Final Control Element
Final Control Element 1 LECTURE OUTLINE 1.The Final Control Element 2. Electric Motor 3. Relay 4. Pneumatic actuator 5. Hydraulics actuator 6. Power electronics 7. SCR Actuator Switching Principles 8. Control valve *Major part of this presentation are from available web resource from corresponding manufacturers 2 The Final Control Element The Final Control Element Carries out control actions through control mechanism produced by actuators. Actuator Actuator A peripheral which converts control signals from the controller to the moving part (mechanically) of the final control element. 3 The Final Control Element.. . Electrical motors, pneumatic actuators, hydraulic pistons, relays, comb drive, piezoelectric actuators, and electro active polymers are some examples of such actuators. 4 The Final Control Element… Motors are mostly used when circular motions are needed, but can also be used for linear applications by transforming circular to linear motion with a bolt and screw transducer. On the other hand, some actuators are intrinsically linear, such as piezoelectric actuators. 5 2. Electric Motor An electric motor converts electrical energy into kinetic energy. The reverse task, that of converting kinetic energy into electrical energy, is accomplished by a generator or dynamo. In many cases the two devices differ only in their application and minor construction details, and some applications use a single device to fill both roles. For example, traction motors used on locomotives often perform both tasks if the locomotive is equipped with dynamic brakes. 6 Motor operation Most electric motors work by electromagnetism, but motors based on other electromechanical phenomena, such as electrostatic forces and the piezoelectric effect, also exist. The fundamental principle upon which electromagnetic motors are based is that there is a mechanical force on any current-carrying wire contained within a magnetic field. The force is described by the Lorentz force law and is perpendicular to both the wire and the magnetic field. Most magnetic motors are rotary, but linear motors also exist. In a rotary motor, the rotating part (usually on the inside) is called the rotor, and the stationary part is called the stator. 7 Motor operation.. The rotor rotates because the wires and magnetic field are arranged so that a torque is developed about the rotor's axis. The motor contains electromagnets that are wound on a frame. Though this frame is often called the armature, that term is often erroneously applied. Correctly, the armature is that part of the motor across which the input voltage is supplied. Depending upon the design of the machine, either the rotor or the stator can serve as the armature. 8 9 ชนิดของมอเตอร์ สามารถแบ่ งตามหลักการทํางานได้ 2 ชนิดคือ Direct Current Motor Alternating Current Motor มอเตอร์ ไฟฟ้ ากระแสตรงแบ่ งออกเป็ น 3 ชนิดได้ แก่ 1. Series Motor 2. Shunt Motor 3. Compound Motor 10 มอเตอร์ ไฟฟ้ากระแสสลับ(Alternating Current Motor) มอเตอร์ ไฟฟ้ ากระแสสลับแบ่งออกเป็ น 2 ชนิ ดได้แก่ 1.มอเตอร์ไฟฟ้ ากระแสสลับชนิด 1 เฟส (A.C. Single Phase) - Split-Phase motor - Capacitor motor - Repulsion-type motor - Universal motor - Shaded-pole motor 2.มอเตอร์ไฟฟ้ ากระแสสลับชนิด 3 เฟส (A.C. Three phase Motor) แบ่งออกตามโครงสร้างและหลักการทํางานของมอเตอร์ ได้ 2 แบบ คือ 1. 3 Phase Induction Motor 2. 3 Phase Synchronous Motor 11 DC Motor…… 12 Wound field DC motor The permanent magnets on the outside (stator) of a DC motor may be replaced by electromagnets. By varying the field current it is possible to alter the speed/torque ratio of the motor. Typically the field winding will be placed in series (series wound) with the armature winding to get a high torque low speed motor, in parallel (shunt wound) with the armature to get a high speed low torque motor, or to have a winding partly in parallel, and partly in series (compound wound) for a balance that gives steady speed over a range of loads. Further reductions in field current are possible to gain even higher speed but correspondingly lower torque, called "weak field" operation. 13 14 Squirrel Cage rotors Most common AC motors use the squirrel cage rotor, which will be found in virtually all domestic and light industrial alternating current motors. The squirrel cage takes its name from its shape - a ring at either end of the rotor, with bars connecting the rings running the length of the rotor. It is typically cast aluminum or copper poured between the iron laminates of the rotor, and usually only the end rings will be visible. The vast majority of the rotor currents will flow through the bars rather than the higher-resistance and usually varnished laminates. Very low voltages at very high currents are typical in the bars and end rings; high efficiency motors will often use cast copper in order to reduce the resistance in the rotor. 15 Squirrel-cage rotor Three-phase wound rotor 16 17 18 19 20 21 22 23 24 Three-phase AC induction motors Three phase AC induction motors rated 1 Hp (746 W) and 25 W with small motors from CD player, toy and CD/DVD drive reader head traverse 25 The speed of the AC motor The speed of the AC motor is determined primarily by the frequency of the AC supply and the number of poles in the stator winding, according to the relation: Ns = 120F / p where Ns = Synchronous speed, in revolutions per minute F = AC power frequency p = Number of poles per phase winding 26 Stepper motors Closely related in design to three-phase AC synchronous motors are stepper motors, where an internal rotor containing permanent magnets or a large iron core with salient poles is controlled by a set of external magnets that are switched electronically. A stepper motor may also be thought of as a cross between a DC electric motor and a solenoid. As each coil is energized in turn, the rotor aligns itself with the magnetic field produced by the energized field winding. Unlike a synchronous motor, in its application, the motor may not rotate continuously; instead, it "steps" from one position to the next as field windings are energized and deenergized in sequence. Depending on the sequence, the rotor may turn forwards 27 or backwards Stepper motors…. The top electromagnet (1) is charged, attracting the topmost four teeth of a sprocket. 28 Stepper motors….. The top electromagnet (1) is turned off, and the right electromagnet (2) is charged, pulling the nearest four teeth to the right. This results in a rotation of 3.6°. 29 Stepper motors…… The bottom electromagnet (3) is charged; another 3.6° rotation occurs. 30 Stepper motors……. The left electromagnet (4) is enabled, rotating again by 3.6°. When the top electromagnet (1) is again charged, the teeth in the sprocket will have rotated by one tooth position; since there are 25 teeth, it will take 100 steps to make a full 31 rotation. Nano-motor Nanomotor constructed at UC Berkeley. The motor is about 500nm across: 300 times smaller than the diameter of a human hair 32 3. Relay A relay is an electrical switch that opens and closes under control of another electrical circuit. In the original form, the switch is operated by an electromagnet to open or close one or many sets of contacts. It was invented by Joseph Henry in 1835. Because a relay is able to control an output circuit of higher power than the input circuit, it can be considered, in a broad sense, to be a form of electrical amplifier. 33 Automotive style miniature relay, Small relay as used in electronics 34 Solid-state relay By analogy with the functions of the original electromagnetic device, a solid-state relay is made with a thyristor or other solid-state switching device. To achieve electrical isolation, a light emitting diode (LED) is used with a photo transistor. (No moving part) 35 Relay contacts Relay contacts can be either Normally Open (NO), Normally Closed (NC), or change-over contacts. Normally-open contacts connect the circuit when the relay is activated; the circuit is disconnected when the relay is inactive. It is also called Form A contact or "make" contact. Form A contact is ideal for applications that require to switch a high-current power source from a remote device. Normally-closed contacts disconnect the circuit when the relay is activated; the circuit is connected when the relay is inactive. It is also called Form B contact or "break" contact. Form B contact is ideal for applications that require the circuit to remain closed until the relay is activated. Change-over contacts control two circuits: one normally-open contact and one normally-closed contact with a common 36 terminal. It is also called Form C contact. Relay contacts.. Circuit symbols of relays. "C" denotes the common terminal in SPDT and DPDT types. 37 Relay contacts…. Since relays are switches, the terminology applied to switches is also applied to relays. According to this classification, relays can be of the following types: SPST - Single Pole Single Throw. These have two terminals which can be switched on/off. In total, four terminals when the coil is also included. 38 Relay contacts…. SPDT - Single Pole Double Throw. These have one row of three terminals. One terminal (common) switches between the other two poles. It is the same as a single change-over switch. In total, five terminals when the coil is also included. DPST - Double Pole Single Throw. These have two pairs of terminals. Equivalent to two SPST switches or relays actuated by a single coil. In total, six terminals when the coil is also included. This configuration may also be referred to as DPNO. DPDT - Double Pole Double Throw. These have two rows of change-over terminals. Equivalent to two SPDT switches or relays actuated by a single coil. In total, eight terminals 39 when the coil is also included. Relay contacts…. A DPDT AC coil relay with "ice cube" packaging 40 4. Pneumatic actuator A pneumatic actuator converts energy (in the form of compressed air, typically) into motion. The motion can be rotary or linear, depending on the type of actuator. Some types of pneumatic actuators include: Tie Rod Cylinders Compact Air Cylinders Rotary Actuators Grippers Escapement mechanisms Rod less Actuators with Magnetic linkage Rod less Actuators with Mechanical linkage Specialty actuators that combine rotary and linear motion-41 frequently used for clamping operations Vacuum Generators Pneumatic cylinder 42 5. Hydraulics actuator In the hydraulic technology we transmit and control forces and velocities by transmitting and controlling pressure and flow. The principles of hydraulic technology are not new. In the 18 Th. century in London a hydraulic press was built and the Eifeltower was adjusted by water hydraulic jacks. About 200 years BC the Greek already used machines that were driven by water hydraulics. 43 Examples of uncertainty statements 44 Variable frequency drive A Variable Frequency Drive (sometimes abbreviated VFD) is a system for controlling the rotational speed of an alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor. A variable frequency drive is a specific type of adjustable speed drive. Variable frequency drives are also known as adjustable frequency drives (AFD), variable speed drives (VSD), AC drives or inverter drives. 45 Variable frequency drive.. 46 Variable frequency drive… Operating principle Variable frequency drives operate under the principle that the synchronous speed of an AC motor is determined by the frequency of the AC supply and the number of poles in the stator winding, according to the relation: 47 Variable frequency drive…. where RPM = Revolutions per minute f = AC power frequency (Hertz) p = Number of poles (an even number) Synchronous motors operate at the synchronous speed determined by the above equation. The speed of an induction motor is slightly less than the synchronous speed. 48 Variable frequency drive….. Example A 4-pole motor that is connected directly to 60 Hz utility (mains) power would have a synchronous speed of 1800 RPM: If the motor is an induction motor, the operating speed at full load will be about 1750 RPM. If the motor is connected to a speed controller that provides power at 40 Hz, the synchronous speed would be 1200 RPM: 49 VFD system description 50 VFD Controller.. 51 Single Phase, Solid State Relays 52 Three Phase Power Controller 53 8. Control valve What Is A Control Valve? The most common final control element in the process control industries is the control valve. The control valve manipulates a flowing fluid, such as gas, steam, water, or chemical compounds, to compensate for the load disturbance and keep the regulated process variable as close as possible to the desired set point. 54 What Is A Control Valve?.. Many people who talk about control valves or valves are really referring to a control valve assembly. The control valve assembly typically consists of the valve body, the internal trim parts, an actuator to provide the motive power to Operate the valve, and a variety of additional valve accessories, which can include positioners, transducers, supply pressure regulators, manual operators, snubbers, or limit switches. 55 Control Valve Terminology Accessory: A device that is mounted on the actuator to complement the actuator’s function and make it a complete operating unit. Examples include positioners, supply pressure regulators, solenoids, and limit switches. Actuator*: A pneumatic, hydraulic, or electrically powered device that supplies force and motion to open or close a valve. 56 Control Valve Terminology.. Capacity* (Valve): The rate of flow through a valve under stated conditions. Control Range: The range of valve travel over which a control valve can maintain the installed valve gain between the normalized values of 0.5 and 2.0. Equal Percentage Characteristic*: An inherent flow characteristic that, for equal increments of rated travel, will ideally give equal percentage changes of the flow coefficient (Cv). 57 Inherent Valve Characteristics 58 Control Valve Terminology… Installed Characteristic*: The relationship between the flow rate and the closure member (disk) travel as it is moved from the closed position to rated travel as the pressure drop across the valve is influenced by the varying process conditions. I/P: Shorthand for current-to-pressure (I-to-P). Typically applied to input transducer modules. 59 Control Valve Terminology…. Positioner*: A position controller (servomechanism) that is mechanically connected to a moving part of a final control element or its actuator and that automatically adjusts its output to the actuator to maintain a desired position in proportion to the input signal. Quick Opening Characteristic*: An inherent flow characteristic in which a maximum flow coefficient is achieved with minimal closure member travel. 60 Control Valve Terminology….. Sizing (Valve): A systematic procedure designed to ensure the correct valve capacity for a set of specified process conditions. 61 Sliding-Stem Control Valve Terminology Control Valve 62 Sliding-Stem Control Valve Terminology.. 63 SlidingStem Control Valve Terminol ogy… 64 Sliding-Stem Control Valve Terminology …. 65 Characterized Cages for GlobeStyle Valve Bodies 66 Rotary-Shaft Control Valve 67 Rotary-Shaft Control Valve. 68 Rotary-Shaft Control Valve… 69 Control Valve Functions and Characteristics Terminology Fail-Closed: A condition wherein the valve closure member moves to a closed position when the actuating energy source fails. Fail-Open: A condition wherein the valve closure member moves to an open position when the actuating energy source fails. Fail-Safe: A characteristic of a valve and its actuator, which upon loss of actuating energy supply, will cause a valve closure member to be fully closed, fully open, or remain in the last position, whichever position is defined as necessary to protect the process. 70 Rotary Actuators 71 Linear Actuators 72 Valve Sizing (Finding Cv) Sizing Valves for Liquids Draft estimate only Q = Cv√(Δp/SG) Where Q : U.S. gallons per minute flow Cv : Valve capacity Δp : Pressure drop across the valve psi SG : Specific gravity of liquid unit less 73 Valve Sizing (Finding Cv).. Example, find the proper Cv for a valve that must allow 150 gal/min of ethyl alcohol with specific gravity of 0.8 at maximum pressure of 50 psi. Q = Cv√(Δp/SG) Cv = Q √(SG/Δp) = 150√(0.8/50) = 18.97 74
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