There are four dominant factors affecting aging of the motor, Thermal, Electrical Ambient and Mechanical or TEAM for short. Understanding team and its consequences for individual machines will help us select the appropriate service or maintenance strategy that will maximize process uptime and help minimize operational expenditure.
Thermal:
Thermal aging happens during normal operation that rated output but is accelerated if the machine runs hot too to overload foul coolers or dirty filters. Thermal aging also occurs during start as a result of long starts, stalling, repeated starts or a locked rotor. if the machine is running ten degrees warmer than specification, the winding lifetime can be reduced by 50% or in other words it can age twice as fast.
Electrical:
Aging can also be the result of electrical issues such as partial discharges in the insulation, even at normal terminal voltages, network harmonics, voltage peaks in the network and phase imbalance.
Ambient:
Ambient or environmental aging can occur through high or low ambient temperature, high humidity, air contaminated with salt, dust orsand or chemicals ranging from acids to alkalize sulfur or ozone.
Mechanical:
Mechanical aging results from vibrations caused by imbalances, misalignment, air gap irregularities, electrical asymmetry, resonance and natural frequencies near the operating speed or vibrations from the driven equipment.
With motors connected direct on line, there is a large inrush current present where 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 the thermal, electrical, and mechanical stresses are significantly higher when starting and during normal operation. As a consequence, repeated starts caused by network disturbances such as a short-circuit or nuisance trips can cause a significant reduction of the machine’s lifespan. these starting stresses can be reduced if the motor is connected to a variable speed drive.
As a rule of thumb, each direct online start will age the machine the equivalent of 20 hours of normal operation, so the equivalent operating hours are the actual operating hours of the Machine plus the number of stars, multiplied by 20. if a machine is started every day, that alone could double the need for maintenance compared toone that runs continuously. this needs to be taken into account when planning the maintenance intervals for direct online motors that start frequently.
