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Mathematical Comparison of CharacteristicsAuthor: E.E. Kimberly For a shunt motor the following equation may be written to relate speed and load defined in terms of armature current: (115) in which n = speed, in rpm; . V = applied voltage; IaRa = resistive voltage drop
in the armature; ϕ = mamfield mix; Z' = number of active armature conductors; k = a constant of the machine. Inasmuch as ϕZf is a constant K_{1} with constant excitation, or (116) This is the equation of a drooping straight line to which the shuntmotor characteristic of Fig. 119 is an approximation. Armature reaction pre Cumulative Compound, and Series Motors, All Referred to the Same Rated Speed and Power
TABELLE 1121 Notes: Starting current is controlled by starting resistor. This table shows general effects, which will vary somewhat for specific ratings, i Courtesy General Electric Co, vents exact similarity between the actual and the theoretical characteristics. In the cumulative compound motor, ϕ is not constant but increases with load. As ϕ of equation (115) increases, the value of the fraction decreases; and there is produced a curve with increasing droop, which approximates the cumulativecompound characteristic of Fig. 119. The amount of the droop depends on the number of ampereturns of the shunt field relative to those of the series field when loaded. In the series motor,
(117) If the saturation of the iron is low, ϕ is approximately proportional to I, and
(118) The speed of a series motor is proportional to and may be controlled by altering either V or ϕ. The applied voltage may be reduced by inserting a resiststance in series with the armature, and ϕ may be reduced by shunting part of the current around the field coils. The interaction of ϕ and Ia produces an approximately hyperbolic speedload characteristic within the operating range of the motor. Tables 111 and 112 give data relating to characteristics and applications of dc motors.


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