Short Answer

DC motors are classified based on how their field winding is connected with the armature. The main types are separately excited DC motors and self-excited DC motors.

Self-excited DC motors are further divided into shunt motor, series motor, and compound motor. Each type has different speed and torque characteristics, so they are used for different applications.

Detailed Explanation:

Types of DC motors

Separately excited DC motor

A separately excited DC motor is a type in which the field winding is supplied from an external source. The armature winding gets power from the main DC supply, while the field winding receives current separately.

This type of motor allows better control over speed because the field current can be adjusted independently. By changing the field current, the magnetic field strength can be controlled, which directly affects the motor speed.

Separately excited motors are used in applications where precise speed control is required, such as in testing equipment, laboratories, and some industrial machines. However, they require an extra power source for field excitation, which increases cost and complexity.

Self-excited DC motors

Self-excited DC motors are those in which the field winding is connected to the same source that supplies the armature. These motors are widely used because they are simple and economical.

They are further divided into three main types:

Shunt motor

In a shunt motor, the field winding is connected in parallel (shunt) with the armature winding. The field winding has many turns of thin wire and carries a small current.

Shunt motors are known for having nearly constant speed, even when the load changes. This makes them suitable for applications like fans, blowers, and machine tools where constant speed is required.

However, the starting torque of a shunt motor is moderate compared to other types.

Series motor

In a series motor, the field winding is connected in series with the armature winding. Therefore, the same current flows through both.

The field winding has fewer turns of thick wire because it carries high current. Series motors produce very high starting torque, which makes them suitable for applications like cranes, hoists, and electric trains.

However, their speed varies greatly with load. At no load, the speed can become dangerously high, so they should never be operated without load.

Compound motor

A compound motor has both shunt and series field windings. It combines the advantages of both types.

Compound motors are further divided into:

• Cumulative compound motor:
  In this type, the series field supports the shunt field. It provides high starting torque and fairly constant speed. This type is widely used in elevators, conveyors, and rolling mills.
• Differential compound motor:
  In this type, the series field opposes the shunt field. This reduces the overall magnetic field and leads to unstable operation. Therefore, it is rarely used in practice.

Importance of classification

The classification of DC motors helps in selecting the right motor for a specific application. Each type has unique characteristics in terms of speed, torque, and performance.

For example, shunt motors are used where constant speed is needed, while series motors are used where high starting torque is required. Compound motors are used where both good torque and stable speed are needed.

Understanding these types helps engineers design efficient systems and choose the best motor for different industrial uses.

Conclusion

DC motors are classified into separately excited and self-excited types, with further divisions like shunt, series, and compound motors. Each type has its own features and applications, making them suitable for different purposes.