Characteristic
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.
When voltage is applied to stator windings an electromagnet with north
and south poles is established. The resultant magnetic field is static
(non-rotational).
For simplicity of explanation, the stator is represented by permanent magnets in the following illustration.
DC Motor Construction
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).
1. Permanent Magnet Motors
Permanent Magnet Motor
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.
2. Series Motors
Series DC motor
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.
Additionally, the speed of a series motor with no load increases to the point where the motor can become damaged. Some load must always be connected to a series-connected motor.
Series-connected motors generally are not suitable for use on most variable speed drive applications.
3. Shunt Motors
DC Shunt Motor
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.
An advantage to a separately excited shunt field is the ability of a variable speed drive to provide independent control of the armature and field.
4. Compound Motors
Compound DC motor
