Permanent Magnets as an Energy Saving Feature of a 3-phase Motor Starter

Recently I pulled a small contactor from DigiKey’s inventory to showcase the features of a reversing three-phase motor starter. Naturally, I did the right thing and disassembled the contactor to determine the general construction and discover why it felt as if it were binding. The armature for most contactors is free to move, but this one was “stuck” requiring a good degree of screwdriver plunger force before it began to move (forcing the relay by pressing on the exposed portion of the armature). Once inside everything became clear as the coil mechanism contains two permanent magnets along with a several metal plates in the immediate vicinity of the coil.

This article describes the structure and purpose of the permanent magnets with an emphasis on the increased armature speed and energy saving features of the design. Before proceeding I encourage you to read the supporting preliminary article concerning flux behavior in a relay. The concept of magnetic flux operating like a constricting rubber band on a balloon is essential to understanding the interaction of the magnetic fields in this contactor.

Relays and contactors with permanent magnets have coil polarity

Let’s start by talking about coil polarity. Most relays are not concerned with the direction of current flow in the coil. However, this contactor’s coil has polarity. The positive terminal must be connected to A1 and the negative to A2. Fail in this, and the relay will not activate.

Things are even more interesting when we connect the coil up backwards and mechanically force the armature. It refused to move while the coil is activated. However, if a manual force remains, the plunger will move when the coil is deactivated. These clues strongly suggest a magnetic field interaction between the electromagnet and the permanent magnets. This reverse polarity experiment suggests that the fields work against each other to increase the speed at which the assembly moves. This is the opposite of the observation, where with reverse polarity, they hold each other. Finally, we observe that the permanent magnet is strong relative to the electromagnet at least when the coil is in the normally off position.

Tech Tip: Many relays and contactors include an integral surge suppression diode. This must be removed before attempting to apply reverse polarity to the coil.

Review of the coil assembly showing coil and the permanent magnets

The coil assembly for the Schneider DPE09BL is shown in Figure 1. This relay features a 24 VDC coil along with a soft iron assembly featuring two permanent magnets. An expanded view is included as Figure 2. This picture provides a top view of the assembly along with the side plates.

Figure 1: Picture of the contactor’s coil assembly featuring permanent magnets. Also includes sketch of the coil shown in the de energized position.

Figure 2: Expanded view of the top side of the coil assembly.

Explanation of the electro and permanent magnet interactions

The operation of this coil assembly isn’t immediately obvious. What follows is my best attempt to describe the operation. I readily admit that this could be incomplete or even missing some essential design aspect. However, I have reasonably high confidence that this information is correct as the previously described reverse polarity produces such as strong holding force. If the permanent and electromagnet hold each other so tightly when the polarity is reversed, they must repel each other with an equal force when normal polarity is applied. Your comments and corrections are most welcome.

The three panels in Figure 3 are an attempt to show the magnetic fields in each of the three relay states including off, time zero energized, and final on position. As we continue, remember that magnetic fields form closed loops. Fields prefer to travel in ferrous material as opposed to air. Finally, the fields seek the shortest path and will move materials to do so.

Figure 3: Sketch showing the interactions between the permanent and electromagnet for each of the three activation states.

From left to right we see:

  • Left: This is the relaxed (off) state of the relay. The armature is pushed up by spring force (not shown). The electromagnet is off. It is represented by a blue bar of ferrous metal. The magnetic flux loops travel up the sides, down through the deenergized coil core, across the lower plate, and then up returning to the south-pole of the magnet.

  • Middle: This panel represents the state of the relay at the moment of electromagnet activation. The electromagnet is now represented by a green bar with the assumed magnetic polarity. The armature is still held up by spring tension. Observe that the permanent and electromagnets are set in opposition. We see this as a south-to-south pole on the lower armature plate. The result is a force pushing down on the armature.

  • Right: This is the energized (on) position for the relay armature. The flux loops move up through the electromagnet, travel down the metal sides, and return to the core. This is a low energy state for the assembly as the magnetic flux has minimized the loop diameter. This was done by moving the armature thereby reducing the air gap. Observe that the permanent magnets are effectively disconnected. Due to distance, they have little influence on the armature.

Physical construction increases armature speed and saves energy

In summary, the permanent and electromagnet field interact to “kick” the armature out of its resting (deenergized) position. Once in motion, the primary flux driven by the electromagnet captures the armature. Like a constricting rubber band on a balloon, the flux minimizes the loop diameter forcing the armature to the on position (Figure 3, right).

The net result is a relay that operates faster than otherwise suggested by the associated coil size. This design should also reduce the coil size and resulting energy consumption.

As for the binding action noted at the beginning of this note, the answer is suggested in Figure 3, left. The flux from the permanent magnet tends to hold the armature in the deenergized state. An operator forcing the relay would observe tension and a sudden release when forcing the contactor. The armature would indeed feel as if it were binding. However, as demonstrated in this article that is not the case. Instead, the apparent “stickiness” is a clever design feature.

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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