Introduction
Start-stop circuits are an essential component in controlling machinery. They allow operators to start or stop a machine safely and efficiently. Understanding the basics of start-stop circuits is crucial for safe operation and diagnosing issues. Additionally, this fundamental skill enhances your career skills and serves as a stepping stone to more advanced electrical concepts.
This article will discuss the essential components of a start-stop circuit and how each component works together to create an electrical system instructing the motor or electrical equipment when to run and stop. We will also cover the electrical supply requirements, the operation of a start-stop circuit, and the different wiring configurations that can be used to control the circuit.
A Quick Overview of the Start-Stop Circuit
A start-stop circuit is an electrical circuit that controls the operation of machines or systems containing electrical motors and control circuits. It is designed to start or stop motors, ladder logic networks, electrical equipment, or components. The circuit comprises several electrical components and wiring, including contacts, relays or contactors, overloads, electrical motors, and buttons. These components work together to create an electrical circuit instructing the motor or electrical component when to run and stop. Start-stop circuits are used in various applications, such as running a conveyor belt or starting a machine with the help of buttons.
We will now discuss the different components used in a start-stop circuit. So, keep on reading to learn more!
Essential Start-Stop Circuit Components
Components in a start-stop circuit play a crucial role in controlling the power supply. In this section, we will discuss the importance of each component in the circuit:
Buttons/Contacts
Push buttons and contacts are essential components in a start-stop circuit. They are responsible for applying and breaking power to the circuit. These components initiate and halt the electrical circuit using switches or push buttons.
Relay/Contactor
Switches, relays, and contactors are devices that open or close an electrical circuit in response to a specific signal. Relays are ideal for smaller current circuits, while contactors are better suited for larger ones.
To close a circuit, contactors need to energize or activate their coils since they are normally open (NO). On the other hand, relays have two states: normally open (NO) and normally closed (NC). By energizing the relay coils, a normally open (NO) circuit becomes normally closed (NC) and vice versa. By utilizing the amplification effect, relays can control larger circuits. To activate them, a fraction of the line voltage must be applied to their coils, which allows for the management of higher-voltage systems.
Motor
Motors convert electrical energy into kinetic energy. They are used in start-stop circuits. The Start-stop control is required for conveyor belts and process machinery that need movement.
Overload
Overload relays safeguard electrical circuits from thermal, power, or electrical overload. They activate only when an overload occurs and usually begin in the normally closed position. Overload relays come with different ratings and applications, and it's crucial to verify their rating before installation to prevent damage to your motors and other accessories.
Electrical Supply Requirements for Start-Stop Circuits
Most control circuits operate at a control voltage of 24V DC. This voltage level is determined by the configuration of the circuit components and how the start-stop circuit is controlled. To control a 24V contactor coil, a start-stop circuit can be used. This will keep the motor's supply voltage separate from the control voltage, thus maintaining a low control voltage. If you have a three-phase motor, the supply is terminated into the contactor, which can be controlled by your 24V start-stop. The 24V coil, controlled by the start-stop circuit, instructs the contactor when to turn on the motor. You can connect this directly to your motor or component using higher-rated contacts. Some systems use 240V contacts that can directly control a single-phase motor.
Understanding the Operation of a Start-Stop Circuit
To understand the functionality of a start-stop circuit, let's look at a circuit that utilizes all the components we've discussed. The blue lines in the circuit diagram represent the current flow. The voltage level of the control circuit can vary from 24V to 400V or more, but 24 volts is typically used for the control side.
In the default state of the start-stop circuit shown in the image above, the relay coil is not powered as the start button has not been pressed, so no current flows through the circuit.
When the start button is pressed, current flows through the circuit and triggers the relay or contactor coil. Contactors are used for controlling motors. The contactor's coil is activated to enable motor operation, allowing current to flow through and run the motor.
When the relay or contactor coil is energized, it activates a contact that latches the circuit. This means you don't have to keep pressing the start button for the current to flow through the circuit. Once the circuit is latched, it will remain operational until the stop button is pressed or an overload fault occurs.
Press the stop button to stop the current flow and de-energize the circuit. This will cut power to the coil and release the latch, making the circuit inactive. To reactivate the circuit, press the "start" button.
Understanding Start-Stop Circuit Wiring Configurations
Controlling the wiring of a start-stop circuit can be achieved using two configurations: two-wire and three-wire control.
Two-Wire Control:
A two-wire control device equipped with contacts can turn a pilot device on or off. Smaller loads that rely on fewer connections must be operated at low current levels to avoid any damage to the circuit. Two-wire controllers are particularly useful for controlling motors or lights. When the button is released, the coil opens, and the closed contact ensures proper operation.
A pressure or limit switch is the most commonly used pilot device for a start-stop circuit. This circuit is used when it must function without any outside assistance. In a power outage, the relays turn on automatically if the switch contacts are closed. When electricity is restored, the circuit is completed. In the 2-wire control, the starter and pilot device are connected using just two wires.
Three-Wire Control:
A 3-wire circuit requires three wires to connect the starter to the pilot device. Momentary switches are used to "start" and energize the starter coils. An external stimulus, such as a brief press of a start button, is required to close the circuit. An auxiliary starter circuit should be connected in parallel to the start button to ensure power is delivered to the motor when the start button is released. In case of power loss, the circuit will be de-energized, and the start button must be briefly pressed again to close the circuit.
Alternative Start-stop Circuits
Additional circuits for starting and stopping can be designed.
Start-stop Jog Circuit
The current flows through the push button and seal-in contact when the start button is pushed. The seal-in contact manages the coil power distribution, so the current flow is not interrupted even when the button is released. The motor's coil can be de-energized in different ways. The contacts will automatically open if the motor overloads, and the coil can be de-energized by pushing the stop button. The control circuit can also be closed by placing the switch in the jog state, which will de-energize the lock. This will make the seal-in contact stop providing power to the device. To re-energize the coil, the start button must be pushed again.
Start-stop Circuits With A Motor Connected
Pressing the button activates the motor, and current flows through the circuit.
The motor will begin running as soon as an electrical current passes through the contactor coil.
When you press the start button, the circuit is complete, and electric current flows through it. The motor will begin to operate and continue running as long as the current flows. However, releasing the start button or pressing the stop button will break the circuit and stop the current flow. As a result, the motor will stop running. It's important to note that the stop button has the same effect as releasing the start button. Without power, the motor will not be able to function.
Conclusion:
Understanding the basics of a start-stop circuit is crucial for safe operation and diagnosing issues with machinery. A start-stop circuit is an electrical circuit that controls machines or systems containing electrical motors and control circuits. The start-stop circuit comprises several components: contacts/buttons, relays/contactors, motors, and overload. These components work together to create an electrical circuit instructing the motor or machines when to run and stop.
The article also discussed the importance of each component in the circuit, the electrical supply requirements, the operation of a start-stop circuit, and the different wiring configurations that can be used to control the circuit. Whether a beginner or an expert, understanding the fundamental concepts of start-stop circuits is crucial for enhancing your career skills and serving as a stepping stone to more advanced electrical concepts.
