A stepper motor or step motor or stepping motor is a brushless DC electric motor that divides a full rotation into a number of equal steps. The motor's position can then be commanded to move and hold at one of these steps without any position sensor for feedback (an open-loop controller), as long as the motor is carefully sized to the application in respect to torque and speed.
Switched reluctance motors are very large stepping motors with a reduced pole count, and generally are closed-loop commutated.
General recommendations Unless you will be using external stepper motor drivers, choose motors with rated current of at least 1.2A, and at most 2.0A for the Duet 0.6 and Duet 0.8.5, or 3A for the Duet WiFi or Duet Ethernet. Plan to run each stepper motor at between 50% and 85% of its rated current. Size: Nema 17 is the most popular size used in 3D printers. Nema 14 stepper motor is an alternative in a highly-geared extruder. Use Nema 23 motors if you cannot get sufficient torque from long Nema 17 motors. Avoid motors with rated voltage (or product of rated current and phase resistance) > 4V or inductance > 4mH. Choose 0.9deg/step motors where you want extra positioning accuracy, e.g. for the tower motors of a delta printer. Otherwise choose 1.8deg/step motors. If you use any 0.9deg/step motors, or high torque motors, use 24V power so that you will be able to maintain torque at higher speeds. If using a highly-geared extruder (for example, an extruder that uses a flexible drive cable to transmit the torque from the motor to a worm reduction gear), use a short low-inductance 1.8deg/step motor to drive it.
See more:http://www.stepperyoyo.com/