> > I have never heard of a transformer burning out with just
> > magnetising current on 50Hz when it would run cool on 60Hz. Did you
> > call Peter Dahl on April 1st?
>
> No, it wasn't April 1. I too have used 50/60 Hz equipment such as the
> SB200 on 50 Hz without any trouble. I've also used 115v/60 Hz. US
> gear with step-down transformers on 230v/50 Hz. However, larger
> transformers can be built for 60 Hz. only and in the case of the
> transformer rated at approximately 3kva CCS which I used, Dahl told me
> that a 50 Hz. rating was neccessary.
That is correct, either for small or large transformers.
You can use a transformer designed for 50 Hz on 60, but not
always the other way unless the transformer has enough headroom
in flux density.
The problem is not a linear problem, it is a saturation problem.
A transformer that behaves quite well and runs very cool on 60 Hz
can overheat on 50 Hz, that is nearly 20% change in frequency.
The effect is almost like having a perfect zener diode in series with
a resistor across a power supply. Below the threshold of the zener
everything is fine, above it the dissipation takes off like a rocketship.
There is a reason this is more common with large transformers.
Large transformers flux density are sometimes moved to near the
limit in an effort to minimize ESR and maximize regulation and
performance. This actually results in a cooler transformer, not a
hotter one, as long as voltage is below an upper limit and frequency
above a lower limit.
Any 60 Hz transformer will make a good 50 Hz transformer at less
than maximum primary voltage, but the voltage has to be reduced
to keep the core below saturation or the transformer will suddenly
show a very large increase in heat without any load.
Conversely a 50Hz 220 volt transformer at the flux density limit
would make a dandy 264 volt primary transformer on 60 Hz, if
insulated well enough.
I use a 50Hz 208 volt design transformer in my PA at 245 volts in
my PA, and it is perfectly fine. The same voltage at 50 Hz would
seriously overheat it.
Most (but not all) small transformers have no need for minimal ESR
is a given physical size, so they rarely have this problem...but they
can.
73, Tom W8JI
W8JI@contesting.com
--
FAQ on WWW: http://www.contesting.com/FAQ/amps
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|