Our company's waste heat power generation circulating water cooling system uses two 75 kW cooling tower fans for circulating water cooling. The transmission part of the cooling fans adopts a traditional equipment drive structure of motor+transmission shaft+reducer, and the input shaft of the motor and reducer is connected by a steel transmission shaft.

(1) The reducer of the cooling fan is installed inside the cooling tower, and the transmission shaft is 4 meters long. Due to the extremely humid environment, the transmission shaft is severely corroded. The transmission shaft loses its dynamic balance due to uneven rust layers, causing severe equipment vibration. The transmission shaft needs to be regularly derusted and calibrated for dynamic balance, which is costly.

(2) As the operating time increases, the cooling fan reducer experiences severe wear on the input shaft oil seal, leading to oil leakage and requiring regular oil replenishment, which also pollutes the environment.

(3) The structure of the reducer design is relatively compact, with high heat generation and weak heat dissipation ability, resulting in high temperature of the reducer, short service life of oil seals and lubricants, and requiring regular maintenance.

(4) The starting method of the driving part motor is direct starting, which results in a low operating load rate of the equipment, with an efficiency of only 0.78 and high energy consumption.

The permanent magnet direct drive motor adopts variable frequency vector control technology, low-speed high torque direct drive technology, and intelligent control technology. Its structure is simplified, and it has the characteristics of high efficiency and low consumption, low-speed high torque, high reliability and maintenance free, high stiffness and fast response, and no need for lubrication. Due to its low speed and smooth operation, low vibration, it greatly reduces the daily maintenance workload and saves labor and material costs. Suitable for use in cooling fan scenarios.

Main advantages:

(1) The permanent magnet direct drive motor directly drives the load, eliminates the need for a gearbox, has a simple and reliable structure, and basically achieves maintenance free operation.

(2) Removing the reducer and transmission shaft, the system's transmission efficiency is 0.93, and it exhibits constant torque characteristics when operating below the rated frequency, meeting the requirements for heavy load starting of the equipment and resulting in more energy-efficient operation.

(3) The equipment occupies a small space and does not consume lubricating oil or spare parts for reducers.

(4) Variable frequency drive control can achieve soft start, reduce the impact on load equipment, and extend its service life.

(5) Under low load conditions, the operating frequency can be appropriately reduced to achieve better energy-saving effects.

3.1 Comparison of power factor and efficiency during load operation

Before the renovation of the cooling tower fan, the power was 75 kW, the rated current was 160 A, the maximum operating current was around 76 A, and the load rate was about 56%. The efficiency and power factor comparison between asynchronous motors and permanent magnet motors under different operating loads are shown in Figure 1.

The permanent magnet direct drive motor is directly connected to the cooling tower fan at the same impeller speed, n0=n=155 r/min (where n is the fan speed). The required shaft power is P=1.732 Uicos πη 1 η 2 η 3=1.732 × 0.38 × 76 × 0.855 × 0.86 × 0.95 × 0.96 ≈ 33.5 (kW), and the required torque is T=9 550P/n0=9 550 × 33.5 ÷ 155=2064 (N · m).

The performance parameters of the permanent magnet direct drive motor are shown in Table 1, and the modified physical product is shown in Figure 2.

3.2 Disadvantages of Permanent Magnet Motors

Permanent magnet synchronous motors generally choose rare earth permanent magnet materials (neodymium iron boron)

 as permanent magnets. Currently, rare earth materials are relatively expensive, and the initial investment cost is high. 

Rare earth permanent magnet synchronous motors may experience irreversible demagnetization due to armature

 reactions caused by surge currents or severe mechanical vibrations when the temperature is too high, leading to 

a decrease in motor performance and even rendering it unusable. So in terms of usage environment, attention 

should be paid to the formulation of motor heat dissipation schemes and measures to prevent vibration.

After using two 55 kW permanent magnet direct drive motors for the cooling tower fan motor, under the same operating conditions, the average hourly current of a single permanent magnet motor decreases by about 35 A in autumn and winter. The measured voltage during operation is about 340 V. Taking U=340 V, cos φ=0.97, and η=0.95, the power is P=1.732 × 0.34 × 35 × 0.97 × 0.95=18.9 (kW), and it is expected to save 18.9 kWh of electricity per hour. In spring and summer, the average hourly current decreases by about 28 A, and the measured voltage during operation is around 370 V. Taking U=370 V, cos φ=0.97, and η=0.95, the power is P=1.732 Uicos φ η=1.732 × 0.37 × 28 × 0.97 × 0.95=16.5 (kW), and it is expected to save 16.5 kWh of electricity per hour. According to the comparison of energy metering data before and after the renovation, a single motor can save 17 kWh of electricity per hour in autumn and winter, and 15 kWh of electricity per hour in spring and summer. Calculated based on a 70% annual operating rate, a single motor can save approximately 98000 kWh of electricity per year.

After the transformation of the cooling tower fan drive system into a permanent magnet motor direct drive system, the problems of equipment oil leakage and large vibration have been solved, the operating efficiency of the equipment has been improved, and the energy-saving effect is significant. The electricity consumption is about 15-17 kWh per hour, with a energy-saving rate of 40%. At the same time, maintenance is reduced, the system operation rate is improved, and the service life of the equipment is extended.

                                                                                                                                              (Editor Zhang Di)

                                                                                                                            Article cites cement magazine