[Boatanchors] 120 - 240vac and output

[WA5CAB at cs.com]

Actually, that isn't generally the case (this 4X figure that has mysterously 
shown up in several posts).  I've been busy and kept thinking that one of the 
other EE's on the list would speak up.  But much of what's appeared in this 
thread simply isn't true.

You don't have twice the voltage drop in the primary .  At least with a 
properly designed transformer, you actually or effectively have half.  Really low 
power applications (like some test equipment) or really cheap designs may have 
a 240 volt primary tapped at 120 volts.  I'm not talking about those.  But 
most from the size of the TCS on up that are designed to run off of 120 and 240 
volts actually have two identical 120 volt primaries.  They are connected in 
parallel for 120 volt operation and in series aiding for 240 volt operation.  If 
the manufacturer sells two variants, one for 120 and one for 240 volts, then 
the primary wiring in the former will (should) have twice the cross section as 
in the latter.

If, for example, the resistance of each primary winding in a dual voltage 
transformer is 1 ohm, the total resistance looking into the primary would be 1/2 
ohm when connected for 120 and 2 ohms when connected for 240 volt operation.  
The same statement would apply to 120 and 240 volt single voltage input 
transformers.  If the primary current for some load was 2 amperes at 120 volts, it 
would be (ideally) 1 ampere at 240 volts.  The primary VA (heat) loss would be 
2 watts in both cases (4 x 0.5 versus 1 x 2).  The current through each 
winding is 1 ampere regardless of which way you connect them (dual windings).  If 
you ignore core (hysteresis and eddy current) losses and secondary losses and 
model the transformer as an otherwise ideal transformer with each primary 
winding in series with a fixed 1 ohm resistor, the voltage drop in each winding 
would be 1 volt (1 x 1) regardless of which way you connected it.  

Assume that the ideal or no load secondary voltage is intended to be 1200 
volts.  The turns ratios primary to secondary would thus have to be 10:1 or 5:1 
depending upon which input voltage it was connected to.  The effective primary 
voltage to apply to the turns ratio would be 119 or 238 volts and the 
secondary voltage would 1190 volts in both cases before you subtract the secondary and 
core losses (which don't care how the primary is connected).

Therefore, as far as a dual voltage transformer is concerned, it makes 
absolutely no difference which way you connect the primary.  Nor does it matter 
between two properly designed single voltage transformers.

However (two or three howevers apply), there are other considerations that 
may effect the overall cost of the system.  The dual voltage transformer will 
cost more, as it has three instead of two windings and six or seven instead four 
or five (full wave or bridge rectifier to be used) output terminals or wires. 
 Ignoring packing factor (how much copper you can put into a given opening 
versus the wire gauge), a straight 120 or straight 240 volt transformer should 
theoretically cost the same (and less than the dual voltage one) as each would 
take the same number of pounds of iron and pounds of copper.  Since we're 
inherently talking about a transformer with higher than line voltages on the 
secondary, insulation (cost) considerations would be the immaterial.  But building 
supply wiring would cost more for the 120 volt than for the 240 volt setup, as 
you would need roughly 3 AWG wire sizes larger wire for the former than for 
the latter.  I.e., if #14 AWG wiring gave acceptable results when operating on 
240 volts, you would need to use #11 AWG for the same overall losses at 120 
volts.  As #11 AWG isn't "readily" available, you'd have to run #10 AWG or two 
#14 AWG.  Both options run up both material and labor costs.

Anyway, there's no reason other than dollars why a transmitter or amplifier 
can't run the same output at the same overall efficiency (all the way back to 
the light bill) regardless of what the primary voltage is.  So if anyone 
advertised a rig as being good for 1000 watts if run off of 120 volts and 1200 watts 
if run off of 240 volts, they cut corners somewhere.

And I wasn't even a Power Major.  :-)

In a message dated 11/26/2004 3:31:09 PM Central Standard Time, 
vic at rakefet.com writes: 
> The PRIMARY voltage drop is 2x, as you say.  But remember that the 
> transformer 
> is now multiplying the voltage from 120 to 3000 (or whatever) instead of 240 
> to 
> 3000.  So any voltage drop in the primary will have twice as much effect on 
> the 
> secondary voltage.  Twice the primary drop thus means four times as much 
> secondary drop.
> 

73
Robert Downs - Houston
<http://www.wa5cab.com> (Web Store)
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