In the motion control industry, step motors have a wide variety of uses. To accommodate these applications, manufacturers offer step motors in various sizes and with various mechanical specifications. In terms of size, step motor manufacturers adhere to various NEMA frame sizes. These sizes range from the miniature NEMA 8 to the large NEMA 42. Each of these motors, in turn, has its own distinct performance characteristics. Smaller motors such as the NEMA 8 or 11, for instance, are meant for high-speed applications that do not require a lot of torque. Larger motors such as the NEMA 34 or 42, on the other hand, are beneficial for applications that require high torque at slower speeds.
Mechanically, the difference between step motors will exist within the motor. This difference will determine whether a motor will operate at either a 0.9- or a 1.8-degree step angle. Similar to frame size selection, users will choose a step motor based on specific needs. A 0.9-degree motor would be better for applications that require high accuracy and a 1.8-degree motor would be more appropriate for applications that require more torque. A motor with a 0.45-degree step angle provides high precision and satisfies applications requiring smooth motion. Considering these performance trade-offs is very important when choosing a motor.
Traditionally, the main difference between one step motor versus another would involve its frame size, step angle and the specific motor manufacturer's capabilities. Some manufacturers go further and create custom motors based on the needs of the specific user.
Different capabilities of the step motor manufacturer, such as engineering, design and product expertise, will affect the types of customizations the manufacturer is able to produce. Not all step motors are created equal, and it is important to note that the same goes for motor manufacturers as well. While some are able to create various types of customizations, others might be limited. The following discusses three types of step motors that one manufacturer, Lin Engineering, produces.
Step motor engineers can face a wide variety of step motor applications for various industries. These industries include: avionics, medical equipment, surveillance, semiconductor, engraving, packaging and so forth. Each industry has its own specific requirements and, in many cases, the step motors have significant visual differences. Engineers at Lin Engineering have characterized these broad requirements into three distinct categories:
||Traditional or Standard (Refer to 1, 2 and 3)
||Motors that are routinely manufactured and can, in essence, be purchased as an “off-the-shelf” type of product.
||Modified (Refer to 4 and 5)
||Motors that require minimal to moderate customizations such as a special shaft, housing, connectors or special materials.
||Ultra-Custom (Refer to 6)
||Motors that require a substantial amount of engineering time and expertise.
A motor developed for an aircraft application required opening and closing an air conditioning (AC) vent. In this application, size, torque, position feedback and price were major concerns. The size constraint called for a NEMA 11 motor but the required torque was a lot higher than what the NEMA 11 can produce. In order to achieve the desired torque with such a small motor, engineers designed a custom gearbox that would both mount onto the NEMA 11 motor frame and adhere to the customer's size constraint.
Since the motor was going to be used to open and close the AC vent a certain distance, position feedback was also necessary. An analog encoder was determined to be a feasible solution, but this device proved to be too costly and would not meet the customer's expense budget. A potentiometer mounted onto the gearbox met both the cost and performance requirements. Spinning the motor would turn the potentiometer and provide a voltage anywhere from 0.1 to 5V dc. Once the correlation between motor steps and potentiometer voltage was determined, positions of the AC vent were then correlated to the voltage. This relationship was then used to move the motor the desired number of steps until a certain voltage amount was read and, in turn, located the AC vent in the desired position. The relationship between motors steps, potentiometer voltage and AC vent position formed the closed loop system the customer desired.
|Frame Size (diameter in inches)
|Holding Torque Range (oz-in)
||3 to 4
||9 to 17
||7 to 20
||8 to 93
||100 to 294
||310 to 1,288
|Max. Radial Load (lb)
|Max. Axial Load (lb)
|NEMA ratings for size 8 to 34 (and even the 42) are essentially the motor diameter times 10. As a result, a higher rating normally means higher holding torque but a range of values provides overlap. Also, the higher rating allows a greater maximum axial load.