DC motors are fundamental to electrical engineering, widely used in applications requiring precise speed control and high starting torque. Understanding their different types is crucial for success in competitive exams like GATE. This module will delve into the classification and characteristics of common DC motor types.

DC motors are primarily classified based on how their field winding (which creates the magnetic field) is connected with respect to the armature winding (which carries the current and experiences the torque). This connection dictates the motor's operating characteristics, such as starting torque, speed regulation, and efficiency.

In a separately excited DC motor, the field winding and armature winding are supplied by independent DC sources. This allows for independent control of field flux and armature current, offering the most flexibility in speed and torque control.

In a shunt DC motor, the field winding is connected in parallel (shunt) with the armature winding. Since the field winding has many turns of thin wire, it has high resistance and draws a small, relatively constant current. This results in a nearly constant field flux, leading to good speed regulation. Shunt motors are known for their stable speed characteristics and are suitable for applications where constant speed is required, like fans and centrifugal pumps.

In a series DC motor, the field winding is connected in series with the armature winding. The field winding has few turns of thick wire to handle the full armature current. This means the field flux is directly proportional to the armature current. Consequently, series motors produce very high starting torque, making them suitable for heavy-duty applications like electric traction, cranes, and hoists. However, their speed varies significantly with load; at no load, the speed can become dangerously high.

A compound DC motor combines features of both shunt and series motors by having both a shunt field winding and a series field winding. The interaction between these two fields determines the motor's characteristics. Compound motors can be further classified into cumulative compound and differential compound motors, depending on the magnetic polarity of the series field relative to the shunt field.

In a cumulative compound motor, the magnetic effect of the series field aids the magnetic effect of the shunt field. This results in higher starting torque than a shunt motor and better speed regulation than a series motor. They are used in applications requiring high starting torque and relatively constant speed, such as rolling mills and presses.

In a differential compound motor, the magnetic effect of the series field opposes the magnetic effect of the shunt field. This configuration is rarely used in practice due to its unstable speed characteristics and is generally avoided for most applications. It is sometimes used in specific applications like arc welding where a drooping voltage-current characteristic is desired.

Focus on the relationship between field winding connection, flux, starting torque, and speed regulation for each type. Remember the unique characteristics and applications of series motors (high starting torque, dangerous at no load) and shunt motors (constant speed). Compound motors offer a blend, with cumulative being more common.