Electric Motors
Electric Motors

Series wound Dc motor (construction, Principle, Speed and Torque)


Series wound Dc motor

Series wound Dc motor as we said before that this type of dc motor is a self-excited Dc motor which 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:

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

And as we also said 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.

series wound Dc motor

Construction of Dc series motor:

Dc series motor has the same construction of any Dc motor, there is: (you can see full details about construction here >>> Construction of dc motor)

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

Construction of Dc series motor

 

However, DC series motor has some distinguishes as there is higher current flows through the field coil we care that:

  • 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 produced because the resistance of both armature and field windings is small, in the same way, this high current  produces a strong magnetic field and a high torque 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.

From previous words, we deduce that in DC series motor we have a maximum speed and torque in starting but with the increase in speed the torque come down because the produced current reduces.

That’s good but 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; as all other Dc motors the field flux is proportional to the field current (Iseα Φ), in this motor we have all currents passes on the circuit are equal:

Ise= Ia =Itotal; so:

Φ α Ise α Ia.

The field flux became 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 produced in the motor

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

And for this reasons series motor becomes essential as the starter motor in industrial applications which deal with heavy mechanical loads like huge cranes or large metal chunks.

characteristics of series Dc motor

Speed Regulation of Series Motor

until now everything going good but 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.

we almost mentioned everything about series wound Dc motor and I’ll be pleased if you solve this example and tell me how you found it.

series wound Dc motor example


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