Speed Control of a DC Series Motor

The dc series motor has its windings and armature connected in series across the supply.

From the figure above, the armature current I_a also supplies the field, hence when I_a is large on starting for example, the magnetic field will be strong, and the torque will be high. As the machine accelerates, the torque, armature current and field strength will all decrease as explained with the help of the graphs below.

Please note this type of motor should never be coupled to its load by means of a belt, because if the belt breaks the required torque from the armature will be removed, the armature and field current will fall, reducing the magnetic field, and the motor will increase in speed until it disintegrates.

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How the speed control of a dc series motor is done

The most effective way of controlling the speed of a dc motor is to vary the strength of the magnetic field on a dc series motor, this is achieved by diverting some of the current through a variable resistor.

Starting a dc series motor

A series motor is started by placing a variable resistor in series with the armature circuit.

Applications of a dc series motor

They are used in applications where heavy masses need to be accelerated from rest, e.g. cranes and lifts.

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