There are many aspects to consider when selecting a motor, such as application, operational, mechanical, and environmental issues. Generally speaking, the choice is either an ac motor, a dc motor, or a servo/stepper motor. Knowing which one to use depends on the industrial application and if there are any special needs required.
A constant or variable torque and horsepower will be required for the motor depending on the type of load the motor is driving. The size of the load, the required speed, and acceleration/deceleration—particularly if it's fast and/or frequent—will define the torque and horsepower that is required. Requirements for controlling motor speed and position also need to be considered.
Motor load types
There are four types of industrial automation motor loads:
Variable horsepower and constant torque applications include conveyors, cranes, and gear-type pumps. In these applications, the torque is constant because the load doesn't change. The required horsepower may vary depending on the application, which makes constant speed ac and dc motors a good choice.
An example of a variable torque and constant horsepower application is a machine rewinding paper. The material speed remains constant, which means the horsepower doesn't change. The load does change, however, as the roll diameter increases. In small systems, this is a good application for dc motors or a servo motor. Regenerative power also is a concern and should be considered when sizing the motor or choosing the energy control method. AC motors with encoders, closed-loop control, and full quadrant drives may be beneficial for larger systems.
Fans, centrifugal pumps, and agitators require variable horsepower and torque. As the motor speed increases, the load output also increases along with the required horsepower and torque. These types of loads are where much of the motor efficiency discussion begins with inverter duty ac motors using variable speed drives (VSDs).
Applications such as linear actuators, which need to move to multiple positions accurately, require tight positional or torque control and often require feedback to verify correct motor position. Servo or stepper motors are the best option for these applications, but a dc motor with feedback or an inverter duty ac motor with an encoder often is used for tight torque control in steel or paper lines as well as similar applications.
Different motor types
While there are two main motor classifications—ac and dc—there are over three dozen types of motors used in industrial applications.
While there are many motor types, there is a great deal of overlap in industrial applications and the market has pushed to simplify motor selection. This has narrowed practical choices for motors in most application. The six most common motor types, which fit the vast majority of applications, are brushless and brush dc motors, ac squirrel cage and wound rotor motors, and servo and stepper motors. These motor types fit the vast majority of applications with the other types used only in specialty applications.
Three main application types
The three main applications for motors are constant speed, variable speed, and position (or torque) control. Different industrial automation situations require different applications and questions and their own set of questions.
For example, a gearbox may be required if the top speed is less than the motor's base speed. This also may allow a smaller motor running at a more efficient speed. While there is a great deal of information online on how to size a motor, users must account for many factors because there are many details to consider. Calculating load inertia, torque, and speed requires the user to know about parameters such as total mass and size (radius) of the load as well as friction, gearbox losses, and the machine cycle. Changes in load, speed of acceleration or deceleration, and the application's duty cycle also must be considered or the motor may overheat.
After the motor type is selected and sized, users also need to consider environmental factors and motor enclosure types such as open frame and stainless housing for washdown applications.
Motor selection: 3 questions
Even after all those decisions have been made, the user needs to address these three questions before making a final decision.
1. Is it a constant speed application?
In a constant speed application, a motor often runs at an approximate speed with little or no concern about acceleration and deceleration ramps. This type of application is usually run using across-the-line on/off control. The control circuits often consist of a branch circuit fusing with a contactor, an overload motor starter, and a manual motor controller or soft starter.
Both ac and dc motors are suitable for constant speed applications. DC motors provide full torque at zero speed and have a large installed base. AC motors are also a good choice because they have a high power factor and require little maintenance. A servo or stepper motor's high performance characteristics, by comparison, would be considered overkill for a simple application.
2. Is it a variable speed application?
Variable speed applications usually require tight velocity and speed changes as well as defined acceleration and deceleration ramps. Reducing the motor speed in application, such as fans and centrifugal pumps often improves efficiency by matching the power consumed to the load instead of running at full speed and throttling or dampening the output. These are very important considerations for conveying applications, such as bottling lines.
Both ac and dc motors with the appropriate drives work well in variable speed applications. A dc motor and drive configuration was the only variable speed motor option for a long time and the components are developed and proven. Even now, dc motors are popular in variable speed, fractional horsepower applications and are useful in low-speed applications because they can provide full torque at low speed and constant torque across a wide range of motor speeds.
Maintenance can be a concern with dc motors, however, because many require brushes for commutation, and they wear out from being in contact with moving parts. Brushless dc motors eliminate this issue, but they are more expensive in upfront costs and the range of available motors is smaller.
Brush wear is not an issue with ac induction motors and a variable frequency drive (VFD) creates a useful choice for applications over 1 hp such as fan and pumping applications, which lead to improved efficiency. The type of drive chosen to run the motor can add some positional awareness. An encoder can be added to the motor if the application requires it, and a drive can be specified to use the encoder feedback. This setup can provide servo-like speed as a result.
3. Is position control required for the application?
Tight position control is accomplished through continuous verification of the motor's position as it moves. Applications such as positioning a linear actuator can use a stepper motor with or without feedback or a servo motor with inherent feedback.
A stepper is designed to accurately move to a position at a moderate speed and then hold the position. An open-loop stepper system offers strong positional control if properly sized. While there is no feedback, the stepper will move the exact number of steps unless it encounters a load disruption beyond its capacity. As the application's speed and dynamics increase, open-loop stepper control may not be able to meet system requirements, which requires an upgrade to a stepper with feedback or to a servo motor system.
A closed-loop system provides accurate, high-speed motion profiles and precise position control. A servo system will provide higher torque at high speeds compared to a stepper, and they also work better in high-dynamic load or complex-motion applications.
For high-performance motion with low-position overshoot, the reflected load inertia should be matched to the servo motor inertia as closely as possible. Up to a 10:1 mismatch will perform adequately in some applications, but a 1:1 match is optimal. Geared speed reduction is an excellent way to solve inertia mismatch problems as the reflected load inertia falls by the square of the gear ratio, but gearbox inertia must be included in the calculations.
Application, motor knowledge
Manufacturers offer a wide selection of motors for industrial applications. Stepper, servo, ac, and dc motors can meet most industrial automation requirements, but the ideal motor depends on the application. Whether it's a constant speed, variable speed, or position control application-users should work closely with the motor and drive the supplier to select the right motor for the application.
Bryan Sisler, product manager, AutomationDirect. Edited by Chris Vavra, production editor, Control Engineering, CFE Media, cvavra(at)cfemedia.com.
Motor selection depends on many facets including application, operational, mechanical, and environmental issues.
Users should determine whether the application is constant or variable speed or if position control is required.
What other considerations should be taken when selecting a motor?