I. Introduction 
For a long time, ball mills have been widely used in the grinding of ceramic raw materials for walls, floors, bricks, and chemical plants. These mills typically employ simple power-frequency control, which easily leads to over-grinding of materials, requires longer grinding cycles, results in lower grinding efficiency, consumes relatively high energy per unit of product, generates large inrush currents, imposes significant stress on both equipment and the power grid, and demands substantial maintenance efforts for mechanical equipment. Moreover, the energy losses are remarkably high. All these factors inevitably cause numerous unnecessary troubles and serious resource waste for manufacturers. Therefore, as the socio-economic environment continues to develop and enterprise production scales expand, the drawbacks of ball mills controlled directly by power-frequency have become glaringly apparent, severely hindering the rapid development of industrial enterprises.

II. Principle of Frequency Conversion Retrofit 
Currently, the commonly used drive system for ball mills is: three-phase AC motor—hydraulic coupling—gear reducer—belt-pulley reducer.

Here, the grinding mill’s cylinder serves as a pulley for the reducer. During the heavy-load start-up of the ball mill, the motor’s starting current can exceed six times its rated current. To mitigate the mechanical shock during start-up, a hydraulic coupling has been incorporated into the transmission system. The hydraulic coupling transmits the motor’s driving energy by controlling the change in the momentum moment of the working oil within its working chamber. The motor drives the input shaft of the hydraulic coupling, causing its active working wheel to rotate and accelerating the working oil. The accelerated working oil then drives the driven working turbine of the hydraulic coupling, transferring the driving energy to its output shaft and ultimately to the load. During start-up, the hydraulic coupling in the transmission system acts as a buffer, enabling the motor to start under light load conditions, thereby reducing the starting current. Once the motor has started, the load is gradually increased, ensuring smooth start-up of the ball mill and minimizing mechanical shock during the start-up phase. In operation, the hydraulic coupling’s speed-regulating function helps the ball mill find its optimal operating speed, thereby improving the mill’s efficiency. Since the ball mill is a constant-torque load, when using a hydraulic coupling for speed regulation, the speed-regulating efficiency equals the speed ratio itself; as a result, a significant portion of the energy is wasted within the hydraulic coupling. This has prompted researchers to develop smoother start-up methods with higher grinding efficiency, greater production capacity, and lower energy consumption—namely, variable-frequency drive (VFD) control.

III. The frequency conversion retrofit of ball mills has the following requirements: 
The primary purpose of using a frequency converter in a ball mill is to achieve energy savings. By leveraging the frequency converter’s multi-speed control and simple PLC functions, the ball mill’s speed can be precisely adjusted, optimizing the grinding process and thereby realizing significant energy savings—typically ranging from 10% to 15%. Additionally, the use of a frequency converter eliminates inrush currents during startup, enhancing the stability of the power grid.

3.1. During operation, the ball mill experiences significant load fluctuations, requiring the inverter to have strong overcurrent, overload, and current-limiting functions, enabling it to maintain long-term stable operation even under substantial load variations.

3.2. When starting the ball mill, the load is extremely high. Generally, the ball mill should be able to start with just a jog start, which means the inverter must provide sufficient low-frequency starting torque.

3.3. During frequency conversion retrofitting, the process requirements of the ball mill must be met to extend the service life of both the ball mill and its motor and reduce maintenance needs.

3.4. The upgraded equipment can achieve automatic control, as well as functions such as manual/frequency-based operation and automatic fault switchover. It can also overcome the voltage rebound caused by the large inertia of ball mills, thereby effectively ensuring the normal operation of the equipment.

3.5. The variable-frequency control system shall be capable of stable, fault-free operation over the long term, with good environmental adaptability and high reliability.

IV. Application Examples and Images of Frequency Conversion Retrofitting 
Based on the above analysis, our company has designed the following renovation plan:

Based on the problems existing in the original operating conditions and in combination with the requirements of the production process, the modified ball mill system should meet the following requirements.

4.1. The modified equipment has sufficient starting torque to meet the requirements of large ball mill loads, and ensures smooth motor operation under variable-frequency operation, thereby guaranteeing that the motor exhibits constant power characteristics.

4.2. By retrofitting the existing ball mill drive system with a variable-frequency speed control system, we can ensure normal operation of the ball mill at low speeds, maintain consistent process control quality, extend the service life of both the ball mill and the motor, and reduce maintenance requirements.

4.3. The upgraded equipment can achieve automatic control, as well as functions such as manual/frequency-based operation and automatic fault switchover. It can also overcome the voltage rebound caused by the large inertia of ball mills, thereby effectively ensuring the normal operation of the equipment.

4.4. Based on the site conditions, we have selected for the customer Powtran’s latest PI500 series products. The PI500 series products feature the following capabilities, which fully meet the customer’s requirements. The key features of the PI500 series inverters are as follows:

Accurate motor parameter self-learning, high speed stability accuracy, wide speed regulation range, high low-speed torque, minimal torque ripple, no downtime during momentary power failures, independent airflow design for operation in 40℃ ambient temperature, next-generation energy-saving operation, three-proof coating treatment, global safety certifications CE/TUV, built-in adaptive PID control module, user-friendly host computer software, I/O expansion capability, integrated braking unit, support for multiple communication protocols, high-protection membrane keypad, multiple installation options—including flange mounting and detachable fan—for easy installation and maintenance, plus the addition of a QUICK multi-function key and keyboard lock function option.

4.5. On-site renovation status:

4.6. Below are pictures of the on-site application retrofit:

V. Effects After Equipment Modification 
The ball mill’s drive system has been upgraded using variable-frequency speed-control technology, meeting the ball mill’s requirements for low-speed operation and high starting torque, and enabling continuous and adjustable speed control of the ball mill. During motor startup, there is no inrush current, yet the starting torque is sufficient, and protection functions have been enhanced. This ensures high-quality process control and reduces maintenance costs. The upgraded equipment now supports automatic control, as well as manual/frequency-based operation and automatic fault switchover features. Moreover, it effectively addresses the voltage rebound caused by the ball mill’s large inertia, thereby ensuring the equipment operates reliably and smoothly.