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f*st, *l .u* 1..,ii{ffirffI i'ryryDemystifying MotorsIntely, I'ue noticed that mywoodworker's brain is a lot likemy shop - more than a littleunkempt, and always too smallfor euerything I try to fi,t insideit. Euen though some things getless than their due attention. I didnotice when the motor on my oldtable saw suddenly "retired" a fewweeks ago.A replacement motor was inorder, but I've always been a littleconfused by the fine print of motorratings and features. After I didsome digging, though, I found areliable set of standards and featuresthat can tell me whether I'mgetting my money's worth. And Ialso began understanding thebasic principles of a single-phaseinduction motor - the kind thatpowers most small-shop woodworkingmachinery.Wild HorsesThe first thing I discovered wasthat the horsepowe rating is thesprint 100 meters on level groundsound-bite of motor specifications. in ten seconds. If he tries to coverNot only does it present an incompletepicture of a motor's quality,it's not always subject to a fixed,independent standard of interpretation.Some manufacturers makeoptimistic claims that don't necessarilyhold up in the shop. Evenworse, this lone number obscurestwo key factors: the motor's abilityto increase its output in responseto the demands made on it, andthe potential for damage if thosedemands repeatedly exceed themotor's intended use.For a sort of reverse analogy,picture a track athlete who canthe same distance up a steepincline at the same pace, he'll haveto develop additional muscle,heart, and lung capacity. Morerealistically, he'Il have to negotiate54-Workbench tlune 1997
Induction Motor BasicsIf youVe ever noticed the way poleson magnets repel and attract eachother, you're already halfivay tounderstanding howinductionmotors work.lnductionmotors rely ontwo types ofmagnets * afreespinningpermanent magnet (the rotor), anda fxed electromagnet (the stator).The stator, a metal cylinderwrapped with windings of copperwire, needs electical current passing through it to become magneticallycharged. Inside the stator areits magnetic poles - stations thattansmit current to the rotor. Thereare usually two or four poles, somotimes six, always arranged in pairsmounted opposite each other.The rotor resembles a squirrelcage, and has no direct connectionto a source of elecbical current.Supported on a bearing-mountedsteel shaft, it nests inside the stator.As electrical current flowsthrough the stator windings, thepoles become magneticallycharged, one negative and one positive in each pair. In response, therotor will make a partial revolutionas its "poles"(the bars in its cage)the grade at a slower pace.An induction motor is differentfrom the runner in two ways.First, the motor's speed is fixedby the number of poles in the statorand by the pulse cycle of theelectrical source. Therefore, itcan't slow down to compensatefor the increased load. Second,the motor can generate additionallorque simply by drawing morecurrent, an option the sprinterdoesn't have.So far it sounds good - themotor can beef up instantly whenit's got a bigger job to do. Butthere's a catch. The motor won'tsustain that power level continuouslywithout generating excessiveheat, the culprit in most motorfailures. This is one of the mostbasic tradeoffs in motor performance,and defines the differencebetween a "continuous-duty" rating(power that can be safely generatedfor extended periods) and a"maximum developed horsepower"rating (a peak level the motorcan generate only intermittentlywithout overheating). The peakhorsepower can be two to threetimes higher than the continuoushorsepower. I think of that burst ofpower as a team of wild horsesthat can pull your wagon out of asteep ravine, but isn't necessaryfor a steady trip cross-country.The bottom line is this: if arepel the like charge and seek theopposite charge in the stator poles.Theoretically, the rotor shouldstop turning once the unlikecharges find each othet but theequilibrium doesn't last. Since thisis an AC motor (alternating current),the currentflow is changingdirection repeatedly, reversingthe charge of each stator polefrom negative to positive and backagain. This generates a rotatingmagnetic field inside the stator,and the constantly changing polarityboth pushes and pulls the rotorin hot pursuit as it produces itsown current. It's sort of like a dogchasing its own tail, except themotor won't get dizry and crashinto your coffee table.machinery or motor manufacfurerphrases horsepower claims withthe word "Develops . . . ." it's a virtualcertainty that the numbergiven is not a continuous-duty ratingand shouldn't be comparedwith one.In any event, the motor's amperagerating, shown on the nameplate,is a more reliable indicator.It states the actual current draw ofthe motor at a given voltage leveland temperature rise.Essentially, horsepower ratingsindicate what's coming out of amotor. Amperage ratings revealwhat's going into it (electrical current),with higher numbers indicatinga more powerful motor.Rotor andshaft assemblyThese are called inductionmotors because the current in therotor is "induced" or generated bythe stator's rotating magneticfield, not provided through wiringor another direct connection.The speed is fxed by the numberof poles in the stator and thecycle frequency (per second) ofthe alternating current. With the6$cycle power in the U. S., a twopolemotor will turn one completerevolution for each full pulse cycle,or 3,600 times per minute. Underload, the speed actually drops toaround 3,450 rpm. The same currentpulse will turn a four-polemotor only half a revolution, yieldinga noload speed of 1,800 rpmand a rated speed of 1,725 rpm.'Workbenchrlune 7997 55
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