Automatic Regulating Equipments Used in Industries | Electrical Engineering

In industrial processes a certain quantity, such as the speed of a motor, the temperature of an oven, the tension in a wire, output voltage of the generator or output current is required to be held at some specified value, usually constant, although in certain cases a predetermined cycle of variation may be required. Electrical devices used for providing such regulation operate on the closed cycle or feedback principle.

Such an arrangement requires essentially:

(i) An actuating device for providing a control signal dependent on the deviation of the controlled quantity from its specified value

(ii) An operating device for controlling the power supply feeding the apparatus to be regulated in such a way as to counteract any deviation and

(iii) A connecting link for transferring the actuating signal in an appropriate manner to the operating device.

The connecting link used for this purpose should be of high power gain and of a very rapid response so that the smallest departure from the specified value will be sufficient to initiate the correcting effect of the operating device and there is no time delay between the occurrence of the actuating signal and the correcting effect.

The thermionic valves, usually of gas filled type, rotating amplifiers (amplidynes) and magnetic amplifiers are extensively used as connecting link.

Equipment # 1. Thyratron:

Thyratron is a hot cathode gas filled triode. Normally it is filled with a small amount of inert gas such as argon, hydrogen, neon or mercury vapour. The fast acting devices do not use mercury vapour as the mercury vapour has the disadvantage of characteristics varying with temperature and it takes time in deionization. The characteristics of hydrogen thyratron are stable and the devices are fast. Plate is a disc of graphite.

The special construction oxide coated cathode is used so that its surface is not damaged due to bombardment of positively charged particles. The control grid has a metal cylinder surrounding the cathode with one or more perforated discs known as grid baffles near the centre. Thus the control grid acts as an electrostatic shield between cathode and anode except for the holes in the grid baffles.

Before the tube fires, its characteristics are similar to those of a vacuum triode i.e., the plate current (small in magnitude) can be varied by variation of potential applied to the grid. After firing of the tube the grid loses control over plate current completely and plate current is limited by the external resistance in the plate circuit i.e., the thyratron behaves like a diode. The introduction of grid in between the plate and cathode makes it possible to control the conditions under which the discharge should start.

Since the only function of the grid is to delay or prevent the starting of the current, the grid is naturally designed to achieve its purpose with as little grid voltage as possible. The grid, therefore, is quite an extensive structure. The critical grid voltage varies with the plate voltage to some extent, and also with the design of the grid.

i. Plate Voltage-Plate Current (or Plate) Characteristics:

These characteristics are drawn for various grid voltages. For a given grid voltage when the plate voltage is increased from zero, initially there is no plate current but when the plate voltage is further increased, a voltage is reached, called the breakdown voltage, the current at once shoots up and is limited only by the external resistance in the plate circuit. If the grid is made more negative, the tube will break down at higher plate voltage, as illustrated in Fig. 2.2.

ii. Grid Control Characteristics:

The curve drawn between plate voltage and critical grid voltage is called the grid control characteristic or starting characteristic or breakdown characteristic. This characteristic can be determined by determining the value of critical grid voltage for different values of plate voltage and plotting them, as shown in Fig. 2.3. The slope of this characteristic is known as control ratio.

Applications:

Thyratron has two main fields of applications:

(i) As an electronic switch to handle heavy currents and

(ii) As a grid controlled rectifier when voltage control is required. The thyratron, when used as electronic switch, keeps the circuit open from the plate to cathode so long as the grid is kept at high negative potential. As soon as the grid potential is brought to such a value that the tube fires, the circuit is closed.

The thyratron switch has two great advantages over mechanical switches:

(i) It is electrically operated and requires only a minute amount of energy to close it, so that it leads itself readily to automatic operation;

(ii) Operation is very fast.

However, it has got some disadvantages also such as:

(a) voltage drop of 8 to 18 volts in the circuit;

(b) it cannot open dc circuit;

(iii) A single thyratron will pass only the positive half waves. A second thyratron connected in parallel with the first one but with opposite polarity may be used to pass the negative half waves. A thyratron can open ac circuit, with a delay that never exceeds one half cycle, so long as frequency is not so high that the time of one half cycle is less than the deionizing time. The thyratron finds wide use in timing and other control applications.

The fact that its time of firing may be controlled makes the thyratron adaptable to heat control in resistance welding. By delaying or advancing the time during the cycle at which the tube fires, the average amount of welding current allowed to flow per cycle may be increased or decreased. In addition to this, the number of cycles that the welding current is allowed to flow may be controlled very accurately.

Thyratron rectifiers are also used in electronic methods of electric motor control for both small and large powers. The main advantages of electronic motor control are that the control can be made to depend on very minute signals such as those obtained from a photocell or even a thermocouple and that where necessary an extremely precise control can be obtained.

Control may be affected in several ways such as:

(i) With direct voltage applied to the plate as well as to the grid

(ii) With alternating voltage to the plate circuit and direct voltage to the grid and

(iii) Alternating voltage applied on both the grid and plate.

In the first case the tube will fire and conduct whenever the grid voltage is made less negative than critical grid voltage and will again stop to conduct when the plate voltage is reduced below striking voltage or the plate circuit is opened. In the second case the grid will regain control when the plate voltage becomes negative, hence conduction will be stopped every half cycle and the tube breakdown will occur each half cycle as the plate voltage becomes positive, the point at which the tube fires depends upon the grid voltage.

The average current flow can be controlled, therefore, by controlling the length of time during the half cycle that the tube is conducting i.e., by controlling the breakdown point. In the last case breakdown point is determined by the phase relation of the two voltages as well as by their magnitudes. There will thus be a point in each positive half cycle of the cathode-plate path when the tube will break down, if the grid voltage has a small negative value than the critical grid voltage with reference to the plate voltage, which will result in ionization and conduction.

By using a phase shifting device to control the phase of grid voltage with respect to the cathode-plate voltage wave the duration of current flow may be controlled from zero to point represented by practically all the positive half cycles of plate voltage.

The thyratron cannot be employed as an amplifier like a vacuum triode because the grid voltage has no control over the magnitude of plate current after firing of the tube.

Equipment # 2. Ignitron:

Ignitron is a gaseous tube and consists of carbon anode, boron carbide ignitor tube and mercury pool cathode. It is a cold cathode tube and does not have any heating element or grid control as in thyratron. Therefore, electrons are to be made available before the tube starts conducting. This is achieved by the ignitor which remains dipped in the mercury pool but being made of boron carbide does not get wet and, therefore, gives high resistance, say 10 to 500 Ω, between the mercury pool and ignitor. On closing of switch S full voltage is impressed between ignitor and mercury pool.

This causes intense electric field which pulls out electrons from mercury pool to start with. The electrons of the arc ionize the mercury vapours if the ignitor current is of the magnitude between 20 and 40 amperes. Positive ions thus released are attracted towards cathode mercury pool and momentum of these mercury ions releases large number of electrons from the cathode pool by secondary emission. These electrons are attracted towards the carbon anode. Now resistance of main path of the current is very low as compared to that of ignitor path and, therefore, ignitor current stops. This process is to be repeated every time anode becomes positive in ac voltage cycle.

Ignitrons can handle current from 40 to 10,000 A. It may also be noted that electronic current flow from ignitor to cathode, spoils the ignitor and, therefore, should not be permitted.

The power output can be controlled by timing the firing by means of the ignitor, usually by shifting the phase of the pulse to the ignitron.

The single units may be connected in 3-phase, 6-phase, or 12-phase. In smaller ratings, sealed-in glass tubes are used; in the intermediate ratings, sealed-in water-cooled metal tanks are used. In the larger power units, pumped all steel water-cooled tanks are used.

Equipment # 3. Amplidyne (Rotary Amplifier):

The amplidyne is a rotary amplifier in which a small amount of power supplied to excite the field is amplified in the generator output. Though the ordinary separately excited generator operated at low flux densities can be used as a power amplifier since the power required for exciting the field is only 1 to 3 per cent of the rated output of the generator and a small change in the power to the field can be made to produce a large proportionate change in the power output. However, much larger power amplification can be had by using the amplidyne, since in this machine power is amplified in two stages. Rotary amplifiers are generally built for outputs up to 100 kW.

Equipment # 4. Magnetic Amplifiers:

In a magnetic amplifier, the basic component is a steel-cored coil with an additional winding energized by direct current. It is intended for controlling alternating current of relatively large power by means of a low power dc.

The simplest practical magnetic amplifier has two identical steel cores with identical ac windings, called the load winding, on each. In addition they have a common dc winding called the control winding. The ac windings are connected together in series or in parallel, and in series with them there is a load. The series connection is used when a short- time response and a high voltage are required, usually in controlling loads requiring low power. The parallel connection, on the other hand, has a slow response of approximate 1 to 3 seconds, but permits control of large current. The control winding is energized by dc.

With no current in control winding, the ac winding has a large inductance, and a high reactance is offered to an ac potential. As a result alternating current to the load is limited to a low value by the inductive reactance, and the load voltage is small. When a dc voltage is now applied to control winding the dc magnetic flux passes through the core and the core approaches magnetic saturation, even when a small value of dc voltage is applied.

This causes the effective inductance of the ac winding to decrease, so that the ac winding impedance drops. Consequently the alternating current flowing through the ac winding (and the load voltage) increases as the flow of direct current through the control winding increases. Thus, varying the magnitude of control current over a small range causes the load voltage to vary over a wide range.

If a magnetic amplifier consisted only of one core with two windings (ac and control), the load current would be noticeably non-sinusoidal. This is one of the reasons why the simplest practical magnetic amplifier is made double. A second reason is that for one core, the control winding would be linked with an alternating magnetic flux, and an alternating emf would be induced in it. This emf could become very significant for a large value of the ratio N_C/N_L. With two cores, the total flux linking the control winding does not have an alternating component and, hence, no alternating emf is induced in it. At present there are a large number of different magnetic amplifier circuits and types of construction.

The output or transfer characteristic curves of a magnetic amplifier are shown in Fig. 2.8 for various load resistances. From the curves it is obvious that with the exception of a small region near the origin the load current is directly proportional to the control current up to a value determined by the load resistance, and secondly that the characteristic curve does not begin from the origin but intersects the ordinate axis above the origin. This is due to the current flowing in ac winding which is determined by the applied voltage and the inductive reactance of the winding. Thus for zero control current, load current, I_L will be equal to E/X_L known as quiescent current.

Magnetic amplifiers have got following advantages over electronic ones:

1. Mechanically sturdy and inherently shockproof.

2. Relatively inexpensive amplifiers can be designed to have very high gain, with low input power requirements, and to be compact and light in weight.

3. It does not have any filament to be heated as is in the case of vacuum tube amplifiers.

4. It can be designed for a wide range of input and output impedances and since the control and load circuits are not conductively coupled, they provide somewhat more flexibility than vacuum tube circuits.

On the other hand magnetic amplifiers have limited frequency response and produce an output that is a modulated ac carrier. They have much lower input impedance than vacuum tubes and provide only a finite power gain whereas vacuum tube may theoretically at least, give an infinite power gain. Other disadvantage is that magnetic amplifier, in general, is required to be designed specifically for each application, since both the supply voltage and the load impedance critically affect the operation.

Magnetic amplifiers are used in electrical systems of control, measurement and automatic regulation as power amplifiers (for example for automatic control of ac and dc motors or in smooth regulation of lighting).

When the ac supply is applied to the motor for starting, the load windings of the magnetic amplifier have a high inductive reactance and only a reduced voltage acts across the stator terminals of the motor. As the motor begins to gain speed, the starting current drops and allows the voltage at the stator terminals to rise. Two of the stator winding terminals are connected to a semiconductor bridge rectifier which serves to excite the control windings of all the three magnetic amplifiers.

As the voltage across the stator terminals rises during starting, the direct current through the control windings also rises. As a result, the inductive reactance of the ac windings is reduced and the voltage at the stator terminals rises. The control winding current, due to rise in stator terminal voltage, increases until it finally saturates the amplifiers. By this time the inductive reactance of the ac winding drops practically to zero and the motor operates at full voltage.

The character of motor current and torque change with time during starting may be adjusted by setting up a suitable ratio of the electromagnetic time constant of the magnetic amplifier control winding circuit to the electromechanical time constant of the drive.