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How to Select a Servo Motor


The simplified definition of a servo system is that it consists of several components which together control or regulate speed/position of a load. The servo motor is one of these components in the system. When it comes time to select an appropriate servo motor for an application some people may be naïve in thinking that they can just check size the motor based on the horsepower rating of the presently installed motor, or exclusively based on the application's torque requirements. The following factors must all be taken into account when selecting the appropriate motor: inertia ratio, speed, and max torque at desired speed.

Any rotating object has a moment of inertia which is a measurement of how difficult it is to change the rotating velocity of that object. Moment of inertia in a servo system can be divided into two parts; load inertia and motor inertia. The motor inertia is part of the servo design and is typically listed in the manufacturers' specification sheet. Load inertia is more complicated because it involves every component that is moved by the motor, and is calculated using proper equations for each component. A typical inertia ratio for most applications is 5:1, but the lower the ratio is, the higher performance will be, and vice versa.

Since there may be a variety of servo motors that meet the required inertia ratio specifications, the next step is to find the smallest, most cost-effective servo motor that will meet the speed and torque demands. Servo motor manufacturers normally provide speed-torque curves for each series of motors, which illustrate several interesting points of the servo motor's characteristics. The speed-torque curve contains two regions; continuous and intermittent, which can translate to correct match or incorrect match (respectively) for the application. If the speed-torque required for a specific application falls into the continuous region of the speed-torque curve, then that motor can produce that torque and speed without overheating. If the speed-torque required for the application falls into the intermittent region of the speed-torque cure, then that motor can only produce that speed and torque for a limited amount of time before overheating.
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