Generally in compound motor both series and shunt field exist. In this type shunt field is much stronger than series field winding. The compound can be classified into two types. They are Cumulative compound motor and differential compound motor. In cumulative compound motor the series field winding aids the shunt field winding. In differential compound motor series field opposes the shunt field. Therefore differential compound motor hardly used because of their meager torque characteristic at large load.
The figure above shows that the circuit diagram of a cumulative DC Compound Motor. In this every pole having both the series and shunt field winding and the series field winding sustaining the shunt field winding. Therefore differential compound.
Armature Torque Vs Armature Current
When the load increases in compound motor, the strength of the series field increases whereas the shunt field residues constant.
Accordingly, the flux in the motor is increased and hence the armature torque.
It might be well-known that torque is always higher in compound motor when compared to shunt motor for a given armature current owing to series field shown in fig.
Speed Vs Armature current Characteristic
As seen above the flux per pole increases whenever the lead increases. As a result the motor speed reduces as the load increases shown in the fig above. It might be prominent that as the load is added the flux increases causes the speed to reduce more than look after the speed of the shunt motor. In consequence the speed regulation is not good as compared to when compared to shunt motor.
Speed Vs Armature Torque Characteristic
In a cumulative compound motor, for a given armature current the torque is less than that of a series motor but more than that of a shunt motor. The speed vs. armature current characteristic curve is shown in the above fig.
With the above seen three characteristic we understood that cumulative compound has characteristics in-between the shunt and series motor. Also we conclude that the motor is not permitted from running away at no-load due to the presence of shunt field and due to the occurrence of series field winding the initial torque is increased.