Our power grid and machines rely heavily on electromechanical relays. These devices sound complex, but electromechanical relay operation is actually simple.
Back in middle school, I was first introduced to electromechanical relay operation. Our science teacher lectured about various circuit components.
He then showed how these circuit components “magically” work together. I had my eyes glued to these demonstrations.
We created various circuit control schemes, depending on how we connected parts together.
I was hooked when I saw these autonomous parts working. Seeing the real-world application of electricity and magnetism was captivating.
At the same time, it was mystifying. Now today, I’m amazed by the great impact of these simple devices.
Enough talk, let’s go over electromechanical relay operation.
What is an electromagnet?
Before we dive headfirst into relays, you need to understand what an electromagnet is.
Let’s first break down the word ‘electromagnet’:
- electro: sounds like electricity
- magnet: as you’d guess, it’s a reference to a regular magnet
So, an electromagnet is a magnet created by electricity.
Important Note: electric current through a wire creates a magnetic field.
The magnetic field generated by the wire coil is like a bar magnet. But also, we place a ferromagnetic material like iron through the center of the wire coil.
This concentrates the magnetic flux and amplifies the magnetic field.
Now, the real-world setup of an electromagnet is simple. You may have even seen an electromagnet experiment in a science class before.
Wrap wire around a big steel nail. Then connect both ends of the wire to the terminals of a battery as seen below.
The electric current then flows from the battery through the wire coil. This creates a magnetic field in the steel nail.
As a result, the steel nail then acts like a magnet. Hence again, the name ‘electromagnet’.
This nail magnet can now even be strong enough to pick up paper clips. I find this to be very cool!
But if you disconnect the wire from the battery, the paper clips will immediately fall.
Important Note: magnets on your refrigerator door are permanent magnets.
Our magnet made from a battery, nail, and wire is an electromagnet. You can switch it on and off.
These magnets run on electricity and become magnetic only when electricity is flowing.
I highly recommend you try this experiment at home. You can easily scrounge up a nail, wire, and battery.
This makes understanding these scientific concepts much more clear. In fact, I find it pretty cool how you can create an electromagnet from a bunch of spare parts.
But NEVER get the wires of an electromagnet near a house outlet. Be safe!
Real-world application of electromagnets
This simple electromagnet is the heart of electromechanical relays.
In recent decades, solid-state relays have become popular. These relays switch the current path through electronics.
We’ll discuss the switching of current paths in greater detail later.
Regardless though, electromechanical relays are still common. Below pictured is a simple electromechanical relay diagram.
In fact, I see electromechanical relays used all over California. Especially in utility facilities.
What’s more, we use electromagnets in motor designs. So, you’re around electromagnets all the time without even knowing it.
Such simple devices, yet so powerful with many real-world applications.
Electromechanical relay operation
Let’s piece everything together now. I’m going to use Schematic #1 in my explanation.
First, on the bottom rung, we have a steel core. Like our steel nail, we wrap a wire around the steel core.
So, when the current flows through this wrapped wire, the steel core becomes magnetic.
Now on the top rung, we have a steel armature that pivots from a point on the left. Then on the right end, we have a set of electrical contacts.
- N.C. contact = Normally Closed contact
- N.O. contact = Normally Open contact
In short, think of the armature like a lever with two positions.
Important Note: when the current doesn’t flow, the relay is in a ‘normal state’. This is the industry standard classification.
In Schematic #1, the steel armature connects with the N.C. contact in the normal state.
The steel armature is also connected to a spring. The spring holds the armature in place against the ‘normally closed’ contact.
This happens when the relay is in its normal state and the spring sits relaxed. In other words, the spring isn’t compressed or stretched out.
So, the spring holds its position when the current doesn’t flow.
Now, imagine current traveling through our top rung only in Schematic #1. The current will flow through the armature and N.C. contact.
Also, you can see an air gap exists between the N.C. and N.O. contacts. For this reason, the current won’t flow through the N.O. contact from the armature. Because a conducting path doesn’t exist.
Electromechanical relay with electric current flow
Using the same schematic as before, we now show the flow of current through our steel core.
The red short arrows in Schematic #2 show the direction of the current flow.
Let’s see what happens step by step in Schematic #2:
- Flowing current will turn our steel core into a magnet.
- The magnet will pull the armature down towards itself through magnetic attraction.
- Spring attached to the armature stretches out.
- The armature then connects with the N.O. contact.
As a result, a new electric path forms for the current to travel. The current will travel through the normally open contact now.
So, whenever current flows through our latch circuit, the contact will be in the N.O. state.
Important Note: the relay’s purpose is to switch the current path in a circuit. To put it simply, it’s an electrically operated switch.
Relays make electric circuits much more versatile through circuit control. In many situations, you don’t want the current to only flow in one path.
Once the current stops flowing, the spring will return the armature to its original position. The armature then once again connects with the N.C. contact.
Electromechanical relay operation for turning on lights
Let’s now go over a real-world example. An electromechanical relay used to power two separate colored lights.
- The N.C. contact has a red light in series with it
- The N.O. contact has a green light in series with it
On the left of the second rung, is a push-button. It’s a regular button that when pressed, allows current to flow through our steel core.
More specifically, when the push-button isn’t pressed, an air gap exists. This air gap prevents electricity from flowing through our wire coil.
With the push-button relaxed, the armature connects with the N.C. contact. As a result, the red light turns on, with the current flowing from our hot to neutral wire.
Without any human intervention, the red light will always remain lit as shown in Schematic #3. While the green light remains off as the current won’t flow through the N.O. contact circuit string.
Of course, as long as we provide power to the circuit.
Now, let’s press the button!
The air gap in the second rung will no longer exist, and the current will flow through our wire coil.
As we’ve discussed already, this magnetizes the steel core. The steel core then pulls down the armature.
Now, the lights swap.
Current no longer flows through the red light, but instead through the green light. Because the armature now makes contact with the N.O. contact.
Once we release the push-button, the circuit returns to its normal state. The green light turns off, and the red light switches back on.
You can see this shown in Schematic #4.
Ladder diagrams with electromechanical relays
Let’s now simplify our previous schematics.
I want to show what you’d actually see on an electrical design drawing. As you’re not going to see a steel core and armature staring back at you on a drawing.
So, we convert Schematic #5 to Schematic #6. They both have the same functionality, but we replace the symbols.
In Schematic #6, the two parallel lines on the top and middle rung are our electrical contacts.
The space between the lines signifies an air gap where the current can’t flow. We have two different contacts shown in Schematic #6.
- On the top rung: a diagonal line runs through the contact. This line represents the armature that closes the air gap when actuated. This creates an electric path for current to flow, to power the red light.
- On the middle rung: the parallel lines remain separated. This is because the armature doesn’t touch the N.O. contact when the relay is in its normal operating state. Thus, the current doesn’t flow to power the green light.
On the third rung, we show the wire around the steel core with the circle on the right. “CR” is our Control Relay.
The CR symbol is our steel core with the wire wrapped around it. And on the left of this third rung is our push-button.
The last two symbols are the red and green lights on rung #1 and #2 respectively.
Now, Schematic #6 is similar in operation to Schematic #3 that we discussed earlier. The red light will remain on as long as the push-button isn’t pressed.
Once you press the push-button, CR charges and the green light turns on while the red light turns off. You can see this in Schematic #7.
Easy way to read and follow electromechanical relay operation in schematics
Let’s go over an industry-standard control relay convention.
Control relays are always shown in their de-energized state on schematics. Let’s look at Schematic #6 again.
The normally closed CR contact in the top rung is closed when CR is de-energized. Likewise, the CR contact in the middle rung is open when CR is de-energized.
These electrical contacts only have two states:
So, switch the state of the electrical contacts when CR becomes energized. This happens when current flows through CR.
In other words, make all the N.O. contacts N.C. contacts. Then make all the N.C. contacts N.O. contacts.
This makes the following of control schemes in schematics much more simple.
All these symbols may look like jibberish, but they all serve a purpose.
Once you get used to them, you’ll be able to set up a control scheme to operate an entire facility.
These cool electromechanical relays have enabled us to lead amazingly comfortable lives today. They’ve pushed open the doors to the digital revolution.
To close out, I want to dedicate this post to Philip J. O’Keefe. His blog and helpfulness, helped me greatly when I was a young engineer. Plus, inspired this very post.
What are your thoughts on the operation of electromechanical relays? Have you ever created an electromagnet?
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Koosha started Engineer Calcs in 2020 to help people better understand the engineering and construction industry, and to discuss various science and engineering-related topics to make people think. He has been working in the engineering and tech industry in California for over 15 years now and is a licensed professional electrical engineer, and also has various entrepreneurial pursuits.
Koosha has an extensive background in the design and specification of electrical systems with areas of expertise including power generation, transmission, distribution, instrumentation and controls, and water distribution and pumping as well as alternative energy (wind, solar, geothermal, and storage).
Koosha is most interested in engineering innovations, the cosmos, our history and future, sports, and fitness.