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Determining choke characteristics...

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Nicko

Tue Sept 08 2009, 10:55AM

Registered Member #1334 Joined: Tue Feb 19 2008, 04:37PM
Location: Nr. London, UK
Posts: 615

In each of the units pictured below, you'll notice two chunky chokes...

They are marked 3.2mH, 2.5A - when I measured them with an LCR meter @200kHz, I got 40uH. I suspect that 200kHz is rather higher than they are designed fpr

They are beasts - the windings are more copper strip than wire - I can provide a close-up if required. The part number is a specific for the customer - the manufacturer won't release the details and I can't get the specs from the customer as they aren't interested.

Where do I go from here? What can I use them for.

Thanks

GeordieBoy

Tue Sept 08 2009, 11:34AM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

Low power LCR bridges (or multimeters with an inductance range) generally don't give accurate readings of inductance for large iron-cored inductors and transformers. The reason is that the LCR meter or multimeter rarely provides enough power to actually magnetise the domains in the iron core.

If you just want to explore the AC behaviour of a power inductor at low frequency (50/60Hz) you are far better off connecting it to the output of a variac and giving it some real power. Connecting to the output of a mains supplied variac and simultaneously measuring I and V over a wide range will let you plot an I-V curve. Since you know the mains supply frequency where you live, this graph will allow you to calculate the inductance of the component at any operating point within the limits of your graph. Provided you have enough EMF available you might also be able to see the onset of saturation in the graph, where the current starts to rise at a rate that is much more than proportional to the applied voltage.

See the ballast inductor section on my web page for some typical curves obtained from a ballast inductor I had made for me:

However, since you specifically said "choke" instead of inductor, I get the feeling that these are designed to smooth DC or at least carry significant direct current. I have no idea what the application is, but those look like some sort of DC bus smoothing units with all the large capacitors, so it may be a PFC choke or a DC smoothing choke. Depending on what you actually intend to use the component for, you might be interested in it's DC characteristics as well as its AC ones. For instance how much the AC inductance falls under DC bias. I'll let you figure out how you might test for that yourself!

-Richie,

klugesmith

Thu Sept 10 2009, 12:34AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

I'll wager that with a DC bias current of 2.5 amperes, the AC impedance will correspond to L of about 3.2 millihenries.

(under "standard" frequency and excitation noted on the chokes datasheet, probably 100-120 Hz and 100 mA or so).

I'm surprised & curious to see flat wire with such a small rated current. What are the wire dimensions?
Can you show us a schematic of how they are connected to the big capacitors, small capacitors, and other components?
What's the DC resistance? (beware of shocks when ohmeter current is disconnected).

Rich
[edit] On closer inspection, the quoted label doesn't -say- the 2.5A is for DC, though that's a commonly rated parameter.
I'd do just what Richie said, chart I vs V at mains frequency. 3.2 mH gives almost exactly 1 ohm at 50 Hz (so use a low voltage transformer between Variac and DUT). But isn't that kind of low for smoothing twice-mains-frequency ripple? Maybe the chokes are intended to block conducted EMI at much higher frequencies. Do they have 4 terminals, suggesting a common-mode choke?

Steve Conner

Thu Sept 10 2009, 09:26AM

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

Sure it's 2.5A and not 25A?

Nicko

Sat Sept 12 2009, 09:33PM

Registered Member #1334 Joined: Tue Feb 19 2008, 04:37PM
Location: Nr. London, UK
Posts: 615

Some better pictures of them:

GeordieBoy

Sat Sept 12 2009, 10:38PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

They look like MOT's that have been rewound with some pretty heavy-duty copper strip. Is there only one winding on each core? It looks like there is a winding of finer copper wire towards the right of the winding window in both pictures? ...or maybe someone has just re-used kapton tape that has previously been or an enammeled copper winding?

There's also no air-gap apparent in the magnetic path as far as I can see from the pictures. So these inductors *may* saturate at a fairly low applied AC voltage, unless the centre limb of the core set is gapped where we can't see it!

-Richie,

Nicko

Sun Sept 13 2009, 06:30AM

Registered Member #1334 Joined: Tue Feb 19 2008, 04:37PM
Location: Nr. London, UK
Posts: 615

GeordieBoy wrote ...

... Is there only one winding on each core? It looks like there is a winding of finer copper wire towards the right of the winding window in both pictures? ...or maybe someone has just re-used kapton tape that has previously been or an enammeled copper winding?

There is only one winding - the appearance of a winding at the right is due to fine re-enforcing strips in the tape.
I'll bring one to Cambridge - They seem too nice to junk.
They were in circuit with the +ve & -ve sides of the cap banks each of which are 12 x 2200uF @ 500vdc in parallel.

GeordieBoy

Sun Sept 13 2009, 03:26PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

Did you plot the I/V curves. I for one, would be interested to see what you came up with.

Proud Mary

Sun Sept 13 2009, 05:22PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Geordie Boy's technique sounds right to me. You can't get a proper reading off a half magnetized core.

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