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Transformer and Inductor Theory Question

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IamSmooth

Fri Nov 03 2006, 01:00AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

I am searching and reading about transformer theory and I am stuck on some points:

If I connect a transformer to a 60hz source with no load barely any current is drawn from the source. The little current that is drawn is the exciting current. The primary of my transformer has an inductance of 0.05H, XL = 19ohms. The unloaded current is about 0.5A; fully loaded it is about 7.5-8.5A. The reactance of the primary limits the secondary's current.

If I connect just an inductor of 0.05H to a 60hz source the current through it is limited by the reactance of the inductor (I = V/XL) and should be about 6.3A

I would think that the reactance of the unloaded transformer's primary would set the current to 6.3 - just like the plain inductor. Or, if one considers the inductor an energy storage device, the current in the plain inductor should be close to zero as it returns the energy back to the source.

My question is why are the two behaving differently? If the primary and the plain inductor have the same inductance why are the currents different when they are connected to a voltage source (for the unloaded transformer)?

Hazmatt_(The Underdog)

Fri Nov 03 2006, 01:56AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
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You need to tell the readers what kind of inductor you're testing, wether it has no magnetic core, or a Ferrite core, or an iron core, and if the core is open or a closed magnetic loop.

IamSmooth

Fri Nov 03 2006, 02:55AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Ok. I have a laminated core with one spool of winding around one of the legs. The measured L = 0.11H. Calculated XL = 42ohms. R is under 0.01 ohms. I put 18.5V across and got I = 0.15A for a total reactance of 121ohms. So, how come the current isn't 18.5/42 = 0.44A?

Steve Ward

Fri Nov 03 2006, 04:52AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
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I think what you are getting hung up on is the fact that the magnetizing inductance (i think you called it exciting current) gets partially (or completely) "shorted out" when you put a load on the transformer. When you short a transformer, Lmag goes to zero, and the current is only limited by Lstray. In the open circuit scenario, Lmag is in series with Lstray.

Why is this so? Well, *transformers* are designed to be energy transfer devices, NOT energy storage devices.

You might want to have a look at this (realizing my short explanation may not satisfy you):

Just study the model, and then you should hopefully gain a better understanding of why your transformer behaves the way it does.

Dr. Slack

Fri Nov 03 2006, 08:07AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
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You say in your first post that the inductance of the primary is 50mH. How did you measure this? Given that you say the primary current drawn on no load is 500mA, this implies the reactance must be around 240ohms, so inductance will be around 650mH, taking the applied voltage and the measured current as a measurement of the inductance (neglecting core losses, which may make a very significant difference on no-load).

Does "fully loaded" mean with a s/c on the secondary, or with its "rated load". If the latter, then the thing principally limiting secondary current is the impedance of the secondary load, the transformer losses will be dropping the seconary voltage slightly. If the secondary is s/c, then the limitation to secondary current will principally be the *leakage* inductance of the trasnformer, ie the couplign defect between primary and secondary (what you would measure the primary as with the secondary s/c, funily enough), and the resistances of primary and secondary.

(I post so infreuwnetly these days that I fall down that login bot-trap every time!)

IamSmooth

Fri Nov 03 2006, 01:01PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Steve, your reference is very helpful as it shows the schematic representation of what is going on with the transformer. Thanks. What I am interested for you or someone elaborating on is how does the Magnetizing inductance "limit" the no-load current. (BTW, I am using fully loaded synonomously with shorted secondary) I guess what I would like help with understanding is what is physically happening as compared with a plain air-core inductor.

This brings me to my inductor question. The 0.05H was measured with an LC meter. The inductor was wrapped around an iron core. The current that I pushed through with the voltage was much less than expected based on the calculated XL reactance. Is it that the core's magnetic field is inducing a back-current from a back-emf, which reduces the measured current?

Steve Conner

Fri Nov 03 2006, 01:15PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Sounds like you measured the magnetizing inductance of the transformer wrongly. It should be of the order of a few Henries: Neil's estimate of 0.65H seems in the right ballpark.

In transformers with no air gap (which is to say the vast majority of them) magnetizing inductance is a highly non-linear function of the amount of voltage you apply. I've seen references that suggest LCR meters can read large iron cored inductors wrongly because they don't supply enough power to magnetize the core properly.

IamSmooth

Fri Nov 03 2006, 02:06PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

Well, that explains the inductor question and the discrepancy with the measured XL and calculated XL.

The last part of my knowledge gap is why there is hardly any current drawn for an open-secondary transformer connected to the mains? How is the magnetizing inductance limiting the primary current when the secondary is open?

Marko

Fri Nov 03 2006, 02:34PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

The last part of my knowledge gap is why there is hardly any current drawn for an open-secondary transformer connected to the mains? How is the magnetizing inductance limiting the primary current when the secondary is open?

Transformer with open secondary is same thing as inductor consisting of the same core and primary winding connected to mains, and it simply limits current by it's Xl.

For 0.65H at 120V 60Hz our transformer would draw exactly 0,489A of current, pretty close to your measure.

''hardly any current' is probably a relative thing to transformer's size..

IamSmooth

Fri Nov 03 2006, 02:52PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Thanks. I forgot that it was mentioned that my measured L with the LC meter was probably inaccurate due to magnetizing the core. It is interesting that at full voltage when I short the secondary the primary current is close to the calculated current using the measured XL. The magnetizing inductance disappears leaving just the series inductance.

Thank you everyone.

As a side note, the method I described in another thread about Power Factors does accurately measured the Inductance and Resistance during open and short circuit conditions.

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