Testing inductors - high power and low !

We make and test a fair number of power magnetics for use in our designs,

from COTS parts for volume stuff, through to 150kVA chokes for high power DAB's at 50kHz.

It is always nice to know how your parts are going to fare - loss wise - before you commit them to a power circuit ( or to a 1000x build ! )

If you have the parts on hand to lash up a choke ( or transformer for that matter ) - testing to discover the true loss is a very useful thing to do indeed.

Similarly for COTS chokes - knowing the delta-B from the design - if you can put in an ac current on the test jig to mimic this - at the same frequency - you can soon tell if the real losses in the real part will be too great.

Resonance testing ( as we call it ) is a great way to test chokes under power conditions.

Shown above is a typical circuit to do just this.

For power testing ( say for losses up to 250W in the choke ), a low loss H bridge is preferably used for the " power amplifier " above, and the feed from the sig gen would then be a timing square wave to set the frequency ( 50% out from the H bridge - with say 3% dead time ).

The choke under test " Lx " is paralleled with an high voltage, low loss capacitor(s) that can handle the full load amps that we will set up in Lx.

L1 is a low loss gapped ferrite and litz wire choke to isolate the driver from Lx and its paralleled cap. It must handle 10 to 20A ac at higher frequencies with very low losses. You may need more than 200uH to really isolate the driver ( two in series for e.g. ) It must also be an high voltage part.

This will give sine wave currents and voltages on Lx - but still gives it the work out we require to gauge losses.

If the choke, Lx, is intended to handle say 10 A rms ac, at 100kHz say, we need to pick a capacitor value that will resonate with Lx // L1 at near to 100kHz.

We can now set the H bridge going at low supply voltage ( say 5 - 10VDC ) and the sig gen for the gate drivers at 150kHz say,

Looking at the current probe, and/or just the scope probe across Lx, as we bring the freq down towards resonance we will see the current and volts increase in Lx ( and the cap too )

We can increase the power supply volts to increase the current also.

When we get to the desired level - here 10A rms, or 28.3A pk-pk in Lx - we can read the power loss in Lx directly off the power supply, assuming the H bridge is low loss ( ZVS if we are just above true resonance ) L1 is low loss - and the wiring etc is low loss.

Secondly - simply sensing the temp rise on Lx will give the designer a pretty good idea of the losses in it !

Looking at the phase difference on the scope between the current probe and the volts allows a direct calculation of losses in the choke ( if the cap is negligible ).

Just a wee note that voltages can get quite high - easily to 1kV pk for really high power testing - so make sure you have a selection of parallel caps to handle this - and HV scope probes !

for example if Lx is 47uH, 100kHz, and 10A rms - this is 295Vac, or 420Vpk.

[ We routinely test to 150A rms in our chokes and transformers ]

It should be evident that testing a shorted transformer in a similar fashion will reveal the total transformer copper losses in very short order. Power capacitors can be tested for loss in a similar fashion.

For plain ol' inductance testing at low power L1 can be an 22k resistor and the sig gen can drive the Lx // Cap parallel combo directly - until resonance is obtained by varying the sig-gen, for a known quality cap, Lx can be found from ( 2.pi.Fo ) = 1 / sqrt(L.C)

happy testing . . .

pwrtrnx.com

Blue = V_Lx = V_C1, Green = I_Lx, I_L1 = I_source = notably lower than I_Lx

F_test = 78.125kHz, i.e. quite close to resonance.

high power testing:

Vdrive = +/-80V, 50kHz, V_Lx = 1100 V peak, I_Lx = 148.5 A rms, I_source = 7 A rms, Voltage across L_iso = 1200 V pk