DC motors
A direct-current motor is a device for converting dc electrical energy into rotating mechanical energy. This process can be reversed, as in a dc generator, to convert mechanical to electrical power. Direct-current equipment lends itself to automatic processes, by giving automated machinery controllable, adjustable speeds, high starting torques, as well as responsive braking. Current from batteries are DC. Alternate-current motors require expensive controllers as well as a source of alternat-current power (like the kind from wall outlets). So unless you want to equip your robot with a very, very long extension cord, I don�t think we�ll be using AC motors. Also, ease of control and high torque capability makes the DC motor attractive for many machinery applications. All motors, both AC and DC, have several basic characteristics in common. They include:
- A stator, which is the frame and other stationary components (provides the fixed magnetic field, could be a permanent magnet or an electromagnet);
- A rotor or armature, which is the rotating shaft and its associated parts (many coils of wire are wound on a cylindrical shaft);
- Auxiliary equipment, such as a brush/commutator assembly for DC motors
There are 4 main types of DC motors:
- permanent-magnet motors
- series-wound motors
- shunt-wound motors
- compound-wound motors
Permanent-Magnet Motors
A permanent magnet motor uses permanent magnets (surprise, surprise) in the stator to create the electric field. When current is passed through the rotor/armature, an electrical field is generated which, through a combination of attractive and repulsive forces with the field of the magnets, the rotor turns. Commutators and brushes make periodic contact with the coils of the armature, which changes the field polarity at the right moment to keep the motor turning the way it should. They are ideal for low-torque applications, and are very cheap. With proper gearing, these are suitable for drive motors.
Series-Wound Motors
A series-wound motor has the armature and field circuit wired in series, as shown in the diagram. It works like a permanent magnet motor, but the stator field comes from the windings. All current is forced to pass through the electromagnets before reaching the armature. This means that if the load increases (which draws more current), the current through the motor also increases, which creates a stronger magnetic field (thus more torque). The motor�s speed varies from very fast at no load to very slow at heavy loads. Since large currents may flow through the low-resistance field, the series motor produces a high-torque output. They are ideal for heavy load applications where speed regulation in not important, such as an automobile starter motor. Large series-wound motors should never be ran under no-load, due to runaway tendency of the motor. The high speed will often cause damage to the brushes or commutator. This problem does not exist for smaller motors due to the internal friction, thus series-wound motors from hand-held power tools such as drills and saws, as well as appliances such as mixers and blenders are perfect for driving weapons. An interesting note is that they also run on AC as well as DC power.
Shunt-Wound Motors
Shunt-wound motors have the wires going to the coils wired in parallel. Thus, the current can choose to go through the stator, or the rotor. This makes for a steadier and more reliable method of speed control, since reducing the current automatically reduces the speed of the motor.
Compound-Wound Motors
Have two coils of wire about the stator. One connected in series, and the other in parallel. This combines the benefits of both series-wound and shunt-wound motors. They are expensive and we will probably never come across them.
Conclusion
- Permanent-magnet dc motors � inexpensive motor with good speed control but with low torque. These are most easily modified to work with robots.
- Series-wound dc motors � high torque but changes in load dramatically change the rotational speed. These are reasonable to work with.
- Shunt-wound dc motors � good speed regulation with slight variations with a changing load, good overall more (widely used in industry). These are difficult to work with.
- Compound-wound motors � has the best characteristics of both the series and shunt-wound motors, but is very expensive.
This information was from: http://www.mae.ncsu.edu/courses/mae732/shih/00_motors.pdf