Control Circuits
Transcription
Control Circuits
Control Circuits INC 102 Instrumentation and Process Control Assoc. Prof. Pakorn Kaewtrakulpong, Ph.D. INC, KMUTT Objectives State the purpose and general principles of control components and circuits Identify various NEC symbols which represent control components Explain a basic line diagram Give examples on how to apply the control circuit for creating control strategies. Control Elements The elements of a control circuit include all of the equipment and devices concerned with the circuit function. This includes enclosures, conductors, relays, contactors, pilot devices, and overcurrent-protection devices. Control Loads and Control Devices The control load is an electrical device that uses electrical power. E.g. Pilot lights, relays, and Contactors. Control devices are used to activate the control load. E.g. Pushbuttons and switches. Electrical Symbols Contact Switch Pushbutton Coil Pilot Light Control Transformer Timing Relays etc Contact used to indicate an open or closed path of current flow. require an action to change their state. can be Normally Open (NO) or Normally Closed (NC). Switch also used to indicate an open or closed path of current flow. require another device or action to change their state. can be Normally Open (NO) or Normally Closed (NC). Switch Selector Switch Two-position Three-position Etc. Selector Switch Pushbutton Push Button Pushbutton (contd.) 2 basic types of pushbuttons: momentary maintained (latches in place when the button is pressed.) can be Normally Open (NO) or Normally Closed (NC). Switch (contd.) Variations of this symbol are used to represent limit switches, foot switches, pressure switches, level switches, temperature-actuated switches, flow switches, and selector switches. Pilot Lights Pilot Lights Pilot Light a small electric light used to indicate a specific condition of a circuit. E.g., a red light might be used to indicate a motor is running. The letter in the center of the pilot light symbol indicates the color of the light. Signaling Column can be mounted locally on individual machines Control Relays Control Relays (contd.) Relays are widely used in control circuits. They are used for switching multiple control circuits and for controlling light loads e.g. starting coils, pilot lights, and audible alarms. Relay Operation Coil used in electromagnetic starters, contactors, and relays. The purpose of contactors and relays is to open and close associated contacts. A letter is used to designate the coil; the associated contacts have the same identifying letter. Coil (contd.) Associated contacts can be Normally Open (NO) or Normally Closed (NC). Coil Example Magnetic Contactors Basic Contactor Operation Magnetic Contactors: Interior Control Transformers Timing Relays / Timers Timing Relays Counter Solenoid Valve & Pneumatic Equipment Line Diagrams also referred to as a ladder diagrams. show the functional relationship of components and devices in an electrical circuit, not the physical relationship. made up of 2 circuits, the control circuit and the power circuit. Line Diagrams (contd.) Electrical wires in a line diagram are represented by lines. Control-circuit wiring is represented by a lighter-weight line and power-circuit wiring is represented by a heavierweight line. A small dot or node at the intersection of 2 or more wires indicates an electrical connection. Line diagrams are read from left to right. Example: Power Circuit and Control Circuit Power circuit Control circuit Connecting Loads and Control Devices Control circuits are made up of control loads and control devices. Two-Wire Control A two-wire control circuit provides low-voltage release but not low-voltage protection. low-voltage release In the event of a power loss the contactor will deenergize, stopping the motor; however, when power is restored, the motor will restart without warning if the control device is still closed. This type of control scheme is used for remote or inaccessible installations such as water-treatment plants or pumping stations. In these applications it is desirable to have an immediate return to service when power is restored. Two-Wire Control (contd.) Holding Circuit Three-Wire Control also called self-holding or self-latching circuit low-voltage protection. If an overload causes the “OL” contacts in the control circuit to open, the “M” coil is deenergized and the motor shut down. When the overload is cleared, the motor will not suddenly restart on its own. An operator must depress the “Start” button to restart the motor. Holding Circuit Three-Wire Control (contd.) Interposing a Relay A 24 VAC coil may not be strong enough to operate a large starter. E.g. the electromagnetic coil of the “M” contactor selected is rated for 460 VAC. The electromagnetic coil of the control relay (CR) selected is 24 VAC. Interposing a Relay (contd.) Schematic Diagram One-Line Diagram Wiring Diagram Some Simple Motor Control Applications Forward-Reverse Motor Control Star-Delta Motor-Starting Control Forward-Reverse Motor Control e.g. Jogging circuits Reversing-after-stop circuits Jogging Circuits using selector switch without shortcircuit protection with shortcircuit protection Reversing-After-Stop Circuits without shortcircuit protection with shortcircuit protection Star-Delta Motor-Starting Control Star-Delta Motor-Starting Control (contd.)