MAGNETIC FIELD INDICATING SHEET GREEN PAPER BASIC INFORMATION AND TUTORIALS

This plastic sheet (usually, but not always, green) is often used to inspect magnetic parts, especially for the transitions between magnetic poles (north-south transitioning to south-north, etc.). Unfortunately, a lack of understanding of the way the sheet is constructed often leads to misinterpretation of the results.

Microscopic flakes of nickel are first coated with an oil, in which a plastic material has been dissolved. The plastic then separates out, forming a skin around the oil drop. The flake is then free to rotate within the shell of plastic, which is invisibly small in diameter.

Many layers of these spheres (perhaps 30) are deposited onto a plastic sheet, which forms a support. The support sheet may be on the order of 0.005 in thick, and the layers of spheres may add on the order of 0.002 in to the total thickness.

When no magnetic field is present, the flakes lie flat in the bottom of their spheres, reflecting upward the color of the plastic sheet. When a magnetic field is present in the plane of the sheet, the brightness is intensified.

On the other hand, if a magnetic field is present which is normal to the sheet—that is, at approximately right angles into or out of the sheet—the flakes stand on end, aligning with the field. When this occurs, the light reflected off them bounces back and forth until it is absorbed, in a manner similar to the light in a metal tube, and no light is reflected (it is black).

It can be seen, then, that the green color means either that no field is present, or that there is a field, in the plane of the sheet. For example, a region in which flux is leaving the sheet at 45° from left to right as it rises will appear to be black when viewed from the right side.

The same region viewed from the left side, however, will appear to be green! In order to have a consistent result from the indications of this material, it must be viewed from directly overhead, not from an angle.

The nickel flakes saturate at a relatively small field. This observer noted a change in color at about 10 G and full transition to black at about 100 G for one type of sheet.

The transition width for two neodymium-iron magnets side by side in air may be from on the order of +4000 G to −4000 G, but the part of this transition which is indicated by the plastic sheet is much narrower—on the order of 1⁄40 as wide.

Based on the indications of the plastic sheet, some have thought they were seeing a very narrow transition between magnet poles, to a degree which is physically impossible.

ELECTRIC AND MAGNETIC FIELD OF A POWER SUBSTATION BASIC INFORMATION ANF TUTORIALS

Electric substations produce electric and magnetic fields. In a substation, the strongest fields around the perimeter fence come from the transmission and distribution lines entering and leaving the substation.

The strength of fields from equipment inside the fence decreases rapidly with distance, reaching very low levels at relatively short distances beyond substation fences. In response to the public concerns with respect to EMF levels, whether perceived or real, and to governmental regulations, the substation designer may consider design measures to lower EMF levels or public exposure to fields while maintaining safe and reliable electric service.

Electric and Magnetic Field Sources in a Substation

Typical sources of electric and magnetic fields in substations include the following:

1. Transmission and distribution lines entering and exiting the substation

2. Buswork

3. Transformers

4. Air core reactors

5. Switchgear and cabling

6. Line traps

7. Circuit breakers

8. Ground grid

9. Capacitors

10. Battery chargers

11. Computers

Electric Fields

Electric fields are present whenever voltage exists on a conductor. Electric fields are not dependent on the current. The magnitude of the electric field is a function of the operating voltage and decreases with the square of the distance from the source. The strength of an electric field is measured in volts per meter.

The most common unit for this application is kilovolts per meter. The electric field can be easily shielded (the strength can be reduced) by any conducting surface such as trees, fences, walls, buildings, and most structures. In substations, the electric field is extremely variable due to the screening effect provided by the presence of the grounded steel structures used for electric bus and equipment support.

Although the level of the electric fields could reach magnitudes of approximately 13 kV/m in the immediate vicinity of high-voltage apparatus, such as near 500-kV circuit breakers, the level of the electric field decreases significantly toward the fence line. At the fence line, which is at least 6.4 m (21 ft) from the nearest live 500-kV conductor (see the NESC), the level of the electric field approaches zero kV/m. If the incoming or outgoing lines are underground, the level of the electric field at the point of crossing the fence is negligible.

Magnetic Fields

Magnetic fields are present whenever current flows in a conductor, and are not voltage dependent. The level of these fields also decreases with distance from the source but these fields are not easily shielded. Unlike electric fields, conducting materials such as the earth, or most metals, have little shielding effect on magnetic fields. Magnetic fields are measured in Webers per square meter (Tesla) or Maxwells per square centimeter (Gauss). One Gauss = 10^–4 Tesla. The most common unit for this application is milliGauss (10^–3 Gauss).

Various factors affect the levels of the fields, including the following:

1. Current magnitude

2. Phase spacing

3. Bus height

4. Phase configurations

5. Distance from the source

6. Phase unbalance (magnitude and angle)

Magnetic fields decrease with increasing distance (r) from the source. The rate is an inverse function and is dependent on the type of source. For point sources such as motors and reactors, the function is 1/ r^2; and for single-phase sources such as neutral or ground conductors the function is 1/r.

Besides distance, conductor spacing and phase balance have the largest effect on the magnetic field level because they control the rate at which the field changes. Magnetic fields can sometimes be shielded by specially engineered enclosures. The application of these shielding techniques in a power system environment is minimal because of the substantial costs involved and the difficulty of obtaining practical designs.

MAGNETIC MOMENT IN ATOMS (BOHR MAGNETON) BASIC INFORMATION

The magnetic moment in various types of materials is a result of the following factors.

● Electron orbit. An electron in an orbit around a nucleus is analogous to a small current loop, in which the current is opposite to the direction of electron travel. This factor is significant only for diamagnetic and paramagnetic materials, where it is the same order of magnitude as the electron spin magnetic moment.

The magnetic properties of most materials (diamagnetic, paramagnetic, and antiferromagnetic) are so weak that they are commonly considered to be nonmagnetic.

● Electron spin. The electron cannot be accurately modeled as a small current loop. However, relativistic quantum theory predicts a value for the spin magnetic moment (or Bohr magneton b).

In an atom with many electrons, only the spin of electrons in shells which are not completely filled contribute to the magnetic moment. This factor is at least an order of magnitude larger than the electron orbit magnetic moment for ferromagnetic, antiferromagnetic, and superparamagnetic materials.

● Nuclear spin. This factor is insignificant relative to the overall magnetic properties of materials. However, it is the basis for nuclear magnetic resonance imaging (MRI).

● Exchange force. The exchange force is an interaction force (or coupling) between the spins of neighboring electrons. This is a quantum effect related to the indistinguishability of electrons, so that nothing changes if the two electrons change places.

The exchange force can be positive or negative, and in some materials the net spins of neighboring atoms are strongly coupled. Chromium and manganese (in which each atom is strongly magnetic) have a strong negative exchange coupling, which forces the electron spins of neighboring atoms to be in opposite directions and results in antiferromagnetic (very weak) magnetic properties.

Iron, cobalt, and nickel have unbalanced electron spins (so that each atom is strongly magnetic) and have a strong positive exchange coupling.

Therefore, the spins of neighboring atoms point in the same direction and produce a large macroscopic magnetization.This large-scale atomic cooperation is called ferromagnetism.

PERMANENT MAGNET DC MOTORS BASIC INFORMATION

Permanent-magnet (PM) motors are available in fractional and low integral-horsepower sizes. They have several advantages over field-wound types.

Excitation power supplies and associated wiring are not needed. Reliability is improved, since there are no exciting field coils to fail, and there is no likelihood of overspeed due to loss of field.

Efficiency and cooling are improved by elimination of power loss in an exciting field. And the torque versus-current characteristic is more nearly linear. Finally a PM motor may be used where a totally enclosed motor is required for a continuous-excitation duty cycle.

Temperature effects depend on the kind of magnet material used. Integral-horsepower motors with Alnico-type magnets are affected less by temperature than those with ceramic magnets because flux is constant.

Ceramic magnets ordinarily used in fractional-horsepower motors have characteristics that vary about as much with temperature as do the shunt fields of excited machines.

Disadvantages are the absence of field control and special speed-torque characteristics. Overloads may cause partial demagnetization that changes motor speed and torque characteristics until magnetization is fully restored.

Generally, an integral-horsepower PM motor is somewhat larger and more expensive than an equivalent shunt-wound motor, but total system cost may be less.

A PM motor is a compromise between compound-wound and series-wound motors. It has better starting torque, but approximately half the no-load speed of a series motor.

In applications where compound motors are traditionally used, the PM motor could be considered where slightly higher efficiency and greater overload capacity are needed. In series-motor applications, cost consideration may influence the decision to switch.

For example, in frame sizes under 5-in diameter the series motor is more economical. But in sizes larger than 5 in, the series motor costs more in high volumes. And the PM motor in these larger sizes challenges the series motor with its high torques and low no-load speed.

LENZ'S LAW BASIC DEFINITION AND TUTORIALS

What is Lenz's Law?

Faraday’s law says that the induced emf is given by

V = - dψ/dt

The direction of the induced emf is given by Lenz’s law, which says that the induced voltage is in the direction such that, if the voltage caused a current to flow in the wire, the magnetic field produced by this current would oppose the change in ψ. The negative sign indicates the opposing nature of the emf.

A current flowing in a simple coil produces a magnetic field. Any change in the current will change the magnetic field, which will in turn induce a back-emf in the coil. The self-inductance or just inductance L (H) of the coil relates the induced voltage to the rate of change of current

V = L dI/dT

Two coils placed close together will interact. The magnetic field of one coil will link with the wire of the second. Changing the current in the primary coil will induce a voltage in the secondary coil, given by the mutual inductance M (H)

V2 = M dI1/dT

Placing the coils very close together, on the same former, gives close coupling of the coils. The magnetic flux linking the primary coil nearly all links the secondary coil. The voltages induced in the primary and secondary coils are each proportional to their number of turns, so that

V1/V2 = N1/N2

and by conservation of energy, approximately

I1/I2 = N2/N1

A two-winding transformer consists of two coils wound on the same ferromagnetic core. An autotransformer has only one coil with tapping points. The voltage across each section is proportional to the number of turns in the section.

AC GENERATOR MAGNETIC CIRCUIT AND MATERIAL BASIC AND TUTORIALS

The magnetic circuit of an ac generator, as with other electric machines, is made up of the air gap, the stator teeth and backiron, the rotor poles, and the shaft section. Each of these elements has an effect on machine rating and operation. The function of the magnetic circuit is to carry flux that links the armature conductors to produce voltage.

Air gap.
The air gap constitutes the division between the rotating part of the machine—the rotor, which carries the field winding—and the stationary part of the machine—the stator, which carries the armature winding. In ac generators, the air-gap dimension is determined by the electrical characteristics of the machine.

There is a trade-off between excitation mmf (toward a small air-gap dimension) and armature reaction flux (toward a large air-gap dimension). This trade-off generally results in an air gap, which is substantially larger than mechanical considerations such as machining tolerances or windage loss would dictate.

Stator Teeth and Backiron.
The armature magnetic circuit carries alternating flux and is always laminated, either with complete ring laminations (for small machines) or with overlapping segmented laminations. The material most commonly used is sheet steel, of an alloy containing about 3.5% silicon, in sheets of thickness between about 0.35 and 0.65 mm.

Grain-oriented steel, with reduced losses and improved permeability in the direction of rolling, is often used in large turbogenerators. Orientation in the circumferential direction is advantageous in such machines because of the large proportion of steel and moderate flux densities in the backiron.

At high flux densities characteristic of the armature teeth, the advantage of grain orientation becomes less pronounced.

The active region of the armature constitutes the alternation of stator teeth and slots carrying the armature winding. The division between teeth and slots is a compromise between flux-carrying capability and current-carrying capability.

The trade-off generally results in a division that is about half slots and half teeth. Flux densities in the stator teeth are usually high enough to result in moderate saturation of the magnetic material.

Rotor Iron.
The magnetic flux in the rotor is nearly constant, varying in the main only slightly with changes in load and terminal voltage and with small higher frequency components due to time and space harmonics of armature flux.

This allows the rotor magnetic circuit to be made of solid steel. In turbine generators, the rotor is typically made of a single-piece forging of steel with slots for the field winding cut by machining.

The losses caused by harmonic driven eddy currents in the solid steel pole faces can be problematic, and are reduced by making the air gap larger, by increasing the number of stator slots and, by choosing a suitable (short-pitch) coil throw for the armature.

Salient-pole machines may have solid or laminated poles. In many cases, laminated poles are necessary to control eddy current losses. Pole laminations are commonly made of low carbon steel, 1.5 to 2 mm thick.

Thinner steel, sometimes with silicon content, may be used where further control of eddy current losses is required. The shaft, or inner portion of the rotor of salient-pole machines, is often a solid forging, or in large machines such as hydroelectric generators may be fabricated from structural steel pieces.

Magnetic Materials. 
Typical magnetization characteristics of steel materials used in the magnetic circuit of ac generators are shown in Fig. 7-14.

Fig 7-14 Magnetization curves of commonly used steels.

MAGNETIC PROPERTIES AND APPLICATIONS BASIC INFORMATION AND TUTORIALS

The relative importance of the various magnetic properties of a magnetic material varies from one application to another. In general, properties of interest may include normal induction, hysteresis, dc permeability, ac permeability, core loss, and exciting power.

It should be noted that there are various means of expressing ac permeability. The choice depends primarily on the ultimate use. Techniques for the magnetic testing of many magnetic materials are described in the ASTM standards.

The magnetic and electric circuits employed in magnetic testing of a specimen are as free as possible from any unfavorable design factors which would prevent the measured magnetic data from being representative of the inherent magnetic properties of the specimen.

The flux “direction” in the specimen is normally specified, since most magnetic materials are magnetically anisotropic. In most ac magnetic tests, the waveform of the flux is required to be sinusoidal.

As a result of the existence of unfavorable conditions, such as those listed and described below, the performance of a magnetic material in a magnetic device can be greatly deteriorated from that which would be expected from magnetic testing of the material.

Allowances for these conditions, if present, must be made during the design of the device if the performance of the device is to be correctly predicted.

Leakage.

A principal difficulty in the design of many magnetic circuits is due to the lack of a practicable material which will act as an insulator with respect to magnetic flux. This results in magnetic flux seldom being completely confined to the desired magnetic circuit. Estimates of leakage flux for a particular design may be made based on experience and/or experimentation.

Flux Direction.

Some magnetic materials have a very pronounced directionality in their magnetic properties. Failure to utilize these materials in their preferred directions results in impaired magnetic properties.

Fabrication.

Stresses introduced into magnetic materials by the various fabricating techniques often adversely affect the magnetic properties of the materials. This occurs particularly in materials having high permeability. Stresses may be eliminated by a suitable stress-relief anneal after fabrication of the material to final shape.

Joints.

Joints in an electromagnetic core may cause a large increase in total excitation requirements. In some cores operated on ac, core loss may also be increased.

Waveform.

When a sinusoidal voltage is applied to an electromagnetic core, the resulting magnetic flux is not necessarily sinusoidal in waveform, especially at high inductions. Any harmonics in the flux waveform cause increases in core loss and required excitation power.

Flux Distribution.

If the maximum and minimum lengths of the magnetic path in an electromagnetic core differ too much, the flux density may be appreciably greater at the inside of the core structure than at the outside. For cores operated on ac, this can cause the waveform of the flux at the extremes of the core structure to be distorted even when the total flux waveform is sinusoidal.

WHAT ARE ELECTROMAGNETS – DEFINITION BASICS AND TUTORIALS

It was noted previously in this section that an electric current flowing through a conductor creates a magnetic field around the conductor. In Fig. 2.9, the shaded circle represents a cross section of a

conductor with current flowing in toward the paper. The current is flowing from negative to positive.

When the current flows as indicated, the magnetic field is in a counterclockwise direction. This is easily determined by the use of the left-hand rule, which is based upon the true direction of current flow. When a wire is grasped in the left hand with the thumb pointing from negative to positive, the magnetic field around the conductor is in the direction that the fingers are pointing.

If a current-carrying wire is bent into a loop, the loop assumes the properties of a magnet; that is, one side of the loop will be a north pole and the other side will be a south pole. If a soft-iron core is placed in the loop, the magnetic lines of force will traverse the iron core and it becomes a magnet.

When a wire is made into a coil and connected to a source of power, the fields of the separate turns join and thread through the entire coil as shown in Fig. 2.10a. Figure 2.10b shows a cross section of the same coil. Note that the lines of force produced by one turn of the coil combine with the lines of force from the other turns and thread through the coil, thus giving the coil a magnetic polarity.

The polarity of the coil is easily determined by the use of the left-hand rule for coils: When a coil is grasped m the left hand with the fingers pointing in the direction of current flow, that is, from negative to positive, the thumb will point toward the north pole of the coil.

When a soft-iron core is placed in a coil, an electromagnet is produced. Of course, the wire in the coil must be insulated so that there can be no short circuit between the turns of the coil. A typical electromagnet is made by winding many turns of insulated wire on a soft-iron core which has been wrapped with an insulating material.

The turns of wire are placed as close together as possible to help prevent magnetic lines of force from passing between the turns. Figure 2.11 is a cross-sectional drawing of an electromagnet. The strength of an electromagnet is proportional to the product of the current passing through the coil and the number of turns in the coil.

This value is usually expressed in ampere-turns. If a current of 5 amp is flowing in a coil of an electromagnet and there are 300 turns of wire in the coil, the coil will have an mmf of 1,500 amp-turns. Since the gilbert is also a measure of mmf and 1 amp-turn is equal to 1.26 gilberts, the mmf may also be given as 1,890 gilberts.  The ultimate strength of the magnet also depends upon the permeability of the core material.

The force exerted upon a magnetic material by an electromagnet is inversely proportional to the square of the distance between the pole of the magnet and the material. For example, if a magnet exerts a pull of 1 Ib upon an iron bar when the bar is f in. from the magnet, then the pull will only be & lb when the bar is 1 in. from the magnet.

For this reason, the design of electrical equipment using electromagnetic actuation requires careful consideration of the distance through which the magnetic force must act. This is especially important in voltage regulators and relays.

INDUCTANCE - BASIC ELECTRICAL PARAMETERS INFORMATION AND TUTORIALS

What Is Inductance?


The basic inductive device is a coil of wire, called an inductor or a solenoid. Its functioning is based on the physical fact that an electric current produces a magnetic field around it.

This magnetic field describes a circular pattern around a current-carrying wire; the direction of the field can be specified with a “right-hand rule.” When a wire is coiled up as shown in figure below, it effectively amplifies this magnetic field, because the contributions from the individual loops add together.

The sum of these contributions is especially great in the center, pointing along the central axis of the coil. The resulting field can be further amplified by inserting a material of high magnetic permeability (such as iron) into the coil; this is how an electromagnet is made.


When such a coil is placed in an a.c. circuit, a second physical fact comes into play, namely, that a changing magnetic field in the vicinity of a conducting wire induces an electric current to flow through this wire. If the current through the coil oscillates back and forth, then so does the magnetic field in its center.

Because this magnetic field is continuously changing, it induces another current in the coil. This induced current is proportional to the rate of change of the magnetic field. The direction of the induced current will be such as to oppose the change in the current responsible for producing the magnetic field.

In other words, the inductor exerts an inhibitive effect on a change in current flow. This inhibitive effect results in a delay or phase shift of the alternating current with respect to the alternating voltage. Specifically, an ideal inductor (with no resistance at all) will cause the current to lag behind the voltage by a quarter cycle, or 90 degrees.

This result is difficult to explain intuitively. We will not attempt to detail the specific changes in the current and magnetic field over the course of a cycle. One thing that can readily be seen from the graph, though, is that the current has its maximum at the instant that the magnetic field changes most rapidly.

As the magnetic field increases and decreases during different parts of the cycle, it stores and releases energy. This energy is not being dissipated, only repeatedly exchanged between the magnetic field and the rest of the circuit. This exchange process becomes very important in the context of power transfer. Because the induced current in an inductor is related to the change in the field per unit time, the frequency of the applied alternating current is important.

The higher the frequency, the more rapidly the magnetic field is changing and reversing, and thus the greater the induced current with its impeding effect is. The lower the frequency, the easier it is for the current to pass through the inductor.

A direct current corresponds to the extreme case of zero frequency. When a steady d.c. voltage is applied to an inductor, it essentially behaves like an ordinary piece of wire. After a brief initial period, during which the field is established, the magnetic field remains constant along with the current.

An unchanging magnetic field exerts no further influence on an electric current, so the flow of a steady direct current through a coil of wire is unaffected by the inductive property. Overall, the effect of an inductor on an a.c. circuit is expressed by its reactance, denoted by X (to specify inductive reactance, the subscript L is sometimes added).

The inductive reactance is the product of the angular a.c. frequency7 and the inductance, denoted by L, which depends on the physical shape of the inductor and is measured in units of henrys (H). In equation form, XL = wL

Thus, unlike resistance, the reactance is not solely determined by the intrinsic characteristics of a device. In the context of power systems, however, because the frequency is always the same, reactance is treated as if it were a constant property.

When describing the behavior of electrical devices in the context of circuit analysis, we are generally interested in writing down a mathematical relationship between the current passing through and the voltage drop across the device. For a resistor, this is simply Ohm’s law, V = IR, where the resistance R is the proportionality constant between voltage and current.

It turns out that the inductance L also works as a proportionality constant between current and voltage across an inductor, but in this case the equation involves the rate of change of current, rather than simply the value of current at any given time. Readers familiar with calculus will recognize the notation dI/dt, which represents the time derivative or rate of change of current with respect to time.

Thus, we write V = L dI/dt meaning that the voltage drop V across an inductor is the product of its inductance L and the rate of change of the current I through it. This equation is used in circuit analysis in a manner analogous to Ohm’s law to establish relationships between current and voltage at different points in the circuit, except that it is more cumbersome to manipulate owing to the time derivative.

ELECTROMAGNETIC FIELD AND HEALTH EFFECTS BASIC INFORMATION AND TUTORIALS

What Are The Health Effects Of Electromagnetic Field?


A current flowing through a wire, alternating at 60 cycles per second (60 Hz), produces around it a magnetic field that changes direction at the same frequency. Thus, whenever in the vicinity of electric equipment carrying any currents, we are exposed to magnetic fields.

Such fields are sometimes referred to as EMF, for electromagnetic fields, or more precisely as ELF, for extremely low-frequency fields, since 60 Hz is extremely low compared to other electromagnetic radiation such as radio waves (which is in the megahertz, or million hertz range).

There is some concern in the scientific community that even fields produced by household appliances or electric transmission and distribution lines may present human health hazards. While such fields may be small in magnitude compared to the Earth’s magnetic field, the fact that they are oscillating at a particular frequency may have important biological implications that are as yet poorly understood.

Research on the health effects of EMFs or ELFs continues. Some results to date seem to indicate a small but statistically significant correlation of exposure to ELFs from electric power with certain forms of cancer, particularly childhood leukemia, while other studies have found no effects.

In any case, the health effects of ELFs on adults appear to be either sufficiently mild or sufficiently rare that no obvious disease clusters have been noted among workers who are routinely exposed—and
have been over decades—to vastly stronger fields than are commonly experienced by the general population.

From a purely physical standpoint, the following observations are relevant: First, the intensity of the magnetic field associated with a current in a wire is directly proportional to the current; second, the intensity of this field decreases at a rate proportional to the inverse square of the distance from the wire, so that doubling the distance reduces the field by a factor of about.

The effect of distance thus tends to outweigh that of current magnitude, especially at close range where a doubling may equate to mere inches. It stands to reason, therefore, that sleeping with an electric blanket or even an electric alarm clock on the bedside table would typically lead to much higher exposure than living near high-voltage transmission lines. Measured ELF data are published by many sources.