Electronics > Beginners
What happens if a transformer that get less voltage then rated for?
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Berni:
Yep transformers can be undervolted as much as you want.

All the numbers are all maxium specs that the transformer has to operate within to work reliably long term.

So half the voltage in means half the voltage out, but the maximum current for both the primary and secondary stay the same. This means you can no longer reach the 400VA spec as you would already start exceeding the maximum currents in the wingdings when you get past 200VA. The transformers are carefully tuned to give the most power at the rated input voltage.

Running at a lower voltage has benefits for low load operation however. The lower voltage produces a weaker magnetic field inside, this makes for lower losses in the core due to magnetization and eddy currents. The current in the primary reduces and as a result also reduces the restive looses in the primary. As a whole the transformer wastes less power for its own operation, consuming less power, running cooler and making less audible hum.
Jwillis:
The VA is calculated from the core area and material used .Since changing the voltage doesn't change the core area or the material the VA remains the same.
The current I in amps is equal to the apparent power S in volt-amps, divided by the voltage V in volts:

I(A) = S(VA) / V(V)

Brumby:

--- Quote from: Jwillis on May 21, 2018, 07:58:24 am ---The VA is calculated from the core area and material used .Since changing the voltage doesn't change the core area or the material the VA remains the same.
The current I in amps is equal to the apparent power S in volt-amps, divided by the voltage V in volts:

I(A) = S(VA) / V(V)

--- End quote ---

The magnetic circuit may be capable of the same VA - but there are still the limitations on the voltage and current-carrying capabilities of the windings.
Jwillis:
Of course the gauge of the winding's determines what the final output characteristics will be.When you measure a core for an application you would allow an over head on the VA depending on application.Typically the rated output is set at 50-80% the measured VA.And the gauge of the winding's is determined by the required output.This is usually 8 -12% the fusible value of the wire.This keeps the heat down to around 25C under normal operation.
Zero999:
Yes, it's fine to run the transformer at a lower voltage. It will also mean that the transformer can be used at a lower frequency. A 240VAC transformer designed for 50Hz operation will be capable of operating down to 25Hz, if run off 120V. The main disadvantage is the current rating will be the same, therefore the VA rating will be halved.

If the frequency is increased, it's also possible to run a transformer at a higher voltage, as long as the voltage rating of the insulation isn't exceeded or the heating due to increased core losses and skin effect, doesn't cause it to exceed the temperature rating, so it's not something which should not be done without caution.
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