How to Select a Comprehensive Protector for Motors

The following key factors need to be considered when selecting a comprehensive protector for motors:

### I. Motor Parameters
1. **Rated Current and Voltage**
- The rated current range of the protector should match the rated current of the motor. For example, for a motor with a rated current of 100A, a protector whose rated current range includes 100A should be selected. Generally, a selection range between 80A and 120A is more appropriate. Meanwhile, the rated voltage of the protector should also be consistent with the rated voltage of the motor. Common rated voltages of motors include 380V, 660V, 1140V, etc. If the voltage doesn't match, the protector may not work properly.
2. **Power and Rotational Speed**
- The capacity of the protector should be selected according to the power of the motor. Generally speaking, the greater the power is, the greater the energy impact will be when faults such as overload and short circuit occur, and the protector needs to have corresponding capabilities to deal with them. For motors with high rotational speeds, due to factors such as centrifugal force during operation, they may be more prone to failures. Therefore, more precise settings (such as the sensitivity of overload protection) may be required in the selection of the protector's functions.

### II. Requirements for Protection Functions
1. **Overload Protection**
- The operating characteristics of overload protection need to be considered. Generally, there are two types: definite time limit and inverse time limit. Definite time limit overload protection operates after a fixed time when the motor current exceeds the set value; inverse time limit overload protection is such that the operating time is inversely proportional to the magnitude of the overload current, that is, the greater the overload current is, the shorter the operating time will be. For example, in some occasions where there are strict requirements for the motor starting time, such as the driving motor of a large conveyor belt, inverse time limit overload protection may be more suitable because it can better adapt to the large current situation when the motor starts.
2. **Short Circuit Protection**
- The quick action of short circuit protection is crucial. The protector should be able to act quickly at the moment of a short circuit to prevent the motor windings from being burned out due to excessive short circuit current. The action current of short circuit protection is generally set to several times to more than ten times the rated current of the motor. The specific multiple should be determined according to factors such as the type of motor and the starting method. For example, for a motor that is directly started, the action current of short circuit protection can be set to 8 - 10 times the rated current.
3. **Phase Loss Protection**
- When one phase of the three-phase power supply of the motor is open-circuited, the motor is in a phase loss operation state, which will lead to unbalanced motor current and serious heating of the windings. A good comprehensive protector should be able to detect the phase loss situation in a timely manner and act. Some protectors can also distinguish whether the phase loss is caused by a power supply phase loss or a broken wire in the motor's internal windings, which is very helpful for troubleshooting.
4. **Unbalance Protection**
- The three-phase currents of the motor should be basically balanced during operation. If the unbalance degree of the three-phase currents exceeds a certain limit (generally 10% - 20%), additional losses and heating will occur in the motor. The unbalance protection function of the protector can monitor the unbalance situation of the three-phase currents and act in a timely manner when the unbalance degree exceeds the set value to protect the motor.

### III. Working Environment
1. **Temperature and Humidity**
- If the motor works in a high-temperature environment, such as the hot rolling workshop of a steel mill, the comprehensive protector should also be able to adapt to high temperatures. Generally, the working temperature range of the protector is required to be between -20°C and 60°C. However, in a high-temperature environment, it may be necessary to select a protector that can withstand higher temperatures, such as a product with a working temperature range between -10°C and 70°C. For environments with high humidity, such as equipment in mines or by the seaside, protectors with a higher protection level should be selected to prevent water vapor from entering the interior and causing failures.
2. **Dust and Corrosive Gases**
- In places with a large amount of dust, such as cement plants and coal mines, the protector should have good dustproof performance. Products with a protection level of IP5X or higher can be selected. If there are corrosive gases in the environment, such as in chemical enterprises, the shell material of the protector should have anti-corrosion ability or a sealed structure should be adopted to prevent corrosive gases from eroding the internal circuits.

### IV. Communication Functions
1. **Remote Monitoring Requirements**
- In some large industrial automation systems or smart grid applications, it is necessary to remotely monitor the operating status of motors. At this time, a comprehensive protector with communication functions should be selected, such as a protector that supports communication protocols such as Modbus and Profibus. Through these communication interfaces, information such as the current, voltage, and fault status of the motor can be transmitted to the remote monitoring center, which is convenient for managers to understand the operating status of the motor in a timely manner and conduct maintenance.
2. **Integration with Control Systems**
- The protector should be able to be well integrated with the control system where the motor is located. For example, on an automated production line, the comprehensive motor protector should be able to communicate with devices such as PLC (Programmable Logic Controller) to realize interlocking control of the motor. When the protector detects a fault, it can send a signal to the PLC in a timely manner, and the PLC will execute corresponding shutdown or alarm operations.

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