Tutorial for the Start, Stop, and Jog Controller

APDahlen · June 2, 2024, 10:38pm

The traditional 3-wire start-stop motor control circuit may be enhanced by adding a jog feature. This preserves the convenient start-stop pushbutton features while allowing the motor to be momentarily activated using an additional jog pushbutton. This engineering brief explores several different ways to construct pushbutton-based start, stop, and jog circuits. It also includes aspects of safety as some seemingly elegant circuit solutions have unexpected (potentially dangerous) operation.

Line drawings are included for each circuit. Short videos are also included to show the circuit in operation and highlighting undesirable behaviors. Note that all experiments were conducted using DigiKey’s small industrial trainer as featured in this article as shown in Figure 1. The trainer is ideal for a learning environment, as students work with real components commonly found in an industrial setting. The 24 VDC system is generally safe and allows students to learn from their mistakes.

Figure 1: The start-stop-jog circuit is built on a Phase Dock 1010 base and features a Schneider motor starter complete with overload and auxiliary blocks. A small 24 VDC fan is used as a dummy load.

What is Jogging?

Jogging is defined as momentary activation of a load such as a motor. The jogging function is generally used for small mechanical movements to position a product or place machine into a specific mechanical position. Examples include momentary operation of a conveyor or brief operation of a horizontal rotary parts tumbler to expose the access hatch. Jogging may also be used to momentarily test the operation of a system, like a pump that requires priming.

The term jogging is synonymous with the term inching. By contrast, jogging is not the same as crawling. The distinction is determined by the available equipment. Jogging (inching) is generally conducted as a full powered activity where the motor starter applies the rail voltage directly to the motor. Crawling, also called turtle mode, is generally reserved for systems that allow partial or variable speed control. A Variable Frequency Drive (VFD) is a requirement to crawl a three-phase motor.

Tech Tip: Repeated jogging places high demands on electrical systems. This is especially true of electric motors when we consider the high startup (inrush) current associated with a full rail-voltage startup. Repeated start-stop cycles can cause the motor windings to overheat. This is mitigated by a trip of the associated motor starter’s thermal overload block. Note that most thermal overload blocks must be manually reset. This allows time for the motor to cool while the technician and operator share some “fun facts” about motor overloads. This assumes that the motor is protected by an appropriately sized and configured motor starter.

What are the behaviors of a start-stop-jog controller?

To my knowledge, the behavior of the start-stop-jog controller is not strictly defined by any standard. A literature search shows a variety of circuits. This suggests that the exact implementation is up to the individual equipment designer.
That’s unsettling, especially when we consider the importance of human factors.

While I am unqualified to provide specific recommendations, we can identify a few guiding principles. We recognize that humans are creatures of habit, we have limited attention spans, we get tired, and we are slow to recognize when things have gone wrong. With that said, we can provide a few guidelines for the ideal start-stop-jog human machine interface that will mitigate our human limitations:

There can be no unexpected operations. Stated another way, the system must always respond the same way for a given set of user inputs. As will be shown, the speed at which the user presses and releases the pushbuttons is a prime consideration.
Single function controls are preferable to multi-function controls. All things being equal, a start pushbutton should be a dedicated solely to the starting; not sometimes for jogging the machine. This is a recognition that multistate machine place additional demands on the user as they require the operator to remember what state the machine is in. This can lead to unexpected operation if the operator is distracted by other tasks.
As a three-state machine, the start-stop-jog interface must have a hierarchical precedence. The stop pushbutton must override all other inputs returning the machine to an off state. The jog pushbutton must have priority as it should break the latch of a running machine. In some ways this is a restatement of principle #1. It ensures that our overly tired operator who presses the jog button is not surprised when the machine continues to operate.
Indicator lamps should reflect the actual state of the machine. For example, a green run panel lamp should not be lit when the motor starter’s thermal overload has tripped.

These guiding principles are directly reflected in the control circuitry. In fact, it is impossible to separate circuit design from these human factors. As will be shown, the designer must consider the complex dynamic interaction of user, relay, and pushbutton as they design. We will see that even simple relay-based designs have complex interactions. In addition, the designer must consider wide variety of components and the aging of those components over the lifetime of the equipment. Ideally, we select circuits that are immune to variations in components. The ideal circuit will use common Commercial Off-The-Shelf (COTS) components and will not be adversely affected as those components age.

What are the challenges with PLC based controls?

Before we proceed, we need to recognize that relay-based control is not the same as Programmable Logic Controller (PLC)-based control. The primary difference is the dynamics of the real-world relays, contactors, and motor starters. These real-world relays have turn on and turn off times measured in milliseconds. By contrast, a PLC’s ladder logic is a memory-to-memory operation that can change state in a single program cycle as measured in microseconds.

These natural delays can cause problems for students who learned industrial controls and automation via a class focused on the PLC. The PLC based class encourages students to think about relays in a uniform and sanitized environment. Students will need to expand and rethink their expectation to match the messy realities of real-world components.

Now that we have defined guiding principles for a Human Machine Interface (HMI) and identified some of the constraints for real world relays, we can explore common implementations of the several start-stop-jog controllers. You are encouraged to physically construct each circuit to better appreciate the unique characteristics of each controller. Hands on activities are the best way to learn the material. This will lead to an intuitive understanding that will help you troubleshoot and design circuits in the future.

Solution 1: The start-stop-job controller with selector switch.

We start our exploration with the controller described in Figures 2 and 3. This start-stop-jog circuit features a selector switch to switch between the run and operating modes. While this circuit has the simplest circuitry it has the most complex operator interface.

It is complex because the circuit has a selector switch to select either jog or run. With the selector switch in the run position, the controller operates identical to the 3-wire motor starter. Pressing the green start switch will latch the circuit on. Pressing the red stop pushbutton will release the latch. If the selector switch is placed in the jog position the motor will run while the green start / jog pushbutton is held.

Based on our previous discussion, this controller is undesirable as the green pushbutton has dual functions. This can lead to mistakes if the operator is not fully aware of the controller’s mode. For example, an operator expecting a short burst may be surprised if the selector switch was in the run position.

Figure 2: Front panel for the start-stop-jog circuit featuring a mode selector switch. The green pushbutton performs double duty as start or jog depending on the user selected operating mode.

The circuit as shown in Figure 3, is similar to the conventions three-wire controller. The only difference is that the mode selector switch disables the latch while in the jog mode. Note that a dual color green/red indicator has been added. The display will be green when M1 is engaged and red in the event of a thermal trip.

Figure 3: Ladder logic for the start-stop-jog controller with a mode selector switch. When the switch is in the job position the M1 latch is disabled.

Video 1: Operation of the start-stop-jog controller with a mode selector switch.

Solution 2: Simple but unreliable and potentially dangerous start-stop-jog controller

A simple unreliable and potentially dangerous start-stop-jog controller is shown in Figures 4, 5, and video 2. It features a simple user interface featuring dedicated pushbuttons for each function including Start, Jog, and Stop.

I’m hesitant to include this circuit as it has very poor performance. Yet, it is commonly seen as you search the web. This is one of those designs that appear to work on paper but fails to account for the natural properties of real-world relays and contactors.

The reason for the unreliable operation is the opening speed of a relay or contactor. Close examination of Figure 5 reveals the problem. On lines 2 and 3 we see the normally open and normally closed contacts of the Jog pushbutton. In theory:

pressing the Jog pushbutton will disable the M1 motor starter latch via the N.C. contacts. Simultaneously the Jog pushbutton’s N.O. contacts will activate M1.
releasing the Jog pushbutton will break open the M1 coil and restore the latch.
The problem is that M1 is very slow especially when compared to the speed at which the Jog pushbutton transitions. Consequently, instead of opening the coil and restoring the latch, the mechanism simply latches an already closed M1. The result is a system that intermittently latches M1 depending on the speed at which the operator releases the Jog pushbutton as shown in Video 2. As previously stated, a system with unpredictable operation is a dangerous system.

Figure 4: Front panel for the start-stop-jog circuit featuring a jog pushbutton.

Figure 5: Ladder logic for a simple but unreliable (dangerous) start-stop-jog controller.

Video 2: Unreliable start-stop-jog controller with sensitivity to user button press speed.

Tech Tip: The circuit as shown in Figure 5 is sensitive to component selection. Pushbuttons with slightly different transition times between the N.O. and N.C. section can improve the circuit performance or make it inoperable with the motor starter latching nearly 100% of the time. Swapping the M1 motor starter for a smaller and faster control relay also improves performance, but never to the 100 % reliable point. Consequently, this circuit should not be used, especially when we consider the long life or industrial equipment and the distinct possibility that component substitutions will be performed.

Solution 3: Conventional start-stop-jog controller

The ladder logic for a conventional start-stop-jog controller is shown in Figure 6. The corresponding control panel is shown in Figure 4. Unlike the previous controller (Figure 5), this new controller is well behaved with the jog operation faithfully responding to the user input. The circuit almost meets the behaviors required for this application. There is a single discrepancy as the jog button does not break the latch for a circuit that is already running. The only other complaint is the system is slow to turn on as both the relay and motor start are involved in the latch mechanism. The start button must be pushed and held deliberately before the system with latch.
This circuit is a bit complicated especially if you approach it from a PLC ladder logic perspective. The interaction between the CR1 contacts violate the rules for PLC programming. The trick to understanding the circuit is to recognize that the latch functionality has been taken over by CR1. Recall that the previous circuit malfunctioned due to natural delays as M1 served as the motor activation mechanism and as a latch.

Start up sequence

The start up sequence is highlighted by the yellow sequence numbers shown in Figure 6. The sequence may be described as:

If not stop.
If start, activate CR1.
When CR1 is activated, the vertical CR1 contacts (between rungs 1 and 2) provides energy to the coil of M1 (line 3).

Latch sequence

If M1 and CR1 are both active, there is continuity through the stop switch and through the rung 2 M1 and CR1 contacts.
Since CR1 is already active, the coil is latched via the vertical CR1 contact.
Rung 2 also provide continuity for the M1 latch

Note that the flow of current through the vertical contacts of CR1 changes. If we assume a conventional current flow (Franklin’s direction) the starting current flows down from rung 1 to rung 2. After both M1 and CR1 are active, the CR1 holding current flows up from rung 2 to rung 1.

Breaking the latch

The latch is broken when the stop button is pressed or when M1 detects a thermal overload. Carefully observe the hold logic on rung 2. Note that both methods will open M1 and both methods will open CR1. This is highly desirable as the circuit avoids the indeterminate condition where CR1 is latched but M1 is deenergized.

Jog operation

The jog pushbutton is located on rung 3. Pressing this button will activate M1 but not CR1 as the vertical contacts between rungs 1 and 2 are open. Should M1 already be latched, the jog pushbutton is redundant and will have no effect on the circuit.

Figure 6: Ladder logic for the conventional start-stop-jog controller. The start sequence is highlighted in yellow.

Video 3: Conventional start-stop-jog controller. Note that the job pushbutton does not release the latch.

Tech Tip: Figure 6 is an aggressive circuit showcasing the advantage of relay bases systems. It is simple and reliable. It also violates many of the ladder logic rules associated with PLC programming. For example, the vertical CR1 contact connection would not be allowed in a PLC software. It’s paradoxical that many PLC textbooks dedicate a chapter to the rules of PLC programming. Presumably, this was done to break the “bad habits” of people accustomed to working with relays. Yet, I wonder if the situation hasn’t changed in the last decade, as many educators place a heavy focus on the PLC and not so much on the relays, contactors, and motor starters.

Solution 4: An improved start-stop-jog controller

Our final start-stop-jog controller is presented in Figure 7. This controller meets all the behavioral requirements identified at the start of this article. The circuit is responsive, predictable, and releasing the jog pushbutton opens the latch.

The circuit consists of a conventional latch for CR1 as shown on rungs 1 and 2 of Figure 7. Careful analysis shows that this latch may be released via the stop pushbutton, the jog pushbutton, or thermal trip of the motor starter’s overload block.

The remainder of the circuit is presented on rungs 3 and 4. Here we see that the motor starter echoes CR1 or the jog command. This circuit is unique as the motor starter is not part of the latching mechanism. Consequently, this circuit is the most responsive to user input as shown in Video 4. Instead of waiting for both CR1 and M1 as in Figure 6, the latch is initiated entirely through CR1.

Figure 7: Ladder logic for an improved start-stop-jog controller. The latch is released when the jog pushbutton is released.

The circuit as shown in Figure 7 closer to what we typically consider “proper” ladder logic. The exception is the merging of the right-hand side of the CR1 and M1 coils. This is an important consideration as an overload event will prevent M1 from operating and release the latch on CR1. Personally, I find the Figure 7 circuit easy to construct. It has a more linear layout with a single “illegal” vertical connection. This is much better than Figure 6 with it’s complex rung to rung connections. In Figure 7 the current flows in one direction while Figure 6 has at least one contact where the direction of current flow is reversed.

Video 4: An improved start-stop-jog circuit that meets all of the defined behaviors.

Tech Tip: There are conventions for the “proper” placement of elements in a ladder diagram. The overload contacts as a good example. In America, the contacts are typically placed to the right of the motor contact. Other regions may place the contacts at the beginning of the rung. Electrically they perform the same function yet, some folks will find the placement objectionable. The “proper” placement is occasionally built into the motor starter with internal connection between the contactor’s coil and the normally closed contacts. These may be internal to the assemble or jumper kit between contactor and overload block. Other motor starters may come prewired such as the Siemens unit shown in Figure 8.

Figure 8: This 14DUD32AC motor starter manufactured by Siemens comes prewired with the thermal overload block directly connected to the coil. Consequently, a thermal event will automatically open the coil.

Conclusion

This engineering brief explores a variety of relay-based start-stop-jog controllers. Armed with this information, you can now describe the controllers in terms of the user interface experience and the underlying circuitry. More importantly, you can begin to describe each controller in terms of human factors. This is a critical but important aspect of electronic design that is often overlooked. Would you agree that human factors are an unwritten language between the designer and operator? Do you agree that the implicit language baked into the user interface can have a profound impact on the safety and reliability of your plant.

You are encouraged to build each of the circuit to better understand the operation and to better appreciate how the user will interact with the controller. You are also encouraged to answer the questions and critical thinking question located at the end of this note.

Please leave your questions and comments in the space below.

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 (partially interwoven with military experience). 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 educational articles about electronics and automation.

Highlighted Experience

Leveraging his military engineering experience, Dahlen provides unique insights into rugged and reliable electronics solutions suited for extreme environments. His articles often reflect the practical, hands-on knowledge gained from his time in the U.S. Coast Guard. You can find over 75 of Dahlen’s industrial control and automation articles in this index.

Questions

Sketch the schematic of the traditional 3-state motor controller?
What is meant by the description “three-state controller”? Which state transitions have the highest priority?
What components are represented by the normally closed overload contact? Hint: It’s one of the components associated with a motor starter.
Identify some of the human limitation that would be considered by a human factors expert.
Why are real-world relays and contactors considered messy when compared to their PLC based analogs.
Why is it generally undesirable to design a control system with dual-use pushbuttons?
With respect to Figure 3:
A) Sketch the ladder logic for the latch. Identify the initiating circuit and then holding circuit.
B) What type of pushbutton is used to start the controller. What type of pushbutton is used to stop the controller. What were these pushbuttons used. Hint: For each case, consider what happens if the wire connecting to the pushbutton is broken.
C) What is meant by AUX and OVLD in the schematic? Hint: You should be able to locate the associated components in Figure 1.
D) What are the symptoms if the wire connecting to the jog pushbutton is broken?
E) Is it possible for both the red and green status lamps to be simultaneously lit?
With respect to Figure 5:
A) What operations are performed by the jog pushbutton?
B) What four actions will release the M1 latch? Hint: How is the unit powered?
C) Why is the controller considered dangerous?
With respect to Figure 6:
A) What is missing from the ideal behavior of a start-stop-jog controller?
B) Describe the sequence of events required to activate and then latch the motor starter.
C) Identify the connections that would be considered illegal in PLC based ladder logic.
D) What components are part of the holding circuit for the latch?
E) Will a thermal trip open the CR1 latch?
F) What impact does the jog pushbutton have on a circuit that is already latched on?
With respect to Figure 7:
A) Assuming CR1 is already latched, what is the relationship between the latch and the jog pushbutton?
B) Why are the CR1 and M1 coils both connected to the N.C. contacts of M1’s overload block?
C) Is M1 part of the latching circuit? Hint: Consider all aspects of a latch including the initiating circuit as well as the holding circuit.
D) What is the significance of the dashed line associated with the jog pushbutton?
E) Figure 5 has a section where the flow of current reverses depending on whether the latch is being initiated or held. Does Figure 7 share this bidirectional current property?
Why is it preferable to drive the green status lamp directly from the motor starter? Here the term directly includes the primary contactor and auxiliary contacts.
Use DigiKey search tools to locate a 22 mm industrial pushbutton assembly featuring independent N.O. and N.C. switch elements. Hint: This will likely require multiple line items.
Construct a table that ranks each of the start-stop-jog circuits in terms of the 4 desired properties as introduced in this article. Also include a comparison for ease of wiring.
Approximately 8% of the population is color blind. Modify Figure 7 to include a time delay (blinking) relay for the red indicator. For full credit use the DigiKey selection tool to locate a suitable relay.

Critical Thinking Questions

Suppose we could purchase a custom jog pushbutton for the circuit shown in Figure 5. Which contact should activate first (N.O. or N.C.). Also, what delay is required for the transitions in this highly specialized switch. Hint: What are the dynamic open and close specifications for the motor starter?
Research to determine if AC driven contactors are faster than their DC driven counterparts. Recommend you focus on 3 to 7.5 hp devices as 24 VDC contactors are generally limited to the lower horsepower applications.
Present the PLC-based ladder logic for a start-stop-job controller. For an additional challenge, describe how you would unwind Figure 6 to fit within the PLC.
Write a short essay that captures the guiding principles of an HMI for industrial control and automations equipment.
We defined several human factor considerations for our controller. What additional factors should we consider?