How does the Armature Current in a Series Motor change with Speed?

The armature current in a series motor varies with speed in the following ways:

The armature current decreases as the motor’s speed increases and vice versa.

The peculiar design of a series motor, in which the armature winding and the field winding are coupled in series, is the cause of this phenomenon. Consequently, the current passing through both windings is the same.

Since the armature at zero speed does not generate a back electromotive force (EMF), the armature current is quite high when the motor is first started from rest. Strong magnetic field created by this high current provides a high starting torque needed to overcome the load and inertia.

The back electromagnetic field (EMF) produced by the rotating armature rises in direct proportion to the motor’s speed. By opposing the applied voltage, this back EMF lowers the net voltage across the field windings and armature.

According to Lenz’s law, the back EMF opposes the change that caused it (in this case, the applied voltage). As the speed increases, this resistance causes the armature current to decrease.

In a series motor, the connection between armature current (Ia) & speed (N) can be written as follows:

Ia = (V - KφN) / (Ra + Rse)


V - The voltage applied is

Kφ - Constant associated with the winding arrangement and flux

N - Motor’s speed.

Ra - Resistance of the armature

Rse - Field winding’s equivalent resistance

This equation shows that the armature current (Ia) drops with increasing speed (N) because of the growing back EMF term (KφN) in denominator.

For series motors, the feature of reducing armature current as speed increases is advantageous since it improves in speed regulation and protects against higher-speed excessive currents that may cause damage to the motor.

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