[Amps] capacitor across mains input to transformer

[Jim Thomson]

Date: Tue, 10 Oct 2017 08:56:07 +1300
From: Steve Wright <stevewrightnz at gmail.com>
To: amps at 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