Motors 101

Types of Motors -

• AC Motors - simply a series of three windings in the exterior (stator) section with a simple rotating section (rotor). The changing field caused by the 50 or 60 Hertz AC line voltage causes the rotor to rotate around the axis of the motor. Cheap e.g. for air conditioners with constant speed. Expensive to control.
  
• DC Motors - Brushes change direction of current and the electromagnetic forces due to the current against the magnetic forces of the magnet + inertia keeps the coil moving.
  
• Brushless DC Motors -
  
• Servo Motors - Consists of a DC motor, a feedback device, and a control board. When given a position command it will actively hold that position and resist movement equal to its torque capacity. In R/C servos the feedback device is a potentiometer (variable resistor). The motor, through a series of gears, turns the output shaft and the potentiometer simultaneously. The potentiometer is fed into the servo control circuit and when the control circuit detects that the position is correct, it stops the motor.
• Brushed DC Servo Motors -
  
• Brushless AC Servo Motors -
  
• Stepper Motors - Picture four coils with a magnet in the center. There are 2 current sources, each going to opposite coils. By varying the direction of flow of the current, it is possible to manipulate the direction of the movement of the center magnet, as well as control its rotational speed.
  
• Linear Motors -
  

Good intro to motors -
http://www.ee.ualberta.ca/~verret/ee401/motors.htm

• In a brushed DC motor, torque control is also simple, since output torque is proportional to current. If you limit the current, you have just limited the torque which the motor can achieve. This makes this motor ideal for delicate applications such as textile manufacturing.

http://www.oddparts.com/acsi/motortut.htm

A drive is the power electronics board which is used to control the speed of a motor. Drives may use various techniques to control speed.

A low-cost form of drive for controlling DC motors makes use of a special type of power transistor called an SCR. Hence the term SCR drive .

An SCR acts as a gate to allow a precise amount of current to flow to the motor. Through feedback systems, this current also controls the voltage allowed to generate the armature field of the motor, and thus the speed is controlled.

SCR drives are most commonly used to control DC motors, but the system is also used in some older AC inverter drives controlling AC motors


http://www.iea.lth.se/sed - good class notes on Torque Control

http://www.mskennedy.com/media/documents/Motor.pdf