How do magnetic fields work in a relay?
Many of us have been told that a relay operates by magnetic attraction. We understand that a magnetic field is developed when a current passes through the coil. This magnetic field then attracts the armature which in turn, moves the contacts.
Fair enough.
However, this model is incomplete and fails to explain the physical structures that we can readily see in relays. In this article we will explore these structures leading to a better understanding of the relay. With this improved knowledge of magnetic circuits, we can better understand advanced relays and expand our understanding to closely related topics such as transformers and motors.
What are the properties of magnetic fields?
Let’s start with two simple axioms:
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There is no such thing as a flux line. Instead of lines, we have loops which are continuous fields that can pass through air but prefer to travel through ferrous materials.
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Magnetic loops “seek” the shortest energy level possible. This has two stipulations. First, traveling within ferrous metal is preferable to air. Second, the flux loops will actively seek to shorten the path by pulling against spring tension and gravity.
With these two properties in mind, we can now improve our conceptual model of a relay by examining the physical structure.
What components are found in a relay?
A close-up picture of a typical relay is included as Figure 1 along with a simplified representation of its magnetic circuit in Figure 2. Let’s focus on the large metal structures associated with the magnetic circuit including the coil’s core, yoke, and armature. Each of these elements are constructed from ferrous material providing a low reluctance path for the magnetic loops. We find a similar structure in nearly every relay.
Figure 1: Close up picture of a relay along with its simplified magnetic circuit with emphasis on the coil, armature, and yoke.
We observe that the metals are heavy, costly, and likely difficult to manufacture when compared to materials such as plastic. We must therefore conclude that the ferrous metal is a necessary component of the relay. We should therefore incorporate this universal construction practice into our conceptual model of relay operation.
What is the role of the magnetic field in a relay?
Figure 2 presents a path for the magnetic flux for an energized but still open and for a closed relay. The driver (external force that gives rise to magnetic flux) is the coil. The strength of the field is proportional to the number of wire turns as well as the current passing through the coil. On the left, the flux loops are seen to travel from the solenoid’s core, yoke, and armature plate. They then jump the air gap arriving back to where they started.
That takes care of our first axiom: magnetic flux travels in a loop with a preference for confinement within the ferrous material. The flux freely travels within confines of core, yoke, and armature until it has no choice. The resulting and shortest path is through the armature to solenoid core air gap.
Figure 2: Picture showing the magnetic path for an initially energized but still open relay alongside an energized and closed relay.
The relay armature closes in response to magnetic loops
We can now apply the second axiom.
Like a rubber band applied to a balloon, the magnetic loops exert a force on all components contained within the loop itself. The effect makes the net circumference as small as possible. In this relay example, the armature is free to move. It will do so when the magnetic field is strong enough to overcome the spring tension. The result is shorter loop length and a corresponding reduction of the energy state as the flux is contained within the ferrous material including the core, yoke, and armature.
Looking back to Figure 1 we see that all three of these components are metal with a good degree of thickness providing a low reluctance path for the magnetic flux. It’s an essential aspect of relay design that you can no longer overlook.
Tech Tip: The concept of a lower energy state can be difficult to comprehend. Please consider our human frame of reference; we know that things fall. Objects are in their lowest energy state when they topple over and lay on the ground. Magnets seem to contradict this observation as they can “pull” up again gravity to, for example, hold a picture on your refrigerator. The magnet will stay in position apparently “working” against gravity. This contradiction is resolved when we consider flux and loop length. The magnet “falls up” because the flux loops have been shortened. We could say that the magnet loops seek the path of least reluctance. On a related note, this is why N and S poles of a magnet seek each other. The loop diameter is smaller, and the system is in a lower energy state.
Review
To say the coil attracts the armature is too simplistic.
Instead, we should say that the flux seeks to minimize the loop length. The relay components are designed to provide a low reluctance for the magnetic flux so that the magnetic flux acting like a constricting rubber band is felt at the distance between armature and coil.
Best Wishes,
APDahlen
About the Author
Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing articles such as this. LinkedIn | Aaron Dahlen - Application Engineer - DigiKey
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