Title: Control Relays
Course: Electrical Motor Controls (Sandbox) > Section: Module 4: Control Relays and Timing Relays, 20 hr

What You Need to Know About Control Relays

The following video explains what a control relay is and what it does.

Lecture Notes

Types of Relays

Relays are switches that open and close circuits electronically.

Like the motor starters you've been using in this course, a relay has a coil and one or more contacts. In fact, a contactor (which is what the motor starter on the trainer is called if you remove the overload module) is a specialized relay with contacts that are made to withstand the high currents and voltages used to run motors.

Your trainers have three relays. Two of them are control relays. They are called control relays, because they are designed for use in control circuits, like the ones you have been building. In industry, they are also called ice cube relays because of how they look.

The third relay on your trainer has a dial on the front of it. This is a timing relay, and we'll talk about that one soon.

Control Relays

Control relays open and close contacts when the coil is energized. It does this electromechanically. This means that when the coil is energized, it creates a magnetic field (electro-) which pulls a mechanical device that physically closes the contact (-mechanical).

Two common uses of control relays:

1. Interface: Relays allow you to control a circuit at one voltage with a circuit of another voltage. Like the motor starters, we control the coil of the relay with our control voltage. When we energize the control relay coil, contacts close and supply the voltage needed to run whatever we're running. A device that is used to control one power source with a different power source is called an interface.
2. Control Logic: Relays are used as logical components of our control circuits, just like buttons, switches, and sensors.

Control relays have a single coil, but different arrangements of contacts. Two of the things that vary between relays are called poles and throws.

A Single Pole (SP) relay has one contact circuit. A Double Pole (DP) relay has two contact circuits.

A Single Throw (ST) relay has normally open contacts only. A Double Throw (DT) relay has both normally open and normally closed contacts.

When specifying these relays, abbreviations are used. For example: A Single Pole, Double Throw relay is specified as a SPDT relay.

Here are the physical arrangements of control relay contacts for the different poles and throws:

Paraphrased from: Rockis, Gary J. and Mazur, Glen A.  (2009). Chapter 15. Electrical Motor Controls for Integrated Systems, 4th Edition.

Unless specified otherwise, any and all work on this page is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.