Servo motors are compact and efficient motors used in a number of industrial applications that require precise position control, such as robotics, conveyor belts, and automatic door openers. Servos are standalone electric motors that push and rotate parts in machines for which a specific task and position is required. Because a servo motor provides great efficiency and power in a relatively small package, you might find it challenging to size one properly.
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Try to Strike a Balance When Sizing a Servo Motor
Selecting a servo motor is a balancing act. If you choose a motor that is undersized, it will not handle load. If you're considering oversizing a servo to make sure you're covered for performance, you're not alone. The U. S. Department of Energy reports that approximately 80% of servo motors in the U. S. are oversized.
But if you do err on the side of oversizing the motor, it might prove to be too expensive in terms of purchase price and operational costs. If you want to avoid this problem and properly size a servo motor, you first need to consider these key sizing factors: inertia, speed, and torque. Other factors come into play in this process such as infrastructure, environment, and physical size limitations, which we'll discuss as well.
Inertia: Knowing the Motor and the Load
Inertia is the tendency of an object to resist changes in speed, and every rotating object has it. A servo motor and the load (or driven components) each have inertia and how similar or how different their inertias are will affect the motor's performance. The ratio of the load inertia to the motor inertia--that is, the inertia of the load(JL) divided by the inertia of the motor(JM)--is one of the most important aspects of servo motor sizing.
The moments of inertia for the motor and load are measurements of this resistance. You can usually find the motor's moment of inertia, which measures the motor's force to overcome the load's resistance, on the manufacturer's data sheet. Determining the moment of inertia of the load is more complex. Components related to the motor's drive and coupling in addition to the external load all must be included in this measurement.
Generally speaking, an inertia ratio of 5:1 is typical. If performance is not crucial, 10:1 or higher would be adequate. For highest performance, you might consider an inertia ratio of 2:1, because control loop tuning becomes easier and because you benefit from increased machine performance at this lower ratio. If all other sizing factors are equal, a lower inertia ratio is preferable. But the downside of choosing an extremely low inertia ratio means that you might end up with a very large, bulky, and costly motor relative to the amount of performance you've gained.
Speed: Know the Motion Profile
Another critical factor in sizing is speed or velocity. For the purposes of sizing a motor, it helps to know how fast and how far the load is moving. It also helps to know the inertia ratio and the motion profile.
If you are working with an application that requires the equipment to perform a repetitive motion, it helps to plot out the required motor speeds throughout the cycle. When doing this, remember to allow for acceleration and deceleration, since servo motors can't make step changes in speed as a stepper motor can. For non-repetitive operations, figure out the peak speed and acceleration that the operation requires.
Torque: How Much Do You Need?
Torque is a measure of how much force it takes to rotate a mechanism. After you learn the load inertia and speed, check the motor's torque-speed curves for required torque values. On a graph, speed is represented on the vertical or y-axis, and torque is represented on the horizontal or x-axis. The calculations you'll need to know are the required continuous torque, peak torque, and maximum motor speed.
The necessary amount of continuous torque must fall inside the continuous operating area of the torque-speed curve. In addition the required amount of peak torque must also fall within the servo system's intermittent operating area of the torque-speed curve, since short bursts of torque don't put the motor in danger of overheating.
Other Considerations for Sizing: Infrastructure, Environment, and Footprint
A relatively straightforward consideration is the infrastructure of the servo motor: that is, the voltage and frequency characteristics of the power source you plan to use. Depending on your application, you can choose an AC servo motor or a DC servo motor. Although AC servos have lower power output, they are more stable and smooth to operate. A DC servo is highly efficient and has higher power output than an AC, but it also more radio frequency noise,
An often overlooked factor for servo motor sizing is the environment in which the motor will operate. The majority of servos are rated for 40 degrees Celsius (104 degrees Fahrenheit) ambient conditions, which is typical for factory and industrial environments. But you should keep in mind the ambient temperature for the entire system, the drive electronics associated with a servo motor, however, may be rated for 25 degrees Celsius (77 degrees Fahrenheit). Similarly, extremely cold temperatures affect the motor's lubricants, causing the motor to stall. If there is dust, moisture, and contamination in the environment, you will want to consider an IP-rated servo.
The limitations of physical size are a practical consideration. Sometimes there is simply isn't enough room for a motor with the frame size you want. If diameter is the main issue, some manufacturers boost the power for a specific frame size by increasing the layers of magnetic lamination stacks in the design of the motor. As a result, the motor generates more power at the expense of being longer but not wider. There are limitations even in this approach, and it further supports the argument for proper servo motor sizing.
A number of factors are involved in sizing a servo motor, and it requires that you take a careful, "big picture" approach to the process. Mader Electric has been servicing and selling motors and pumps for over 30 years and can help you select the right servo motor for your application. For more information, contact us today.