Wind Turbine Main Gearbox Test Stand
GE Wind Energy, Erie, PA
This test stand was designed for post-overhaul testing of the 1.5mW gearbox. It determines vibration and acoustical characteristics as well as the verifying that the output speed and load requirements are within specifications. The unit under test (UUT) is connected to an identical gearbox that runs in reverse and simulates the wind turbine torque. The entire test stand is driven and loaded by four GE traction motors and controlled by a Lab View® Data System.
From designing industrial turbine test cells to developing alternative fuel power plants, to making Wind Turbines more efficient, Emprise engineers are respected for combining the best in electrical and mechanical design to develop cost effective solutions. Emprise works with manufacturers of utility power plants, such as gas turbines and fuel cells, to successfully solve all kinds of testing problems.
Brief Description of the Project Problem Definition
As GE Wind Energy expanded their wind turbine operations, they decided to locate a key gearbox assembly operation in an existing GE Transportation plant in Erie, Pennsylvania. While this plant is well suited to this purpose, no facility existed there that was capable of testing the finished product.
Prior to shipment and installation atop a wind turbine tower, the gearbox’s performance must be verified. This requires gearbox operation at full power and speed. Where specific speed/torque test points and accurate data on vibration, noise, lube oil temperature and gear mesh geometry can be obtained.
The problem was further complicated by need to move the test gearboxes quickly thru the test process. Further, the test site was to be on the main manufacturing floor. Noise interference between the two activities would be an obstacle as would the lack of a dynamic block or machine base for mounting the test equipment. The plant utility system would require extensive upgrade to provide the drive power should the full 1.5 mW be required.
When complete, the test facility would be operated by skilled GE technicians but not by computer engineers. Thus, the computer controlled data acquisition and control system must be robust and user friendly.
Technical Solutions
The most critical technical issue for the gearbox test stand was the problem of simulating the 578,000 lb-ft, 18 rpm input of the wind turbine. The wind turbine gearbox is one of the few devices available with this duty rating. Therefore, using another, identical gearbox (operating in reverse) proved to be an ideal source of this speed/torque combination.
The test gearbox is connected to a second, permanently mounted wind turbine gearbox at the input shaft. A 1.5 mW motor system drives the output shaft of the permanent gearbox which drives the input of the test gearbox at the desired speed and torque. A 1.5 mW generator system is connected to the test gearbox output shaft and simulates the desired load.
These motors are variable frequency, A.C. machines controlled by solid state drives. These drives generate AC power by inverting DC power, thus the DC circuits of the two drives could be connected so that the power generated as load could be returned to the motor inverters and used to simulate the wind turbine input power.
GE Transportation also manufactures the wheel motors and drives used in large open pit mining trucks and locomotives. Analysis revealed that two of the GEB-16A4 model wheel motors would provide the correct speed, torque and power required for the proposed test. Likewise, the breaking function of these motors was sufficient to simulate the generator load.
Figure 1 illustrates this closed loop concept. The high torque is produced by the reciprocal gearbox and the power circulates from the generator to the motor thru the solid state drives. System losses are compensated by a 600 kW converter connected between the utility power line and the drive DC buss.
Figure 1 – Closed Loop Drive/Load System
The two gearboxes are normally mounted in elastomeric bushings to minimize noise and vibration. The test fixture duplicates this resilient mounting feature. Unfortunately, these soft mounts created an alignment problem between the test and reciprocal gearbox.
Since the test gearbox must be installed and removed quickly, a sliding gear coupling was needed between the two boxes. While this type coupling provided the high torque coupling very quickly, it could not transmit the bending moment needed to support the shaft ends.
An alignment pin mounted on a screw jack solved this problem. When the test gearbox was in place, the tapered pin was inserted between the two coupling halves thus supporting the overhung moment of the two gearboxes. The gear coupling was then connected using face keys and bolts to carry the shaft torque.
Installation time was also reduced by using a hydraulically activated shrink disk (by Ringfedder) to attach the gear coupling to the test gearbox input shaft.
The floor space allocated to the test stand presented two more problems. First, no dynamic block existed and due to underground utilities, one could not be installed. Secondly, the area is an active wheel motor assembly line and noise interferences were a significant problem.
A massive steel frame resting on isolation pads directly on the factory floor proved to be an excellent solution to the foundation problem. It was installed in two days and provided the firm, vibration free support required by the drive line. Figure 2 illustrates this concept.
Note also on this figure that the motors are mounted in acoustically treated enclosures and connected to an exterior mounted forced air cooling system. This feature protects the test and the factory floor from the noise and cooling function of the motors.
Figure 2 – General Arrangement
The data acquisition system consisted of a National Instruments FieldPoint system coupled with a Brejer & Kiel sound and vibration system. Both of these systems included custom software that allows a high level interface to the operator. The GE test technicians quickly mastered the controls and are able to efficiently run the acceptance tests and document the results.
Results
The regenerative load system minimizes the net energy needed for the test while providing full load capability. The free-standing support frame simplified installation and future facility relocation. The quick-change coupling/alignment pin method proved ideal and met project time targets.
The computerized control system speeds test sequencing and provides accurate, repeatable test data.
Innovative Application of Existing Techniques
Emprise has provided several computer controlled gearbox test facilities but the Wind Turbine project is the largest. Several of our gas turbine projects have used free-standing bases but this approach is very unusual for high torque gearbox testing.
The high torque, quick-change coupling is perhaps the most innovative feature. Gear couplings are in common use in such applications but we believe the rigid alignment pin scheme is a unique design. This approach provides the needed high torque transmission, facilitates rapid installation and allows the elastomeric mounting to be used in the test environment.
Complexity
The most complex achievement of the project was the adaptation of off-highway wheel motors and drives to the wind turbine gearbox test application. This approach required significant hardware and software modifications. Motors are operated in tandem and the drives work from a common DC buss. A DC resistor bank provided emergency braking.
In the final configuration, the motor/drive combination provided accurate torque and speed control that was stable over the specified operating range.
Meeting and Exceeding Client/Owner’s Needs
The project goal involved testing a newly fabricated gearbox in accordance with the client’s test procedure. This procedure involved loaded operation simulating actual wind turbine experience. While these speeds and torques were applied, data was collected on vibration, noise, oil temperature pressure and cleanliness level. Gear mesh patterns, were also checked.
While these “needs” were challenging, it was also necessary to meet initial delivery schedules. Thus, the test facility and the first production gearbox were being fabricated at the same time. Emprise engineers, working closely with GE engineers and other consultants, were successful in achieving concurrent completion and an on-time first test. The project was also completed on budget.
