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Managing the reliability of VSDs

01 June, 2018

Martin Richardson, ABB’s water framework manager for the UK, examines and assesses various approaches to maintenance to determine which is the most cost-effective for variable-speed drives.

Variable-speed drives (VSDs) play a major role in motor-driven systems. Not only do they run motors at the right speed to optimise energy use, they can also detect excessive motor temperatures and extreme load conditions, improving reliability. However, they can only make this contribution if they are themselves reliable.

So, what are the causes of failures in VSDs? Some 80–90% of failures are the result of excessive temperatures or unsuitable environmental conditions.

These, in turn, can be the result of blocked cabinet filters, failed cooling fans and contamination from dust, dirt or chemicals. Other factors include moisture, and incorrect installation leading to poor ventilation, earthing and motor and cable faults.

Mitigating these failures comes down to maintenance, which has three strategic forms: corrective; preventive; and predictive.

Corrective maintenance is essentially fixing a system when it breaks. Because this means that no routine maintenance is carried out, the risk of VSD failure is higher. It also entails high costs, both in labour and time and unexpected lost production. Corrective maintenance is simply not sustainable or cost-effective.

Preventive maintenance is based on interventions at fixed time intervals or after a set number of running hours. This is done to prevent potential breakdowns. Regular tasks should include changing the filters in the motor control centre (MCC), vacuum-cleaning the heatsink, and changing VSD cooling fans. As these are the only moving part in the VSD, they are particularly susceptible to failure.

Preventive maintenance carries much less risk of breakdown, but costs may be higher than necessary because maintenance may be carried out more frequently than is really required.

Predictive maintenance uses data and statistics to predict abnormal running. It seeks to determine the optimum time to conduct preventative maintenance. Although it carries a slightly higher risk of breakdown than purely preventative maintenance, it achieves the lowest overall cost.

Predictive maintenance can be particularly effective when it is based on remote condition-monitoring. Smart drives that can monitor both themselves and connected components, such as motors, can communicate with systems that can analyse this data. This is used to predict where problems might occur and to recommend actions to prevent them.

Just how effective predictive maintenance can be was demonstrated recently at a remotely monitored plant in the US that uses a VSD to run a pump. A temperature difference was detected between two power modules. On investigation, this was found to be due to a build-up of dust near a heatsink, and a cooling fan not running at the correct speed. The situation was corrected, saving the plant up to five hours of unscheduled down time that would have cost $500,000.

Keeping VSDs reliable is the key to maintaining the reliability of the plants where they operate, and predictive maintenance is the most cost-effective way of achieving this.

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