Types of DC Motors - Series, Shunt and Compound Wound

According to the type of connection of the field winding with the armature, the DC motors are classified as follows −

• Permanent Magnet DC Motors
• Separately-Excited DC Motors
• Self-Excited DC Motors
  • Series Wound DC Motor
  • Shunt Wound DC Motor
  • Compound Wound DC Motor

Permanent Magnet DC Motor

A DC motor in which the main field flux is produced by the permanent magnets is known as permanent magnet DC motor. In this type DC motor, there is only one external source of DC supply is required, for supplying electrical power to the armature. The permanent magnet DC motors are mainly used in small scale application like in toys.

Separately-Excited DC Motor

In a separately-excited DC motor, the main field winding is excited by an external source of DC supply. The separately-excited dc motor is a doubly-excited motor, in which two sources of DC supply are required, one for armature and the second for the excitation of field winding.

Here,

$$\mathrm{Armature\:current,I_{a} = I_{s}}$$

$$\mathrm{Supply\:voltage,\:V_{s} = E_{b} + I_{a}R_{a}}$$

$$\mathrm{Electric\:power\:developed\:in\:armature = E_{b}I_{a}}$$

Series Wound DC Motor

A DC motor in which the field winding is connected in series with the armature winding is known as series wound DC motor. Since the series field winding carries the whole armature current. Therefore, the series field winding has a small number of turns of thick wire and should possess a low resistance.

Here,

$$\mathrm{Armature\:current,\:I_{a}= I_{se}}$$

$$\mathrm{Supply\:voltage,\:VS \:= \:E_{b} + I_{se}R_{se} + I_{a}R_{a} \:= \:E_{b} + I_{a}(R_{a} + R_{se})}$$

Applications – The series DC motors are variable speed motors i.e. their speed is low at high torque and vice-versa. Although, at no-load or light load, the motor attains dangerously high speed. The series motors have high starting torques. Therefore, they are used in following applications -

• Used where large starting torque is required like in elevators, electric tractions, cranes, etc.
• Used where load is subjected to heavy fluctuations and the speed is required to be automatically regulated according to load requirements.
• Also used in air compressors, vacuum cleaners, hair driers, sewing machines etc.

Shunt Wound DC Motor

A shunt wound DC motor is the one, in which the field winding is connected in parallel with the armature winding. The shunt field windings are designed to have high resistance, i.e., have a large number of turns of fine wire so that the shunt field current is relatively small as compared to the armature current.

Here,

$$\mathrm{Armature \:current,\:I_{a} \:= \:I_{s} − I_{sh}}$$

$$\mathrm{Shunt \:field \:current,\:I_{sh}\: =\:\frac{V_{s}}{R_{sh}}}$$

$$\mathrm{Supply \:voltage,\:V_{s} \:= \:E_{b} + I_{a}R_{a}}$$

Applications – The shunt motors are constant speed motors. Therefore, they are used in following applications -

• Where speed is required to remain constant form no-load to full load.
• Used in lathes, drills, sharpers, spinning and weaving machines, boring mills etc.

Compound Wound DC Motor

A DC motor in which both the series field and shunt field are combined is known as compound wound DC motor. There two types of compound DC motors as −

Short-Shunt Compound Motor

In a short-shunt compound motor, the shunt field winding is directly connected in parallel with the armature winding.

Long-Shunt Compound Motor

When the shunt field winding is connected in parallel with the series combination of armature winding and the series field winding, then the motor is known as long-shunt compound motor.

Note – When the series field flux aids the shunt field flux, i.e., both are in same direction, then the compound motor is known cumulative compound motor whereas when the series field opposes the shunt field, i.e., both are in opposite direction, the motor is known as differential compound motor.

Important – The compound DC machines (generator or motor), are always designed such that the magnetic flux produced by shunt field winding is greater than the flux produced by the series field winding.

Applications – The differentially-compound motors are rarely used due to their poor torque characteristics. However, the cumulatively-compound motors are used in the constant speed applications with irregular loads or suddenly applied heavy loads like presses, reciprocating machines and shears etc.