There are a wide variety of stepper types, some of which require very specialized drivers. For our purposes, we will focus on stepper motors that can be driven with commonly available drivers. These are: Permanent Magnet or Hybrid steppers, either 2-phase bipolar, or 4-phase unipolar.

Motor Size

One of the first things to consider is the work that the motor has to do. As you might expect, larger motors are capable of delivering more power. Stepper motors come in sizes ranging from smaller than a peanut to big NEMA 57 monsters.

Most motors have torque ratings. This is what you need to look at to decide if the motor has the strength to do what you want.

NEMA 17 is a common size used in 3D printers and smaller CNC mills. Smaller motors find applications in many robotic and animatronic applications. The larger NEMA frames are common in CNC machines and industrial applications.

The NEMA numbers define standard faceplate dimensions for mounting the motor. They do not define the other characteristics of a motor. Two different NEMA 17 motors may have entirely different electrical or mechanical specifications and are not necessarily interchangeable.

Step Count

The next thing to consider is the positioning resolution you require. The number of steps per revolution ranges from 4 to 400. Commonly available step counts are 24, 48 and 200.

Resolution is often expressed as degrees per step. A 1.8° motor is the same as a 200 step/revolution motor.

The trade-off for high resolution is speed and torque. High step count motors top-out at lower RPMs than similar size. And the higher step-rates needed to turn these motors results in lower torque than a similar size low-step-count motor at similar speeds.

Gearing

Another way to achieve high positioning resolution is with gearing. A 32:1 gear-train applied to the output of an 8-steps/revolution motor will result in a 256 step motor.

A gear train will also increase the torque of the motor. Some tiny geared steppers are capable of impressive torque. But the tradeoff of course is speed. Geared stepper motors are generally limited to low RPM applications.

Backlash is another issue with geared motors.  When the motor reverses direction, it needs to take up any slack there may be in the gear-train.  This can affect positioning accuracy.

Shaft Style

Another thing to consider is how the motor will interface with the rest of the drive system. Motors are available with a number of shaft styles:

• Round or "D" Shaft: These are available in a variety of standard diameters and there are many pulleys, gears and shaft couplers designed to fit. "D" shafts have one flattened side to help prevent slippage. These are desirable when high torques are involved.
• Geared shaft: Some shafts have gear teeth milled right into them. These are typically designed to mate with modular gear trains.
• Lead-Screw Shaft: Motors with lead-screw shafts are used to build linear actuators. Miniature versions of these can be found as head positioners in many disk drives.
  

Wiring

There are many variations in stepper motor wiring. For our purposes, we will focus on steppers that can be driven with commonly available drivers. These are Permanent Magnet or Hybrid steppers wired as 2-phase bipolar, or 4-phase unipolar.

Coils and Phases

A stepper motor may have any number of coils. But these are connected in groups called "phases". All the coils in a phase are energized together.

Unipolar vs. Bipolar

Unipolar drivers, always energize the phases in the same way. One lead, the "common" lead, will always be negative. The other lead will always be positive. Unipolar drivers can be implemented with simple transistor circuitry. The disadvantage is that there is less available torque because only half of the coils can be energized at a time.

Bipolar drivers use H-bridge circuitry to actually reverse the current flow through the phases. By energizing the phases with alternating the polarity, all the coils can be put to work turning the motor.

A two phase bipolar motor has 2 groups of coils. A 4 phase unipolar motor has 4. A 2-phase bipolar motor will have 4 wires - 2 for each phase. Some motors come with flexible wiring that allows you to run the motor as either bipolar or unipolar.