Output power, which you can find on the manufacturer’s data sheet, is the chief design criteria when trying to get the best price and performance of a stepper motor. Be aware that there are serious differences in performance depending on the stepper motor driver. Measuring stepper motor pull-in and -out torque is rather difficult because it’s easily influenced by inertia and various resonances in the system — not to mention that the application’s inertia and damping are typically different, resulting in pull-out curve data that isn’t necessarily correct for a specific application.
You should also take mechanical aspects into consideration when choosing a stepper motor. The stepper motor’s weight and dimensions are important considerations as well. Smaller stepper motors often allow for compact mechanical designs, making for easier motion system design.
When you need a long operating life you’ll need a stepper motor with ball bearings. Smaller permanent-magnet (PM) motors generally use slide bearings whereas hybrid motors use ball bearings in order to maintain narrow air-gaps. Some manufacturers offer PM motors with optional ball bearings, but they are typically prohibitively expensive. Ball bearings are especially important if your stepper motor directly drives a belt gearing or transmission; they prolong the motor’s life in addition to reducing torque loss resulting from bearing friction caused by the belt’s tension.
The cost of your step motor will depend on the type and size of the motor (note: resistance and type of winding do not influence the cost). Hybrid motors are generally more expensive than their PM counterparts. Generally stepper motor cost increases with the size of the motor. Motor production volumes and the number of manufacturers creating a specific type/size of motor will also affect the cost of your stepper motor, meaning that the more popular motor model is often the best choice — even when the output power is higher than necessary.
The final thing to consider when selecting a stepper motor is customization. Many manufacturers allow you to customize the stepper motor’s shaft, winding, rotor, and lead wires in medium- and high-volume applications. Some manufacturers allow you to customize bearing types, mounting flange, and shaft diameter in high-volume applications.