Direct Current
Direct current, on the other hand, magnetizes the entire cross section more or less uniformly in the case of longitudinal magnetization, and with a straight line gradient of strength from a maximum at the surf ace to zero at the center of the bar in the case of direct contact (circular) magnetization. Magnetic fields produced by direct current penetrate deeper into a test object than fields produced by alternating current, making detection of subsurface discontinuities possible.
In the presence of direct current fields, dry powder particles behave as though they were immobile, tending to remain wherever they happen to land on the surface of the test object. This contrasts what happens with dry powder particles in the presence of alternating current and half wave fields. In these fields, the particles have mobility on a surface because of the pulsating character of the fields. Particle mobility aids considerably the formation of particle accumulations (indications) at discontinuities.
Pure direct current can be obtained from automotive type storage batteries, but today this method is seldom used, except occasionally in emergencies when a battery may be used to power a handheld magnetizing device. The disadvantages of using batteries are their weight when a number of them must be used to obtain high amperage currents, the frequent charging and maintenance required and their limited life and replacement cost.
Alternating currents used in magnetic particle testing are at low voltages. Current amperages range from about 100 A up to about 20 000 A, depending on the test object to be magnetized and the method of magnetizing. The higher amperages are obtained by using stepdown transformers that reduce power line voltages, while at the same time increasing current at about the same ratio. Exceptions are the much lower amperages drawn by handheld devices that operate from standard 120 V outlets. Alternating current and half wave direct current for magnetizing test objects are obtained from single phase systems or from one phase of three phase systems. Full wave direct currents are usually obtained from three phase systems using full three phase bridge rectifiers.
The primary method for obtaining direct current for magnetic particle testing is through rectification of alternating current using silicon rectifiers. A rectifier, or diode, is a device that allows electric current to flow through it in only one direction. By proper connection of rectifiers, the back and forth flow of alternating current is converted to a current flow in only one direction. This is a
form of direct current. A rectifier circuit that converts both half cycles of alternations (back and forth flow) of the alternating current to one direction of current flow is called a full wave rectifier.
Single phase alternating current also can be rectified using a full wave rectifier circuit to obtain direct current for magnetic particle testing. Since three phase power is so readily available in industry, direct current for magnetic particle testing units is usually obtained using three phase full wave rectifiers.
Half Wave Direct Current
Half wave direct current provides the greatest sensitivity for detecting discontinuities that lie below the surface, particularly when using dry powder and the continuous method. The pulsations of the half wave current vibrate the magnetic particles, thereby aiding their migration across a surface to form indications at discontinuities. This particle mobility, which is very pronounced when dry magnetic powder is used, contrasts with the immobility of the powder when pure direct current is used.
There is some skin effect when half wave current is used, caused by the pulsating magnetic fields produced by this current. However, the effect on field penetration is small at the usual power frequencies of 50 and 60 Hz.
Figure shows the waveform for half wave direct current. When single phase alternating current is passed through a simple rectifier, the reversed flow of current is blocked or clipped. This produces a series of current pulses that start at zero, reach a maximum point, drop back to zero and then pause until the next
Half wave direct current waveform
positive cycle begins. The result is a varying current that flows only in one direction.
Half wave direct current has penetrating power comparable to single phase full wave direct current. Half wave current has a flux density of zero at the center of a test object, and the density increases until it reaches a maximum at the test surface. The pulsing effect of the rectified wave produces maximum mobility for the magnetic particles; dry method tests are enhanced by this effect.
Another distinct advantage of half wave direct current is the simplicity of its electrical components. It can be easily combined with portable and mobile alternating current equipment for weld, construction and casting tests.
One of the disadvantages of half wave magnetization is the problem in demagnetization: the current does not reverse so it cannot be used for demagnetizing. Alternating current can be used to remove some residual magnetism, but the skin effect of alternating current and the deeper penetration of half wave direct current cause incomplete demagnetization.
Full Wave Direct Current
It is possible for electrical circuitry to not only block (or rectify) the negative flowing current, but to invert it so that the number of positive pulses is doubled. Figure 8.3 shows the waveform of single phase full wave rectified alternating current. The resulting current is usually called single phase full wave direct current.
Single phase full wave direct current has essentially the same penetrating ability as three phase full wave direct current. The current fluctuation causes a skin effect that is not significant. It is also possible to incorporate switching devices in the circuitry that
Single phase full wave direct current waveform
reverse the current flow. This permits built in reversing direct current demagnetization. The initial cost of single phase full wave direct current equipment is much less than that of three phase full wave equipment because of its simpler components. One disadvantage of single phase units is the input power requirement. Single phase equipment requires 1.73 times more input
current than three phase units. This becomes very significant at higher magnetizing currents where input values can exceed 600 A.
Three Phase Full Wave Direct Current
Commercial electric power can be provided as three phase alternating current, with each phase providing part of the total current. Figure a shows the waveform of three phase alternating current. Three phase full wave magnetic particle equipment rectifies all three alternating current phases and inverts the negative flow to a positive direction, producing a nearly flat line direct current
magnetizing current. Figure b shows the waveform of three phase full wave direct current.
Three phase full wave direct current has all of the advantages of single phase full wave direct current, plus some additional benefits.
The current draw on the power line is spread over three phases, reducing the demand by nearly half. The demand on the line is also balanced, with each leg providing a portion of the current (single phase pulls all of the current from two legs, resulting in an unbalanced line load). Many power companies charge a higher rate to customers with unbalanced, high current requirements.