Fundamentals of HVACR: Understanding Relays and Contactors. I promised last week to reveal my secret for teaching relays. I know I sound like a broken record, but the first step is making sure that the students have the necessary electrical foundation knowledge. Before introducing relays and contactors, students must clearly understand circuits, loads, and switches. A relay is essentially an electrically operated switch. A contactor is really just a bigger relay. The key to understanding relay operation is realizing that a relay is actually two electrical devices: a set of contacts and a coil. The contacts are a switch. They are wired in circuits exactly like switches and they behave like switches because they are switches. Relay coils are loads; specifically, solenoids. They wire in circuits like a solenoid and they behave like solenoids because they are solenoids. The purpose of the relay coil is to operate the relay contacts. Energizing or de- energizing the relay coil makes the contacts open or close, but the coil and contacts are almost always in separate circuits. 2009 – January WG I-01 Report - Understanding microprocessor-based technology applied to relaying 2 relays that introduced many changes to the industry. Despite their advantages, the new technology was competing. 7052 Control Relays and Timer Circuits (Course) In CBT 104, you will learn how to read. Confused by logic diagrams and SELOGIC control equation settings? CBT 104: Understanding SEL Relay Logic. What they see is a box with a lot of electrical connections. To demonstrate how a relay works I ask the student to wire a toggle switch to control a light. Then I ask them to wire another circuit to a relay with a 1. Make sure and reinforce the idea that the coil is a load. The student should operate the circuit and hear the relay click when the switch is closed. Have the student check the continuity across the contacts with the coil de- energized, then have them operate the relay coil and check the continuity again. Industrial Training Programs. Hands-on learning results in an understanding of the. Relay Tech Job Postings. Do you have a clue how to test specific relays? If you are using a relay with both normally open and normally closed contacts they should check both. Now have the student combine the circuit to the light they wired earlier with the relay by replacing the toggle switch controlling the light with the relay contacts. This illustrates that the contacts are just a switch. Next, mention that the coil and contacts do not have to be the same voltage. Have the student wire a toggle switch and a 2. Then have them use the relay contacts to control a 1. If you want to extend the HVAC/R component analogy you can replace the toggle switch with a thermostat, explaining that a thermostat is simply a switch. Before leaving the exercise, find a schematic diagram and show the circuit with the relay coil and the circuit with the relay contacts to illustrate that they are in separate circuits. You can point out that the labeling identifies which relay contacts are controlled by which relay coils. Figure 3. 2- 2. 3 in Fundamentals of HVAC/R shows a simple diagram with a 2. The figure shows that the coil in the 2. Building the circuits to the relay in steps helps students understand how relays operate by relating the relay coil and the relay contacts to components the student already understands. There are many resources for teaching relays and contactors in Fundamentals of HVAC/R. Contactors are discussed and illustrated in 3. Contactors in Unit 3. Diagrams using relay and contactor circuits are discussed extensively throughout Unit 3. Relay logic - Wikipedia, the free encyclopedia. Relay logic is a method of controlling industrial electronic circuits by using relays and contacts. Intelligent Protection & Control Relays. We offer Electrical and Mechanical Engineers a complete. For practicing engineers and technicians looking for a hands on element to training and protection relay understanding.Ladder logic. A relay logic circuit is an electrical network consisting of lines, or rungs, in which each line or rung must have continuity to enable the output device. A typical circuit consists of a number of rungs, with each rung controlling an output. This output is controlled by a combination of input or output conditions, such as input switches and control relays. The conditions that represent the inputs are connected in series, parallel, or series- parallel to obtain the logic required to drive the output. The relay logic circuit forms an electrical schematic diagram for the control of input and output devices. Relay logic diagrams represent the physical interconnection of devices. Each rung would have a unique identifying reference number and the individual wires on that rung would have wire numbers as a derivative of the rung number. Thus, if a rung was labelled as 1. A wire would be named for the top most rung to which it connected, even if it branched to lower rungs. When designing a system, it was common practice to skip numbers for the rungs to allow later additions as required. When the rack was manufactured, as a wire was installed, each end would be marked with wire labels (a. This also applied for pulling wire into the factory through conduit or in trays where each wire would have corresponding numbers. Wire labels were typically pieces of white tape with numbers or letters printed onto them and collected in small, pocket sized booklets. A number strip would be peeled out and wrapped around the wire near the end. Wire numbers were made up of a series of the number strips so wire 1. There are also pocket sized printers that print onto an adhesive backed label that can be wrapped around the wire. The basic format for relay logic diagrams is as follows: 1. The two vertical lines that connect all devices on the relay logic diagram are labeled L1 and L2. The space between L1 and L2 represents the voltage of the control circuit. Output devices are always connected to L2. Any electrical overloads that are to be included must be shown between the output device and L2; otherwise, the output device must be the last component before L2. Control devices are always shown between L1 and the output device. Control devices may be connected either in series or in parallel with each other. Devices which perform a STOP function are usually connected in series, while devices that perform a START function are connected in parallel. Understanding Electrical Diagrams Training. Understanding Electrical Diagrams and Control Circuits. Both of these programs are our most popular companion programs! Control Panel Optimization. Changes to Your Automation Programs Happen. Understanding Relays. Electrical devices are shown in their normal conditions. An NC contact would be shown as normally closed, and an NO contact would appear as a normally open device. All contacts associated with a device will change state when the device is energized. Figure 1 shows a typical relay logic diagram. In this circuit, a STOP/START station is used to control two pilot lights. When the START button is pressed, the control relay energizes and its associated contacts change state. The green pilot light is now ON and the red lamp is OFF. When the STOP button is pressed, the contacts return to their resting state, the red pilot light is ON, and the green switches OFF. Relay logic design. In general, the following suggestions apply to designing a relay logic diagram: 1. Define the process to be controlled. Draw a sketch of the operation process. Make sure all the components of the system are present in the drawing. Determine the sequence of operations to be performed. List the sequence of operational steps in as much detail as possible. Write out the sequence in sentences, or put them in table form. Write the relay logic diagram from the sequence of operations. Applications. This safety critical application uses interlocking to ensure conflicting routes can never be selected and helps reduce accidents. Elevators are another common application - large relay logic circuits were employed from the 1. Relay logic is also used for controlling and automation purposes in electro- hydraulics and electro- pneumatics. Systems using relay logic diagrams in other forms include the Vernam cipher machine, the many 2. Harvard Mark II. Design tools for these include Karnaugh maps and Boolean algebra.
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