Design changes address issues with rotating rectifiers

An issue with the diode ratings and the lack of surge protection in one design of rotating rectifier has been experienced by several rail operators, causing trains to be taken out of service. Working with its rail industry customers, maintenance engineering specialist Sulzer has recognized the common fault and developed a solution to the issue. The result is a faster repair that is now reducing the downtime for affected trains and helping to maintain scheduled services.

Trains play a key part in our lives. Commuters and travelers alike rely on trains to get them to their destinations on time. Whether this is for work, visiting friends and family or going on holiday, this service depends on trains that are reliable. One of the key components onboard a train is the auxiliary generator which incorporates a rotating rectifier. The alternator powers the auxiliary circuits such as lighting, battery charging and air conditioning.

However, when this well-known issue with the rotating rectifier occurs, the train must be taken out of service and there is a considerable mechanical and electrical task involved in repairing and getting the train back to operational status. In order to ensure scheduled services for passengers, the hope is that the proposed solution will be adopted by more companies so that the impact of this issue can be minimized.

The alternators are located within the bogie assemblies of the trains and they are responsible for regulating the auxiliary power for the trains. The rotating rectifier is an integral part of the brushless alternator and any faulty component means that the engine has to be taken out of service for repairs. Hence when they fail, service is disrupted.

Due to their location, the repair can be a time-consuming task. With that said, it is easier to replace the rectifier than remove the entire alternator, so many train operators, or their maintenance contractors, are holding several rectifiers in stock.

However, as maintenance engineers correlated information, it became clear that the issue is not related to time in service. The failures can occur at any time, which means a recently installed component has the same chance of failing as an old one. It is this repetitive, but still random, nature of the issue that is causing so much concern for the rail operators because even shortening a planned maintenance schedule will not affect the reliability of these components.

Sulzer’s experts in Nottingham conducted an investigation into the rectifier failures and proposed an improved design that would be fully interchangeable with the original. With decades of experience in rail engineering, Sulzer’s service center in Nottingham is renowned as a center of excellence for the rail sector.

The investigation into rectifier reliability discovered that the current rating of the diodes appeared to be inadequate, particularly for transient and field forcing conditions. In addition, the mechanical clearances between the diodes and the earthed hub were insufficient, offering the potential for short-circuit failures. The engineers also found that there was no surge or spike suppression fitted, which could lead to diode failures due to surges transmitted to the rotor caused by external faults or load changes.

The engineers proposed a design that used higher rated devices as well as the addition of transient suppression to protect against electrical surges. The new rectifier also has increased mechanical clearances to prevent short circuit failures.

Typically, rail operators expect this type of component to perform without any maintenance for four years, after which the regular service routine will instigate an inspection. Sulzer’s new design of rectifier is expected to far outlast the current equipment, in fact, there is very little reason for it to fail at all.

This is just one example of Sulzer’s engineering expertise being applied to the rail industry. Another notable project involves the repair or redesign of automatic voltage regulator (AVR) cards, which also pose a considerable reliability issue within the industry.

Ultimately, Sulzer’s aim is to work with rail operators and their maintenance providers to develop solutions that improve reliability and efficiency, making the business of operating trains more cost-effective. This in turn will make more trains available for operation, helping to ensure that we all reach our destinations on time.