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Series Wound DC Motor

July 12, 2020

Series wound DC motor is a self-excited Dc motor that converts electrical power into mechanical power based on electromechanical law. But in this motor, the field winding is connected in series to the armature winding.

In this motor we have a large amount of starting torque so it’s the best with (applications of DC series motor):

  • Small electrical appliances.
  • Traction applications.
  • Crane applications.
  • Elevators.
  • Winching systems.
  • And versatile electric equipment.

And also the advantages of the series motor are:

  • High starting torque for a given power rating.
  • Simple construction.
  • Easy to design.
  • Easy maintenance.
  • It works with both alternating or direct current power source.
  • And effective cost.

Construction of DC series motor

DC series motor has the same construction of any DC motor, there is:

The stator is considered the house of field winding and made up of two or more electromagnetic pole pieces.
The rotor carries the armature conductors, the commutator(brush segments), and all fundamental components in other motors.

However, DC series motor has some distinguishes as there are higher current flows through the field coil.

The field coils are wound with relatively fewer but thicker turns to be able to support the highest current produced. We use heavier wires to provide minimum electrical resistance.

The principle of operation of DC series motor

Initially, when we connect a voltage source to the motor, a high amount of current is produced because the resistance of both armature and field windings is small. In the same way, this high current produces a strong magnetic field.

High torque is provided to the armature shaft so the motor rotates at its maximum speed in starting, and in the presence, a counter EMF resulted and limits the produced current.

The DC series motor has a maximum speed and torque in starting but with the increase in speed, the torque comes down because the produced current reduces.

We agree that equations are the best to make everything clear and easy; so we will start with the current:

Voltage and Current Equation of Series DC Motor

The total current passes on the motor are:

Itotal= Ise=Ia.

And from the basic voltage equation:

E=Eb+Ise.Rse+Ia.R.a

And as it’s a series motor and the series current equals the armature current (Ise=Ia) so;

E=Eb+Ia. (Rse+Ra).

Also, like all other DC motors, the field flux is proportional to the field current (Iseα Φ), in this motor all currents pass on the circuit are equal:

Ise= Ia =Itotal; so:

Φ α Ise α Ia.

The field flux becomes proportional to the armature current or the total supply current so we say that the series motor flux is strong enough to produce sufficient torque.

We should mention that the torque of the series motor is proportional to the square of the field current so we have this high starting torque.

Characteristics of series wound DC motor

Speed & Torque of Series DC Motor

The most important thing in any machine is the torque and the speed and that’s what we need from any machine.

In series motor, the torque has a linear relationship with the current produced in the windings. If a heavy current flows throw the windings, a large torque is produced in the motor.

The speed of series motor depends on the torque, as a heavy electromagnetic torque produced a high strong speed. This speed is able to lift heavy load overcoming its initial inertia of rest.

And for this reason, a series motor becomes essential as a starter motor in industrial applications that deal with heavy mechanical loads like huge cranes or large metal chunks.

Speed Regulation of Series Motor

When we add external load to the shaft, the speed of the motor automatically reduces and the motor doesn’t have the ability to bring back the reduced speed to its original value. And with this reduction in speed (N) we will have a back EMF

Eb=P.Q.Z.N/60.A.

This EMF decreases and will help to increase the net voltage which in turn increases the field current to be:

Ise=E-Eb/(Ra+Rse).

And as the current increases, the magnetic core of the field tends to saturate so the magnetic flux linking the coils increases. However, the motor isn’t able to provide the necessary amount of force to bring back the speed to its previous value.

Disadvantages of DC series motor

Unfortunately series DC motor has some small disadvantages as:

  • Poor speed regulation, as when we increase the load the speed of the motor decreases.
  • Not suitable for many applications because it requires being loaded before starting.
  • The torque is proportional to the speed so when the speed decreases the torque drops sharply.