Primary coil

For the measurement of magnetisation in hysteresis-loop measurements, a search coil consisting of several-turn windings around a specimen is coimected to a fluxmeter whose output can be recoded on thejy-axis of an x,y plotter. The x-axis records the field strength of the primary coil around the specimen. The whole procedure can be done conveniendy in a hysteresigraph, whereby the induction is recorded automatically as the field is varied continuously. 

Figure 2-68 illustrates the basics of a transformer. First there is a core, usually constructed of a material of high magnetic permeability to achieve a high magnetic flux density. The core has two windings of conductors, a primary coil (designated as... 

N in the figure) and dt secondary coil (designated as N, ). Electric current through the primary coil causes a magnetic flux in the core and at the same time an impedance to the current and therefore an induced emf across the primary. The magnetic flux in the core in turn induces an emf across the secondary coil, causing a current to flow. The relation between the emf induced in the primary coil (note that this is not the source emf) and the emf induced in the secondary coil is given by... 

The two secondary windings are connected in series opposition so that the output voltage is the difference of the voltages in the secondary windings (i.e. V0 = V, - V2, Vt > V2). A ferromagnetic core moves inside the primary coil and thus varies the mutual inductance between the primary and secondary coils which in turn varies the secondary voltages. The displacement of the pressure sensor (capsule, bellows or... 

The primary coil is wound around the center of the tube. The secondary coil is divided with one half wound around each end of the tube. Each end is wound in the opposite direction, which causes the voltages induced to oppose one another. A core, positioned by a pressure element, is movable within the tube. When the core is in the lower position, the lower half of the secondary coil provides the output. When the core is in the upper position, the upper half of the secondary coil provides the output. The magnitude and direction of the output depends on the amount the core is displaced from its center position. When the core is in the mid-position, there is no secondary output. 

Induction furnaces are optimal for the melting operation. Their good temperature control permits accurate adjustment of the degree of oxidation. Since the melt is held in constant turbulent motion by the magnetic field produced in the primary coil, it is well mixed, even for short melt times. In comparison to the most frequently used resistance furnaces, plant cost and power consumption are higher. 

Testing procedure. The magnetic permeability measurement is performed on the samples formed to the shape of toroids wrapped uniformly with two sets of wire windings. The primary coil is excited with a sine wave from a function generator. The voltage induced in the secondary coil is measured with a lock amplifier at a frequency range of 0.5 Hz to 120 kHz. ... 

A sinusoidal frequency between 60 Hz and 20 kHz is used as the primary-coil excitation. The alternating magnetic field induces nearly equal secondary voltages Uce and uje the output voltage is Ucd = 1 06 i ed- When the slug is symmetrically placed, the two secondary voltages are equal and the output signal is zero. 

The alternating magnetic field can be combined with an admixture of the constant field generated by a DC current. The magnetic moment is induced in a sample through the application of the DC bias field, produced either by the primary coil or by a superconducting magnet. 

POTENTIAL TRANSFORMER - A voltage transformer. The voltage supplied to a primary coil induces a voltage in a secondary coil according, to the ratio of the wire windings in each of the coils. 

FIGURE 3.2 Illustration of the basic operating principle of linear variable differential transformer displacement sensors. When the core is positioned at the exact center between secondary coils 1 and 2, the differential voltage between the two secondary coils, V, is zero. Movement of the core creates an imbalance, increasing V. The signals at the primary coil, secondary coils 1 and 2, and the differential voltage Vare also shown. 

Figure 6 (A) A series-tuned, balanced-matched, inductively coupled sample (primary) coil. This example consists of a single loop of conductor with two symmetrically positioned fixed tuning capacitors, 2Q, and a variable capacitor, Q, for fine adjustment of the resonance frequency. The series tuning capacitors lower the coil voltage and should reduce dielectric losses. A secondary (impedance-matching) coil is required for coupling the primary to the spectrometer. (B) A circuit for a balanced-matched, inductively coupled surface coil. Lg is the inductance of the sample (primary) coil, which is tuned by Q and Q. Lm is the inductance of the matching (secondary) coil. The impedance matching can be fine-adjusted using Cm. (Reproduced with permission from Cady EB (1990). Magnetic Resonance Spectroscopy. New York Plenum Plenum.)...

If the secondary coil is connected to a load, a current I2 flows in the load and a corresponding current I flows in the primary coil. Since energy is conserved, the current transformation ratio is the inverse of the voltage transformation ratio ... 

Unless there is current flowing in the primary coil of a transformer, there will be no current in the coil. 

In general, the coil into which power flows is called the primary coil, whereas the other coil is called the secondary. 

The primary coils are located just above where it is desired to maintain the vapor zone as is shown in Figures 1.10 through 1.15, 1.19, and 1.21. 

Connect the other end of that same clip lead to either one of the two secondary wires on the transformer, which both have thin yellow plastic insulation on them. Do not use any of the black wires of this transformer, either the thinly insulated "center tap" of the secondary coil or the two thickly insulated black wires of the "primary" coil. (This experiment can be done either with or without a long "power cord" and plug attached to the primary. )... 

The reader probably knows from high school science courses that the primary coil of this transformer usually has several hundred "turns" of wire in its coil, although the transformer symbol used in this book only shows 3 turns. The secondary would have only one tenth as many turns, but for simplicity, each of the "windings" is shown here as having 3 turns. In this experiment the windings are not being used as a transformer — we are merely using one part as a simple inductor. 

If a portable AM/FM radio receiver is available, place it about three feet from the inductor and battery. (If a small radio is not available, the inductor and battery can be taken outside, near an automobile radio.) Turn on the radio, set it for AM, and tune the frequency dial to a number where there are no nearby radio stations, so only a very low level of background noise is audible. Turn the volume up somewhat. Disconnect the neon tester, and then make repeated sparks with just the inductor and battery, as shown in Fig. 1.2, but using the primary coil (heavily insulated black wires). Loud clicks will be heard from the radio loudspeaker, each time a spark is made. The high voltage pulse generates a radio wave, which travels through the air to the radio antenna. (Further analysis of these various aspects of radio theory will be explained in later chapters.)... 

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