Bad transformer?

[SiliconArtist]

      I bought a new 200VA 230/230V transformer but I think it is something wrong with it. I draws ~370mA without a load (I connected a 100 resistor in series and it drops 37V on it). It happens the same if I supply the secondary side. Both primary and secondary have 8 . The magnetization current can be so high?!
     The transformer has an electrostatic shield. Could it be shorted in a way that it behaves like a continuous loop?

https://adajusa.es/es/transformadores-monofasicos/5150-transformador-200va-230230vac-ultra-aislamiento-ip-20-8435532851509.html

[Sorama]

What is the reasoning behind putting a resistor in the primary??
Of course you will have a voltage drop across it.

[SiliconArtist]

What is the reasoning behind putting a resistor in the primary??
Of course you will have a voltage drop across it.

I added the resistor because the transformer was making strange noises. Like an intermittent buzz. So I wanted to have some current limitation in case there is a short inside. This is just for testing, I will remove it of course. But in this way I discovered the large voltage drop across the resistor. Without a load.

[coppercone2]

it should not increase the standby draw of the transformer to have a resistor there

[Sorama]

it should not increase the standby draw of the transformer to have a resistor there

Yes it should, given the 8 ohm and thus a voltage divider.

Edit; not increase the current, but change accordingly.

[Benta]

The magnetizing current is plausible though high for a 200 VA type. It's almost purely inductive, so it costs you nothing. Modern transformers are run very close to saturation (to save copper and iron).

[Xena E]

Does it overheat?

You can ring test the windings to check for a shorted turn.

[RoGeorge]

By its size, looks more like 20VA transformer rather than 200VA.  What is the size of the magnetic core?

There was an empirical formula to approximate the required section of the magnetic core by the total power.  Vaguely recall

S = 1.5 * sqrt(P)
Where
- S is the core section in square centimeters (by core section meaning only the metal encircled by the coils, so only the middle leg of the E shaped core)
- P is the transformer power, here 200VA

For example, for a 200VA transformer, the expected core section should be about 20cm² or more, so the thickness of the E stack multiplied with the width of the middle leg of a single E (all in cm) should be about 20 or bigger for a 200VA mains transformer.

Most probably I recall that formula wrong... , better search online for mains transformer design, particularly for the step where the section of the core is estimated starting from the desired total power.

The no load current seems large to me, I guess that is because the magnetic core enters into saturation (too much voltage).  Seeing USA mentioned on the label, makes me wonder why would they sell 230V transformer in USA, where the mains AC is 110V.  Maybe that transformer was mislabeled, and it is in fact a 110V transformer? 

[SiliconArtist]

Does it overheat?

You can ring test the windings to check for a shorted turn.

What is ring test? Is the measuring of the Q of each winding? My transformer has Q=21 and inductance of 2H @ 100Hz for both primary and secondary. Are these reasonable values?

I removed the primary resistor and added a 4.6k load on the secondary. The current in the primary was 0.44A and in the secondary 0.05A. Secondary voltage was 225V. There is a huge difference between the two currents. Is this normal for a 1:1 transformer?

[Xena E]

  Seeing USA mentioned on the label, makes me wonder why would they sell 230V transformer in USA, where the mains AC is 110V.  Maybe that transformer was mislabeled, and it is in fact a 110V transformer? 

It says "Por Adajusa" not USA, it's the distributors name.

[SiliconArtist]

By its size, looks more like 20VA transformer rather than 200VA.  What is the size of the magnetic core?

There was an empirical formula to approximate the required section of the magnetic core by the total power.  Vaguely recall

S = 1.5 * sqrt(P)
Where
- S is the core section in square centimeters (by core section meaning only the metal encircled by the coils, so only the middle leg of the E shaped core)
- P is the transformer power, here 200VA

For example, for a 200VA transformer, the expected core section should be about 20cm² or more, so the thickness of the E stack multiplied with the width of the middle leg of a single E (all in cm) should be about 20 or bigger for a 200VA mains transformer.

The thickness of the laminated stack is 37mm. I can't measure more because it is covered in a resin. But according to your formula the middle section should be 57mm in width. And this is not possible given the total width of the core (95mm).

I think I will return it, it is something fishy with this transformer

[Xena E]

Does it overheat?

You can ring test the windings to check for a shorted turn.

What is ring test? Is the measuring of the Q of each winding? My transformer has Q=21 and inductance of 2H @ 100Hz for both primary and secondary. Are these reasonable values?

I removed the primary resistor and added a 4.6k load on the secondary. The current in the secondary was 0.44A and in the secondary 0.05A. Secondary voltage was 225V. There is a huge difference between the two currents. Is this normal for a 1:1 transformer?

A ring test is to see if the transformer I'd being damped by a shorted turn or similar... I'll try to find a link to an example... back in a mo!

[RoGeorge]

The thickness of the laminated stack is 37mm. I can't measure more because it is covered in a resin. But according to your formula the middle section should be 57mm in width. And this is not possible given the total width of the core (95mm).

My formula is most probably wrong, Ive just strikeout that formula moments ago in my previous post, sorry for that, but the idea still stands.  Find out the area and deduce the max power.  Better find some online calculator.

Usually the width of middle leg of the E core is a certain ratio of the E size, see if you can identify the width of the middle leg from the datasheet (random example for mains transformers E+I core materials/sizes https://www.lodestonepacific.com/wp-content/uploads/2021/08/Bobbin-E-Core-Lam-Cross-Reference.pdf )

[SiliconArtist]

The thickness of the laminated stack is 37mm. I can't measure more because it is covered in a resin. But according to your formula the middle section should be 57mm in width. And this is not possible given the total width of the core (95mm).

My formula is most probably wrong, Ive just strikeout that formula moments ago in my previous post, sorry for that, but the idea still stands.  Find out the area and deduce the max power.  Better find some online calculator.

Usually the width of middle leg of the E core is a certain ratio of the E size, see if you can identify the width of the middle leg from the datasheet (random example for mains transformers E+I core materials/sizes https://www.lodestonepacific.com/wp-content/uploads/2021/08/Bobbin-E-Core-Lam-Cross-Reference.pdf )

Good idea! According to that table the width of the middle section is 31mm. This result in a cross section of 31 x 27 = 1147 mm^2. Depending on the source, for 200VA the section should be between 1600 to 1900 mm^2 ...

[tooki]

The no load current seems large to me, I guess that is because the magnetic core enters into saturation (too much voltage).  Seeing USA mentioned on the label, makes me wonder why would they sell 230V transformer in USA, where the mains AC is 110V.  Maybe that transformer was mislabeled, and it is in fact a 110V transformer? 

Also, a reminder that the voltage in USA has been 120V since the 1950s. It’s not 110V now, and hasn’t been for a very long time.

[Zero999]

      I bought a new 200VA 230/230V transformer but I think it is something wrong with it. I draws ~370mA without a load (I connected a 100 resistor in series and it drops 37V on it). It happens the same if I supply the secondary side. Both primary and secondary have 8 . The magnetization current can be so high?!
     The transformer has an electrostatic shield. Could it be shorted in a way that it behaves like a continuous loop?

https://adajusa.es/es/transformadores-monofasicos/5150-transformador-200va-230230vac-ultra-aislamiento-ip-20-8435532851509.html

It's obviously bad. Return it.

By its size, looks more like 20VA transformer rather than 200VA.  What is the size of the magnetic core?

Did you read the specification linked to in the original post?

2.9kg seems about right for  200VA transformer.

[SiliconArtist]

I changed my mind, I think the transformer is fine. I let it run for 1h with a 125mA DC load (after rectification) and it is barely warm. That large primary current I measured must contribute only to reactive power.

[Benta]

I changed my mind, I think the transformer is fine. I let it run for 1h with a 125mA DC load (after rectification) and it is barely warm. That large primary current I measured must contribute only to reactive power.

Q.E.D.

[soldar]

I changed my mind, I think the transformer is fine. I let it run for 1h with a 125mA DC load (after rectification) and it is barely warm. That large primary current I measured must contribute only to reactive power.

Everything in that transformer looks perfectly good to me, magnetizing current included.