Nondestructive Testing Encyclopedia- Magnetic Particle Inspection - Alternating Current (AC)
 Alternating Current (AC) :Alternating current is current that reverses its direction of flow at regular intervals. Such current is frequently referred to as AC. Full-Wave Rectified Single-Phase AC:This rectified alternating current for which the rectifier is so connected that the reverse half of the cycle is “turned around”, and fed into the circuit flowing in the same direction as the first half of the cycle. This produces pulsating DC, but with no interval between the pulses. Such current is also referred to as single-phase full-wave DC. Full-Wave Rectified Three-Phase AC:When three-phase alternating current is rectified the full-wave rectification system is used. The result is DC with very little pulsation - in fact only a ripple of varying voltage distinguishes it from straight DC. Half-Wave Rectified AC:When a single-phase alternating current is rectified in the simplest manner, the reverse half of the cycle is blocked out entirely. The result is a pulsating unidirectional current with intervals when no current at all is flowing. This is often referred to as “half-wave” or as pulsating direct current. Single-Phase Alternating Current:This term refers to a simple current, alternating in direction. Commercial single-phase current follows a sine wave. Such a current requires only two conductors for its circuit. Most common commercial frequencies are 25, 50 and 60 cycles per second. Three-Phase Alternating Current:Commercial electricity is commonly transmitted as three single-phase currents, that is, three separate currents following separate sine curves, each at 60 cycles (or other frequency) per second, but with the peaks of their individual curves one-third of a cycle apart. At least three (sometimes four) conductors are required for three-phase alternating current. Pulsed and Impulse Current - Magnetization:A magnetization technique utilizing short circuited AC or condenser discharged DC. Very high magnetizing currents are possible for short durations. ( 1/100 to 1/1000 sec.) without the use of transformers. A Pulsed magnetization applies high fields for brief periods. A slight variation of pulsed magnetization is impulse magnetization. Betz called this flash magnetization. Associated with electrical methods of magnetization is the heating involved when current passes through a metal. Heating may limit the field intensity achievable. To overcome this difficulty a method of short duration high intensity fields was developed. Thin walled materials can be tested without the risk of heating at contact points. This is normally used for the residual method and allows for even higher field applications as they are applied once and briefly. In the early days of MPI it was thought that DC from storage batteries provided best test results. Since then it has been shown that variations of the AC supplies are as effective and in some cases even more desirable. An important feature of alternating current is the skin effect. The options of current available from AC and DC generators are all that is now required. In addition to the commutator that provides us with “full wave rectified” AC, there is electronic circuitry, called rectifiers, that provide us with the other modifications to the types of currents used. Only storage batteries (and Faraday’s Dynamo) provide true DC, all other sources we use are AC derived. Commutators and rectifiers limit or reverse negative flow so current, although its amplitude may fluctuate, flows only in one direction. For this reason rectified alternating current is considered direct and may sometimes be termed “half wave DC” and “full wave DC”. various types of currentAdvantages & Disadvantages of Various Types of Magnetizing Current Direct Current (DC) Skin Effect