A Method for Evaluating the Application of Variable Frequency Drives with Coal Mine Ventilation Fans

The adjustable-pitch setting on an axial-flow fan is the most common method of controlling airflow for primary coal mine ventilation. With this method, the fan operates at a constant speed dictated by its motor design. The angles of the blades are adjusted to change the amount of airflow and pressure to meet ventilation requirements. Typically, the fan does not operate at its optimum efficiency, which only occurs in a narrow band of air pressures and quantities. The use of variable frequency drives (VFDs), which control fan speed, provides a solution to this problem. VFDs are already used in various similar applications such as pumping and building ventilation. New technology now enables efficient VFD operation in medium voltage (2,300 – 6,900 V) fan applications. The primary benefit of a variable frequency drive is that it allows motors to operate at reduced speeds, and thus at a lower power, without a loss of torque. VFDs also allow for efficient operation over the entire life of the fan. The technical considerations of using a VFD are presented in this work, along with a method for choosing and modeling a variable speed fan to achieve maximum energy savings. As a part of this research, a spreadsheet program was developed that will calculate the optimum fan operating speed based on given fan data and specified operating conditions. A representative room and pillar coal mine is modeled to illustrate the selection and modeling process and as an example of the economic implications of using a VFD. The use of VFDs is shown to potentially yield large energy savings by increasing the fan efficiency over the life of the mine. Although there are definite power savings while using variable speed fans, the magnitude of these savings is specific to an individual mine and the operating conditions encountered. The determination of whether the use of VFDs is economically feasible requires analysis for the specific mine and its operating conditions. This work provides the background and a method for such an evaluation.