[Prev][Next][Index][Thread]
Re: new power transformer (variac as ballast)
>kept reasonably low. In a test I did, I drew only (less than) 3 amps
through
>a 20A variac, but it saturated, but I was using only about 10 turns or so
of
>the variac, so maybe the volts per turn went too high. But then another
>possibility is that the current peaks were a lot higher than 20 amps.
>In any case, the
A 20-amp variac used as a variac will put out 20amps for the following
reason:
it is a transformer. Power is delivered by the magnetic flux. If the
output would
load the variac more the input power would rise. That means also that if
the
input current rises (magn flux rises) the output will draw current and
reduce
the flux.
The main result of operating it as a transformer is that the only current
causing
magnetic flux in the core is the magnetizing current of the transformer
which
is rather small. However, in order to put say 20 amps through the variac
the
conductors still have to be heavy enough for that current.
On the other hand, if you put 20 amps through the variac using it as an
inductor
magnetic field can be easily calculated from
B = u N I /le where B=field, u=permeability of core, N=turns, I=current,
le=lenght
of magnetic path.
That means you will get higher field if you rise the current. Because the
magnetizing current is small the device is rated not to saturate at a low
current.
That's why you usually cut slots (ie. a gap) in the core. That will lower
the
apparent permeability of core and keep the magnetic field reasonable.
--
Harri Suomalainen mailto:haba-at-cc.hut.fi
We have phone numbers, why'd we need IP-numbers? - a person in a bus