Introduction to the 3-Wire Start-Stop Circuit

What is the 3-wire start-stop circuit?

The 3-wire start-stop circuit is typically used to remotely start and stop large industrial motors. The minimalist circuit consists of two pushbuttons and a motor starter. Although this definition is valid, the 3-wire circuit is so much more. In fact, we can argue that the principles embodied by this circuit are essential to understanding the greater field of industrial control systems. Second only to the construction of the continuous circuit, this is a circuit that all industrial control and automation students must master.

In this article, we will explore the 3-wire circuit using the Phase Dock trainer presented in Figure 1. It features the stop (red) and start (green) pushbuttons along with a small 3-phase motor starter.

Figure 1: The Phase Dock trainer holds the components for the 3-wire motor starter circuit.

Additional Information about the motor starter

This article is part of a larger work that introduces the motor starter and explores the important and at time subtle applications. Please refer to these related posts for more information:

Why is the circuit important?

It’s important because the 3-wire circuit is a common and well-known building block. It appears in industrial settings as a twin pushbutton start-stop motor starter. Perhaps more importantly, it is one of the building blocks upon which the Programmable Logic Controller’s (PLC) Ladder Logic (LL) language is built. Knowing this circuit sets the stage for follow-on work in industrial automation and control equipment.

How does the circuit work?

The wire diagram for the circuit is included as Figure 2. A close-up image of the pushbuttons is included as Figure 3. Observe that there are three general components:

  • Stop pushbutton: The red (stop) momentary pushbutton features a set of normally closed contacts. This can clearly be seen in Figure 3 with the red switch block labeled NC for normally closed.
  • Start pushbutton: The green (start) momentary pushbutton’s normally open contacts can be seen in Figure 3 with the NO label.
  • Motor Starter: The motor starter consists of three components which can be seen in figure 1. This includes the base three-phase contactor, the overload block (front), and the auxiliary contact block (top). A close inspection of Figure 1 reveals a small red and a small blue square pushbutton, labeled “stop” and “reset,” respectively. These are used to test the overload block and to reset the device if the motor has an overload event.

Figure 2: Wire diagram for the 3-wire start stop circuit developed using KiCad.

Returning to Figure 2, observe that the motor starter components are depicted in three places. The coil, labeled M1, is near the center of the diagram, the overload block is on the right, and the normally open auxiliary contact is in parallel with the start pushbutton. Also observe that rung continuity is only possible if there is no overload, and the stop pushbutton is not being pressed.

Memory is an essential component of the 3-wire circuit

This circuit features a latch — a mechanism to hold something in place. It’s a form of feedback or memory where a relay provides power to its own coil via its normally open contacts — the relays latches itself.

Coil activation

Looking back to Figure 2 we see that the coil M1 will be activated when there is continuity across the rung. Using the words of Boolean logic, we would say the coil M1 activates when “not stop, start, and not overload.” Once the coil M1 is activated the M1 auxiliary contacts (in parallel with the green start pushbutton) will close providing an alternate path for powering the M1 coil. This path remains active after the operator releases the start pushbutton.

Coil release

The M1 coil will remain active as long as there is continuity across the rung. Observe that the stop pushbutton will cause the system to deactivate. Likewise, continuity is broken if a motor overload is detected by the overload block.

Together these actions form a type of primitive memory. A single bit of information is held in the M1 contacts. The memory bit “remembers” the last pushbutton that was pressed.

Tech Tip: With every control system we need examine how it responds to unpredicted or deliberate operator misuse. We must prioritize safety and actively mitigate against mistakes. In this example, the operator may press both the start and the stop pushbuttons. Close inspection of Figure 2 reveals that the stop pushbutton has priority.

Where are the three wires?

The circuit derives its name from the number of wires passed to the switch assembly. The three wires are clearly shown in Figure 3. Using the wire designation from Figure 2:

  • Rung 1, Wire 1 connects the stop pushbutton contact to 24 VDC.
  • Rung 1, Wire 2 jumpers the stop pushbutton to the start pushbutton and to the aux normally open contact of the aux coil.
  • Rung 1, Wire 3 jumpers the start pushbutton and the motor starter aux coil (memory) to the M1 coil.

Tech Tip: In theory, it only takes three wires to remotely control the motor starter. In practice, four wires are required especially if a 120 VAC system is used. The fourth wire is the safety ground used to ground the metal section of the remote box. Without this ground, a fault within the box could expose the operator to high voltage. Be sure to follow all applicable state and federal regulations when wiring your control panels including the remote devices.

Figure 3: Close-up image showing the three wires leading to the normally open (green start) and normally closed (red stop) pushbuttons.

Tech Tip: It is possible to construct a two-wire motor control circuit. Instead of using the momentary pushbuttons, a selector switch is used. This switch is like the wall-mounted light switch in your home with one position for on and one position for off.

The two-wire control is a simple reliable circuit but can lead to confusion when the circuit malfunction. For example, consider the overload contacts on Figure 2. These will open thereby disabling the motor controller to protect the motor. With the two-wire control we have a discrepancy between the physical “on” switch position and the reality of the disabled motor. The three-wire switch does not suffer this problem as there can be no discrepancy. Another way to think about this situation is to say that the primitive software is in control of the circuit. If necessary, the software can and will change the state of the motor. As you continue you studies this distinction will become more important especially when you consider the complexity of the human machine interactions.

Conclusion

The three-wire start stop circuit is a fundamental building block for industrial control and automation system. It is a circuit that every student should be able to identify, quickly construct, and troubleshoot. In this example, we used a small motor starter as it represents the purest application. However, the operation would be the same if we had used a SPDT relay. In fact, I encourage you to construct the circuit using a variety of contactors and relays. In the process, you will certainly induce a few wiring errors. Real learning occurs as you work out the bugs. This will serve you well as you transition to PLC based circuit using ladder logic.

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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