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'[OT] off topic AC Power -- plus DC'
1998\02\01@065225 by Morgan Olsson

picon face
At 23:19 1998-01-31 -0500, you wrote:
>From my recollection my 4th year power systems engineering course,  the
>reasoning for the choice of 60Hz was to minimize the losses incurred in the
>transformer cores (ie: magnetic heating of the iron, etc.)  Apparently
>somebody did some math (and a lot of laboratory testing to demonstrate that
>the losses in transformation are at a minimum at approximately 57 Hz.
-snip-
>Tom

I believe the size and power rating of transformer, generators and motors
strongly sets what frequency give best performance. Large units = low
frequency. Also the materials play a great role.

Higher frequency will give less losses for the end-user (smaller motors
etc), while lower frequencies will give lower losses in the power plant and
distribution.

However what is optimum has probably changed with time, as materials and
demands change. Another issue is noise. While higher frequenzy is easier to
filter electrically in the aplliances, lower frequenzies make less noise
for our ears. Radiation from power lines also invrease with frequency, as
well as impedance! (they are a lot of miles...)

An example of very large power unit operating at relatively high frequency
is the 600MW generators at the double nuclear power plant Barseback near
Malmo, Sweden.  Each one is conneted directly to the steam turbine shaft in
each plant, running at 300rpm.  They are synchronous three-phase liquid
cooled types. (If I remember correct, the windings are made of copper tubes
in which the coolant is pumped.)
The size of that 600MW generator is only about 2,5m diameter and 4m long!

Another example is the 100 000 rpm class generators used in some hybrid
cars, driven by combustion turbine.

An exapmle of low frequency is the Swedish railroad that uses 16 2/3 Hz (if
I remember correct) = 50Hz/3.  There is a lot of standards, in different
countries, however.

As example a modern switching power supply use frequencies about 30-50kHz.
Some resonant systems up to 1MHz.  That makes very small transformers with
very low winding losses.  A 50kHz 200W push-pull transformer is about the
same size as a 50Hz 3W transformer!

In Sweden long before I was born we also had 127V DC, 127V AC, 220V AC and
probably a lot of local variations.  Lots of home appliances and radions
could work on both 127V DC and 127 and 220V AC using a switch.  However, I
have never heard of anything else than 50Hz used on AC public power net in
Sweden.

For long distances high voltage DC givs the least total losses, since the
power lines at DC always work at optimum, but AC has two problems: 1) high
peak voltage requiring more isolation, 2) A lot of the time in each
conductor the voltage is low (sinewawe) so very little power is transferred.

Modern technology make AC/CD transformations more efficient. In future
maybe all distance high power lines will be DC for less line losses. Then
the power plant also can then vary the generator rpm to optimum at each
time beeing.

Already today there are large gearless flat generator wind power plants
operating at always optimum rpm using AC/DC/AC converters. (the AC->DC
stage is to adapt to local AC lines)

I read that former ASEA (Sweden), now Brown-Boveri (Sweden+Switzerland) has
developed very high power semiconductors that can operate at 700¡C (!) and
very high voltage and current.

/Morgan
Morgan Olsson, MORGANS REGLERTEKNIK, Sweden, ph: +46 (0)414 70741; fax 70331
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