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Measuring inductance of isolation transformer

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Ash Small
Fri Dec 19 2014, 10:04PM Print
Ash Small Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
I picked up an RS isolation transformer today. The core is 114mm x 95mm x 64mm, yet it is only rated for 200W max @ 250V.

I expected it to have higher Ohmic resistance than 4 Ohms, so I'm wondering why it is rated so low.

I want to mearsure the inductance of primary and secondary, just for fun. What's the best way to do this, ie if I measure the inductance of the primary using a known capacitance and sweep to find resonance, should I have the secondary open circuit or short circuit, or is it best to just connect primary and secondary together before measuring?

Thanks in advance.


1419026649 3414 FT0 Isolating Xformer
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...
Fri Dec 19 2014, 10:26PM
... Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
If you want to know the primary inductance you should leave the secondary open and apply a stimulus to the primary. To measure the secondary you apply a stimulus to the secondary and leave the primary open (for an isolation transformer these should be the same of course). To measure the leakage inductance you short the secondary and apply a stimulus to the primary.

Note - since that is an iron cored transformer you should keep your stimulus at a relatively low frequency, ideally 60hz. In fact, the easiest way to figure out the primary inductance is to simply hook it up to the wall and measure the magnetizing current, the inductance is just L = z/(2*pi*f). Since the primary impedance is going to be very high you can ignore the real (resistive) component.

200w does sound a little conservative for a transformer that large, it will only be dissipating about 10w based on your 4 ohm dc resistance measurement. It can probably handle a kw for short periods before melting. The conservative rating may have been calculated based on some sort of droop based measurement (assuming no leakage inductance, a 200w load would correspond to about a 3% droop of the output based on your DC measurements).
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Steve Conner
Sat Dec 20 2014, 02:43PM
Steve Conner Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I have a 500VA isolating transformer and it is about twice the size of this one.

You should find that the primary and secondary have different resistances. One is wound outside the other so will have a greater length of wire for the same number of turns. I'd expect the 4 ohm one is the shorter, and the other would be about 6 ohms, for a total DCR of about 10 ohms.

A transformer has two inductances, magnetising and leakage,
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Ash Small
Sat Dec 20 2014, 03:01PM
Ash Small Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Steve Conner wrote ...

I have a 500VA isolating transformer and it is about twice the size of this one.

You should find that the primary and secondary have different resistances. One is wound outside the other so will have a greater length of wire for the same number of turns. I'd expect the 4 ohm one is the shorter, and the other would be about 6 ohms, for a total DCR of about 10 ohms.

A transformer has two inductances, magnetising and leakage,

The secondary did measure a bit higher, but I'd not cleaned the connections yet so didn't expect a great deal of accuracy.

I'll try and get a 'scope and sig gen set up later. I know I have 1uF and 10uF capacitors that are within a few percent.

I'm not expecting it to be straightforward for an iron core of this size, and I've no idea of what the inductance might be.

What's the simplest (read 'safest') way to measure the magnetising current?

Site transformers of a few kVa can have cores smaller than this one, and they do the same thing. I realise it's because it's due to it being 'test equipment'.

EDIT: Would you not use a bi-filar winding in something like this, so that pri and sec are identical and have the best coupling?

I don't see a 'screen' connection on this xformer.
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TwirlyWhirly555
Sat Dec 20 2014, 06:37PM
TwirlyWhirly555 Registered Member #4104 Joined: Fri Sept 23 2011, 06:54PM
Location: Uk .
Posts: 122
I think been a isolation transformer the primary and secondary are kept separate for that reason to provide less risk to primary / secondary shorts

The screen connection will be attached to the earth if its there .
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Ash Small
Sat Dec 20 2014, 06:43PM
Ash Small Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
TwirlyWhirly555 wrote ...

I think been a isolation transformer the primary and secondary are kept separate for that reason to provide less risk to primary / secondary shorts

The screen connection will be attached to the earth if its there .

That makes sense, being as it is an isolation transformer. Separate windings give better isolation.
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