Tiêu đề Notes (Automation Chapter-1) (1).pdf ✅

Digital instrumentation & Relay Logic Control MD.Amirul islam What is relay logic control? Relay logic control refers to a type of electrical control system that uses relays (electromechanical switches) to implement logic functions for controlling machinery or processes. In this system, the relays are used to open or close electrical circuits based on input signals, which allows the creation of logical sequences of operations. Key Components of Relay Logic: 1. Relays: Electromechanical devices that control circuits by opening or closing contacts when energized. They act as switches and can be configured to create logical conditions. 2. Contacts: Each relay has contacts that either open (break the circuit) or close (complete the circuit) depending on the relay's state. 3. Coils: The electromagnets inside relays that, when energized, change the state of the relay (open or close the contacts). 4. Inputs/Outputs: Inputs are typically switches or sensors, while outputs are devices like motors, lights, or alarms. Relay logic has largely been replaced by digital control systems like PLCs in more advanced or larger automation systems, but it is still used in simpler applications where reliability and cost are key considerations. isolator: The isolator is a mechanical switch that is used for reliably cutting off current and isolating a circuit when maintenance and repair work is going on. These switches are used in many commercial and industrial settings to create safe maintenance and repair work environments. Circuit breaker: A circuit breaker is a safety device used in electrical systems to protect the circuit from damage caused by overloads or short circuits. When the current flowing through the circuit exceeds a safe level, the circuit breaker automatically interrupts the flow of electricity, preventing potential damage to wires, devices, or other components in the system. Short circuits: A sudden flow of a very large current due to direct contact of a live wire and a neutral wire is called short-circuiting. resistance almost zero in this situation. Reasons of short circuit: 1. If phase + neutral wire are short 2. One phase + aonther phase wire are short 3. Phase + ground wire are short 4. Loose connections
Digital instrumentation & Relay Logic Control MD.Amirul islam Overload: An electric overload occurs when too much current passes through electric wires. The wires heat and can melt, with the risk of starting a fire. Overloading: When too many devices are connected to the same circuit and are operated at time. There are different types of circuit breakers, including: 1. Thermal Circuit Breakers: These respond to heat. If the current exceeds a preset threshold, the breaker trips due to the heat generated by the overload. 2. Magnetic Circuit Breakers: These use a magnetic field to trip when the current exceeds a specified limit. The stronger the current, the stronger the magnetic field generated, which triggers the breaker to open. 3. Residual Current Circuit Breakers (RCCBs): These detect imbalances between live and neutral wires, often due to leakage currents. They are commonly used for safety, preventing electrocution. MCB: MCB stands for Miniature Circuit Breaker. MCB is a small electrical device used to protect electrical circuits from overload and short circuits. Miniature circuit breakers Ratings: 6A,10A,16A, 20A, 25A, 32A, 40A, 50A, 63A Types of miniature circuit breakers (MCBs): Types Trips at current Tripping times Applications B 3-5 times the rated current flow then 0.04s -13s Commercial & residential C 5-10 times the rated current flow then 0.04s -5s Fluorescent lighting, small motors D 10-20 times the rated current flow then 0.04s -3s UPS, transformers, X-ray machines K 8-12 times rated current flow then 0.1s. very sensitive Inductive loads Types of miniature circuit breakers (MCBs) by number of poles: 1. Single pole 2. Double pole 3. Three poles 4. four poles What is breaking capacity of MCB: The breaking capacity of a Miniature Circuit Breaker (MCB) refers to the maximum fault current that the MCB can safely interrupt without being damaged. Types of Braking capacity of a MCB: Low – 6000A, Medium – 10000A & high – 25000A.
Digital instrumentation & Relay Logic Control MD.Amirul islam Energy limiting class of a MCB: The "energy-limiting class" of a Miniature Circuit Breaker (MCB) refers to the ability of the MCB to limit the energy released during a short-circuit event. MCBs are generally categorized into different classes based on their energy-limiting properties: Class 1 (Standard MCB): These MCBs do not limit energy in an efficient way and are more likely to allow a higher level of fault energy through. Class 2 (Energy-Limiting MCB): These MCBs are designed to limit the energy delivered during a fault. They provide better protection and minimize the effects of short-circuit faults on the electrical system. Class 3 (High Energy-Limiting MCB): These offer superior energy-limiting characteristics, reducing the fault energy even further. These are generally used in circuits where sensitive equipment needs additional protection. How to read nameplates of a MCB: Symbol of MCB: Overload Short Circuit