Thursday, June 10, 2010

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. This is difficult for many students to understand. 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 120 volt coil the same way, with a toggle switch controlling the relay coil. 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. 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. Now have the student operate the relay to see it control the light. Next, mention that the coil and contacts do not have to be the same voltage. Have the student wire a toggle switch and a 24 volt transformer to control a 24 volt relay coil. Then have them use the relay contacts to control a 120 volt light. 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 32-23 in Fundamentals of HVAC/R shows a simple diagram with a 24 volt contactor controlling a compressor. The figure shows that the coil in the 24 volt circuit is controlling the contacts in the 230 volt circuit. 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. Unit 29 contains good discussion and illustrations of how relays operate in 29.7 Relays and Contactors. Contactors are discussed and illustrated in 32.16 Contactors in Unit 32. Diagrams using relay and contactor circuits are discussed extensively throughout Unit 33.

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