Why do some variable frequency motors require two junction boxes for their independent fans?
To ensure effective ventilation and heat dissipation , enclosed large motors often employ independent fans. These are called independent fans primarily because the fan cannot share a power supply with the motor. The fan must operate independently according to actual needs; that is, regardless of power control or mechanical connection, the fan maintains a completely independent relationship with the motor. Therefore, the fan must have its own independent power supply, meaning it must be equipped with an independent wiring system. Mechanically, to ensure effective heat dissipation across the entire frequency range, the fan speed is constant, and its rotating parts are completely independent of the motor.
In most cases, a wind turbine only needs to be equipped with one junction box, but why do some wind turbines need to be equipped with two junction boxes?
By carefully analyzing the motor's configuration requirements, we can understand the purpose of adding a junction box. During the operation of some variable frequency motors, depending on the control requirements of the supporting equipment, it may be necessary to collect some status parameters of the motor during operation, requiring the addition of data acquisition and monitoring devices such as encoders, temperature sensors, and overspeed switches. When the wiring of various additional devices is integrated into the independent fan, it simplifies the design and also improves the safety and reliability of the motor during stable operation. Of course, this is only one way to achieve motor status monitoring. Many motor manufacturers will concentrate the wiring of all status monitoring and control devices in a dedicated junction box, that is, add a junction box to the main body of the motor, while keeping it relatively independent from the fan.
In short, the initial purpose of a dual-junction box fan was simply to facilitate the wiring of additional devices such as encoders and overspeed switches inside the independent fan casing. Integrating the wiring of temperature protection switches and temperature and vibration sensors would not significantly change the wiring configuration but would greatly simplify the structure. Installing other additional devices would increase the axial length of the motor. Whether this would alter the overall heat dissipation effect should be evaluated beforehand based on existing test data to determine if it is necessary to increase the air pressure and air volume beyond the standard fan specifications.
Based on comprehensive test data and theoretical analysis, the allocation of independent fans for the same frame size should be based on the principle of meeting the worst-case scenario for heat dissipation, i.e., equipping the fans according to the most severe heating state of the motor windings. Which frequency band causes the motor windings to heat up most severely? Ideally , this should be verified through testing on as many typical frequency bands as possible across the entire frequency domain. Some motor manufacturers only have testing capabilities at the 50Hz power frequency, and the test results are insufficient to support estimation of the motor's full-frequency operating state. In such cases, electromagnetic calculations should be performed on as many typical frequency bands as possible across the entire frequency domain, simulating the actual operating conditions of the motor. The maximum heat generation of the motor should be estimated by comparing the test data, and the minimum guaranteed values for fan pressure and airflow should be determined.
With the development of automation and intelligence of driven equipment, the application of variable frequency motors to replace ordinary motors and energy-saving renovation projects are increasing. The independent fans that are matched with them are gradually becoming more standardized and serialized on a higher level. In particular, when the wind pressure and flow provided by the independent fan are matched with the motor airflow, the motor efficiency and temperature rise index will form a virtuous cycle of mutual improvement.
XINDA