How to measure the leakage inductance in a transformer?

[joao_lima]

Hello,

I am trying to measure the leakage inductance in each winding of a monophasic transformer. I tried to measure it at the same frequency that the transformer operate, 50 Hz, but the value keeps changing very fast. Does anyone have a suggestion to measure this value?

Regards,

joao_lima

 

[FvM]

I presume, there's no consistent definition of leakage inductance for a transformer with more than two windings. In this case, you'll probably measure self and mutual inductance of all windings.

You'll probably measure it by shorting all other windings. So why shout the value "keep changing"? Because you are using an unsuitable instrument?

You'll either need a LCR meter that can be switched to 50 Hz (or 100 Hz, which is quite near), or a 50 Hz source and a setup that allows to determine the complex impedance of the winding under test. If you understand AC networks, a reference resistor and an AC voltameter would be sufficient (consider vectorial addition of voltages).

In electrical engineering, the respective measurement is known as Xk/Rk measurement.

 

[The Electrician]

You need to give more details of your situation.

How many windings does the transformer have?

What kind of instrument are you using to make the measurement?

What procedure are you following to make the measurement?

Any other information that might be helpful.

 

[smijesh]

Short the secondary and measure primary inductance. You can get the primary leakage inductance. For 50Hz application lamination type cores are using. The permeability of such cores are not constant it will vary hence the variation in leakage inductance.

 

[joao_lima]

You need to give more details of your situation.

How many windings does the transformer have?

What kind of instrument are you using to make the measurement?

What procedure are you following to make the measurement?

Any other information that might be helpful.

Hello The Electrician,

I am working with a 4.5 VA transformer 230/18 V, with a rated current of 19.57 mA in the primary and 250 mA in the secondary.
I have stipulated the number of winding of 9598 based on the resistance measured using a DC sign of a LCRmeter. With the resistance value I calculated the copper length and the windings.
I am using a LCRmeter PM6304 Fluke device to make the measurements.
To measure the leakage inductance I was short circuiting one winding and measuring the other winding leakage inductance.

Regards,

joao_lima

 

[FvM]

I have no doubt that PM6304 can measure the leakage inductance with reasonable accuracy. May be some settings are inappropriate.

I don't believe that a DC resistance measurement gives a reliable windings count, but the point hasn't to do with leakage inductance anyway.

 

[joao_lima]

I have no doubt that PM6304 can measure the leakage inductance with reasonable accuracy. May be some settings are inappropriate.

I don't believe that a DC resistance measurement gives a reliable winding count, but the point hasn't to do with leakage inductance anyway.

Do you have a reliable way to measure the winding without opening the transformer?

Also can you suggest a way to measure the leakage using the PM6304?

Thank you

Regards

joao_lima

 

[FvM]

If the transformer is molded, you won't be able to determine the absolute windings count (actually I don't understand why it would be of interest), but the windings ratio can be measured as open circuit voltage ratio.

I don't use PM6304, but similar industry standard instruments, e.g. from Agilent. If you don't get fairly stable readings, I presume a defective instrument or an operator fault.

 

[joao_lima]

If the transformer is molded, you won't be able to determine the absolute windings count (actually I don't understand why it would be of interest), but the windings ratio can be measured as open circuit voltage ratio.

I don't use PM6304, but similar industry standard instruments, e.g. from Agilent. If you don't get fairly stable readings, I presume a defective instrument or an operator fault.

The number of winding is important because I am modeling it in a LT Spice environment and a few of the parameters require the number of windings in the high voltage side.

I would consider a bad use of the instrument by myself, I am trying to get to a reliable way to make the measurement, again if someone can point me a away and also explain why this is a good way or point me a direction to find a way I will be very grateful.

Regards,

João

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Add:

I am measuring a capacitance between the two windings of this transformer, and at the operational frequency it also changes very fast, and if raise the frequency it goes down a bit then up again but stays in a range between 22pF-27pF.

What I did here was to put the LCR meter and raise step by step the frequency from 50 hz to 100kHz. But I would like to know if this is a reliable method to discover the capacitance between to windings.

I need this values to model this transformer in LT Spice.

 

[The Electrician]

You say that the reading is changing very fast, but you haven't said how many digits you are seeing in the display.

When I measure the leakage inductance of a similar transformer with a Wayne-Kerr 6440B, I see 5 digits in the display. It is possible to set the instrument to fast, medium, and slow speed. When set to fast, the last 2 out of 5 digits are changing rapidly. This may be what you are seeing.

How many digits are displayed, and how many of them are changing rapidly? Can you find a setting so the meter is in averaging mode, or in a slow mode?

When measuring winding to winding capacitance, you should short each winding, and then measure from winding to winding.