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An Introduction to Motors and How To Use Themby Al Margolis, founder www.HobbyEngineering.comChapter 1: Basic Components and ConceptsA motor is a machine which converts energy into rotating motion. The dictionary definition of motor is broader than that but when engineers and mechanics talk about motors they are almost always talking about rotating motion. There are different names for devices which convert energy into other types of motion. The hub or drive shaft is the rotating output of the motor. A DC motor is a motor that uses direct electrical current (DC) as the source of its energy. An AC motor is a motor that uses alternating electrical current (AC) as the source of its energy. AC current is the type of electricity provided by household wall outlets. DC current is the type of electricity provided by batteries. A gear motor is a motor with an attached set of gears driving a secondary drive shaft. Practical motor designs result in motors that spin too fast for most uses. As a result, almost all gear sets are used to "gear down" the motor. The geared down drive shaft spins slower than the direct motor drive shaft. The geared down drive shaft also spins "harder". Motor speed is generally measured in revolutions per minute (RPM). Rotating force is called torque and for hobby motors is generally measured in inch-ounces or centimeter-grams. We will discuss torque more in a future chapter of this tutorial. For now, just remember that the higher the number the harder the motor turns. Gearing down a motor reduces its RPMs (speed) but increases its torque. Conversely, gearing up a motor increases its RPMs but decreases its torque. Gears are generally contained within a housing that protects the gears from interference and which provides a bearing surface for the various gear shafts and drive shafts. The term gear box generally refers to the entire system of gears, shafts, bearings and housing. When you apply energy to a motor it spins as fast and hard as its design allows for that energy level and output load. If you increase the energy supply it spins faster and harder. If you attach a load the motor will slow down. If you continue increasing the load it slows ever more until the motors capability to work is exceeded. When the extreme load causes the motor to stop it is said to be stalled. Reducing the load causes the motor spin faster. If you entirely remove the load the motor is said to be "free running" and operates at its maximum speed for that input energy level. Because of the influence of the load, you cannot absolutely control the speed of the motor by controlling the input energy level. If you need to precisely control the motor speed you must consider the load. Because it is difficult to measure load, the most practical way to precisely control motor speed is to directly measure the speed. A device which detects the rotation of a shaft is called a shaft encoder. By counting rotations for a period of time you can determine motor speed. A servo is a motor with a brain. Instead of blindly taking input power and working as hard as it can, it takes requests for specific movement and attempts to satisfy the request. Some servos accept speed commands and attempt to move at the requested speed regardless of load. Other servos accept location commands and attempt to rotate the output shaft to a particular position. A "servo motor" is generally a neat package of motor, gear box and regulating electronics. A servo motor has two inputs: an energy source for the motor and a command source to tell it what to do. End of ChapterForward to Chapter 2: Interpreting Motor Specifications 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. |
The content on this page was updated on 09/27/2004