[Amps] Transformers

[Joe Isabella]

Since you mentioned that, why does the front of the SB-220 say "2KW LINEAR AMPLIFIER" if it was designed as a 1kW input amp?

I've often wondered that...

Joe, N3JI


----- Original Message ----
From: Tom W8JI <w8ji at w8ji.com>
To: amps at contesting.com
Sent: Friday, July 14, 2006 11:01:33 PM
Subject: [Amps] Transformers


> The Heathkit 220 and a few other models had around 15% 
> regulation which is right on the edge of
> saturation (they ran a high flux density to save on weight 
> and cost).

The SB220 transformer was designed for a kilowatt INPUT DC, 
which was the legal input when it was designed. This was 
about 450 mA at about 2200 volts. It was a good conservative 
design for that power level. At rated power on a good power 
line it sags less than 10%.

The problem is most people seem to think it was designed for 
1000-1300 watts CW output, and they expect a 1000 watt INPUT 
power design to be stellar at two or three times the rated 
CW dc input.

>They don't saturate, but under full
> load they're cutting it pretty close.

A regular power transformer certainly does not work the way 
that statement might lead us to believe.

Maximum flux density, which means closest operation to 
saturation, occurs with NO load. As load is increased flux 
levels do not increase. Increased current, because of 
resistive losses in the primary circuit, actually causes the 
transformer's flux level to decrease. When a transformer is 
designed the highest primary voltage under no load is used 
to set flux density at a safe level.

The actual mechanism inside a transformer is the secondary 
develops a counter-MMF. This opposing flux would reduce flux 
density, but primary current increases in order to try and 
maintain the **same**  flux density. When mains and primary 
resistances carry more current from increased load, the 
primary voltage drops slightly. This REDUCES flux density, 
moving the transformer further from saturation. Not closer 
to it.

If the transformer had a separate secondary winding with 
constant current you would actually see the voltage on it 
decrease with any increase in load on the secondary with 
variable load. This voltage decrease is because flux density 
decreased.

73 Tom 


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