Electrical And Electronics For U

The types of DC motor can be listed as follows
•DC motor
  • Permanent Magnet DC Motor
  • Separately Excited DC Motor
  • Self Excited DC Motor
       • Shunt Wound DC Motor
       • Series Wound DC Motor
       • Compound Wound DC Motor
              • Cumulative compound DC motor
                      • Short shunt DC Motor
                      • Long shunt DC Motor
              • Differential Compound DC Motor
                      • Short Shunt DC Motor
                      • Long Shunt DC Motor

Now let’s do a detailed discussion about all the essential types of dc motor.

As the name suggests, in case of a separately excited DC motor the supply is given separately to the field and armature windings. The main distinguishing fact in these types of dc motor is that, the armature current does not flow through the field windings, as the field winding is energized from a separate external source of dc current as shown in the figure beside.

From the torque equation of dc motor we know Tg = Ka φ Ia So the torque in this case can be varied by varying field flux φ, independent of the armature current Ia.

The permanent magnet DC motor consists of an armature winding as in case of an usual motor, but does not necessarily contain the field windings. The construction of these types of DC motor are such that, radially magnetized permanent magnets are mounted on the inner periphery of the stator core to produce the field flux. The rotor on the other hand has a conventional dc armature with commutator segments and brushes. The diagrammatic representation of a permanent magnet dc motor is given below.

The torque equation of dc motor suggests Tg = Ka φ Ia. Here φ is always constant, as permanent magnets of required flux density are chosen at the time of construction and can’t be changed there after.

For a permanent magnet dc motor Tg = Ka1Ia

Where Ka1 = Ka.φ which is another constant. In this case the torque of DC Motor can only be changed by controlling armature supply.

In case of self excited dc motor, the field winding is connected either in series or in parallel or partly in series, partly in parallel to the armature winding, and on this basis its further classified as:-

Let’s now go into the details of these types of self excited dc motor.

In case of a shunt wound dc motor or more specifically shunt wound self excited dc motor, the field windings are exposed to the entire terminal voltage as they are connected in parallel to the armature winding as shown in the figure below.

To understand the characteristic of these types of DC motor, lets consider the basic voltage equation given by,

[Where E, Eb, Ia, Ra are the supply voltage, back emf, armature current and armature resistance respectively]

Now substituting Eb from equation (2) to equation (1) we get,


The torque equation of a dc motor resembles,

This is similar to the equation of a straight line, and we can graphically representing the torque speed characteristic of a shunt wound self excited dc motor as

The shunt wound dc motor is a constant speed motor, as the speed does not vary here with the variation of mechanical load on the output.

In case of a series wound self excited dc motor or simply series wound dc motor, the entire armature current flows through the field winding as its connected in series to the armature winding. The series wound self excited dc motor is diagrammatically represented below for clear understanding.

Now to determint the torque speed characteristic of these types of DC motor, lets get to the torque speed equation.

From the circuit diagram we can see that the voltage equation gets modified to

Where as back emf remains Eb = kaφω

Neglecting saturation we get,

From equation (5) & (6)

From this equation we obtain the torque speed characteristic as

In a series wound dc motor, the speed varies with load. And operation wise this is its main difference from a shunt wound dc motor.

The compound excitation characteristic in a dc motor can be obtained by combining the operational characteristic of both the shunt and series excited dc motor. The compound wound self excited dc motor or simply compound wound dc motor essentially contains the field winding connected both in series and in parallel to the armature winding as shown in the figure below:

The excitation of compound wound dc motor can be of two types depending on the nature of compounding.

When the shunt field flux assists the main field flux, produced by the main field connected in series to the armature winding then its called cumulative compound dc motor.

In case of a differentially compounded self excited dc motor i.e. differential compound dc motor, the arrangement of shunt and series winding is such that the field flux produced by the shunt field winding diminishes the effect of flux by the main series field winding.

The net flux produced in this case is lesser than the original flux and hence does not find much of a practical application.

The compounding characteristic of the self excited dc motor is shown in the figure below.

Both the cumulative compound and differential compound dc motor can either be of short shunt or long shunt type depending on the nature of arrangement.

If the shunt field winding is only parallel to the armature winding and not the series field winding then its known as short shunt dc motor or more specifically short shunt type compound wound dc motor.

If the shunt field winding is parallel to both the armature winding and the series field winding then it’s known as long shunt type compounded wound dc motor or simply long shunt dc motor.

Short shunt and long shunt type motors have been shown in the diagram below.