Effect of Transient Conditions On Application of D-C. Compound Motors

When a d-c. compound motor is started, the rise of current in the series field induces a voltage in the shunt field counter to that applied, and the shunt field current may reach several times normal value in a negative direction. This current reversal gives, in effect, a differentially compounded motor during the starting period and causes the starting torque to be materially reduced. In many applications of compound motors, therefore, predetermined performance is not obtained. In particular cases where the motor is continually started and stopped, the effect may be so serious as to dictate the choice of another type of motor, or one of a special design to minimize the undesirable action. In case the compound motor is disconnected from the line while running, and reconnected with no external resistance, the delay in establishing a flux due to the damping action of the shunt field may permit excessive currents to flow. In other d-c. motors, currents induced in the solid iron portions of the frame and poles exert a damping effect similar to that of the shunt field in the compound motor, which may at times be undesirable. A study of such transients leads to means by which they may be miinimized and provides calculation methods for predetermining motor behavior.