Experience Based Fast Calculation of Protective Device for Three-Phase Electric Motors
This article introduces an efficient calculation method for protective devices for three-phase electric motors based on experience. This method is suitable for calculating a new protective device under the same motor working condition. The calculation method is simple and easy to use, which is faster than the conventional theoretical calculations.
The purpose of the protective device is to protect the motor from damage or accidents by cutting off power supply when the motor has an abnormal situation, such as phase-to-phase short-circuit, single-phase opening, etc. Therefore, the first consideration in calculating the protective device is safety. The safety value of the protective device should exceed the rated current of the motor by about 50%.
The second is the protective device’s performance. The protective device of the three-phase electric motor should be able to accurately judge the abnormal conditions of the motor, and have a fast response speed and sufficient breaking current to respond quickly and reliably to abnormal voltages.
The traditional method for calculating the protective device for three-phase electric motors is to use theoretical values. This method is suitable for calculating the basic parameters of the protective device. However, the calculation time is usually too long, which can lead to an inefficiency.
Experience based fast calculation of protective device for three-phase electric motors is an effective alternative to the traditional calculation method. This experience gained from previous calculations can be used as a reference and can be used to quickly calculate the parameters of the protective device such as breaking current, making current, tripping time and contact gap. First, the basic parameters of the three-phase electric motor, such as the power, voltage, rated frequency, and winding number, are selected. Depending on the rated current and the operating frequency of the motor, the specific parameters of the protective device can be determined.
For example, assuming a three-phase electric motor with a rated power of 0.75KW and a rated current of 5A, when the power supply has a frequency of 50Hz, the specific parameters of the protective device can be determined as follows:
The breaking current of the protective device should be 8A and the current setting value can be 8.5A. The making current should be 3.5A and the current setting value can be 4A. The trip time should be 0.1s and the contact gap should be 5mm.
These parameters are general parameters based on experience and can be used to determine the parameters of the protective device. However, it should be noted that when the load factor of the motor is different from that used in the calculation, it is necessary to adjust the parameters accordingly, otherwise the accuracy of the protective device will be affected.
In conclusion, experience based fast calculation of protective device for three-phase electric motors can accurately and quickly determine the parameters of the protective device. This method is suitable for calculating a new protective device under the same motor working condition. It is simpler and faster than traditional theoretical calculations. It provides an efficient and accurate alternative for calculating the protective device.
