Current Waveforms and Electrical Measurements

Outcomes

After this chapter, the learner should be able to:

• compare AC, DC, HWR and FWR magnetising behaviour;
• link skin effect and particle mobility to discontinuity location;
• distinguish peak, RMS and mean current values;
• perform common waveform conversions;
• explain why meter basis must match a calculation; and
• identify electrical measurement and equipment errors.

5.1 Alternating current

At a 50 Hz supply, sinusoidal AC completes 50 cycles per second and reverses direction every half-cycle - 100 reversals per second. The changing field promotes particle mobility and creates eddy currents that oppose penetration. The resulting skin effect concentrates useful magnetisation near the surface of a ferromagnetic component.

AC is therefore highly effective for fine surface-breaking discontinuities and generally supports easier demagnetisation and portable equipment. It is a poor choice when the inspection objective depends on deeper near-surface response.

5.2 Direct current

DC flows in one direction with little or no intentional pulsation. It produces a field that penetrates farther through the component section than AC. Because it does not vibrate particles in the same way, particle mobility is lower. DC or a rectified form is selected when near-surface sensitivity is needed and the geometry and discontinuity orientation are favourable.

Battery DC can support portable work but introduces capacity and maintenance limits. Rectified equipment is heavier and more complex. DC also tends to leave a stronger residual field, which can help hold an indication but makes demagnetisation more demanding.

5.3 Rectified waveforms

Half-wave rectified current (HWR) passes one half of each single-phase AC cycle. The result is unidirectional pulses separated by zero-current intervals. It combines useful penetration with particle agitation and is often paired with dry powder for near-surface examination.

Full-wave rectified current (FWR) inverts the negative half-cycle so current remains unidirectional. Single-phase FWR is pulsating without the long zero interval of HWR. Its field penetrates better than AC and particle motion is generally lower than with AC or HWR. Three-phase full-wave rectification produces a smoother output suited to fixed equipment.

Waveform labels on equipment are not enough. Confirm whether the indicated value is peak, RMS or mean and whether the output waveform under load matches the basis used by the procedure.

5.4 Peak, RMS and mean

Peak is the maximum instantaneous current. RMS is the effective heating-equivalent value and is commonly used for time-varying magnetisation calculations. Mean is the time average over the specified waveform interval. For ideal waveforms represented in the supplied training source:

Conversions must use the factor for the actual waveform:

• RMS = peak x RMS factor
• peak = RMS / RMS factor

Example 1: 1,000 A peak sinusoidal AC corresponds to 1,000 x 0.707 = 707 A RMS.

Example 2: 600 A RMS HWR corresponds to 600 / 0.5 = 1,200 A peak.

Real equipment output can deviate from an ideal waveform. Calibration and approved measurement practice take precedence over a theoretical conversion.

5.5 Resistance, impedance and Ohm's law

Resistance opposes DC. Impedance is the broader opposition to AC and includes resistance plus frequency-dependent effects. Both are expressed in ohms. In a DC circuit:

R = V / I

Low-voltage, high-current MT circuits need secure contacts and correctly sized cables. A small rise in contact resistance can create local heating and arcing even when the equipment operates at low voltage.

5.6 Ammeter verification

An ammeter that reads lower than the true current encourages an operator to apply excessive current. A meter that reads high can leave the component under-magnetised. Verification normally compares the equipment indication with a calibrated instrument suitable for the waveform and connected by an approved method.

Check:

• instrument identification and calibration status;
• waveform compatibility;
• range and resolution;
• connection method and polarity where relevant;
• reading stability under load;
• acceptance tolerance in the controlling procedure; and
• the response across the working range, not at only one convenient point.

Do not use an instrument that merely calculates RMS or peak from an assumed waveform when the procedure requires direct measurement of the actual output.

5.7 Field distribution in conductors

For current through a solid conductor, the circular field is zero at the centre and increases toward the surface. Outside similar conductors carrying the same current, H in air is governed by the current path and distance; within the material, B also depends on permeability. AC concentrates the effective response toward the surface. A central non-ferromagnetic conductor through a hollow ferromagnetic part establishes a strong field at the bore without passing current through the component.

5.8 Common traps

• Writing a setting without the waveform and meter basis.
• Multiplying RMS by 0.707 when converting RMS to peak; the operation should be division for sinusoidal AC.
• Assuming a theoretical factor proves actual equipment output.
• Selecting DC for depth while using a particle system with poor mobility and no performance check.
• Ignoring cable and contact heating during repeated shots.

Chapter summary

Waveform controls both magnetic penetration and particle movement. AC excels at surface response; DC and rectified currents extend near-surface capability. Current values are only meaningful when waveform, basis, calibration and load behaviour are known.