D.C. Compound Motor

The compound motor consists of part of the field winding connected in series and part of the field winding connected in parallel with armature. It is further classified as long shunt compound and short shunt compound motor.

1.1 Long Shunt Compound Motor

In this type, the shunt field winding is connected across the combination of armature and the series field winding as shown in the Fig. 1.

Fig. 1   Long shunt compound motor

Let Rse be the resistance of series field and R_sh be the resistance of shunt field winding. The total current drawn from supply is I_L.

So      I_L =  I_se +  I_sh
But    I_se = I_a
.^..               I_L =  Ia+  I_sh
And  I_sh = V/R_sh
And    V = E_b  + I_a R_a + I_se R_se + V_brush
But as      I_se  = I_a ,
.^..               V = E_b  + I_a (R_a + R_se) + V_brush 

1.2 Short Shunt Compound Motor

In this type, the shunt field is connected purely in parallel with armature and the series field is connected in series with this combination shown in the Fig. 2.

Fig. 2   Short shunt compound motor

I_L =  Ise

The entire line current is passing through the series field winding.

and          I_L = I_a + I_sh

Now the drop across the shunt field winding is to be calculated from the voltage equation.
So           V = E_b   + I_se R_se + I_a R_a + V_brush 
but         I_se = I_L 

.^..                   V = E_b   + I_L R_se + I_a R_a + V_brush 

.^..     Drop across shunt field winding is,

=  V – I_L R_se  = E_b  + I_a R_a + Vbrush

Apart from these two, compound motor can be classified into two more types,

i) Cumulatively compound motors and ii) Differential compound motors.

Note :  If the two field windings are wound in such a manner that the fluxes produced by the two always help each other, the motor is called cumulatively compound. If the fluxes produced by the two field windings are trying to cancel each other i.e. they are in opposite direction, the motor is called differential compound.

A long shunt compound motor can be of cumulative or differential type. Similarly short shunt compound motor can be cumulative or differential type.