Classification of DC Motor

The DC Motor is an electrical machine which converts mechanical energy into electrical energy. The basic principle of working is  based on production of induced emf.  The DC Motors can be classified into following categories.

Separately Excited DC Motor

The supply to the armature and field windings in separately excited DC Motors is provided separately. The field windings are separately excited through a DC supply source. Therefore, the armature current will not flow through the armature windings.

From the torque current equation of DC Motor we have

T_g = K_a φ I_a

From the above relation we can conclude that the torque is not dependent upon the armature current and can be varied only by varying the field flux.

Permanent Magnet DC Motor

These types of motors have armature windings as the basic DC motor but will necessarily not have any field windings. The field flux in such motors is produced by the radially magnetized permanent magnet which is mounted on the periphery of the stator core. The relation between the torque and current for permanent magnet DC Motor is as below

T_g = K_a1 I_a

From the above relation we can conclude that the armature supply can be controlled through the torque.

Self Excited DC Motor

In this type of motors the field windings are connected such that they can be in parallel, series or partially parallel or series. The types of self excited DC Motor are 

1)    Shunt motor – In the illustration shown below the field winding is connected in parallel with the armature.

The input current I = I_a + I_sh

For field winding, V = I_shR_sh

For armature, V = E_b + I_aR_a

I_a = armature current

I_sh = shunt field current

V = input voltage

I = Input current

Power Input = Mechanical power + losses in armature + losses in field

VI = P_mech +  I_a²R_a + I_sh²R_sh

P_mech = VI – VI_sh – = I_a²R_a

                 = V(I – I_sh) – I_a²R_a

         = VI_a – = I_a²R_sh = (V – I_aR_a)I_a

P_mech = E_aI_a

2)    Series motor – In this, the field winding is connected in series with the armature.

I = I_a = I_se

V = E_a + I(R_a + R_se)

Then from above equation

VI = E_aI + I²(R_a + R_se)

VI = P_mech + I² R_a + I²R_se

P_mech = E_a I

3)    Compound Motor – It is a type of dc motor which has both shunt and series field windings.

V = E_a ± I_a (R_a + R_se)

I_L = I_a ± I_F

They are basically of 2 types- long shunt and short shunt. 

Short Shunt DC Motor

When the armature winding and shunt field windings are only parallel to each other and to the series field winding. Such types of motors are also called short shunt DC Motors. 

Long Shunt DC Motor

When the armature winding is parallel to shunt field windings and to the series field winding. Such types of motors are also called long shunt DC Motors.