Metodologías ágiles

10.16.1 Specific apparent power – volt amps per kg – VAs

Designers of machines are interested not only in the power loss arising when steel is cycled through its peak operating induction, but in the amount of magnetomotive force required to do that. To take an extreme example, use of air and no iron at all would eliminate iron loss but require enormous currents to create useful flux levels.

So the permeability of the steel matters as well as its power (core) loss. If two steels of

equal power loss at the same frequency and peak induction require widely differing magnetising currents, then the losses happening in the copper windings used will differ widely also.

Power dissipated in a winding varies as I²R and if more current is demanded this loss rises. A useful measure of this penalty is the so-called specific apparent power or volt–ampère product. If steel in an Epstein square or other test device is excited to a given peak induction then the product of the voltage associated with the number of turns and the RMS (root mean square) current required will be indicative of the V × A product or apparent power demanded.

For any electrical device the watts consumed = V × I × power factor. If the power factor is low, the number of real heat dissipating watts is low but the current flowing in the winding can be much higher than provision of that number of watts would imply. V × A is the volt-ampere product and for a set supply voltage shows what the current drawn will be, and via the circuit ohmic resistance, what the I²R losses will be.

When a sinusoidal induction is enforced by the use of a feedback amplifier the RMS value of the voltage at the square secondary if multiplied by the RMS value of supplied current gives an accurate value for the VA product.

Note: Average rectified voltage is noted for ˆB setting, but RMS for VA calculation.

The RMS value of current supplied may be noted from an RMS-sensitive ammeter suitably connected. If the number of turns on the secondary winding differs from that on the primary, suitable arithmetic adjustments must be made to the V × A product.

Then when V × A is divided by sample mass, a VA/kg value for specific apparent power emerges.

10.16.2 Permeability

While the VA product involves the permeability of the steel the specific quotient B/H is of interest for some applications. For thin material at power frequencies a plot of ˆB versus ˆH yields a curve not dissimilar to the traditional normal induction curve and represents a rapid method of getting a view of what a normal induction curve would be like. Particularly above 1.0 T the similarity is good.

The H peak can be determined by the use of:

• average rectified voltage sensing of an H coil output (may need amplification)

• use of a peak reading ammeter in the magnetising current feed line. This could be a peak reading voltmeter in parallel with a precision resistor.

Plots of power loss versus ˆB and of power loss versus frequency give an excellent insight into the properties of an electrical steel. Further, plots of B/H and of VA versus

ˆB complete the picture.

10.16.3 Direct current measurements

The tests so far described are all related to AC conditions. Electrical steels are used in DC applications also. These may include relays, nuclear particle accelerator magnets and the like. For these applications power loss is not a prime consideration, but other

Testing and measurement 147 parameters are important. These may be:

• coercive force

• saturation induction

• remanent magnetism

• normal induction curve shape

• maximum and initial permeabilities.

These are set out in Figure 10.31.

The relevance of coercive force may be illustrated as follows. Small alternators used in portable generators and the like are expected to self-excite at up. At start-up the rotor winding is fed with DC derived from rectification of stator output which is present due to the remanent magnetism in the machine flux. To work well a high remanent magnetism (associated with a high coercive force) is needed. Stator steel needs to have low power losses, so low hysteresis, low coercive force metal is used.

However the rotor may need to be made of a higher coercive force material so that excitation will occur. The need is greater in more recent years where rectifiers have changed from germanium to silicon, which has a higher forward breakover voltage.

The design features and metallurgy which allows (economically) the stator and rotor laminations to be derived from the same feedstock are covered by commercial confidentiality.

A range of techniques is available to measure these properties. The ring method and Epstein frame may be used to apply DC tests to samples as well as specialist permeameters. A permeameter is an arrangement of sample, coils and yokes specially adapted for DC tests. There are many types and varieties of permeameter. Figure 10.32

Initial

Figure 10.31 Key DC properties of an electrical steel evident from a BH loop

Sample Flux closure yoke

B winding

Magnetising coil (sectional view)

Figure 10.32 Diagram and photograph of a permeameter

shows one. Plate samples are placed in the coil system and yokes closed up to complete the magnetic circuit. The coils involve:

(a) main magnetising coils

(b) compensating windings to apply extra mmf to offset leakage at the yoke–sample joint

(c) B coils wound onto the sample itself, or on a separate former (d) H coils if these are used

(e) RCP system to assess exactness of compensation applied.

Testing and measurement 149 Magnetising fields can be applied via the main magnetising coils and if need be compensation via the compensating coils. A range of values of applied field can be used depending on the measurements required.

(a) A sample is placed in the coils and the yokes closed up.

(b) A strong magnetising field is applied to take the sample close to saturation.

This is then reversed in polarity several times by a manual or automated switch to set the material into a reproducible cyclic magnetic state. This process is repeated for progressively decreasing values of applied field until the sample may be considered to be demagnetised. Alternatively a slowly decreasing AC field could be used for demagnetisation. However the high inductance of the windings of some permeameters may make this difficult. A freshly annealed sample may not require specific demagnetisation.

(c) A low value of applied field is selected, e.g. 40 A/m and a cyclic state established first by repeated reversals. Then the total flux change arising from reversing this field H1is noted by the charge circulating in a charge integrator connected to the B coils of the permeameter. This corresponds to a magnetisation change of 2B1. (d) This procedure is repeated for higher values of H : H2, H3, etc. giving corre-sponding values of 2B2, 2B3, etc. From these values the tips of loops are used to plot out a normal induction curve.

At each point a check may be made to see that flux leakage compensation is exact if a facility is provided for using compensating windings.

The charge integrator used to determine B values can be a ballistic galvanometer or an electronic charge integrator. This will be calibrated such that with a knowledge of the sample cross-sectional area the number of B coil turns and the size of deflection produced, a value for the size of B reversal can be determined.

When samples are thick, e.g. 3+ mm, operations must be carried out slowly enough to allow full flux penetration into the sample (delay due to Lenz law effects).

The permeameter might be configured so that some of the procedure is automated.

Ballistic galvanometers and charge integrators can be calibrated by use of charge delivered from standard capacitors or mutual inductors [10.8].