[Boatanchors] 120 - 240vac and output

Gary Schafer garyschafer at comcast.net
Fri Nov 26 23:51:26 EST 2004


I don't think that most were thinking in terms of the transformer 
resistance. Mostly just the 120 or 240 volt house mains line delivering 
the primary power to the amp.

Also I believe that most were using the same size wire for the mains 
whether it be 120 or 240 volts in the examples.

Let's take an amp that draws 1200 watts from the mains:
We will use a plate transformer that delivers 1200 volts at it's secondary.

So 10 amps on a 120 volt (say #14 wire) would have 2X the voltage drop 
on it as the same size wire would on 240 volts with 5 amps on it. Same 
power. Both delivering 1200 watts.
That is where one of the 2X comes from.

The other 2X is the fact that the transformer ratio for a given plate 
voltage has a 2X ratio difference between the two.
The 120 volt to 1200 volt transformer has a 10:1 ratio.
A 240 volt to 1200 volt transformer has a 5:1 ratio.
The difference between the two ratios is 2X.

If the #14 wire in the house had 1 ohm of resistance, when used at 120 
volts with 10 amps that would cause a 10 volt drop in the voltage 
delivered to the primary. Being that the 120 volt transformer has a 10:1 
ratio to the secondary that 10 volt primary drop would be seen as a 100 
volt drop at the secondary. Voltage would be 1100 volts rather than 1200 
volts.

With the same #14 wire with the same 1 ohm resistance on the 240 volt 
circuit there would only be 5 amps of current for the same amount of 
power. 5 ohms at 1 amp would be 5 volts drop seen at the primary.
Since the ratio of the 240 volt transformer is only 5:1, that would be a 
25 volt drop at the secondary. The secondary voltage would be 1200 - 25 
volts or 1175 volts.

That makes 100 volts drop on 120 volts and 25 volts drop on 240 volts. 
The ratio of 100:25 is 4:1 or 4X the voltage drop.

So from the above for a given amount of total power drawn by an amp we 
can say that there will be 4 times greater voltage drop on the secondary 
side of the transformer because of mains voltage drop when run on 120 
volts verses 240 volts.

This does not necessarily mean that the TOTAL plate voltage drop will be 
1/4 less when run on 240 volts. There is still going to be the factors 
of the secondary circuit voltage drop. Transformer resistance, capacitor 
size, type of rectifier circuit etc.


Now keep in mind that if a heavy enough mains line is used the voltage 
drop may be very small and not worth the bother.

73
Gary  K4FMX


WA5CAB at cs.com wrote:
> 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)
> <wa5cab at cs.com> (Primary email)
> <wa5cab at houston.rr.com> (Backup email)
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