A DC compound motor is a type of DC motor that combines the features of both a shunt motor and a series motor. It has both a shunt field winding and a series field winding. The shunt field winding is connected in parallel (shunt) with the armature winding, while the series field winding is connected in series with the armature winding.
The working principle of a DC compound motor can be understood as follows:
- Stator: The stator of a DC compound motor consists of three sets of windings: the armature winding, the shunt field winding, and the series field winding. The armature winding is connected in parallel with the shunt field winding, while the series field winding is connected in series with the armature winding.
- Armature: The armature is the rotating part of the motor, consisting of a coil of wire wound around a core. When a DC voltage is applied to the armature winding, a magnetic field is created.
- Shunt Field Winding: The shunt field winding is connected in parallel with the armature winding. It generates a relatively constant magnetic field that remains nearly constant regardless of the load.
- Series Field Winding: The series field winding is connected in series with the armature winding. It generates a magnetic field that varies with the armature current.
- Magnetic Field Interaction: The shunt field winding and series field winding both produce magnetic fields that interact with the magnetic field produced by the armature current. The series field winding strengthens the magnetic field as the armature current increases.
- Speed and Torque Characteristics: The combination of the shunt field winding and series field winding in a DC compound motor allows for good speed regulation and high starting torque. The shunt field provides a nearly constant magnetic field, while the series field strengthens the magnetic field with a growing armature current, providing additional torque.
DC compound motors can be classified into two types: cumulative compound motors and differential compound motors.
- Cumulative Compound Motor: In a cumulative compound motor, the series field winding is wound in the same direction as the shunt field winding. As a result, the series field adds to the shunt field, increasing the overall magnetic field strength. Cumulative compound motors have a higher torque than shunt motors but still maintain relatively good speed regulation.
- Differential Compound Motor: In a differential compound motor, the series field winding is wound in the opposite direction to the shunt field winding. The series field weakens the shunt field, reducing the overall magnetic field strength. Differential compound motors have a lower torque than shunt motors and tend to exhibit poorer speed regulation.
DC compound motors find applications in situations where both high starting torque and good speed regulation are required, such as in elevators, rolling mills, and heavy industrial machinery.