Inductive load

Load inductance Load resistance Load parallel capacitance Load parallel resistance... 

If/ . L of cable, transformer and load inductances) and C are the switching circuit parameters then... 

Fig. 40. Block diagram of measuring circuit. P, potentiometer L, load inductance R, small resistance D, dual scoper O, audio frequency oscillator A, sawtooth wave generator B, tuned amplifier K, oscilloscope equipped with amplifiers Q, gain control of cathode follower to read Q...

Tests of the cross-line type of inductance sw itch were made using the loop circuit of Fig. 6. A current split of 8 to 1 was obtained, which could have been increased if the load inductances had been reduced. 

Considering an ungrounded generator, that feeds an electrical load, as shown in Fig. 3.3.1. The generator model is represented by a source and an inductance in series. The cable and the load inductance are illustrated too. Also it represents the cables capacitance, the capacitors banks and the surge arresters. 

The cavity design shown in the figure is set up to be sHghtly shorter than a fuU 1/4-wavelength at the operating frequency. This makes the load inductive and resonates the tube s output capacity. Thus, the physically foreshortened shorted transmission Hne is resonated and electrically lengthened to 1/4-wavelength. 

Power factor is an important, but often misunderstood, characteristic of AC power systems. It is defined as the ratio of true power to apparent power, generally expressed as a percentage. (PF also maybe expressed as a decimal value.) Reactive loads (inductive or capacitive) act on power systems to shift the current out of phase with the voltage. The cosine of the resulting angle between the current and voltage is the power factor. 

A particularity of ohmic-inductive loads like magnetostrictive or magnetorheological actuators is that they - especially during dynamic operation mainly require reactive power and only a little active power. Usually, the current is given and the voltage required for operation is determined by the time derivative of the current multiplied with the load inductance. Since the control field is generated by a coil, the copper resistance must also be taken into accoimt. 

Determination of Key Data. The parameters of the actuator to be driven form the basis for the choice of the appropriate power amplifier. The amplifier must be capable of providing the required current. The necessary operating voltage is determined by means of the greatest incline in the current-time signal that the amplifier has to produce and the load inductance. To this end, one applies a tangent to the geometric curve or differentiates it mathematically. The procedure is similar to the one described for power amplifiers used to drive capacitive loads in the examples in Fig. 6.150 to the left simply replace V hy I and vice versa and C by L. 

The principle of pulse-width modulation is shown in Figure 10.11. The same circuit as shown in Figure 10.9 is used. In the positive cycle, only switch D is on all the time, and switch A is on intermittently. When A is on, current builds up in the load. When A is off, the current continues to flow, because of the load inductance, through switch D and the free-wheeling diode in parallel with switch C, around the bottom right loop of the circuit. 

Induction furnaces utilize the phenomena of electromagnetic induction to produce an electric current in the load or workpiece. This current is a result of a varying magnetic field created by an alternating current in a cod that typically surrounds the workpiece. Power to heat the load results from the passage of the electric current through the resistance of the load. Physical contact between the electric system and the material to be heated is not essential and is usually avoided. Nonconducting materials cannot be heated directiy by induction fields. 

Utdity power distribution grids normally operate at a fixed frequency of 50 or 60 Hz. These frequencies can be utilized directiy for the induction process if the load characteristics are appropriate. If they are not, specific appHcations can be optimized by the use of variable and higher frequencies produced by soHd-state frequency power converters connected between the supply and the load. 

The efficiency of an induction furnace installation is determined by the ratio of the load usehil power, P, to the input power P, drawn from the utihty. Losses that must be considered include those in the power converter (transformer, capacitors, frequency converter, etc), transmission lines, cod electrical losses, and thermal loss from the furnace. Figure 1 illustrates the relationships for an induction furnace operating at a constant load temperature with variable input power. Thermal losses are constant, cod losses are a constant percentage of the cod input power, and the usehd out power varies linearly once the fixed losses are satisfied. 

Fig. 2. Induction heating cod and load showing (a), current distribution in load, and (b), reference depth.

Power Supplies and Controls. Induction heating furnace loads rarely can be connected directiy to the user s electric power distribution system. If the load is to operate at the supply frequency, a transformer is used to provide the proper load voltage as weU as isolation from the supply system. Adjustment of the load voltage can be achieved by means of a tapped transformer or by use of a solid-state switch. The low power factor of an induction load can be corrected by installing a capacitor bank in the primary or secondary circuit. 

A special coil configuration is used to heat thin strips of metal that caimot be heated efficiently with a coil that encircles the load, as the strip thickness is small compared to the depth of penetration. The transverse flux induction coil is positioned on either side of a strip to produce a uniformly heated strip with good efficiency in a much smaller space than conventional radiant or convective strip heating furnaces (6). 

Process-flow control and buffer-gas control have been discussed under Variable Nozzles and Buffer-Gas System respectively. Speed is usually self-controlled by a matching speed-sensitive load such as a compressor or a pump. If the load is an induction or svn-chronous generator feeding into a stable ac system, the system frequency fixes the speed. Otherwise, the speed can be controlled by a conventional governor. 

This is why an induction motor ceases to run at synchronous speed. The rotor, however, adjusts its speed, N such that the induced e.m.f. in the rotor conductors is Just enough to produce a torque that can counter-balance the mechanical load and the rotor losses, including frictional losses. The difference in the two speeds is known as slip. S, in r.p.m. and is expressed in terms of percentage of synchronous speed, i.e. 

The declared efficiency and power factor of a motor are affected by its loading. Irrespective of the load, no-load losses as well as the reactive component of the motor remain constant. The useful stator current, i.e. the phase current minus the no-load current of a normal induction motor, has a power factor as high as 0.9-0.95. But because of the magnetizing current, the p.f. of the motor does not generally exceed 0.8-0.85 at full load. Thus, at loads lower than rated, the magnetizing current remaining the same, the power factor of the motor decreases sharply. The efficiency, however, remains practically constant for up to nearly 70% of load in view of the fact that maximum efficiency occurs at a load when copper losses (f R) are equal to the no-load losses. Table 1.9 shows an approximate variation in the power factor and efficiency with the load. From the various tests conducted on different types and sizes of motors, it has been established that the... 

This is a very useful nomogram to determine the performance of a motor with the help of only no-load and short-circuit test results. In slip-ring motors, it also helps to determine the external resistance required in the rotor circuit to control the speed of the motor and achieve the desired operating performance. Slip-ring motors are discussed in Chapter 5. The concept behind this nomogram is that the locus of the rotor and the stator currents is a circle. Consider the equivalent circuit of an induction motor as shown in Figure 1.15, where... 

Due to excessive starting and braking heat losses it is not advisable to switch an induction motor ON and OFF frequently. The number of starts and stops a motor is capable of performing will depend upon its working conditions such as type of switching, braking and load demand etc. and can be determined from... 

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