A compound wound DC motor or rather a DC compound motor falls under the category of self excited motors, and is made up of both series the field coils S1 S2 and shunt field coils F1 F2 connected to the armature winding as shown in the figure below.
Both the field coils provide for the required amount of magnetic flux, that links with the armature coil and brings about the torque necessary to facilitate rotation at desired speed.
As we can understand, a compound wound DC motor is basically formed by the amalgamation of a shunt wound DC motor and series wound DC motor to achieve the better off properties of both these types. Like a shunt wound DC motor is bestowed with an extremely efficient speed regulation characteristic, whereas the DC series motor has high starting torque
So the compound wound DC motor reaches a compromise in terms of both this features and has a good combination of proper speed regulation and high starting toque.
Though its staring torque is not as high as in case of DC motor, nor is its speed regulation as good as a shunt DC motor. Overall characteristics of DC shunt motor falls somewhere in between these 2 extreme limits.
Types of Compound Wound DC Motor
The compound wound DC motor can further be subdivided into 2 major types on the basis of its field winding connection with respect to the armature winding, and they are:
Long Shunt Compound Wound
DC Motor In case of long shunt compound wound DC motor, the shunt field winding is connected in parallel across the series combination of both the armature and series field coil, as shown in the diagram below.
Short Shunt Compound Wound DC Motor
In case of short shunt compound wound DC motor, the shunt field winding is connected in parallel across the armature winding only. And series field coil is exposed to the entire supply current, before being split up into armature and shunt field current as shown in the diagram below
Compound DC Motors Advantage
Inexpensive controls for speed regulation
Generally a low-cost motor option
Easily pairs with gear reducers
Compound DC Motors Disadvantage
Cogging at speeds of less than 300 rpm
Significant power loss on full wave rectified voltage