Date: Tue, 10 Oct 2017 08:56:07 +1300
From: Steve Wright <stevewrightnz@gmail.com>
To: amps@contesting.com
Subject: [Amps] capacitor across mains input to transformer
<Can someone direct me to reading material on this?
<I've wound a HT transformer and it's got a decent hum to it and draws
<about 1A @230V input with no load.
<Thanks,
<Steve
### That 1 A of current you are seeing on the primary...with no load on the
sec, is
called...magnetizing current. If you dont use enough primary
turns.....keeping the
turns ratio constant, it will result in higher magnetizing current.
Microwave oven
transformers typ have high magnetizing currents.
## Flip side is.... with more turns on the primary side, you will now require
more turns on the sec !
IE: 100 turns on primary and 1000 turns on sec...... vs 200 turns on
primary + 2000 turns on sec.
## In the above case, the 200 turn primary will result in lower magnetizing
current. Problem is, all this extra
wire may not fit on the core. So typ what happens is even smaller gauge wire
is used on the sec..or pri..or both.
Problem with that is, then you end up with higher dc resistance on the pri,
sec, or both. High resistance secs
is a disaster with high C filters on B+ supplies.
## I measured aprx 1.9A mag current on my 120 lb hypersil pole pig, 10 kVA
CCS. My 253 lb dahl
measures 1.5A. Mag current on the dahl primary will increase to 1.6A , if I
feed 240 vac into the 230 vac tap.
Mag current will decrease to just 1.4A, when feeding 240 vac into the 250 vac
tap.
## I do have a smaller dahl 46 lb xfmr. But I dont have notes on what mag
current it draws.
## I also noticed that when I fed 120 vac into the 240 vac primary of my 10
kva pole pig.... my Mag current dropped
like a rock, like down to just 40 ma.
## Microwave oven xfmrs run hot with no load on em.... due to excess mag
current. Cheap design, typ.
## as long as ur xfmr does not run hot..with no load on the sec, you are
probably ok.
## as a side note, I tried a cap across the primary of the xfmr to improve
the power factor. Doesn’t work, just makes it worse.
It also increases the current on the input side, but that is not an increase
in mag current. Its current through the cap. B4 Mr Dahl
passed away, he told me Im wasting my time with power factor correction caps,
cant be done. The typ ssb + cw load is bouncing
up and down like a yo-yo.
later.... Jim VE7RF
Jim VE7RF
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