[Abstract] This paper mainly introduces the application of the transformation of permanent magnet clutches for air preheater motors carried out by Guodian Huangjinbu Power Generation Co., Ltd., and compares their reliability. The results show that the transformed permanent magnet clutches for air preheater motors feature high reliability, soft start, maintenance-free operation, on-site maintenance and on-line automatic switching control.
[Keywords] Air Preheater; Permanent Magnet Transmission Technology; Clutch; Automatic Switching
1 Overview
Guodian Huangjinbu Power Generation Co., Ltd. is located in Huangjinbu Town, Yugan County, Jiangxi Province. Its designed planned capacity is 4×600 MW, and the current construction involves 2×600 MW generating units. Unit #1 and Unit #2 successfully completed the 168-hour trial operation on March 15, 2007 and July 31, 2007 respectively. At present, each of the two boilers for Unit #1 and Unit #2 is equipped with two three-compartment rotary air preheaters of model 2-32-VI(T) –2400 QMR, which are manufactured by Shanghai Boiler Works Co., Ltd. The air preheaters adopt the band drive mode, and their speed reducers are driven by air motors, auxiliary motors and main motors.
An air preheater is a heat transfer surface installed in the flue duct at the tail of a boiler. It uses the flue gas passing through internal heat dissipation plates to preheat the incoming air to a certain temperature before the air enters the boiler. It serves to improve the heat exchange efficiency of the boiler and reduce energy consumption. A malfunction of the air preheater will directly cut the power generation load of the power plant by approximately half. The main and auxiliary motors of the air preheater are core equipment, so renovation work on them requires solutions with a high safety level. To ensure uninterrupted operation of the air preheater, the main motor and auxiliary motor are engaged through gears and stand by at all times. As a result, the auxiliary motor will rotate along with the running main motor. Overrunning clutches were traditionally used for separation and engagement, yet such clutches are prone to damage and require time-consuming and labor-intensive maintenance, leaving no ideal solution available so far.
2 Determination of Adjustment Methods for Permanent Magnetic Clutches
Currently, most retrofits in the market replace overrunning clutches with permanent magnetic clutches, which offer the following advantages:
1. Soft start capability;
2. Low failure rate;
3. It can perform the functions of an overrunning clutch and prevent motor runaway.
However, as power plant equipment is optimized and becomes increasingly automated, traditional manual switching can no longer meet on-site practical requirements. The solution equipped with electric actuators enables remote automatic switching with a switching time of approximately 60 seconds and supports DCS control. Nevertheless, the switching time required for the main and auxiliary motors of most air preheaters is much shorter, within 20 seconds or even 10 seconds, making electric actuators unable to satisfy the time requirement.
After extensive research, we found that Anhui Wofu Permanent Magnet Technology Co., Ltd. provides a type of permanent magnetic clutch that fully complies with the above requirements, enabling the switching between main and auxiliary motors within 10 seconds. This product features a simple structure, low failure rate, low full-lifecycle maintenance cost and high reliability. It delivers cushioned start and boasts a long service life, and can operate continuously in ambient temperatures ranging from -20°C to +60°C. Incorporating all essential retrofit features and being easy to operate, it stands as the optimal choice for air preheater retrofits. Taking all its strengths into comprehensive consideration, we have selected the permanent magnetic clutch manufactured by Anhui Wofu Permanent Magnet Technology Co., Ltd. for the retrofit project.
3 Principle of Permanent Magnetic Clutch
Permanent magnetic transmission technology is a revolutionary transmission device that transmits torque through air gaps. There is no mechanical connection between the rotating shafts of the motor and the load equipment. When the motor rotates, it drives the conductive disc to cut magnetic lines of force in the strong magnetic field generated by the disc equipped with powerful rare-earth magnets, thereby producing eddy currents in the conductive disc. These eddy currents create a reaction magnetic field on the conductive field, which drives the relative motion between the conductive disc and the magnetic disc, realizing torque transmission between the motor and the load. Different from the conventional rigid mechanical connection, the torque transmission between the motor and the load is completed via air gaps.
4 Actual Renovation Plan
The schematic diagram of the air preheater at Guodian Huangjinbu Power Plant is as follows:
There are three sets of motors, namely the pneumatic motor, auxiliary (small) motor and main (large) motor. A fluid coupling is generally used to connect the main motor and the load. The pneumatic motor is also known as the maintenance motor.
The Voofu WF-CL370S (servo motor controlled) permanent magnetic clutch is installed between the auxiliary motor and gearbox of the air preheater. The original low-speed auxiliary motors on Side A and Side B of Unit #1 and #2 are replaced with standby motors having the same power as the main motors. Welded bases for the motors are newly fabricated. The motors are remotely controlled to slide via servo motor slides. The permanent magnetic clutch consists of two discs, which are mounted on the load-side shaft and motor shaft respectively. The two discs have no direct contact and transmit torque through the air gap.
In view of the operating status and requirements of the air preheater drive unit, the on-site renovation objectives are as follows: realize automatic engagement and disengagement of the auxiliary motor when the main motor is in operation; resolve the issue of unavailable spare parts for the overrunning clutch; and enable the auxiliary motor to operate at full power. To address the above problems, the original auxiliary motor, reducer and overrunning clutch are removed, and replaced with a motor combined with a permanent magnetic clutch. Considering the higher torque demand of the auxiliary motor, its power is upgraded to 37kW.
When the main motor malfunctions, the DCS sends a signal to the servo motor of the auxiliary motor. The auxiliary motor starts to rotate (taking about 2 to 3 seconds), and then engages with the air preheater (taking about 8 to 10 seconds).
Schematic Diagram of Auxiliary Motor Renovation:
On-site renovation completed
5 Benefit Analysis after Renovation
After being retrofitted with permanent magnet clutches, the system operates stably. There have been no further instances where the air preheater failed to function due to motor damage, which enhances system reliability. In addition, the equipment is completely maintenance-free, cutting down labor costs and requiring no lubrication. It also isolates vibration transmission between the motor and the reducer, providing effective protection for the motor. The overall operation is highly safe and dependable.
6 Conclusion
After the permanent magnet technology transformation of the main and auxiliary motors for the air preheaters of Units 1 and 2 at Guodian Huangjinbu Power Plant, the equipment has been operating excellently with no defects or faults occurring over the past year. The clutch can keep the air preheater in normal working condition and enable no-load startup. This transformation requires little modification to the original system. The equipment features a simple structure, low failure rate, low subsequent maintenance costs, high reliability and long service life. Currently, Anhui Wofu is the only enterprise that applies the servo-controlled permanent magnet clutch technology, and it has achieved satisfactory operational results.
