Overload Protection Mechanism of Magnetic Starter

The magnetic starter plays a crucial role in motor control, and its overload protection mechanism is like a security guard. An in-depth exploration of this mechanism can ensure the stable operation of the motor and prevent accidents.

I. Protection by Thermal Relay
Principle Analysis:
The thermal relay operates based on the thermal effect of current. When the motor current is too large, the current flowing through the thermal element of the thermal relay generates heat, causing the bimetallic strip to bend due to heat. Due to the difference in the expansion coefficients of different metals, it pushes the actuating mechanism, and the normally closed contact opens, cutting off the control circuit of the magnetic starter and stopping the motor. For example, when the overload current reaches 1.2 times the rated value and lasts for a certain period of time, the thermal relay will operate.
Its operating characteristic curve has an inverse time characteristic. The higher the degree of overload, the shorter the operating time; when the overload is slight, the operating time is long, which can avoid malfunction during the momentary large current when the motor starts. For example, in the case of a slight overload, it may operate after several minutes, while in the case of a severe overload, it will respond within a few seconds.
Parameter Setting:
Determine the rated current of the thermal relay based on the rated current of the motor. Generally, a value slightly larger than the rated current of the motor is selected. For example, if the rated current of the motor is 10A, a thermal relay with a rated current of 11 - 12A can be selected to ensure that it will not operate during normal operation and will respond when there is an overload.
The operating current can be fine-tuned. By adjusting the knob or DIP switch on the thermal relay, the operating current can be accurately set according to the actual load characteristics and operating environment of the motor. For example, the operating current can be set to 1.05 - 1.2 times the rated current.

II. Electronic Overload Protection Module
Working Principle:
Real-time current monitoring. It uses a current transformer to collect the current signal of the motor during operation. After being converted and amplified by the signal conditioning circuit, it is input into a microprocessor or a dedicated chip for analysis and processing.
Comparison and judgment with preset values. The microprocessor compares the collected current value with the overload current threshold stored internally. Once it exceeds the threshold, it outputs a control signal according to the set logic, triggering the operation of a relay or a thyristor to cut off the power supply circuit of the magnetic starter. For example, if the preset overload current is 15A and the measured value exceeds this value, it will respond rapidly.
Integration of multiple protection functions. Besides overload protection, it can also achieve short-circuit, undervoltage, and phase loss protection, etc. In the case of a short circuit, it will cut off the circuit instantaneously; when there is undervoltage or phase loss, it will implement protection by delaying or immediately according to the set strategy, comprehensively ensuring the safety of the motor.
Advantages:
High precision and fast response. Compared with the thermal relay, the electronic module is more sensitive to current changes, and its operating time can be accurate to the millisecond level. It can quickly protect the motor at the early stage of overload and reduce equipment damage.
Flexible parameter and function setting. It can conveniently adjust the overload current threshold, operating time, protection mode, etc. through software programming or a human-machine interface to meet the needs of different motors and application scenarios. It can also enable remote communication and monitoring to achieve intelligent management.

III. Q&A Session
(I) Question: Can the thermal relay and the electronic overload protection module be used simultaneously?
Answer: Yes. The combination of the two can provide dual protection. The thermal relay serves as the basic overload protection, and the electronic module enhances the protection precision and functions, improving the reliability of the system. However, it is necessary to reasonably coordinate the parameter settings of the two to avoid conflicts.
(II) Question: How to quickly resume operation after the overload protection operates?
Answer: For the thermal relay with manual reset, it is necessary to manually check and eliminate the fault and then press the reset button. For the automatic reset type, it will automatically resume operation after the fault is eliminated, but it is necessary to confirm that the fault has been eliminated. For the electronic module, it may be necessary to manually reset the control signal or automatically reset according to the type of fault. Before resetting, it is necessary to identify and solve the cause of the overload.

IV. Overload Protection Process Steps
(I) Process of Thermal Relay Protection
Install the thermal relay and set the parameters according to the rated current of the motor.
When the motor is running, the thermal relay continuously monitors the current. When the current is too large, the heat generated by the thermal element causes the bimetallic strip to operate.
The operation of the bimetallic strip causes the normally closed contact to open, cutting off the control circuit of the magnetic starter and stopping the motor.
After the fault is eliminated, restore the normal state of the thermal relay according to the reset mode (manual or automatic).
(II) Process of Electronic Overload Protection Module
Install the electronic module and set parameters such as the overload current threshold and operating time.
The current transformer collects the current signal of the motor, which is then sent to the microprocessor for analysis after processing.
If the current exceeds the threshold, the microprocessor outputs a control signal, triggering the protection element to operate and cut off the power supply circuit.
After dealing with the fault, restore the operation according to the reset requirements of the module. It can be reset manually or automatically, and some can be remotely operated.

V. Summary
The overload protection mechanism of the magnetic starter includes the thermal relay and the electronic overload protection module. The thermal relay has a simple principle, low cost, and high reliability; the electronic module has high precision, fast response, rich functions, and is intelligent. Each has its own advantages and jointly safeguards the operation of the motor. By understanding the knowledge in the Q&A session and following the standardized protection process steps, we can better utilize the overload protection mechanism to ensure that the motor works efficiently in a safe state, extend the service life of the motor and the magnetic starter, and improve the stability and reliability of the electrical system.