## Using 240 volt transformer at 120 volts

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grant fair
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### Using 240 volt transformer at 120 volts

I have a surplus toroidal transformer specified at 240 volts 50/60 hz primary, and 62 volts secondary. Is there any reason not to use it with 120 volts supplied to the primary?

I understand I will then wind up with 31 volts at the secondary. It is rated at 5 amps at 240 volts primary. What current can I expect with the primary voltage halved at 120?

Grant
Grant

Edd
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### Re: Using 240 volt transformer at 120 volts

Since you halved your power input, that current spec will be halved or more.
core is not being magnetically driven to its design spec.

73's de Edd
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The very fastest way to find something you've lost is to replace it.

Chris Smith
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### Re: Using 240 volt transformer at 120 volts

IE, to continue on Eds summary,....you wont get perfect specs! Just most of them?

philba
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### Re: Using 240 volt transformer at 120 volts

so, edd, when you say "current spec" do you mean that it would be a 2.5A, 120 VAC primary? That's 1/4 of the power (1KVA vs 250VA). [Though, I suspect the secondary is rated at 5A so it would 300VA vs 75VA.] I can understand halving the VA of transformer minus some efficiency loss but don't understand 1/4. could you explain that?

jwax
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### Re: Using 240 volt transformer at 120 volts

Half the applied voltage will draw half the current, thus 1/2 X 1/2= 1/4 Power.
You'll have 31 volts at 10 amps available on the secondary. Ideally.

Carl Pugh
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### Re: Using 240 volt transformer at 120 volts

Current rating will be the same as at 240 volt.
Voltage rating will be 1/2.
Power rating will be 1/2.
% sag will be twice what it was at 240 volt.
Sag probably will probably not be a problemill.

MrAl
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### Re: Using 240 volt transformer at 120 volts

Hello there,

Sorry Edd, that's not correct.
The output current is based mostly on the
wire size of the secondary (and primary).
Since the transformer was designed to handle
5 amps on the output already then reducing the
input voltage wont change that because the wire
diameter is still the same (on both primary and
secondary). This of course means the transformer
will still put out 5 amps only with the voltage
reduced to 31 volts.
The core, being excited less, will be just fine,
and the output will be cleaner than before because
of the reduced odd harmonics due to less B peak.
Also, if there was any humming at 240v, that
should be greatly reduced at half the voltage
input.
The only drawback is that the primary and secondary
turns were calculated to handle twice the voltage,
so that means there are more turns than would
be necessary for a 120v transformer that put
out 31 volts. This means the wire resistance
is not optimal for the voltage it has to
support, which means a little more voltage sag
under load then a more perfect design, but
since the resistance is probably quite low anyway
there will still only be a little sag under load.

Take care,
Al
LEDs vs Bulbs, LEDs are winning.

jwax
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### Re: Using 240 volt transformer at 120 volts

I misread 5 amps as the primary current rating.
You'll still have 5 amps at the secondary, 31 volts by feeding 120 volts to the primary.

grant fair
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### Re: Using 240 volt transformer at 120 volts

Thanks to you all for your responses. I guess I will break down and look for some power resistors to actually measure the output current. Ten amps would be great but I am voting for 5. I'm away for a few days but when I am back and have some results I will post.

Grant
Grant

Edd
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### Re: Using 240 volt transformer at 120 volts

Okay , continuing on the sole comment…of one mere elongated sentence. Which initially was started on and then was impinged upon being disrupted at a filing time of 53 minutes after the hour. That info being made by an on screen prompting that the server that feeds work stations was about to shut down in 2 more minutes.

The first comment was merely on the transformers primary and there being application of only half voltage to its winding., and to be expecting a like approximate halving of its current demand. The qualifier there was that I was being a bit more liberal by virtue of the utilization of a toroidal design of transformer.
Thereby, not knowing if its core was using a width gradiated spiral laminar wound tape or a solid core., but EITHER of which utilized , would be a smidge more load performance respected than the E-I or cross interleaved E-I lamination configuration of a common transformer.
That seems to really start coming into play when the transformer gets on up toward the high current end of its spec.

Moving from the primary on to the transformers secondary, we can expect an ~ quarter of the inputted primary power to be available there, that with the transformers full design specs being unknown.

The mere mention of Grant’s acquiring a toroidal design of power transformer keyed me in to the fact that I remember storing two units away that I had salvaged from a ’70 vintage GE X-RAY Standby LV linear power supply. They were about half of Grant’s mentioned transformers voltage output . I was initially hoping for future location of others up in the 60-0-60 volt range for utilization in a brute strength linear power supply for a higher output SS audio amplifier, but that was not to be accomplished with that low of a derived supply voltage.

Putting one on the bench for evaluation would give some degree of ratiometric comparison to Grants unit, even if working in his same manner of operating a unit at half of its design center voltage. In this case mine were for 125VAC input , therefore a like evaluation would be operating at 62.5 VAC.
Findings were made using a Dale 47 ohm at 25 watt wirewound load resistor, as that was the only unit at hand at that time.

125VAC OPERATION:

Placing 125VAC into the transformer primary recorded a current consumption of 200 maAC making a 25W power consumption.

Reading the secondary voltage showed 34.7VAC at a current consumption of 715maAC for a 24.8W power consumption.

62.5 VAC OPERATION:

Placing 62.5 VAC into the transformer primary recorded a current
consumption of 110 maAC making a 6.875 W power consumption.

Reading the secondary voltage showed16.83VAC at a current consumption of 315maAC for a 5.3W power consumption.

If you will then compare the secondary power sag with the 62.5 VAC input voltage , it is already more severe than was experienced with the transformer operating at its spec’d input voltage, and that only being with a fourth of the current loading with the primary at the 62.5 voltage level.

No problem with the initial transformers selected wire gauging. If it was proper at full line voltage , its for sure that any larger gauging will not be required at reduced voltage operation. How far that core is in towards its saturation with that full spec’d voltage core design as compared to the 1/2 voltage primary activation comes more into play in being a bit more deficient approaching the max current consumed end of the available output.

Whenever I get a surplus mystery power transformer (nearly always E-I construction, RARELY a toroidal design..…for 60~ use that is) that I need to get in the ball park on its ratings I typically take an unloaded voltage reading and then load it down to the common unloaded/loaded to spec ratio that I find on some old workhouse….Meissner, Thordarson or Triad brands. Now try that on some Radio Shack, et al power transformers and you will really see some current headroom sag as well as heating up of those units.

Grant, hope that your unit will fulfill your application adequately. The unit sizes of mine are about 2 in high by a 5 in diameter on the donut. I have used several freebie industrial application control transformers (E-I style) with 110,220V as well as a 440V winding in the same reduced input manner with the same minor shortcomings.

73's de Edd
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MrAl
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### Re: Using 240 volt transformer at 120 volts

Hello again,

I did some tests using a spice model of a simple
transformer and got the following results.
The transformer had a 5:1 turns ratio, and
i used 'round' numbers for the input voltage
to make things clearer.

Turns ratio=5:1
Load current=5 amps for both 200 and 100 volt inputs