As you already know, there are two electrical elements of a DC motor, the field windings and the armature.The armature windings are made up of current carrying conductors that terminate at a commutator.
DC voltage is applied to the armature windings through carbon brushes which ride on the commutator. In small DC motors, permanent magnets can be used for the stator. However, in large motors used in industrial applications the stator is an electromagnet.
For simplicity of explanation, the stator is represented by permanent magnets in the following illustration.
The field of DC motors can be:
- Permanent magnet (Permanent magnet stator),
- Electromagnets connected in series (Wound stator),
- Shunt (Wound stator), or
- Compound (Wound stator).
Let’s see the basics of each type as well as their advantages and disadvantages.
The permanent magnet motor uses a magnet to supply field flux. Permanent magnet DC motors have excellent starting torque capability with good speed regulation. A disadvantage of permanent magnet DC motors is they are limited to the amount of load they can drive.These motors can be found on low horsepower applications.
Another disadvantage is that torque is usually limited to 150% of rated torque to prevent demagnetization of the permanent magnets.
In a series DC motor the field is connected in series with the armature.The field is wound with a few turns of large wire because it must carry the full armature current.
A characteristic of series motors is the motor develops a large amount of starting torque. However, speed varies widely between no load and full load. Series motors cannot be used where a constant speed is required under varying loads.
Series-connected motors generally are not suitable for use on most variable speed drive applications.
In a shunt motor the field is connected in parallel (shunt) with the armature windings. The shunt-connected motor offers good speed regulation. The field winding can be separately excited or connected to the same source as the armature.
The shunt-connected motor offers simplified control for reversing. This is especially beneficial in regenerative drives.
Compound motors have a field connected in series with the armature and a separately excited shunt field. The series field provides better starting torque and the shunt field provides better speed regulation.
DC Motor – Explained (VIDEO)
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Reference: Basics of DC Drives – SIEMENS (Download)