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16 September, 2019

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Why electric motors need servicing

25 April, 2019

Rob Wood, ABB’s UK sales manager for LV motors, examines the factors that can cause motors to age.

The diagram below shows the typical lifecycle of a motor. The grey curve shows the condition and performance of the machine which, over time and without a maintenance strategy, declines mainly due to ageing. How fast the performance declines depends on how well the machine copes with operational stresses, and how well it has been maintained. Machines with an appropriate maintenance strategy in place will generally last longer than those that don’t.


The red curve shows the operating expenditure (Opex). Installation and commissioning is a critical step, because complications introduced here will impact on the reliability of the motor in the future.

 

Without a proper maintenance strategy, the motor may fail, and the Opex will increase dramatically due to the costs of emergency repairs, urgent spares and the loss of production.

 

The four main factors affecting ageing are thermal, electrical, ambient and mechanical ­­- or Team. Understanding Team and its consequences for individual machines, helps to select the appropriate service or maintenance strategy that will maximise uptime, and help to minimise Opex.

 

Thermal ageing happens during normal operation at rated output but is accelerated if the machine runs hot due to overloading, fouled coolers or dirty filters. Thermal ageing also occurs during start-up as a result of long starts, stalling, repeated starts or a locked rotor. 

 

Electrical ageing includes partial discharges in the insulation, even at normal terminal voltages, network harmonics, voltage peaks in the network and phase imbalance.

 

Ambient ageing can occur as a result of high or low ambient temperatures, high humidity or air contaminated with salt, dust, sand or chemicals.

Mechanical ageing results from vibrations caused by imbalances, misalignment, airgap irregularities, electrical asymmetry, resonance at natural frequencies near the operating speed, or vibrations from the driven equipment.

 

With motors connected direct-on-line (DOL), there is a large inrush of current when the electrical supply is first applied to the machine. The size of this current, coupled with the way the rotating stator field interacts with the stationary rotor, means that thermal, electrical and mechanical stresses are significantly higher when starting than during normal operation.

 

Repeated starts caused by network disturbances such as a short-circuit or nuisance trips, can therefore shorten the machine’s lifespan significantly. These starting stresses can be reduced if the motor is connected to a variable-speed drive.

 

If a machine is started every day, that alone could double the need for maintenance compared to one that runs continuously. This needs to be considered when planning the maintenance intervals for DOL motors that start frequently. 

 

For more information, you can view this video:  http://bit.ly/2SQYeR2




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