Abstract:

This article provides a detailed explanation of the working principle, circuit design, components, and practical applications of the starting, protection, and stopping circuit of a three-phase asynchronous motor. By delving into this circuit, the aim is to help electrical engineers, automation students, and related technical personnel gain a comprehensive understanding of its operation, enabling them to apply this knowledge proficiently in design, maintenance, and troubleshooting.

1. Introduction

The starting, protection, and stopping circuit of a three-phase asynchronous motor is one of the most fundamental and essential circuits in the field of industrial electrical control. Through simple yet effective circuit design, this circuit controls the motor's startup, keeps it running, and stops its operation, with additional protection functions such as overload and short-circuit protection. A deep understanding of its working principle, mastery of circuit design key points, and the ability to accurately troubleshoot and maintain the system are crucial for ensuring the smooth operation of industrial production, improving equipment reliability, and safeguarding safety.

2. Working Principle of the Starting, Protection, and Stopping Circuit of a Three-Phase Asynchronous Motor

(1) Startup Process

When the start button is pressed, the control circuit is energized. Current flows through the start button and the contactor coil, completing the circuit. When the contactor coil is energized, its main contacts close, connecting the three-phase power supply to the motor windings, causing the motor to start rotating. Simultaneously, the auxiliary normally open contacts of the contactor close, providing a self-locking function. Even if the start button is released, the current can still pass through the closed auxiliary contacts, keeping the contactor coil energized and ensuring continuous operation of the motor.

(2) Stopping Process

When the stop button is pressed, the control circuit is interrupted. The contactor coil is de-energized, and its main contacts open, disconnecting the motor from the three-phase power supply and stopping the motor. The auxiliary normally open contacts of the contactor also open, preparing the system for the next startup.

3. Components and Functions of the Starting, Protection, and Stopping Circuit

(1) Circuit Breaker

The circuit breaker is used to connect and disconnect the motor's main power supply. It provides overload and short-circuit protection. When an overload or short-circuit fault occurs in the circuit, the circuit breaker automatically trips, cutting off the power supply and protecting the motor and other electrical equipment from damage.

(2) Contactor

The contactor is one of the key components in the starting, protection, and stopping circuit. Its main contacts control the on/off operation of the motor's main circuit and can withstand large current loads. The auxiliary normally open contacts implement the self-locking function, while the auxiliary normally closed contacts can be used for other control logic or signal feedback. The contactor uses electromagnetic force to control the closing and opening of the contacts, ensuring quick and reliable operation.

(3) Thermal Overload Relay

The thermal overload relay is mainly used for motor overload protection. It monitors the motor’s current and, if the current exceeds a set value for a certain period, the bimetallic strip inside the relay bends due to heat, causing the normally closed contacts to open. This disconnects the contactor coil circuit, stopping the motor to prevent damage due to prolonged overload.

(4) Start and Stop Buttons

The start button is a normally open button that closes when pressed and opens when released, used to initiate the motor's operation. The stop button is normally closed and opens when pressed, stopping the motor's operation.

4. Applications of the Starting, Protection, and Stopping Circuit in Industry

(1) Machine Tools

In various machine tools, such as lathes, milling machines, and drilling machines, the starting, protection, and stopping circuit is used to control the operation of the spindle motor, feed motor, and others. With precise control of starting, protection, and stopping, the circuit enables various machining operations, such as cutting, feeding, and rapid movement, improving machining accuracy and production efficiency.

(2) Ventilation and Air Conditioning Systems

Ventilation fans, air conditioning compressors, and other equipment are commonly driven by three-phase asynchronous motors. The starting, protection, and stopping circuit can automatically control the motor's startup and shutdown based on parameters such as environmental temperature and humidity, adjusting ventilation and cooling/heating effects, thus providing a comfortable working and living environment.

(3) Conveying Equipment

In various material handling systems, such as belt conveyors and chain conveyors, the starting, protection, and stopping circuit controls the motor's operation, ensuring smooth material transportation. The motor's start and stop functions can be flexibly adjusted to meet different material flow requirements in production.

5. Troubleshooting and Maintenance

(1) Common Faults and Causes

· Motor fails to start: This could be caused by power supply issues, circuit breaker tripping, contactor failure, faulty start button, or broken control circuits.

· Motor stops suddenly after running: This could be caused by thermal overload relay activation, accidental stop button press, poor contact of the contactor main contacts, or motor winding short circuits.

· Abnormal motor operation, such as excessive vibration, noise, or unstable speed: Possible causes include damaged motor bearings, unbalanced three-phase power supply, single-phase operation, or overloaded motor.

(2) Troubleshooting Methods

· First, check whether the power supply is normal, including voltage stability and three-phase balance.

· Verify that the circuit breaker is in the ON position. If it has tripped, check for short-circuit or overload faults.

· Inspect the contactor's working status, including whether the coil is energized and whether the main and auxiliary contacts are in good contact. Use a multimeter for testing.

· Check whether the thermal overload relay's set value is correct and if it has been activated due to overload. If it has, identify the overload cause, eliminate the fault, and manually reset the relay.

· For motor-related faults, such as winding short circuits or bearing damage, professional motor testing equipment is required for detection and repair.

(3) Maintenance Considerations

· Regularly check the connections of electrical components to ensure there are no loose connections. Tighten any loose connections promptly.

· Clean the contact points of the contactor, circuit breaker, and other components to prevent contact failure due to dust, oil, or other contaminants.

· Periodically verify the set values of the thermal overload relay to ensure its protective function remains effective.

· Monitor the motor's operating condition, including temperature, vibration, and noise, to detect potential faults early and address them promptly.