The rotary selector switch is a common pilot device for industrial control panels. They are available in many different configurations available including on-off-on and mom-off-mom types. This engineering brief will explore these switches using a small example from the Altech catalog as shown in Figure 1.
This material is generally applicable to all switches. However, it is focused on industrial switches with block type switch contacts like the red (normally closed) and green (normally open) blocks as shown in Figure 1.
Figure 1: Excerpt from the Altech catalog describing the SaS2 series of selector switches.
Configuration of the switch contact blocks
Most industrial switches feature two slots for switch contact blocks as shown in Figure 2. These contact blocks are then configured to meet the needs of your design. In some cases, a single green Normally Open (N.O.) or a single red Normally Closed (N.C.) switch block is sufficient. In other cases, the switch blocks may be stacked with a combination of N.O. and N.C. elements. The plunger from the lower switch will activate the plunger for the upper switch.
Figure 2: A collection of Altech switch assemblies. Note that the switches may be stacked.
Relationship between switch contact blocks
This is a source of confusion as the blocks’ on and off states are dependent on the switch body type. For example, the Altech 2AS2-1 maintain off-on switch operates differently than the Altech 2AS2-4 spring-loaded mom-off-mom switch.
We can better understand this relationship by exploring the switch bodies shown in Figure 3. From left to right, we see:
- 2AML4 emergency stop
- 2AS2-1 off-on maintained selector switch
- 2AS2-3 on-off-on maintained selector switch
- 2AS2-4 mom-off-mom center spring return selector switch
The operation is best visualized by focusing on the body of the emergency stop switch. In Figure 3, we see a black ring in the lower left portion of the picture. When the emergency stop is pressed this ring moves to engage the plunger on the associated N.C. switch block. Referring back to Figure 1, we can see the plunger for each switch block. These are the mechanical elements that must be forced inward to activate the switch element.
With respect to activating the plunger, we observe that the emergency stop plunger is symmetrical and will move down in response to an operator press to activate the switch contacts. Consequently, all switch elements installed in the assembly will be simultaneously activated.
Tech Tip: All of the switches featured in this article use the same green N.O. S1 and red N.C. S2 switch contact blocks. These are sold separately from the switch bodies.
Figure 3: A collection of Altech switch bodies mounted on a Phase Dock switch plate.
Contrast the emergency stop with the two switches on the right-hand side of Figure 3. Here, we see the green colored cam. Careful analysis of the Figure 1 plunger mechanism will show that rotating the cam will activate the switch contact. However, only one switch can be activated at a time. A clockwise rotation will activate the left-side switch.
Tech Tip: The relative terms “left” and “right” can add confusion when describing a switch block. For example, the panel is Figure 3 is upside down to maintain the switch order with the emergency stop on the left. In a real control panel, up would be preserved thereby swapping left and right.
Maintained vs spring return to center
The two right-hand switches in Figure 3 appear to be identical. They differ in the internal construction of the mechanism.
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The rightmost switch is a spring return to center, also known as a momentary-off-momentary or simply a mom-off-mom switch. The internal spring will always move the cam to the resting position as shown in Figure 3. Therefore, it rests with both switch contact blocks in the non-plunged position be they green N.O. or the red N.C.
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The second switch from the right is a maintained on-off-on mechanism otherwise known as a center-off switch. Like its counterpart, only one switch block (stack) may be activated at a time. However, this switch body contains a spring and detent to hold the switch in one of three positions. There is a good amount of tension in this notch to prevent the switch from vibrating loose.
The second-from-the-left switch is also maintained. A careful inspection of Figure 3 shows that this switch has a symmetrical cam. Consequently, it will simultaneously activate both the left and right-side switch blocks (stacks).
Where are rotary switches used?
This section explores general uses for rotary selector switch in an industrial setting.
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The off-on maintained selector switch is the quintessential switch used in thousands of machines. It’s a simple and intuitive control with clockwise for on and counterclockwise for off.
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The on-off-on switch provides a classic method for transfer of control between the local control panel and a remote operator. We often see this labeled as hand-off-auto (HOA) or local-off-remote. The switch is installed on a remote-control panel or on the equipment itself. It allows an operator or technician to take control of the system and manipulate the process from the local control panel. It also allows control to be transferred to remote which often implies control using a Supervisory Control and Data Acquisition (SCADA) system.
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The mom-off-mom switch and the off-mom switches provide a convenient way to initiate a latch or to jog the system. With care, the mom-off-mom switch could be used to jog a three-phase motor. Here the term care implies local control with a trained operator or time delays to prevent instantaneous changes in motor direction.
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The momentary switches provide an excellent way to interface with a Programmable Logic Controller (PLC). For example, the mom-off-mom could be used to control the direction of a three-phase motor. From the user perspective, it’s a simple command to move the motor either forward or reverse. In the background, the PLC would take care of the timing delays to prevent the high current associated with instantaneously reversing a motor.
Tech Tip: Who and what is in control is a very important aspect of industrial control. We could argue that maintained switches are undesirable as they retain control unto themselves. For example, in the case of a power failure, upon power restoration, equipment guarded with a simple on-off switch could automatically restart. This could be very dangerous for some types of equipment.
On a related note, there are times when the equipment faults out or is remotely secured. When this happens, we have a mismatch between switch position and equipment operation. The on-off switch may be in the on position, yet the equipment may be off.
The 3-wire start stop circuit is a classic solution to this problem. The machine shouldn’t auto start, and a latch release will stop the machine. In both cases, the logic is in control of the machine not the switch.
For an added layer of protection, be sure to use a safety relay.
Tech Tip: Transferring between local and remote control can be problematic. Ideally, we want a bumpless transfer of control. This implies that the machine or process continues in its current state as control is transferred. This could be as simple as the user placing the control in the correct position or as complex as two or more PLCs coordinating their activity over a network.
Parting thoughts
The rotary switch is a foundational pilot device used to provide a satisfying tactile and natural user interface. This brief explored a few common switch configurations. In truth, we have barely scratched the surface of industrial controls. There are so many topics for us to explore.
Please leave your feedback and suggestions in the comments section.
What should we explore next?
Best wishes,
APDahlen
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About this 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.