Well there are five main parameters you really need to worry about.
1. Motor type, bipolar/unipolar.
2. Physical size, see "What does Nema stand for". To help you out, Nema23 is used for most hobby machines and Nema34 for bigger stuff.
3. Rated voltage. This voltage is what can be applied to the motor windings without damage being caused due to over-current.
4. Maximum current rating
5. Holding torque.
I will consider 3 and 4.
Now it is common for people to think that they should buy a stepper motor with a rated voltage equal to their supply voltage. This is of course not going to do the motors any harm if you do and in some instances such as printers it is sensible as it keeps the price down as no current limiting hardware is required. If on the other hand you are not a printer manufacturer then it is much better to run the motors at up to 20X there rated voltage. That might sound a terrible thing to do however if you buy a driver with a current limit (often known as a chopper driver) then the current can be kept to below the maximum current rating.
What does this mean? Well volts equate to speed effectively. If you have a high voltage your motors can spin quicker. Current equates to torque but too much current can damage the motor. A higher voltage supply and the current limit gives the best of both worlds.
So how do I choose a motor. Well if you are the average builder then once you have a rough idea of the physical size of the motor and the required torque (another FAQ required) then you should try and get a motor that suits your supply or a motor with a low rated voltage so that when you build your supply the output voltage can be lower for good speed. The current to some extent will be limited by your drivers. Most hobby ones are around the 2 amp mark. Getting the highest current that your driver can handle will get you the highest torque but it will depend on the specific motor.
I hope this is accurate and at least slightly helpful if a little basic
for a lot more detailed coverage of stepper motors (bottom of page)
Last edited by ynneb; 08-28-2004 at 09:22 AM. Reason: Just a pretty picture to suit the post.