When specialist equipment, such as a frequency converter, needs to be refurbished, it is important to work with experts who have the experience and facilities to complete the project efficiently. With a state-of-the-art high-voltage (HV) coil development lab, Sulzer was ideally placed to support a partner during the rewind of a large 20-metric-ton common-shaft frequency converter.
Also known as motor generator sets, rotary frequency converters are used in many industrial applications to provide an electrical supply with a precise frequency, usually different from the local grid supply. Their robust design makes them particularly well-suited to operating in outdoor locations, and with proper enclosures they can provide long and reliable service.
Frequency converters consist of a motor, which is coupled to drive the generator. The main principle behind their operation is the difference in windings between the motor and the generator, which allows the frequency of the supply to the motor to differ from the output of the generator.
These specialized pieces of equipment are used for applications with large starting motor loads or where a particularly clean supply, that is, free from harmonic distortion and noise, is required. They are often seen in marine, offshore, and military applications, as well as in the rail sector.
Despite their rugged design, many years of service can take a toll on this type of equipment, and there comes a time when most units need to be refurbished, as was the case for a recent project that was undertaken by Sulzer. In this instance, the project was in partnership with Karsten Moholt, which has worked with Sulzer on larger rewind projects for many years.
Routine maintenance of the frequency converter had revealed low insulation resistance readings on the windings of the generator, and Karsten Moholt was contracted to complete the repair. In projects involving larger pieces of equipment, there is a longstanding arrangement between the companies to have Sulzer assist with repairs through its HV coil center of excellence in Birmingham, England.
1. The original copper bars were reinsulated and consolidated before being reinstalled in the frequency converter. Courtesy: Sulzer
The process began with the stator being disassembled, unsoldering the coil joints and removing all of the original insulation using a controlled burn-off oven, before cleaning and polishing the copper bars. The coils were then reinsulated using resin-rich tapes (Figure 1) and consolidated in a heated press to produce the precise coil slot dimensions.
“In this case, Sulzer reinsulated the generator side of the frequency converter using the existing copper bars, which had a Kaiser transposition as part of the design. This is where two pieces of copper run side-by-side and cross over one another within the slot section. This is more commonly seen in traction motor coils, which are also manufactured at the Birmingham Service Center,” a technician explained.
2. Sulzer also designed and manufactured a new blocking system using Class F materials. Courtesy: Sulzer
As part of the project, the engineers in Birmingham also designed and manufactured a new blocking system to replace the original components (Figure 2), which had begun to crack and deteriorate due to age. The new blocks were manufactured to meet the Class F insulation standards and will provide a much-needed extension in the service life of the components.
In many cases, customers often come to the Birmingham Service Center to review progress and as part of the customer acceptance tests. Recent investment by Sulzer has allowed HV coil testing capacity to significantly increase. It is now possible to prove insulation systems quicker, as well as watch live tests from a remote location, making it easier for customers to witness tests and check the results.
“In all, the whole project was completed within six weeks and Karsten Moholt was able to return the completed frequency converter to the end customer on time,” the technician said. “As a direct result of this project, Sulzer has also been considered for the refurbishment of the complete drive-train for a similar machine, which would include both stators from the motor and generator sides as well as the associated rotors.”
—Benny Hinchliffe is head of International Sales with Sulzer.