Electromechanical Relay Operation: Step by Step

Electromechanical relay operation is made possible by several simple parts. Together, these parts enable our power grid to function.

We’ll discuss these parts, in order to gain a better understanding of how electromechanical relays operate.

What is an electromagnet?

The word ‘electromagnet’ is defined as follows:

  • electro: sounds like electricity
  • magnet: refers to a regular magnet

So, an electromagnet is a type of magnet, which requires electricity. The ‘mechanical’ portion of the relay refers to the moving parts. In the coming sections, we’ll visually see how everything relates.

Important Note: Electric current flowing through a wire creates a magnetic field. The magnetic field generated by the wire coil is similar to a bar magnet. To amplify the magnetic field, a ferromagnetic material such as iron, can be placed within the wire coil. This concentrates the magnetic flux.

To create your own electromagnet, wrap a wire around a big steel nail. Then, connect both ends of the wire to the terminals of a battery as shown below.

electromagnet schematic with battery wire nail

The electric current flows from the battery through the wire coil, which magnetizes the steel nail. The nail can now pick up paper clips.

Once you disconnect the wire from the battery though, the paper clips will immediately fall. I highly recommend you try this experiment at home to better understand the scientific concept.

Important Note: Magnets on your refrigerator are permanent magnets. Our fabricated magnet made from parts is an electromagnet, which you can switch on and off. It becomes magnetized only when electricity flows through it. 

Real-world application of electromagnets

The electromagnet is the heart of electromechanical relays. Below pictured is an example of an electromechanical relay diagram.

In recent decades, solid-state relays have become more popular. These relays switch the current path through electronics. But I still see electromechanical relays installed in all types of industrial facilities.

electromechanical relay parts
Electromechanical relay parts (Photo Credit: David Boettcher)

Electromechanical relay operation

We’ll now piece everything together, starting with Schematic #1 below.

Schematic #1

electric relay control schematic 1

On the bottom rung, is a steel core, similar to our steel nail wrapped with a wire. When the current flows through this rung, it becomes magnetized.

Next, on the top rung, there’s a steel armature, which has a pivot point on the left. On the right end, there’s a set of electrical contacts. Think of the steel armature as a lever with the following two positions:

  • N.C. contact = Normally Closed contact
  • N.O. contact = Normally Open contact

The steel armature also connects to a spring. In its relaxed state, the spring holds the armature in place against the ‘normally closed’ contact. When current flows, it travels through the armature and N.C. contact.

There’s also an air gap between the N.C. and N.O. contacts. This prevents current flow through the N.O. contact from the armature in the relaxed spring state.

Important Note: When current doesn’t flow, a relay is said to be 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. 

Electromechanical relay with electric current flow

Using the previous schematic, we now show the current flow through the steel core. The red short arrows in Schematic #2 below, indicate the direction of the current flow.

Schematic #2

electric relay control schematic 4

The following happens step by step in Schematic #2:

  1. A flowing current turns the steel core into a magnet
  2. The magnet pulls the armature down toward itself, through magnetic attraction
  3. The spring attached to the armature stretches out
  4. The armature connects with the N.O. contact

A new electric path forms, where the current will travel through the N.O. contact. Once the current stops flowing, the spring will return the armature to its original position, connecting with the N.C. contact.

Important Note: A relay’s purpose is to switch the current path in a circuit. It’s an electrically operated switch, making electric circuits more versatile through circuit control. 

Electromechanical relay operation for turning on lights

As a real-world example, we’ll see how an electromechanical relay powers two lights of different colors.

Schematic #3

electric relay control schematic 2

  • 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, there’s a push button. When pressed, current flows through the steel core. In the unpressed position, an air gap forms, preventing current flow. As a result, the armature connects with the N.C. contact, turning on the red light. You can see the current flow with the indicating red arrows, traveling from the hot to neutral wire.

Without any intervention, the red light will remain lit as shown in Schematic #3.

Once you press the button, the air gap in the second rung closes. Current flows through the wire coil, magnetizing the steel core. The steel core then pulls down the armature, causing the lights to switch. You can see the new circuit in Schematic #4 below, with the green light lit.

When you release the push button, the circuit returns to its normal state. The green light turns off, and the red light switches back on.

Schematic #4

electric relay control schematic 5

Ladder diagrams with electromechanical relays

To simplify the previous schematics, we’ll go over what you’ll find on a real design drawing. The armature and steel core were physical representations to explain the concept.

Looking below, we’ll convert the previous schematic #5 into Schematic #6.

Schematic #5

electric relay control schematic 2

Schematic #6

electric relay control schematic 8

In Schematic #6, the two parallel lines on the top and middle rung are the electrical contacts.

  • On the top rung: a diagonal line runs through the contact. This line represents the armature, which closes the air gap when actuated. This creates an electric path for current to flow, powering 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. So, the current doesn’t flow to power the green light.

On the third rung, the wire wrapped around the steel core is the ‘CR’ circle. ‘CR’ is Control Relay. Note how the contacts also have ‘CR’ labels, indicating they’re associated with the ‘CR’ control relay.

The push button remains as previously shown, and the last two symbols are the red and green lights on rungs #1 and #2 respectively.

Now, Schematic #6 operates similarly to Schematic #3, which we previously discussed. 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 and the red light turns off. Schematic #7 below illustrates this.

Schematic #7

electric relay control schematic 9

How read and follow electromechanical relay operations in schematics?

Per industry standards, control relays are shown in their de-energized state on schematics. Take a look at Schematic #6 again.

The normally closed CR contact in the top rung is closed when CR is de-energized. Similarly, the CR contact in the middle rung is open when CR is de-energized. So, electrical contacts have the two following states:

  • Open
  • Closed

Now, simply switch the state of the electrical contacts when CR becomes energized. This happens when current flows through CR. The N.O. contact becomes a N.C. contact, and the N.C. contact becomes a N.O. contact.

Electromechanical relay operation wrap-up

Once you understand electromechanical relay operation, you can create many cool designs. You can make machines operate any way you wish, without any computers.

Interestingly, this capability fueled the innovation we see today. Power grids were able to efficiently operate, leading other industries to grow.

To close out, I want to dedicate this post to Philip J. O’Keefe. His blog and helpfulness greatly assisted me when I was a young engineer and inspired this post.

What are your thoughts on the operation of electromechanical relays? Have you ever created an electromagnet?

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