Hi there,
The conclusion is that we can drive the transformer in this way, but
only if the output current is still restricted to the original 10 amps
as with the 'normal' connections.
It is possible to wire the two primaries in series and supply with
120vac, which should not be a problem. As mentioned in the
previous thread, the core losses go down as the excitation voltage
goes down.
There is a loss of output power however, in that the available output
power is reduced by about one half, and trying to draw more power
than the original full load current will cause more heating in the
windings.
The following set of numbers tells the whole story. These are taken
from a transformer like the one described with 2.5 ohms series
resistance in EACH primary winding, and 0.1 ohms series resistance
in the total secondary (from end to end, not looking at the center
tap). This would be a typical set of resistances for a transformer
like this.
From the numbers we see that we can go up to 10 amps on the
output but that's about it. Maybe a little higher, but if we try to
get the original power output at the reduced voltage we will most
likely burn up the transformer.
The 'normal' power in the transformer is about 16 watts, but if the
secondary is loaded to 20 amps (with series 120vac input) the power
in the transformer goes up to more than 4 times this normal operating
power dissipation, which is very very bad.
Also note however that loading to 10 amps is just fine.
Code:
Voltage (no load)
Parallel 120vac input: 25.2v out
Series 240vac input: 25.2v out
Series 120vac input: 12.6v out
Power
Parallel 120vac input:
10 amps, Primary 5.6 watts, Secondary 10 watts, Load 236 watts, volts 23.6v
Series 240vac input:
10 amps, Primary 5.6 watts, Secondary 10 watts, Load 236 watts, volts 23.6v
Series 120vac input:
5 amps, Primary 1.4 watts, Secondary 2.5 watts, Load 59 watts, volts 11.8v
10 amps, Primary 5.6 watts, Secondary 10 watts, Load 110 watts, volts 11v
20 amps, Primary 11 watts, Secondary 40 watts, Load 189 watts, volts 9.5v