Start Stop Circuits: A Brief Introduction Into Its Components, Working, And Control

Start Stop Circuits are fundamental components in electrical and electronic systems, playing a crucial role in controlling the operation of various devices and machines. These circuits provide a simple yet effective means of initiating and terminating the operation of electrical equipment, ensuring safety, convenience, and energy efficiency. In this comprehensive article, we will delve into the components, working principles, and control mechanisms of Start Stop Circuits, offering readers a thorough understanding of this essential technology.

Components of Start Stop Circuits

Start Stop Circuits comprise several key components that work together to achieve the desired control functionality. Let's explore these components in detail:

Pushbuttons

Pushbuttons are the primary interface between the user and the circuit. They come in two main types:

1. Start Button (Normally Open): This button initiates the operation of the controlled device.
2. Stop Button (Normally Closed): This button terminates the operation of the controlled device.

Contactors

Contactors are electromechanical switching devices that handle the main power flow to the controlled equipment. They consist of:

• Main Contacts: These carry the load current.
• Auxiliary Contacts: These are used for control and signaling purposes.
• Coil: The electromagnet that operates the contactor.

Overload Relay

The overload relay protects the circuit and the connected equipment from excessive current draw. It typically consists of:

• Bimetallic strips: These bend when heated by current flow, triggering the relay.
• Adjustable current setting: Allows customization based on the specific load requirements.

Control Transformer

In many industrial applications, a control transformer is used to:

• Step down the main supply voltage to a lower, safer level for the control circuit.
• Isolate the control circuit from the main power circuit.

Indicator Lights

Indicator lights provide visual feedback on the circuit's status:

• Green Light: Typically indicates that the circuit is running.
• Red Light: Usually signifies that the circuit is stopped or an error has occurred.

Fuses or Circuit Breakers

These protective devices safeguard the circuit against short circuits and overcurrents:

• Fuses: One-time use devices that melt when excessive current flows.
• Circuit Breakers: Reusable devices that can be reset after tripping.

Timing Relays (Optional)

In some applications, timing relays are incorporated to:

• Delay the start or stop of the circuit.
• Implement sequential operations in multi-motor systems.

Here's a table summarizing the key components and their functions:

Working Principle of Start Stop Circuits

The operation of a Start Stop Circuit follows a logical sequence of events, utilizing the interplay between its various components. Let's break down this process step by step:

1. Initial State

In the initial state:

• The main contactor is de-energized.
• The Stop button is closed (being normally closed).
• The Start button is open (being normally open).
• The overload relay contacts are closed (assuming no overload condition).

2. Starting the Circuit

When the Start button is pressed:

1. A current path is created through the control circuit.
2. The contactor coil is energized.
3. The main contacts of the contactor close, supplying power to the load.
4. Auxiliary contacts on the contactor close, creating a "seal-in" or "latching" circuit.

3. Maintaining the Run State

After the Start button is released:

• The seal-in circuit maintains current flow to the contactor coil.
• This keeps the main contacts closed and the load powered.

4. Stopping the Circuit

The circuit can be stopped in several ways:

1. Manual Stop: Pressing the Stop button breaks the control circuit, de-energizing the contactor coil.
2. Overload Trip: If an overload occurs, the overload relay contacts open, breaking the control circuit.
3. Power Loss: Any interruption in the main power supply will de-energize the contactor, stopping the circuit.

5. Reset After Stop

After a stop condition:

• The contactor de-energizes, opening its main and auxiliary contacts.
• The circuit returns to its initial state.
• If stopped due to an overload, the overload relay may need to be manually reset.

This working principle can be visualized in the following table, showing the state of key components during different phases of operation:

Control Mechanisms in Start Stop Circuits

Start Stop Circuits incorporate various control mechanisms to enhance functionality, safety, and versatility. Let's explore some of these key control features:

1. Seal-in (Latching) Control

Seal-in control ensures that the circuit remains operational after the Start button is released. This is achieved through:

• An auxiliary contact on the contactor, wired in parallel with the Start button.
• When the contactor energizes, this contact closes, maintaining the current path to the coil.

2. Jogging (Inching) Control

Jogging allows momentary operation of the equipment, useful for positioning or testing. It's implemented by:

• Bypassing the seal-in contact.
• The equipment runs only while the Start button is held down.

3. Multiple Start-Stop Stations

For large equipment or processes, multiple control stations may be required:

• Start buttons are wired in parallel.
• Stop buttons are wired in series.
• This ensures the system can be started from any station but stopped from all stations.

4. Interlocking

Interlocking prevents conflicting operations in systems with multiple motors or processes:

• It uses auxiliary contacts from one contactor to control the operation of another.
• This ensures that certain operations occur in a specific sequence or are mutually exclusive.

5. Time-Based Control

Timing relays can be incorporated to add time-based functionality:

• On-Delay: Delays the start of the equipment after the Start button is pressed.
• Off-Delay: Keeps the equipment running for a set time after the Stop button is pressed.

6. Automatic Start-Stop

In some applications, automatic control based on external conditions is required:

• Sensors or switches replace or supplement manual Start-Stop buttons.
• Examples include level controls in pumping systems or temperature controls in HVAC systems.

7. Emergency Stop

Emergency Stop (E-Stop) functionality is a critical safety feature:

• Uses a special, latching pushbutton that, when activated, immediately cuts power to the system.
• Often implemented with redundant, force-guided contacts for increased reliability.

8. Monitoring and Indication

Advanced Start Stop Circuits often include monitoring and indication features:

• Pilot lights to show system status (running, stopped, faulted).
• Meters or displays showing current draw, run time, or other parameters.

Here's a table summarizing these control mechanisms and their primary purposes:

Applications of Start Stop Circuits

Start Stop Circuits find extensive use across various industries and applications due to their simplicity, reliability, and effectiveness. Here are some key areas where these circuits play a crucial role:

1. Industrial Machinery

• Machine Tools: Lathes, milling machines, drill presses.
• Conveyor Systems: In manufacturing and logistics.
• Pumps and Compressors: In fluid handling systems.

2. HVAC Systems

• Fans and Blowers: For air circulation and ventilation.
• Chillers and Cooling Towers: In large-scale air conditioning systems.
• Heat Pumps: For both heating and cooling applications.

3. Automotive Industry

• Assembly Line Equipment: Robotic arms, welding machines.
• Paint Booths: For controlling ventilation and painting processes.
• Test Benches: For quality control and performance testing.

4. Power Generation and Distribution

• Motor-Generator Sets: For backup power systems.
• Switchgear Operations: In electrical substations.
• Renewable Energy Systems: Solar inverters, wind turbine controls.

5. Agricultural Equipment

• Irrigation Systems: Pumps and valves control.
• Grain Handling Equipment: Conveyors and elevators.
• Milking Machines: In dairy farming.

6. Building Automation

• Elevator Systems: For motor control and door operations.
• Parking Systems: Barriers and ticket machines.
• Lighting Control: In large commercial buildings.

7. Water and Wastewater Treatment

• Filtration Systems: Control of pumps and valves.
• Chemical Dosing Equipment: Precise control of treatment processes.
• Aeration Systems: In wastewater treatment plants.

8. Mining and Mineral Processing

• Crushers and Mills: For ore processing.
• Conveyor Systems: For material transport.
• Ventilation Systems: In underground mines.

9. Food and Beverage Industry

• Mixing and Blending Equipment: In food processing.
• Bottling Lines: For filling and packaging.
• Refrigeration Systems: In storage and transportation.

10. Pharmaceutical Manufacturing

• Tablet Press Machines: For medication production.
• Sterilization Equipment: In clean room environments.
• Packaging Lines: For medical supplies and drugs.

This table summarizes the applications across different industries:

Advantages and Disadvantages

Start Stop Circuits offer numerous benefits but also have some limitations. Understanding these can help in determining their suitability for specific applications.

Advantages

1. Simplicity: The straightforward design makes them easy to understand, implement, and maintain.
2. Reliability: With few components, these circuits are less prone to failure compared to more complex control systems.
3. Cost-Effectiveness: The basic components are relatively inexpensive, making them an economical choice for many applications.
4. Safety: Incorporates features like emergency stop and overload protection, enhancing operational safety.
5. Versatility: Can be easily modified or expanded to suit various control requirements.
6. Manual Override: Allows for direct human intervention when needed, which is crucial in many industrial settings.
7. Energy Efficiency: Enables easy shutdown of equipment when not in use, conserving energy.
8. Quick Response: Provides immediate control over the connected equipment.
9. Standardization: Widely used and understood across industries, facilitating easy integration and maintenance.
10. Scalability: Can be designed for both small and large-scale applications.

Disadvantages

1. Limited Complexity: Not suitable for applications requiring sophisticated control algorithms or extensive automation.
2. Wear and Tear: Mechanical components like contactors and pushbuttons can wear out over time, requiring replacement.
3. Noise: The operation of contactors can produce audible clicks, which might be undesirable in some environments.
4. Space Requirements: In applications requiring multiple circuits, the physical size of components can become a constraint.
5. EMI Generation: The switching of contactors can generate electromagnetic interference, potentially affecting sensitive equipment nearby.
6. Limited Remote Control: Basic circuits don't inherently support remote or networked control without additional components.
7. Inflexibility in Sequencing: Complex start-up or shutdown sequences can be challenging to implement without additional control devices.
8. Potential for Human Error: Reliance on manual operation can introduce the risk of operator mistakes.
9. Limited Diagnostics: Basic circuits don't provide detailed feedback or diagnostics about system status or faults.
10. Power Consumption: The continuous power draw of the contactor coil, while small, contributes to overall energy consumption.

Here's a table summarizing the key advantages and disadvantages:

Troubleshooting Common Issues

Even with their relative simplicity, Start Stop Circuits can encounter various issues. Knowing how to troubleshoot these problems is crucial for maintaining efficient operations. Here are some common issues and their potential solutions:

1. Circuit Fails to Start

Possible causes and solutions:

• Open circuit in control wiring: Check for loose connections or broken wires.
• Faulty Start button: Test and replace if necessary.
• Blown control fuse: Inspect and replace fuse, investigating the cause of the blow.
• Tripped overload relay: Check for overload condition, reset relay.
• Defective contactor coil: Test coil resistance, replace contactor if faulty.

2. Circuit Won't Stop

Potential issues and remedies:

• Welded contactor contacts: Inspect contacts, replace contactor if welded.
• Shorted Start button: Test button functionality, replace if faulty.
• Control wiring short circuit: Inspect wiring for damaged insulation or incorrect connections.

3. Contactor Chatters

Causes and fixes:

• Low control voltage: Check control transformer output, replace if faulty.
• Loose connections: Tighten all control circuit connections.
• Dirty or pitted contacts: Clean or replace contactor.
• Mechanical binding: Check for proper contactor alignment and free movement.

4. Overload Relay Trips Frequently

Possible reasons and solutions:

• Incorrect overload setting: Adjust setting to match motor full load current.
• Overloaded motor: Check for mechanical issues or excessive load on the motor.
• Single phasing: In three-phase systems, check for loss of a phase.
• Defective overload relay: Test and replace if faulty.