Extending the life of large motors and generators

Paul Boughton

Richard Emery looks at the issues and some potential resolutions

When addressing the operating costs of a modern heavy industrial production facility, most industrial processes will involve electric motors and all will need the services of a generator, either directly on site or indirectly through the electricity supplier. Prolonging the service life of these vital pieces of equipment can deliver significant improvements in their life cycle cost as well as operational savings for the production facility.

Life cycle cost (LCC) is a vital driver in modern engineering and comes to the fore when the initial purchase cost of large rotating machinery has been amortised. Running costs and avoiding unscheduled downtime are then at the top of an operations team’s agenda. Sulzer, as a global repairer and 24hr service provider with its own worldwide network of service centers for both motors and generators of any size as well as turbomachinery and pumps is in an ideal position to provide some guidance on improving LCC.

The designs of multi-megawatt motors and generators have many similarities; they are classed as large rotating machines that have a stator and a rotor both of which contain windings that need to be insulated and bearings that need to be lubricated. Obviously the major difference is one generates the electricity while the other consumes it. However, by looking at the similarities it is possible to employ similar methods to prolong the service life of both.

In order to determine the best course of action for improving service life it is important to first understand the causes of component failure and so begin to address them.

Causes of reduced life expectancy

By far the biggest concern for both motors and generators is excessive heat, in fact many of the other issues contribute to this problem as well. It is often stated that for every 10°C increase in operating temperature, the life of the winding insulation is halved.

It is therefore crucial to maintain the optimum operating temperature and put in place a maintenance regime that can resolve issues such as dirt build-up, poor ventilation and poor lubrication. Each of these contributes its own issues to the performance of rotating equipment as well as contributing to an increased operating temperature.

Another major cause of motor breakdown is bearing failure which can cause overheating, damage to insulation and also cause secondary damage to other motor components. In more severe cases the motor may have to be replaced, so it is important to employ suitable techniques to maintain and monitor bearing performance.

Condition monitoring

On larger machines, the use of vibration monitoring equipment and thermal imaging cameras can provide vital information and indicate the early signs of a bearing beginning to fail. Both of these predictive maintenance methods can be used without having to stop the equipment and they can provide regular data for a preventative maintenance programme.

For the smallest of electric motors, a bearing failure and any subsequent damage can put it beyond economical repair, while a winding failure on a medium sized motor can have the same result. When it comes to the larger motors and most generators, it is usually more time and cost effective to have the windings replaced as opposed to buying a new piece of equipment.

Winding insulation degradation

The electrical insulation applied to motor and generator windings degrades over time and according to the operating stresses it is subjected to. Voltage imbalance, over and under-voltage, voltage disturbance and temperature all play a role in the degradation of the insulation. Using a technique such as partial discharge analysis can provide vital information about the condition of the stator.

Partial discharges (PD) are essentially sparks occurring in the voids within the electrical insulation system itself or adjacent to the insulation of high voltage stator windings. The deterioration of the stator insulation is a gradual process, occurring over a period of years and so a programme of periodic PD measurements is an ideal method of condition monitoring. The data provided allows the machine operator to maximise the machine life to the point where a critical condition is reached and the windings need to be replaced. This helps to avoid unexpected, catastrophic failure and allows the repair to be scheduled with a minimum of disruption.

Improved reliability and efficiency

Improved production techniques and insulation technologies allow modern coils to improve the efficiency and longevity of existing equipment. Throughout the manufacturing process however, quality control is essential to maintain the production of highly reliable, uniform coils. Sulzer’s Birmingham Service Centre for example uses CAD design, modern insulation technologies and testing, combined with its own copper rolling mill to provide high quality coils that can be produced and installed in a very competitive time frame.

Starting with the controls over raw material quality, through measuring every coil for dimensional accuracy, to the final electrical tests, every process is checked against the original drawings and specifications to ensure that the finished product can be easily assembled and - given an appropriate operating environment, will deliver reliable and efficient service for many years to come.

Best practice advice

Employing a range of condition monitoring equipment, combined with suitable analysis techniques, can provide an accurate assessment of the status of motors and generators, allowing operators to avoid unplanned downtime and lost production. The use of vibration analysis, combined with thermal imaging, which is used to identify imminent bearing failure, poor electrical connections and the imbalance of phase loadings, can produce an accurate indication of the overall status of the plant.

Additional testing of the electrical windings, especially partial discharge analysis, can also provide very useful information on the overall condition of the equipment as well as the remaining lifetime for the equipment.

Richard Emery is Head of Technical Services at the Sulzer Birmingham Service Centre

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