In practical DC motors, the magnetic field is produced by electromagnets rather than permanent magnets. DC motors are then classified based on the connection of field winding in the motor circuit. On this basis, DC motors are classified into the following two types −

• Separately Excited DC Motors

• Self-Excited DC Motors

Separately Excited DC Motor

A DC motor whose magnetic field winding is excited from an independent source of DC electric supply like a battery is called a separately excited DC motor. Figure-1 shows the connection diagram of a separately excited DC motor.

The speed of a separately excited DC motor depends upon the supply voltage and field current, i.e. magnetic flux in the machine. However, the separately excited DC motors are rarely used in practical applications because these require an external source of DC power for field excitation.

Self-Excited DC Motors

The type of DC motor whose magnetic field winding is excited from the same power supply from which the armature is supplied, is known as a self-excited DC motor.

Depending upon the manner in which the field winding is connected with the armature winding, self-excited DC motors are classified in the following three types −

• Series DC motor

• Shunt DC motor

• Compound DC motor

Series DC Motor

A DC motor whose field winding is connected in series with the armature winding so that whole armature current passes through the field winding is called a series DC motor. Figure-2 shows the connection diagram of a series DC motor.

In case of a series DC motor, the field winding carries the whole armature current, thus it is made up of thick wire with less number of turns so that it possesses minimum resistance.

The following are some important expressions for the series DC motor −

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

Where, $mathit{I_{se}}$ is the series field current and $mathit{I_{s}}$ is the supply current.

$$mathrm{mathrm{Supply:voltage},mathit{V_{s}}:=:mathit{E_{b}+I_{a}left ( mathit{R_{a}+R_{se}} right )}}$$

Where, $mathit{E_{b}}$ is the back EMF, $mathit{R_{a}}$ is the armature circuit resistance, $mathit{R_{se}}$ is the series field resistance.

Shunt DC Motor

A DC motor whose field winding is connected in parallel with the armature winding so that total supply voltage is applied across it, is known as a shunt DC motor. Figure-3 shows the connection diagram of a shunt DC motor.

In a shunt DC motor, the shunt field winding has a large number of turns of thin wire so that it has high resistance, and therefore only a part of supply current flows through it and the rest flows through the armature winding.

Following are the important expressions of a shunt DC motor −

$$mathrm{mathrm{Armature:current,}mathit{I_{a}}:=:mathit{I_{s}-I_{sh}}}$$

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

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

Compound DC Motor

A compound DC motor is one which has two sets of field windings on each magnetic pole – one is in series and the other is in parallel with the armature winding.

Compound DC motors are sub-divided into the following two types −

• Short-shunt compound DC motor

• Long-shunt compound DC motor

A short-shunt compound DC motor is one in which only shunt field winding is in parallel with the armature winding as shown in Figure-4.

A long-shunt compound DC motor is one in which shunt field winding is in parallel with both series field winding and armature winding as shown in Figure-5.

The following are the important expressions for compound DC motors −

For short-shunt motor,

$$mathrm{mathrm{Armature:current,}mathit{I_{a}}:=:mathit{I_{s}-I_{sh}}}$$

$$mathrm{mathrm{Series:field:current,}mathit{I_{se}}:=:mathit{I_{a}}}$$

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

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

For long-shunt motor,

$$mathrm{mathrm{Armature:current,}mathit{I_{a}}:=:mathit{I_{s}-I_{sh}}}$$

$$mathrm{mathrm{Series:field:current,}mathit{I_{se}}:=:mathit{I_{s}}}$$

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

$$mathrm{mathrm{Supply:voltage},mathit{V_{s}}:=:mathit{E_{b}+I_{a}left ( R_{a}+R_{se} right )}}$$