Considering that an induction motor is a simple designed and rugged construction machine, its life time will exclusively depend on the quality level of the insulation materials. Motor insulation is affected by several factors including moisture, vibration, corrosive environments and others. Among all these factors, operating temperature of the insulating materials is the most critical.
The motor life time is reduced by half when subject 8% to 10 oC in operation above the rated temperature of the class of insulating material. To ensure a longer lifetime for the electric motor, the use of thermal sensors is recommended for the winding protection.
When we refer to motor life time reduction, we do not refer specifically to excessively high temperatures resulting in sudden insulation burn out. Insulation life time ( in terms of operating temperature much below than the one affecting the insulation ) refers to permanent aging of the insulation material which becomes dry and loses its insulation properties. As a result, it will not withstand the voltage applied to it, thus causing short-circuit.
If operating temperature is kept below its limit, experiences have proved that the motor insulation can practically last for ever. Any increasing value above such limit will reduce insulation life time proportionally. Such limit of temperature is much lower that the temperature that causes insulation burn out and it depends on the type of used material. This limit of temperature refers to insulation hottest spot and not necessarily to the whole insulation. On the other hand, a single weak spot in the insulation is enough to damage the winding completely.
With increasing use of frequency inverters for the speed control of induction motors, other application criteria must also be considered for the preservation of the insulation system.
INSULATION MATERIAL
The insulation material prevents, limits and directs the electric current flux. Although the insulating material is primarily intended to block the current flux from a cable to ground or to the lowest potential, it also serves to provide mechanical support, protect the cable from degradation caused by environment influences and to transfer the heat to the external environment.Based on system requirements, gases, liquids and solid materials are used to insulate electric equipment. Insulation systems affect the quality of the equipment, and type and quality of the insulation affect the cost, weight, performance and its useful lifetime.