During mine mining, as the underground roadways extend and the number of working faces in the mining area increases, the resistance of the ventilation network will also continuously increase. Furthermore, with increased production, the amount of gas emitted from the mine also increases. To adapt to these changes, the operating point of the fans should be adjusted as needed. It is well known that there are three common methods for adjusting the operating point of centrifugal fans: changing the impeller speed, adjusting the guide valve, and adjusting the gate throttling method. The gate throttling method is generally performed after the fan is put into operation, and the adjustment range is very small, so the effect is not obvious. Currently, most of the 4-72 series fans operating in mines do not have guide valves, so the operating point cannot be adjusted using guide valves. The main measure for adjusting the operating point of centrifugal fans is changing the impeller speed. General textbooks only vaguely introduce that centrifugal fans can change the impeller speed by replacing the pulley or motor to achieve the purpose of adjusting the operating point, rarely providing specific details on the methods and principles of replacing the pulley or motor. In addition, mining motors are relatively expensive to manufacture, and blindly replacing them will cause waste or economic losses to the enterprise. Therefore, it is necessary to discuss this in order to fully explore the potential of the equipment and save resources. The following example illustrates the specific process and effect of changing the fan's operating point. Before changing the impeller speed, theoretical calculations are required. 1. Determining the Impeller Speed ​​at the New Operating Point There are two methods to determine the fan impeller speed at the new operating point. One is that after network characteristic measurements in the mine, the network characteristic curve can be plotted on the individual characteristic curve of the ventilation fan, and the speed at the new operating point can be determined based on the requirements of air volume and air pressure. If the operating point is located between two speed lines, interpolation can be used to determine the speed. Secondly, if it is not possible to measure the network characteristic curve, the proportional law can be used for estimation. 2. Replacing the Pulley After determining the impeller speed, the next step should be to consider replacing the motor pulley to meet the requirements. According to transmission theory, the diameter of the motor pulley should be increased, and the diameter of the drive pulley should not be decreased. 3. Verifying Motor Power When verifying motor power, the calculation should be performed using the motor currently in use. The spare capacity can be disregarded initially. If the calculation results show that the motor is slightly overloaded, do not blindly replace the motor. Instead, analyze the motor's operating data and the error factors between the calculated data and the actual operating data. A comprehensive analysis and comparison are necessary to determine an economically reasonable solution. Example: A mine under Zhengzhou Coal Group urgently needs to increase its ventilation volume due to increased raw coal production (this mine has a single-wing production system with one exhaust fan). Based on the gas emission rate, the required ventilation volume is determined to be 65 m³/s, with a total pressure of 1.47 hPa. The relevant technical data for the mine's ventilation equipment are shown in Table 1 below. (l) Based on the above conditions, the speed is estimated using the proportional law. From n1/n2＝Q1/Q2, we get n2=n1. Q2/Q1＝627.5 r/min. Table 1 Fan model 4-72-11-NO.20B Motor model JS126-8 Impeller speed (r/min) 560 Rated voltage (V) 380 Measured air volume (m3/s) 58 Rated current (A) 210 Measured total pressure (kPa) 1.176 Rated power (kw) 110 Pulley diameter (mm) D2=800 Rated speed (r/min) 730 The speed is determined to be 630r/min. (2) Calculate the diameter of the enlarged drive pulley D=D2n2/n＝690mm (3) Verify the power of the motor. According to the adjusted parameters Q=65m3/s, total pressure 1.47kP. Given the fan efficiency η = 0.82 and transmission efficiency ηc = 0.95, the motor power is calculated as: N = HQ / (102ηηc) = 123 kW > 110 kW. Based on this calculation, the motor is overloaded and should be replaced. (4) Analysis of the actual operation of the fan According to the actual measurement: Q=58m3/s, H=1·17kPa (5) Calculate the motor power N_li = HQ/(120ηηc) = 88kw When the fan is running, the motor current I = 120A, the voltage U = 80v, the power factor cosφ = 0.9 Calculate the motor power: Nλ = kw Therefore, there is an error between the input power of the motor and the calculated power, which is √3cosφUI = 71kw The ratio of the actual input power of the motor to the calculated power is: Nλ/N_li = 71/88x100% = 81% If we consider it proportionally, the input of the motor at the new operating point is NλxN_li = 99.6kw < 110kw, so it can be seen that the original motor can meet the requirements without overloading. 5. Actual Operating Results Based on the above analysis, the mine continued to use the original electric motor, but the motor pulley was changed from 610mm to 690mm. After replacing the pulley, the fan operated well. After adjusting the operating point, the measured air volume increased by 500 m³/min, and the total pressure increased by 96 Pa, meeting the mine's needs.