Application of Puchuan Technology's Frequency Converters in Rotary Kiln Main Drives

Abstract: Variable frequency speed control technology has been widely used in industrial production due to its superior control characteristics and energy-saving effect. This paper introduces the application of Puchuan frequency converters in the main drive of rotary kilns, replacing the original DC speed control system. The advantages and energy-saving effects of the frequency converter speed control system are explained.

Keywords: Variable frequency speed control, DC speed control, rotary kiln

1. Introduction

In industries such as cement, refractory materials, and metallurgy, rotary kilns are key large-scale core equipment, and their operating status directly affects their service life, product quality, and output. Traditionally, the main drive of rotary kilns has been driven by DC motors or AC motors with hydraulic couplers for speed regulation. However, due to the drawbacks of DC speed control systems, such as high maintenance workload, high accident rate, and limited operating environment, and the advantages of variable frequency speed control (VFD) such as smooth starting, good torque control, and energy saving, the adoption of VFD systems to replace the original DC speed control systems or hydraulic couplers has become a trend in recent years. Puchuan Technology Co., Ltd.'s high-performance vector frequency converters have been widely used in the aforementioned industries, achieving excellent process requirements and energy-saving effects. This article uses the rotary kiln retrofit of a company in Wuhai as an example to illustrate the successful application of Puchuan Technology's frequency converters in the main drive.

Problems with the original speed control system of the 2nd rotary kiln

The main drive at this site originally used a DC speed control system, but after a long period of operation, many problems were found with this speed control system:

1. Poor reliability of DC motor speed control. The speed control method for DC motors is field weakening, which involves connecting a variable resistor in series with the motor's magnetic windings to reduce the current in the windings, thus reducing the magnetic field and allowing the motor to accelerate through field weakening. The biggest drawback of this method is that if the variable resistor's sliding contact fails to make contact, gets stuck, or breaks during sliding, it can cause abnormal speed control. Customers frequently encounter this problem in actual production. Furthermore, the secondary circuit wiring of DC speed control systems is complex, using various relay combinations. Some relays are not sensitive enough to small current changes in the excitation circuit. Therefore, the accuracy and reliability of the control and protection components are low, and stalling and other faults have occurred multiple times during the original startup and operation of the equipment.

2. DC power supply equipment is bulky, prone to failure, and uneconomical. DC motors require dedicated power supplies, and the original rectifier transformers, power units, and power distribution equipment require dedicated distribution rooms. These systems are large, heavy, and contain numerous components. According to on-site maintenance personnel, there are hundreds of failures and repairs annually. Troubleshooting is cumbersome, spare parts consumption is high, and the rectifier transformer itself consumes electricity. The heat generated during operation must be forcibly cooled by dedicated fans. This results in significant annual wear and tear on the components, reportedly costing customers tens of thousands of yuan annually.

Specifications and characteristics of 3 rotary kilns

The technical parameters of this rotary kiln are as follows: external dimensions: 4 meters x 60 meters; main drive motor model: Y2-355L-6; motor power: 250KW; rotational speed: 990r/min

Rotary kilns are constant torque loads with high dynamic and static resistance torques, making them suitable for heavy-load startup. Their load characteristics are:
1) Constant torque, resistance is independent of rotational speed;
2) It has a large starting torque and often needs to be started under full load conditions. Since no oil film is formed on the roller, the resistance torque is relatively large, generally 3.5 to 4 times that of normal operation.
Therefore, when selecting a frequency converter, it is required that it has stable high starting torque at low speeds and strong overload capacity.

Due to the unique load characteristics of rotary kilns, the selection of frequency converters and motors is far more complex than for other loads. Choosing an excessively high power rating, while ensuring smooth start-up and operation, increases initial investment and energy consumption, leading to long-term underutilization of the motor's capacity. Conversely, selecting power based on actual operating conditions, while efficient and requiring less investment, often results in overload during startup, preventing normal operation. Therefore, the selection of the frequency converter's power rating is crucial.

4. Points to note during inverter configuration and commissioning

Rotary kilns have high starting torque at low speeds and are considered high-inertia load production equipment. After startup, the driving energy required to maintain normal operation becomes relatively small. Therefore, using an AC frequency converter for control and properly allocating the power between startup and normal operation is crucial for rotary kiln frequency conversion control. Given this characteristic of rotary kilns, it is necessary to appropriately increase the capacity of the frequency converter when selecting one. In this case, a Puchuan Technology PI7800 280H3 frequency converter with constant torque characteristics is used to drive the motor.
1) Since the installation location of the frequency converter is about 100 meters away from the kiln head control room, in order to prevent external electromagnetic interference, current signal speed regulation is adopted. A signal transmitter is installed at the kiln head, and a 4-20mA current signal is used to control the speed regulation of the frequency converter.
2) To reduce interference and control the harmonic components of the frequency converter, input and output reactors should be installed at the input and output terminals of the frequency converter.

3) According to the general settings of the frequency converter, it starts normally when there is no material, but after feeding material for a period of time, when it stops and restarts, an overcurrent occurs, preventing it from starting. Considering that the rotary kiln only experiences high resistance during startup, but once it starts rotating, it becomes a normal constant torque load, the acceleration time is appropriately extended. The torque boosting function of the Puchuan frequency converter is applied, and the optimal curve is selected from the 61 V/F curves provided by the frequency converter. This ensures smooth startup.

Not only does it have sufficient torque, but the slope of the curve is also reduced, making the start-up smoother.

The diagram shows the main drive system of the rotary kiln before and after its modification.

5. Inverter parameter settings

6. Application Features and Usage Effects
1. The motor starts without impact and runs smoothly. When the main drive of the rotary kiln is controlled by DC speed regulation, it cannot operate at very low speeds, resulting in significant equipment vibration during startup. With AC frequency conversion speed regulation, the motor starts from zero speed, with a small initial starting torque and a smooth, gradual increase in speed, reducing the impact on the equipment, lowering the failure rate, and creating the necessary conditions for stabilizing the thermal parameters of the rotary kiln. The rotary kiln speed regulation is 0-50Hz, with a rotational speed of 0-975 r/min. During normal operation, it operates at only 700-800 r/min, with an operating frequency of approximately 40Hz, thus fully meeting the requirements of the rotary kiln's operating conditions.
2. The frequency converter has complete protection functions. If overcurrent, overload, overvoltage, phase loss and overheating occur during operation, the frequency converter can protect the motor in time and display the cause of the fault, thus shortening the maintenance time.
3. Improved power factor, resulting in significant energy savings: The power factor before the upgrade was 0.8, and after the upgrade it reached over 0.95; the annual power consumption after the upgrade was reduced by more than 40% compared to before the upgrade.

4. The frequency converter has not experienced any malfunctions in the months since it started operating, which has greatly reduced the amount of maintenance work and improved the operating rate of the rotary kiln.
5. Reduced maintenance costs: The original DC motor system cost about 20,000 yuan per year to maintain. However, after adopting frequency conversion speed regulation, due to the strong protection function and shock-free operation of Puchuan Technology's frequency converter, the service life of the mechanical equipment is extended and the maintenance-free period is long, saving 20,000 yuan in maintenance costs per year.

6. After installing variable frequency speed control, the actual output current of the frequency converter is 300-350A and the voltage is 260-280V, which is significantly more energy-saving than the original system, and the indirect economic benefits are more considerable. Therefore, it can operate stably for a long time, reduce production losses caused by downtime due to failure, and improve the yield and quality of the fired materials to a certain extent.

7 Summary
1. The rotary kiln is driven by a frequency converter and a three-phase AC motor. Since there are no faults caused by brushes and commutators as in DC motor drives, the reliability is greatly improved. It is also easier to maintain and the economic benefits are very obvious.

2. By adopting AC speed regulation, not only is the motor capacity reduced and energy-saving operation achieved, but the rotary kiln's operating rate and production capacity are also significantly improved compared to general DC devices, ensuring stable operation under fluctuating process conditions. Generally, rotary kilns have a large margin in their power drive configuration; the conventionally selected DC motor drive can be completely replaced by an AC motor of the same capacity without increasing the power.