DC motors are rotary motors that operate on electrical energy. They are primarily used for converting electrical energy into mechanical energy. Most of the DC motors rely on magnetic fields to generate torque inside them. DC motors' most prominent feature is that it contains a mechanism to periodically change the current direction. This is done to maintain the unidirectional nature of current in parts of the motor. Small DC motors find use in many toys, tools and appliances.
DC motors can be differentiated based on the type of windings they use or the operating mechanism they employ. Some common types of DC motors are described below.
In these motors, a different electrical source is used to power the field and the armature windings. The most important feature is that the armature current does not flow through the field windings. A separate DC source usually operates field windings. This source is external to the motor itself but acts in accordance with it. The torque of a separately excited DC motor can be changed by changing the field flux. This can be done independently of the armature current.
A permanent magnet DC motor does not have field windings on the stator frame. The magnetic field is instead provided by the magnetic field exerted by the permanent magnet. Windings are used in series with the armature. This is generally used on large motors, and it improves the commutation under the load of the motor.
Since the magnetic field generated by a permanent magnet does not change with time, the field of a permanent magnet DC motor is fixed. Therefore it is not possible to vary or control the speed of this motor. However, these motors are useful in miniature form because they eliminate the power consumption due to field windings. In addition, the motor does not heat up unnecessarily, which also reduces the fire hazard due to overheating.
The armature and field windings in series DC motors are connected in series with a common DC power source. This means that a common current flows through both armature windings and the field windings. The motor speed of such motors changes non-linearly with load torque and the armature current. This leads to slower speed with a high load, but conversely, it rotates very fast with no load.
The common current in the stator and rotor leads to the motor exhibiting current squared behaviour. The series DC motor has a very high starting torque. That is why it is used to start motors with a very high inertial load. Examples can be trains, elevators and hoists. The properties that the series DC motor has with respect to speed and torque are very useful in dragline excavations.
A series DC motor should never be started with no load. This can lead to overspeeding in the rotor, and as a result, the motor may end up getting damaged. This condition is called the runaway condition.
Shunt DC motors have armature and field windings connected in parallel, which is also called a shunt, with the external DC source. As in series DC motors, the external power source is common to both armature and the field windings. Shunt DC motors give good speed regulation. This is independent of the change in load. This makes them useful in situations where high-speed control is desired, for example, industries where the machines and humans work in close proximity.
Shunt motors do have a very high starting torque. They are, therefore, not suited to be operated in motors with a high inertial load. They are most commonly used in industrial adjustable speed appliances like machine tools, winding and unwinding machines and tensioners.
Compound DC motors possess the feature of both series DC motors and parallel DC motors. In the compound, DC motors, the armature windings and the field windings are connected in series and shunt to give the motor high starting torque and good speed regulation. This is very useful in situations where high initial torque and speed regulation are very important. In addition, a compound DC motor gives flexibility in how the user desires to operate the motor in a specific situation.
Compound DC motors have two different arrangements. The first arrangement is called cumulative arrangement. In this arrangement, the series field is connected to help the shunt field. This gives a very high starting torque and speed regulation to a lesser extent. The second arrangement is called the differential arrangement. This arrangement provides good speed regulation and is commonly operated at a constant speed.