A family tree may not be the first descriptor to come to mind when discussing motor designs. However, with such a diverse range of models, organizing motor designs into an approachable “tree” can help guide a discussion on application usages. Much like members of a family, each motor design comes with its own character traits and strengths. There is not one universal design that works for all applications. The application itself should drive the motor design selection. There are three key factors to consider when moving through the selection process.
Constant Power Speed Range (CP speed range) is an important metric for understanding the relationship between the motor model and its intended application. The concept of CP speed range can be defined in different ways. However, in this context, this concept can be summarized as “the ratio of the highest speed at which rated power can be developed in high-speed mode, while operating within current rating, divided by the base speed.”
Motors with large CP speed ranges provide continuous torque up to base speed. With moderate drops in torque, the motor can rapidly get up to very high speeds with the large CP speed range. This end of the CP speed spectrum is typically seen in applications requiring significant traction. Motors with smaller CP speed ranges cannot jump to high speeds as quickly, but this feature is not necessary for all applications. Smaller CP speed ranges are more appropriate for applications such as driving hydraulic pumps.
Another key factor to consider is protection. This is an area that should be addressed early on in the motor selection process. Looking at the application and working backward, ask yourself, what protection concerns need to be accounted for in the motor design?
A common protection concern is water exposure. If an application will expose a motor to water, then a closed motor design may be necessary to ensure functionality. Selecting the correct protection will also help to mitigate future maintenance costs in the field. Closed motors may be more appropriate for an outdoor application, but their higher cost would not be justified for all indoor applications. Open drip proof motors (ODP), for example, are cheaper and are well-suited for applications such as washing machines or indoor air compressors.
A third important consideration in motor selection is an understanding of Duty Cyles and which cycle category is the best fit for your application. The International Electrotechnical Commission (IEC) identifies eight duty cycle designations to describe electrical motors operating conditions. Three of the more common designations are the following:
- S1 Continuous Duty: Motor can operate for an unlimited time at the rated load
- S2 Short-Time Duty: Motor can operate for a pre-set period of time at the rated load before reaching the maximum permissible temperature
- S3 Intermittent Periodic Duty: Motor operates at predetermined run and rest cycles, staying below the maximum permissible temperature
Knowing the properties of each duty designation can inform OEMs on the best choice for a specific application. Similar to the protection consideration, duty cycles involve different costs for implementation and maintenance. An escalator, for example, typically requires a continuous duty motor as it is expected to run non-stop. For a short duty operation, alternatively, a packaging line for food production may be a more appropriate fit.
There are even more considerations than discussed here, and the sheer number of variables can be overwhelming to navigate. The best place to start is to understand the requirements of your application and then sorting these into flexible and non-flexible categories. There will be compromises with any selection. However, you must understand the non-negotiable aspects of your particular project, which will help guide you to the best possible motor solution.