Operating an electric motor always gives rise to undesirable energy conversion. This in turn leads to heating up which, above all, strains the winding insulations. The service life of a machine is decisively influenced by its inside temperature. Thermal overloading can engender operational disturbances. Estimates indicate that a temperature increase of 8K reduces machine life by 50 per cent. Heating up results first and foremost through energy passage in the windings. The designation W = I2• R • t shows that the conversion into heat and the related temperature rise are determined by the current flow and its duration. Temporary overloading is permissible as, due to thermal inertia, the temperature increase remains insignificant.
A torque increase for the work unit, respectively a mass inertia when starting or braking give rise to greater losses in the motor through the flow of higher starting or braking currents. Load starting and braking thus exert an influence on the degree of heating up. Consequently, for reasons related to thermal load, electric motors must be aligned to the load rhythm of the work unit.
Mode of operation relates to the nature and sequence pattern, equally the duration of standstill and idling times, also to the nominal load of electrical machines.
[Ukrainian] [English] [Russian]