A simplified definition of a servo system is that it consists of several components that work together to control or regulate the speed/position of a load. A servo motor is one of these components in a system.
When it comes time to select the right servo motor for an application, some may think they can simply size the motor based on the horsepower rating of the currently installed motor or based solely on the torque requirements of the application.
Instead, the following factors must be considered when selecting the right motor:
Ratio of inertia
Speed
Maximum torque at desired speed
Ratio of inertia
Any rotating object has a rotational inertia, which is a measure of how difficult it is to change the rotational speed of that object. The moment of inertia in a servo system can be divided into two parts: load inertia and motor inertia.
Motor inertia is part of the servo design and is usually listed in the manufacturer's specification sheet. Load inertia is more complex because it involves each component moved by the motor and is calculated using the appropriate equations for each component.
Load inertia to motor inertia is the inertia ratio of the system. For servo motor applications, we recommend an inertia ratio of 30:1 or less. The lower the ratio, the better the motor response.
Speed and Torque
Since there may be a variety of servo motors that can meet the required inertia ratio specifications, the next step is to find the smallest, most cost-effective servo motor that can meet the speed and torque requirements.
Servo motor manufacturers typically provide speed-torque curves for each series of motors, and these curves illustrate several interesting points about servo motor characteristics. The speed-torque curve contains two regions; continuous and intermittent, which can translate into a correct match or an incorrect match (respectively) for the application.
If the speed-torque required for a particular application falls into the continuous region of the speed-torque curve, the motor can produce that torque and speed without overheating. If the speed-torque required for an application falls into the intermittent region of the curve, that motor can only produce that speed and torque for a limited time before overheating.
Other Factors
When selecting a motor, be sure to consider any other environmental requirements or space constraints your application may have. For example, your system may require a certain IP rating in a harsher environment, or if you have a small footprint, you may need an integrated system.