Example B-H Curves

Two additional results from the circuit for plotting B-H curves are given below -

Square 50 tape wound core

The BH curve for a nickel iron core A nickel iron core used to obtain its BH curve This toroid core uses a nickel-iron (50% nickel, 50% iron) alloy tape.

Circuit parameters for tape core measurement.
Parameter Value Unit
R2 10 Ω
Primary turns 24 Tn
'scope horiz. scale (1 div) 24.0 Am-1
Secondary turns 27 Tn
Integrator time constant 0.00101 s
'scope vert. scale (1 div) 0.37 T
Cyclic power loss 161.4 Jm-3
Saturation 1.4 T
Coercive force 28.9 Am-1
Device data for a tape wound nickel-iron core.
Parameter Value
Manufacturer Telcon
Distributer -
Type number 1a
Material R50SQ
le / mm 49.9
Ae / mm2 6.1
Packing factor 0.6

The data provided by Magnetic Metals for 50% nickel-iron at 60 Hz test frequency agrees well with this curve. Interestingly, it also has a curve taken at 'DC' which shows very much steeper sides and a value of HC about 1/3 of the above. Telcon quotes the hysteresis loss as 65 Jm-3 which agrees with the lower value of HC.

It will clearly be interesting to perform a measurement at lower frequency to see if the result changes. Eddy currents in the core might be to blame for this effect, but the strip thickness is not above 0.013 millimetre; where the eddy currents at 50 Hz should be very small.

6VA mains transformer

The BH curve for a small mains transformer A silicon steel transformer used to obtain its BH curve This is a conventional, small, 'silicon steel', E-I laminated core mains transformer.

Circuit parameters for mains transformer measurement.
Parameter Value Unit
R2 0.1 Ω
Primary turns 233 Tn
'scope horiz. scale (1 div) 569 Am-1
Secondary turns 1 Tn
Integrator time constant 0.00101 s
'scope vert. scale (1 div) 0.508 T
Cyclic power loss 1,410 Jm-3
Saturation 1.6 T
Coercive force 200 Am-1
Device data for a small mains transformer.
Parameter Value
Manufacturer Walsall ?
Distributer RS
Type number 196-296
Material Silicon steel
le / mm 81.9
Ae / mm2 199
Packing factor 0.93 ?

Dimensions of the core of a a small mains transformer As with most such 'off the peg' transformers, you are provided with no information regarding the number of turns it comes with. The workaround is to apply an additional winding of one turn onto the core by threading a short length of thin, insulated, solid core wire through the gap between the existing winding and the core. Even a single such turn provides sufficient induction to measure the voltage ratio with the original windings.

The easiest and safest way to stimulate the core is to drive the original secondary via a 240 to 18 V, 20 VA isolation transformer and a variac. The one-turn winding was used to feed the integrator.

The value for Bsat is lower than suggested by some published figures elsewhere (about 1.9T). A noticeable feature of this curve is that the saturation point is ill defined. This is typical of B-H curves for these components. The reason for this may be due to the lamination geometry. The E and I shapes are stamped out and then stacked up together to a limited precision, with the result that there is often a small gap between the pieces where, ideally, they should butt against each other.

At flux densities below 50% of the material Bsat the core reluctance is almost unaffected. This is because the flux can cross the gap using a route through the interleaved laminations. The sectional area of those lams is, of course, 50% of the core total. Once the 'bridging' lams have saturated then you are left with (effectively) air gaps approaching, perhaps, half a millimetre or so. That may not sound a lot, but might easily add as much reluctance as a whole metre or two of iron.

The shallow rise in flux seen at extreme values of H could be the result of additional flux finally being forced across the gaps.

Of more concern is HC. This is given as 40 Am-1 in some tables for non grain oriented materials, or 6 Am-1 for GO steel. HC is usually very dependent upon composition and treatment. Small mains transformers are less likely, it appears, to use orientation.

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See also -
[Electronic Transformers]

E-mail: R.Clarke@surrey.ac.uk
Last modified: 2008 May 10th.