How to Keep Motors Turning
Step motors can be infamous for failing or stalling, right when you need them to work the most. Although it might be easy to assume that the motor went bad and start the trial and error process during motor selection, doing some troubleshooting and looking a little deeper oftentimes goes a long way into solving the problem. It’s important to note that, similar to any kind of mechanical or electrical device, step motors have inherent performance efficiency and performance limitation thresholds. The trick is to properly utilize the right step motor and then operate it at its maximum efficiency without driving it past its performance limitations. This process might be a little difficult to master at first, but asking for help never hurts.
Lin Engineering Inc., the Step Motor Specialists, have coined the term “Maximize Torque at Desired Speed” in order to simplify this process. The term essentially describes selecting a motor that will operate at its most efficient and output the maximum available dynamic torque, while running within the user’s crucial operating speed. Doing so often times avoids experiencing a motor stall, under perform, or otherwise fail. Sounds easy, but most step motor manufacturers continue to do trial and error in finding the right motor for the application.
So how much torque is really needed and how fast am I going? In general, step motor users tend to have difficulty in trying to determine their operating speed range and the torque required to move the associated load. Calculating and understanding these two parameters is essential when selecting the correct motor. It can easily make the difference between selecting a motor that will end up failing because it was overdriven and selecting a motor that will hum like a bird because it’s running at its most efficient state. A few things to keep in mind:
1. Speed should always be referred to as a rotational speed, like revolutions per second (RPS) or revolutions per minute (RPM). Pulses per second (PPS) is also acceptable as long as microstep resolution information is provided as well. This eliminates many follow up questions that may come up.
2. An easy torque calculation that can be used is to determine your inertial load, and multiply it by how quickly you’d like to move this load. In other words, torque equals inertial load times acceleration. Inertia can be estimated by taking half the mass of an object, multiplied by the radius squared, or at least some kind of variation to this equation.
Once the speed and torque requirements have been determined, a step motor can be wound with the right amount of coils in order to output the best performance while using the minimum amount of power. An efficiency curve exists for every motor, and an excellent step motor manufacturer’s job is to select the best motor for the task at hand.
The image above depicts three different motor efficiencies when plotted across a speed range. Some motors, such as #1, perform at their best only at low speeds while others, such as #3, perform better at high speeds. Although one might be able to use motor #3 at a low to mid speed range and generate enough torque to satisfy the application, more power than necessary will be used and the life of the motor will decrease because it is being overdriven. As demonstrated here, efficiency should be something that is considered.
Also, some users might find it difficult to find a motor that will operate at their desired speed while also outputting an adequate amount of torque. This is one area where the term “Maximize Torque at Desired Speed” can be of immense help. Recently a customer approached Lin Engineering when they had gone through some trial and error while selecting a motor but could still not seem to get the right one without having to change the amount of current or voltage being run through their system. After going through some calculations, it was determined that the customer’s operating speed was at 5 revolutions per second (or 1,000 PPS full stepping). The existing motor was producing around 5 oz-in of dynamic torque while the torque required to move the load was about 10 oz-in. Using the practice of “Maximize Torque at Speed”, engineers at Lin Engineering were able to select a winding performed at its most efficient state while running at the customer’s operating speed.
If step motor users experience torque issues, it might just be that the wrong motor was selected for the job. Just remember that every motor has a predetermined set of characteristics and there is a right motor for every job. In a matter of minutes, the correct motor selection process can save you months of qualifying a step motor.
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