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DC Motors- are seldom used in ordinary industrial applications where alternating current is readily available, but they are larger and more expensive than AC induction motors. DC motors are the obvious choice in applications where DC sources are all that is available. However for some applications it is advantageous to transform the alternating current into direct current in order to use DC motors. One advantage of DC machines is the ability to precisely control speed and/or torque of the application.
DC motor characteristics:
- Adjustable motor speed over a wide range of applications.
- Constant mechanical torque.
- Rapid acceleration or deceleration.
- Responsive to feedback signals.
Primary DC motor parts:
Field- is the stationary outer part of the motor. This is the winding which has the DC current flowing through it.
Armature- is the rotating inner part of the motor. This contains the windings which carry the alternating current in them.
Commutator- is the structure which changes the direction of the armature currents depending on their alignment relative to the stator field. It is made of conducting bars which are electrically insulated from each other. The copper windings for each bar adjacent to each other are wound in opposite directions from each other.
Brushes- transfer voltage from the DC power source to the armature via the commutator. They ride on the commutator. They are shaped like a paint brush and made of semiconductor carbon.
DC motor types:
- Wound Field motor (Brush motors) - typically a self excited series DC construction. The field winding and armature are connected in series. The field current equals the armature current. The field is constructed of a few turns of large gauge wire which has less resistance and results in higher current draw. Higher current equals higher torque. For applications requiring a high starting torque, high accelerating torque, and/or high speeds with lighter loads. As the load decreases, speed increases.
- Shunt Connected Field motor – typically a self excited construction with the field windings in parallel with the armature. The field is constructed of many turns of small gauge wire. Ideal for applications requiring constant speed with changing loads.
- Compound Field motor – typically a self excited construction, it has both a series and shunt field. Both fields are wound in the same direction to aid each other. A rheostat controls current going to the shunt field. Ideal for sudden changes in load it maintains a reasonably constant speed.
- Permanent Magnet DC motor – The field winding electromagnet is replaced by a permanent magnet(s). An inexpensive reliable construction for small loads.
Note: Self excited motors require a residual magnetism to be present in the motor iron to start the self excitation process. DC motors with separately excited fields utilize an independent voltage source to excite the field windings. These are utilized in feedback control systems where control of the armature voltage over a wide range is required.
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