  I sure did a
double take when I saw my meter pin on the 250 vac scale on the 120
volt line.

Bill
(wtm@neoucom.UUCP)