Machine Ratings

Machine Ratings –

A name-plate fixed to the outside frame of an electrical machine records the data pertaining to
its rating. A machine rating species the voltage, current, speed, excitation, pf, efliciency, power output etc. under which it can operate satistactorily. Here satisfactory operation implies that temperature rise of the machine above ambient (or surrounding) temperature does not exceed a specified temperature when machine operates in accordance with the data on the name-plate. For all types of A.C. motors and D.C. machines, output power rating is given in kW (kilowatt). Older practice was to specifty the power output of A.C, and D.C, motors in horse-power (1 h.p. = 746 watts). For A.C. generators, rating is in kVA or MVA.
Electrical machines are rated on the basis of their temperature rise resulting from the power losses in iron and conductor. The temperature rise mentioned on the name-plate is the temperature difference between the hottest part of the winding under specified conditions of load, speed, voltage, excitation, cooling and the ambient temperature. For reliable and satisfactory operation of an electrical machine, it should be ensured that its temperature rise remains within specified limits. The temperature rise not only affects the insulation of an electrical machine but also its mechanical parts ; however, the extent of damage is more detrimental to insulating materials than to the mechanical parts.
Deterioration of insulation depends on the temperature as well as the time. It has been found that time to failure for organic insulation is reduced to half for every 8 to 10° C rise in machine temperature.