“ Series motors are never used unless they are directly connected to load”- why?

In a series motor the flux depends upon the current in the series field, which is the same current that flows through the armature. At light loads the armature current is small; therefore the flux and torque are small. An increase in the load increases both the armature current and the flux.

The basic speed equation

S = Vt - IaRa - IaRa

KФ

Since the current in the series field is the armature current, the equation may be rewritten substituting Ia for Ф.

S = Vt - IaRa - IaRa

KIa

With no load connection to the motor the value of I is very small. The numerator of the speed equation is large, while the denominator is very small, indicating that the speed of the motor will be extremely large. In fact, the speed of the motor may reach such a value as to cause the centrifugal force to destroy the motor. It is this reason that series motors are never used unless they are directly connected to the load.

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