Specific Electric Loading in Electrical Motors

The specific electric loading is abbreviated as AC in italics ‘ac’ and it is defined as the RMS ampere conductors per meter of armature periphery at the air gap surface.

Mathematical formula for ac:

ac= (Total number of armature ampere conductors)/(Armature periphery at air gap)

The selection of a particular value of specific electric loading dictates three factors:

  • Voltage rating
  • Temperature rise
  • Overloading capacity

Voltage rating

In case of low voltage ratings smaller insulation space is required, however as the voltage rating increases and moves towards high voltage, more insulation space is required and slot factor will be reduced accordingly.

If the Design Engineers opt for high ac, the larger armature diameters becomes necessary which in turn increases the size of motor.

Temperature rise

The larger the value of ac is, the increased will be the temperature rise. That is, since for larger values of ac, there are increased armature copper losses which result in increased temperature rise.

Overload Capacity

The larger the value of ac is, the reduced is the overloading capacity of machine. With an increased value of ac, there is an increase in turns per phase and henceforth the leakage reactance which in turn decreased the ac.


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