Instrumentation Sensors Book

Figure 13.18 (a) TRIAC power control circuit, and (b) the circuit waveforms.

load from 0% to 100%, by controlling the trigger points with respect to the ac sine wave as the ac voltage increases from zero. The capacitor C is then charged via R1 until the breakdown voltage of the DIAC is reached, and the TRIAC is triggered on both the positive and negative half-cycles, as shown by the waveforms in Figure 13.18(b).

Example 13.4

A TRIAC is used to supply 750A to a load from a 120V supply. What is the maximum power that can be supplied to the load, and the power loss in the TRIAC? Assume the voltage drop across the TRIAC is 2.1V.

Power loss in TRIAC = 2.1 × 750W = 1.575 kW

Power from supply = 750 × 120W = 90 kW

Power to load = 90 − 1.575 kW = 88.425 kW

This example illustrates that the efficiency of the switch is greater than 98%, and the high dissipation that can occur in the switch and the need for cooling fins with low thermal resistance. Precautions in the design of power switching circuits, choices of devices for specific applications, and thermal limitations are outside the scope of this book. Device data sheets must be consulted and advice obtained from device manufacturers before designing power controllers [3].

When opening and closing switches with voltage applied in power control circuits, problems can occur, such as power surges that cause large current transients in the supply line. These transients produce unwanted RF interference and potentially damaging high voltage inductive transients. A solution to this problem is to fire the thyristor when the supply voltage is at or near zero. Several commercial devices are available for this function. These devices are called zero-voltage switches (ZVS). These devices control power by eliminating cycles. Figure 13.19(a) shows a zero-voltage crossover switch driving a TRIAC. The waveforms are shown in Figure 13.19(b). Power is supplied in complete cycles, and one power cycle for three line cycles, or 33% power to the load, is shown in the figure.

Figure 13.19 Zero-voltage crossover switch (a) driving a TRIAC, and (b) associated waveforms.

Zero-voltage crossover switches are suitable for controlling heating elements, valve control, and lighting, but are not ideally suited for motor speed control, since motors under heavy load tend to slow down, and the missing power cycles tend to cause vibration.

Power devices that are turned On and Off by the input are:

1.BJTs, which are current-controlled devices. Power bipolar devices have low gain, so are normally used in a Darling configurations to give high current gain and the ability to control high currents with low drive currents [4].

2.Power MOSFETs, which are voltage-controlled devices designed for high-speed operation, but their high saturation voltage and temperature sensitivity limit their application in power circuits.

3.IGBTs [5]. An MOS transistor, as opposed to the Darlington bipolar configuration, controls the power bipolar output device, making it a voltage-controlled device. The IGBT has fast switching times. Older devices had a high saturation voltage, and newer devices have a saturation voltage about the same as a BJT.

4.MCTs are voltage-controlled devices with a low saturation voltage and medium speed switching characteristics.

A comparison of the characteristics of power devices is given in Table 13.2. These devices are used for power and motor control. Applications include rectification of multiphase ac power to give a variable voltage dc power level output, or the

Table 13.2 Comparison of Power Device Characteristics

These field windings are connected directly or amplified, and then fed to three identical field windings in the remote synchronous motor, which will replicate the magnetic field in the local motor. Because the two rotors are fed from the same ac supply, the rotor in the slave will seek the same position as the rotor in the master. Typically, synchronous motors have many applications when duplicating position in formation to remote locations. The output from the master synchronous motor can be digitized in a synchro to digital converter (SDC) for transmission and processing. The signal can be converted back to its original format using a digital to synchro converter (DSC), for use by the slave synchronous motor.

13.6Application Considerations

13.6.1Valves

The selection of control valves for a particular application depends on many variables, such as the corrosive nature of the fluid, temperature of operation, pressure of the fluid, velocity of the flow, volume of the flow, and the amount of suspended solids.

Valves are the final element in a control loop, and are critical in providing the correct flow for process control. The valve is subject to operation in very harsh conditions, and is one of the most costly elements in the process control system. Their choice and correct installation require both knowledge and experience. Careful attention must be made to the system requirements and manufacturers specifications, before a careful valve selection can be made. Additional information can be obtained from the ISA 75 series of standards.

Some of the factors affecting the choice of valves are:

1.Fail-safe considerations for two-way and three-way types of valves;

2.Valve size from flow requirements, avoiding both oversizing and undersizing;

3.Materials used in the valve construction, ranging from PVC to brass to steel, based on considerations of pressure, size, and corrosion;

4.Tightness of shutoff, as classified by quality of shutoff by leakage at maximum pressure (Valves are classified into six classes depending on leakage, from 0.5% of rated capacity to 0.15 mL/min for a 1-in diameter valve);

5.Level of acceptable pressure drop across the valve;

6.Linear or rotary motion of type of valve (e.g., globe, diaphragm, ball, or butterfly valves).

The type of valve or plug depends on the nature of the process reaction. In the case of a fast reaction with small load changes, control is only slightly affected by valve characteristics. When the process is slow with large load changes, valve characteristics are important. If the load change is linear, then a valve with a linear characteristic should be used. In the case of a nonlinear load change, a valve with an equal percentage change may be required. In some applications, valves are required to be completely closed when Off. Other considerations are: maintenance; service-

ability; fail-safe features; pneumatic, hydraulic, solenoid, or motor control; and the need for feedback. The above is a limited review of actuator valves, and, as previously noted, the manufacturers’ data sheets should be consulted when choosing a valve for a particular application.

Electrically operated servo or stepper motors normally control position and speed. In applications such as pumping, compressors, or conveyer belts, three-phase motors are normally used.

13.6.2Power Devices

Power switching devices, from contactors to solid state devices, will be chosen from considerations of power handling, switching speed, isolation, and cost. Some of the considerations are:

1.For low-speed operation, mechanical relay devices give isolation, relatively low dissipation, and are low cost.

2.Light control and ac motor control can use SCRs and TRIACs, which are available in a wide range of packages, depending on current handling and heat dissipation requirements [8].

3.For power control, multiphase motor control, and high-speed switching applications, BJT or IGBTs can be used. These devices also come in a variety of low thermal resistance packages.

4.MOSFET devices can be used in medium power applications, since they the advantage that control circuits can be integrated on to the same die as the power device [9].

13.7Summary

Regulators and valves are available in many shapes and sizes, and since they are one of the most expensive and most important components in a process control system, great care has to be taken in their selection. The various types of regulators, including internal and external connected regulators, were discussed. Regulators can be loaded using spring, weight, or pressure. More expensive devices use pilot devices in the feedback loop for higher system feedback gain, which gives better regulation, control, and flexibility. The most common valve is the globe valve. This device is available in many configurations, having many types of plugs to give fast opening, linear, or equal percentage characteristics. Valve sizes depend on rates of flow and acceptable losses. Materials used depend on pressure, temperature, and resistance to corrosion. Globe valves can be configured as two-way or three-way fail-safe modes, split body for ease of maintenance, and so forth. Other types of valves are the butterfly, diaphragm, ball, and rotary plug valves. Actuators can be controlled pneumatically or electronically. The more common electronic power handling device types are the SCR, TRIAC, and IGBT. Electronic control devices have fast operation, are robust, and can handle large amounts of power for control. Devices can operate from an ac or dc supply. Actuator positions can be controlled by stepper motors or motors using feedback. Other types of motors for controlling position are synchronous motors.

References

[1]Battikha, N. E., The Condensed Handbook of Measurement and Control, 2nd ed., ISA, 2004, pp. 223–238.

[2]Johnson, C. D., Process Control Instrumentation Technology, 7th ed., Prentice Hall, 2003, pp. 331–340.

[3]Chan, C. C., “An Overview of Electric Vehicle Technology,” Proceedings of the IEEE, Vol. 81, No. 9, September 1993, pp. 1302–1313.

[4]van de Wouw, T., “Darlingtons for High Power Systems,” P. C. I 88 Conference Proceedings, Vol. 15, June 1988, pp. 204–213.

[5]Yilmaz, H., et al., “50A 1,200V N-channel IGT,” IEE Proceedings, Vol. 132, Part 1, No. 6, December 1985.

[6]Jurgen, R. K., Automotive Electronics Handbook, 2nd ed., McGraw-Hill, 1999, Chapter 33.

[7]Humphries, J. T., and L. P. Sheets, Industrial Electronics, 4th ed., Delmar, 1993, pp. 464–474.

[8]Schuster, D., “Know Your Power,” Sensors Magazine, Vol. 16, No. 8, August 1999.

[9]Dunn, B., and R. Frank, “Guidelines for Choosing a Smart Power Technology,” P. C. I 88 Conference Proceedings, Vol. 15, June 1988, pp. 143–157.