What happens if a transformer that get less voltage then rated for?

[FriedMule]

What would happen if lets imagine, you have a 400VA 200V main in to 40V 10A secondary out.
And you only supply it with 100V. What would that give?  20V ?A secondary out?

[Doctorandus_P]

Ratio between input and ouput voltage wil stay the same.
So, if you halve the input voltage, the output voltage wil also be halved.

The opposite is not true.
Most (mains) transformers are driven pretty close to saturation of the core, and if you put more voltage into a transformer winding than it is rated for you are likely to destroy it.
Destruction is not instant though, but it wil overheat (too much current) after a short time (can be minutes).

Because (usually) transformers are so close to the maximum voltage they can handle, they lose a bit of linearity between input and output voltage. So it is possible that your transformer will output 21V or 22V.

Current rating of the transformer wil stay the same. So output will be 20V,10A instead of 40V,10A.

Also note that the output voltage is usually specified under full load. Output voltage will be a few percent higher for "big" transformers under a light or no load, and for small transformers this can be upto about 20%.

[Electro Detective]

Are you limiting the input voltage with a variac ? please explain the conditions

[FriedMule]

My idea was to get a 600VA 240V in 2 x 30 out transformer but then I read about flux, saturation and heat.
That gave me the idea to use a 300V in 2 x 37.5V out or if you recalculate it (300V - 70V ) / 1.25 = 240V - 60V

[Brumby]

My experience is that people with capacity concerns simply upgrade the VA rating.

In your example of 600VA 240V in 2 x 30 out I'd go for 750VA (or higher) 240V in 2 x 30 out.

But derating a higher voltage transformer is also valid.

[FriedMule]

the price difference between a 600VA and a i.e. 750VA seems really high.
But you would mean it is equally good solutions 600VA 300V and 750VA 240V?

[Doctorandus_P]

Substraction
When working with ratio's, do not add / subract.
It's more like:

octave:1> 240/300*37.5
ans =  30

Your calculation is off:
octave:2> (300 - 70 ) / 1.25
ans =  184
octave:3> ans+60
ans =  244 (Not 240V as you thought).



A long time ago (25+ years) I saved a 0.7A variac from the dump.
Combined it (wen I was still young) with a 2x40V transformer and a bunch of diode's / elco's / fuses / switches to make an un-regulated variable (AC & DC) power supply.
Works beautifully (Within it's limit's).

I have also used this for measuring the characteristics of unknown transformers.
With this power supply I can (safely, the 4x40V transformer's secundary is isolated) put a variable AC voltage on an unknown transformer winding, and make some notes while slowly increase the voltage.
Once you get near (over) the working voltage for the unknown transformer the current increases non-lineair.

The biggest disadvantage of running a transformer on a voltage lower than it's rated voltage is that a part of the iron is "not used", but the weight is still there.

[IanB]

If you take a transformer and supply it with one quarter of the designed primary voltage, won't this reduce the magnetic flux in the core by a significant amount? And won't this reduce the VA rating of the transformer proportionally? I do not know the theory behind this, but surely something has to give in this scenario?

[FriedMule]

Substraction
When working with ratio's, do not add / subract.
It's more like:

octave:1> 240/300*37.5
ans =  30

Your calculation is off:
octave:2> (300 - 70 ) / 1.25
ans =  184
octave:3> ans+60
ans =  244 (Not 240V as you thought).

You are with garanti correct! My formula was properly wrong but I am glad that we arrived at the same target voltage:-)
End goal 240V in 60V out as 2 x 30, but I put 1.25 more in and therefor will i get 1.25 more out.

[Brumby]

the price difference between a 600VA and a i.e. 750VA seems really high.
But you would mean it is equally good solutions 600VA 300V and 750VA 240V?

Ignoring efficiencies for the moment:
 
A 600VA 300V primary transformer will have a maximum primary current of 2A.  When run at 240V, this maximum current should not be exceeded, as that has been the design value for the wire in the primary.  This effectively gives you a 480VA transformer.

Secondary voltages also need to be adjusted.  If you want 30V off your (nominal) 300V transformer, then you need to have nominal secondary voltages of 37.5V  (30 * 300/240).

For this derated transformer (480VA) supplying two 30V secondaries, you will only get 8A out of each of them.  If this matches you needs, then by all means you can run with it.

[FriedMule]

If you take a transformer and supply it with one quarter of the designed primary voltage, won't this reduce the magnetic flux in the core by a significant amount? And won't this reduce the VA rating of the transformer proportionally? I do not know the theory behind this, but surely something has to give in this scenario?

You are correct, something gives.
If you put less voltage in, you get less voltage out but the current stay the same.

i.e. a transformer to 240V -12V 2A is a ratio of 1:20  (240V/12V = 20) so 120V (120V/20 = 10V) but it can only deliver the same amount of amps so in the end, you only gets half of the power out with halv voltage.

[FriedMule]

the price difference between a 600VA and a i.e. 750VA seems really high.
But you would mean it is equally good solutions 600VA 300V and 750VA 240V?

Ignoring efficiencies for the moment:
 
A 600VA 300V primary transformer will have a maximum primary current of 2A.  When run at 240V, this maximum current should not be exceeded, as that has been the design value for the wire in the primary.  This effectively gives you a 480VA transformer.

Secondary voltages also need to be adjusted.  If you want 30V off your (nominal) 300V transformer, then you need to have nominal secondary voltages of 37.5V  (30 * 300/240).

For this derated transformer (480VA) supplying two 30V secondaries, you will only get 8A out of each of them.  If this matches you needs, then by all means you can run with it.

My thought was this:
600VA with 240V in and about 60V 10A out, split in two = 30V x 2.
Therefor 600VA 300V in 75 out, split in two = 37,5V about 10A.

Is that correct?

[tautech]

My thought was this:
600VA with 240V in and about 60V 10A out, split in two = 30V x 2.
Therefor 600VA 300V in 75 out, split in two = 37,5V about 10A.

Is that correct?

Where will you obtain 300V from when Denmark mains is 230VAC ?

What do you need 2x 37.5V 10A supplies for ?

Please tell us more.

[FriedMule]

Where will you obtain 300V from when Denmark mains is 230VAC ?

What do you need 2x 37.5V 10A supplies for ?

Please tell us more.

There is no ware in Denmark that you cab get 300V just out of the wall, the standard is 240V.
What I am doing, are to order a transformer that have 1/4 time larger in and out, then I need.
And that way, try to avoid heat, saturation and other of the bad side effects that else would be if you run it by nearly max voltage.

[Brumby]

Starting with a transformer rated at 300V primary and 37.5V secondaries.

Run it at 240V primary voltage, secondaries now deliver 30V.

However, to get 10A out of both secondaries, the primary current would have to exceed 2A - which for a 600VA, 300V transformer is more than the design current for the primary.

[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]

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)

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.

[FriedMule]

As I understand it from all your great explanations, I have to calculate the transformers VA by specifying output V and A.
So if I want 2 x 30V (= 60V) 10A out it would demand a 600VA transformer. And if I reduce voltage in, I reduce voltage out and the VA ration of the transformer.

So to get a 300V in to 2 x 37,5 (= 75V) 10A out but are only supplying 240V in, I should get a 10A x 75A = 750VA transformer to end up with a 600VA transformer?

[Brumby]

Yes - that is how I would work it out if I were to do what you are considering.

[C]

Break it down in to simple parts
Changing the Voltage does not change,

1. The mass or iron of the transformer.
2. The wire size of the primary.
3. The wire size of the other windings.
4. The number of turns of the windings.

Simple fact is that you can not get more out then you put in.

You only get x amps on the secondary due to the input which is voltage & current.

So 300 Volts input and 10 amps on secondary will only happen at 300 volts input.

240 volts is 0.8 of 300 volts, to keep all the ratios, the secondary can only output 8 amps or less.

AT 240 volts, What you get is,
 A lower VA as you can not reach the designed max VA.
 A lower resistance secondary due to the wire being larger.

Not reaching the designed limits, lets you have a DC current on the secondary with out reaching the limits of the magnetic field.

C