Series Wound DC Motor or DC Series Motor

A series wound DC motor is a self-excited DC motor, like a shunt wound DC motor or a compound wound DC motor. Its field winding is in series with the armature winding, so the field carries the full armature current, unlike a shunt motor.

Construction of Series DC Motor

A series wound DC motor uses the same main parts as other types of DC motors: a stator field winding plus a rotor with armature conductors and the commutator or the brush segments with brushes.

The field and armature coils are wired differently from shunt and compound machines.
The field coil is in series with the armature winding, so a large current flows in the field. The field is built for that current, as below.

1. The field coils of a DC series motor have fewer turns because the armature current flows through them, needing fewer turns to produce the required magnetic force.
   
2. The wire is heavier, as the diameter is considerable increased to provide minimum electrical resistance to the flow of full armature current.
3. In spite of the above mentioned differences, about having fewer coil turns the running of this DC motor remains unaffected, as the current through the field is reasonably high to produce a field strong enough for generating the required amount of torque. To understand that better lets look into the voltage and current equation of DC series motor.

More DC motor questions are in the Electrical MCQs.

Voltage and Current Equation of Series DC Motor

The circuit of a typical series wound DC motor is shown below.

Let supply voltage and current at the motor terminals be E and I_total.
That line current flows through both the armature and the field.

where I_se is the series field current and I_a is the armature current.
The voltage equation of the DC motor is then

where E_b is back emf,
R_se is series-field resistance and R_a is armature resistance.
Because I_se = I_a,

That is the voltage equation of a series wound DC motor.
Field flux in a DC series motor, as in other DC motors, is proportional to field current below saturation.

But since here

so field flux follows the armature current, and thus the total supply current, until the iron saturates. That large flux still gives useful torque even though the field coil has few turns.

Speed and Torque of Series DC Motor

Below saturation, series-motor torque is roughly proportional to the square of current, because flux also rises with armature current. After saturation, torque rises more nearly with current alone. The thick series field still carries a large current, so starting torque is high enough to move a heavy load from rest. Series motors are used on cranes, hoists, traction and engine starters. They must run with a mechanical load. With little or no load the speed can rise to a damaging value. Engine-starter duty is only a few seconds; traction and hoist series motors are built for their own duty cycle and are not limited to a few seconds of running.

Speed Regulation of Series Wound DC Motor

Unlike a DC shunt motor, a DC series motor has poor speed regulation: added shaft load leaves speed well below the previous value.
Any DC motor slows at the instant load is added. Speed regulation is the later recovery toward the earlier speed. A series motor does little of that recovery. Lower speed N reduces back emf,


Lower E_b raises the net voltage E – E_b, so series field current rises,

That extra current also raises field magnetic flux until the iron saturates. Speed of a series motor is roughly proportional to voltage over flux, so more flux holds speed down instead of restoring it. After saturation, flux rises more slowly with current, as in the figure below.
The magnetic field does not return the machine to the previous speed.
A series wound DC motor therefore suits high-torque variable-speed work under load including starting heavy machinery. It must not be run unloaded.