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An Introduction to Motors and How To Use Themby Al Margolis, founder www.HobbyEngineering.comChapter 2: Interpreting Motor SpecificationsA data sheet is a document that defines the characteristics of a component. The information on a data sheet varies depending on the nature of the component, the traditions and standards of the industry in which the component is commonly used and the operating style of the manufacturing company. Some data sheets are little more than marketing fluff while some are complex and detailed technical documents. It is up to the reader -- that's you -- to interpret the specifications and determine if the component is appropriate for your intended use. When interpreting the specifications you must consider both the scientific meaning of the terms and the methods used to determine the published numbers. Many published specifications are determined using methods that are highly optimistic and rarely achieved in the real world. Some published specifications are just plain wrong. The bottom line is that ultimately the designer / builder -- again, that's you -- must test every component to verify that it performs as needed for your intended usage. Sometimes you will be disappointed. That is part of the cost of building any project. A data sheet is a legal document of sorts. Manufacturer's assume that they will some day be in court for a product liability claim with the plaintiffs lawyer waving the data sheet before the jury. A data sheet which says "precisely xx under any and all circumstances" makes it very hard to justify any variance in performance. As a result, companies make data sheets as vague as they can get away with in their industry. That gives defense lawyers the opportunity to say "you thought rated-life referred to the motor? We were referring to the fool who bought the motor." High value products often have very detailed specifications which include ratings, maximum variances and testing methods. Low value products tend to have vague specifications or no published specification at all because the manufacturer can't afford to make a guarantee. If the specification doesn't include variance (maximum plus and minus percentage difference from the published specification) and testing method the manufacturer can call almost any level of actual performance "close enough". Key DC Motor SpecificationsNominal voltage is the input voltage used when determining the other published specifications. Ratings for speed, torque and other critical motor characteristics are meaningful only for a specific input voltage and load. The numbers you see are based on some specific voltage -- the nominal voltage. Some motor data sheets have detailed charts that help you predict performance over a broad range of input voltages and loads. That can be too much detail for a manufacturer to guarantee. Lower cost motors tend to have listed ratings at only one or two specific input voltages and just the minimum and maximum load.Operating voltage range defines the minimum and maximum voltages at which the motor can be expected to perform usefully. In theory, you can apply whatever voltage you want to a motor. At lower voltages the motor spins slower and with less torque. At some low voltage it does not have enough torque to turn at all. At higher voltages the motor spins faster with more torque but also generates more heat which eventually causes the motor to fail. I jokingly tell customers that if they want they can apply 10,000 volts to our 5v (nominal) motors so they can spin extremely fast for an extremely short period of time -- just before spectacularly burning out in a ball of flames. The operating voltage range defines the lowest voltage that will reliably spin the motor with enough torque to be useful and the highest voltage that provides a reasonable operating life and safe operating conditions. Torque defines the "pushing power" or force of the motor. Force can only be measured when pushing against something that pushes back equally hard. If that sounds odd, consider how easy it is to hit a punching bag "hard" and how difficult it is (impossible really) to punch into the air with the same force. The motor torque specification is really a measurement of the force pushing back against the motor. The torque specification is for the maximum torque provided by the motor at the nominal voltage. This torque is provided just as the load, the force pushing back, causes the motor to grind to a halt. This is called stalling the motor. The unit of measurement for torque has two components: distance and force. You may want to think of the force component as weight. Your physics instructor will slap you across the knuckles for thinking that way, but I'll allow it as long as you promise to use your motors only on the surface of planet earth. The distance component is required because transferring rotational force involves leverage. A motor can push something close to its hub harder than something further away. If you put tiny wheels on vehicle it can carry nearly the weight (sorry prof) of its motor's torque specification on a smooth level surface. It will move very slowly because the small wheels have a small circumference so they don't go very far in each revolution. If you put large wheels on the vehicle it will go much farther with each revolution of the wheels, but it will stall with a lighter load because the pushing force against the ground is being applied further from the motor hub. Operating Current defines the amount of electrical energy actually used by the motor. Free running current is the current drawn when the motor is held in the air with no load pushing back against the motor. Stall current is the current drawn when enough load is applied to just bring the motor to a halt. This is the load referenced in the torque specification. Note that the motor is not actually moving when it provides its maximum torque and draws its maximum current. This is also when a motor is generating its maximum heat. That is why stalled motors are often associated with blown fuses, damaged control electronics and the acrid smell of burning motor windings. Speed states the motor's speed in revolutions per minute (RPM). Motor speed varies with both input voltage and load so high intensity graphing is required to fully define a motor's speed characteristics. This complexity is often abandoned in favor of a single number which which is generally the free running speed at the nominal voltage. Gear ratio describes the degree of "gearing down" (or up) that occurs in the gear box. The gear ratio is only relevant for gear motors and is one of the least useful specifications that is commonly published. The ratio is usually in a form like 240:1 which indicates that for every 240 revolutions of the motor output shaft the gearbox output shaft will make one revolution. This is usually the only specification for gear motors that specifically addresses the raw motor output instead of the gearbox output. The gear ratio can be used to make some educated guesses about the pitch of a motor's noise and its low speed characteristics but there are enough exceptions that the guesses do not appear to have a very high level of education. End of ChapterBack to Chapter 1:Basic Components and Concepts This tutorial is a work in progress. If you found it interesting, awful or incomplete please let me know by sending email toal@biznode.com. Chapter three will be published as soon as I can manage. |
The content on this page was updated on 10/10/2004