Category: General
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29.4 Proportional-only control
Imagine a liquid-level control system for a vessel, where the position of a level-sensing float directly sets the stem position of a control valve. As the liquid level rises, the valve opens up proportionally: Despite its crude mechanical nature, this proportional control system would in fact help regulate the level of liquid inside the process vessel. If…
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29.3 On/off control
Once while working as an instrument technician in an aluminum foundry, a mechanic asked me what it was that I did. I began to explain my job, which was essentially to calibrate, maintain, troubleshoot, document, and modify (as needed) all automatic control systems in the facility. The mechanic seemed puzzled as I explained the task…
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29.2 Diagnosing feedback control problems
Negative feedback systems, in general, tend to cause much confusion for those first learning their fundamental principles and behaviors. The closed-cycle “loop” formed by the interaction of sensing element, controller, final control element, and process means essentially that everything affects everything else. This is especially problematic when the feedback control system in question contains a fault and…
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Chapter 29 Closed-loop control
Instrumentation is the science of automated measurement and control. Applications of this science abound in modern research, industry, and everyday living. From automobile engine control systems to home thermostats to aircraft autopilots to the manufacture of pharmaceutical drugs, automation surrounds us. This chapter explains some of the fundamental principles of automatic process control. 29.1 Basic feedback…
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28.6 Metering pumps
A very common method for directly controlling low flow rates of fluids is to use a device known as a metering pump. A “metering pump” is a pump mechanism, motor, and drive electronics contained in a monolithic package. Simply supply 120 VAC power and a control signal to a metering pump, and it is ready to…
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28.5 Use of line reactors
Regulating the electric power sent to an electric motor is a task performed by high-speed switching transistors inside a motor drive, modulating the pulse-width of a high-frequency square wave to the motor. The high-speed switching happening inside of a motor “drive” circuit results in the drive drawing current from the AC power source as high-frequency…
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28.4 Motor drive features
Modern DC and AC motor drives provide features useful when using electric motors as final control elements. Some common features seen in both VSDs and VFDs are listed here: Speed limiting Torque limiting Torque profile curves (used to regulate the amount of torque available at different motor speeds) Acceleration (speed rate-of-change) limiting Deceleration (speed rate-of-change)…
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28.3 AC motor braking
There are several different methods useful for causing an AC induction motor to brake, or slow down: DC injection Dynamic braking Regenerative braking Plugging DC injection uses the technique of energizing the stator windings with low-current DC instead of high-current AC as is the case when the motor runs. Dynamic braking works the motor as a generator, dissipating energy…
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28.2 AC motor speed control
AC induction motors are based on the principle of a rotating magnetic field produced by a set of stationary windings (called stator windings) energized by AC power of different phases. The effect is not unlike a series of blinking “chaser” light bulbs which appear to “move” in one direction due to the blinking sequence. If sets of wire coils…
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28.1 DC motor speed control
DC electric motors generate torque by a reaction between two magnetic fields: one field established by stationary “field” windings (coils), and the other by windings in the rotating armature. Some DC motors lack field windings, substituting large permanent magnets in their place so that the stationary magnetic field is constant for all operating conditions. In…
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Chapter 28 Variable-speed Motor Controls and Drives
An alternative to control valves for adjusting fluid flow is to regulate the speed of the machine(s) motivating fluid to flow. In the case of liquid flow control, this would take the form of variable-speed pumps. In the case of gas flow control, it would mean varying the rotational speed of compressors or blowers. Flow…
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27.14 Common Control valve problems
Control valves are subject to a number of common problems. This section is dedicated to an exploration of the more common control valve problems, and potential remedies. 27.14.1 Mechanical friction Control valves are mechanical devices with moving parts, and as such they are subject to friction, primarily between the valve stem and the stem packing. Some degree…
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27.13 Control valve characterization
Control valves are supposed to deliver reliable, repeatable control of process fluid flow rate over a wide range of operating conditions. As we will soon see, this is something of a challenge, as the rate of fluid flow through a control valve depends on more than just the position of its stem. This section discusses…
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27.12 Control valve sizing
When control valves operate between fully open and fully shut, they serve much the same purpose in process systems as resistors do in electric circuits: to dissipate energy. Like resistors, the form that this dissipated energy takes is mostly heat, although some of the dissipated energy manifests in the form of vibration and noise27 . In…
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27.11 Split-range control
There are many process control applications in industry where it is desirable to have multiple control valves respond to the output of a common controller. Control valves configured to follow the command of the same controller are said to be split-ranged, or sequenced. Split-ranged control valves may take different forms of sequencing. A few different modes of…