Document jNnJdzeX7V90NyjKr6bpqpB32

be slightly lower because of the In* creased current On on occasion I had a now motor in which the air gap was so smalt that, when the rotor expanded with heat, . it rubbed on the stator aod the motor aieUed. Only remedy was to take a smell cut off the rotor. Alfred Si. U. rent, Qaebee Power Co, St, Raphael, Quebec. Don't Do It Baroas ddcuuinc what happens in a squirrel-cage motor after the rotor is turned down, let's review some magnetic theory. Air has a reluctance about 1000 to 2000 times greater then iron or steel. Therefore, considerably more ampere turns are needed to force a given amount of flux through air than ueel If a motor is wound for a given numher of ampere turns and the rotor is machined down, it requires more current to force the same flux through the larger eir gap. Motor elip would also increase if rotor diameter were reduced. If the rotor of a motor is removed and full voltage applied to the stator, it draws a large current from the line. Turning the rotor would have a similar effect, and the Increased current would cause greater heating. The heating produced by the excess current does not Increase in direct proportion, but as the equare of the current If the cur. rent is doubled, the heating is increased four times. Power factor of the motor would decrease along with ita efficiency. If a rotor has become damaged bfr cause of rubbing, smooth it up with a file but don't turn it down. lames J Anderson, St. Paul, Mtnn. Stator Winding Overheats In Cchehal, h is not advisable to take a cut from the rotor of e squirrel-cage motor. A cut Increases the air.gap clearance. This results In a reduced starling torque, efficiency and power factor. Increaaed stator current would cause overheating of windings to pull the load. These conditions are brought about by the high reluctance of the afr gap. Higher current is required to force the magnetic flux through the longer gap. Unless the manufacturer is consulted, or auitable calculations made to show the effects of increased air*gnp clear* once, the cut should not be taken. Better bearing maintenance and re* pair Is on easier and more effective method of eliminating alr-gnp difficul ties. An integrated Inspection, main tenance and repair program prevents most bearing failures. Time spent on such work is well spent, since rotor re* pairs are reduced. Tyler G Hicks, New York Giy. Turning Down the Rotor Will Increase Heating HE aSks'adout turning down motor ro tors. My advice is don't do lL Turning down tho rotor of en induction motor interferes with the electrical perform ance in several ways. Cutting action of the tool peena over edges of laminalions so they make better electrical contacts with one another. This in creases iron losses of the tolar and tends to cause more heating. This effect can be reduced by grinding with a fine wheel, bat even this is to be discouraged. Cutting down the rotor also length ens the air gap. This increases the m&gneiltlng current end might make the motor useless. Turning down the copper or aluminum rings increases ro tor resistance. This decreases motor efficiency and causes local heating. If HE has rotors that have been dam aged es he describes, his money will bo well spent if he throws them owsy end buys new ones. Earl E Nomua, man ager, City Light and Water Utilities, KsUinotoo, Mich. Turning Induction Rotor Increases Starting Torque HE is plirtinc with trouble if he mokes e general practice of truing squirrelcage rotors by taking off o cut. No one but the designer can actually tell how much the performance will be altered by machining the rotor. It may be pos sible to advise HE whet to expect from e theoretical standpoint, but this may not always apply. If HE't application isn't critical, he can turn down the rotor. 1 suggest, however, that ho attempt to do his tru ing by reoonditionlng the bearings. Cutting down rotor bars and end rings also reduces copper cross-sectional area, and at the same lime increases the air gap. Decreasing copper area increases the rotor resistance, resulting in a greater HR loss. With greater loss tho operating temperature is higher. The increased HR loss results in a low er efficiency. Maximum torque occurs at higher va lues of slip because'of the increased rotor resistance. The starting torque will be relatively greater since at stand still slip U maximum. The rotor reac tance is lower because of the larger air gap end the increased rotor resistance. v^curs when nuj sislancc ts equal to rotor reactant condition is approached when |u"v^ Is turned down because the origW?5 science is decreased and the sielance is Increased. K B electrical engineer, Tenatly, ft Consult tho Manufectui Tiitnt 18 wo OBJKCTtow to tumlnr &> the rotor if HE is prepared to the changes In operating cbarieterS! Increasing tho air gap will locretiS no-load current, slip end power f and also decrease the torque pti pere. Tho most satisfactory way to pp4i the problem is to present ell <Wj|f the manufacturer, who can stated^ itely what the actual results will' C O von Dannenbevg, Brooklyn, R/y i Avoid Reducing Air Dap f Induction motoos ora designed with smell an air gap between rotor end 2 tor as is consistent with good prt By "good practice," I mean enough clearance between the two : ments so there is no danger of i nibbing on stator because of wes the heerings. The current in the K mary of an induction motor predv: a rotating field, which travels i the stator. This flux links with t ductora on the rotor ond pulls t along, thus causing rotation. The sir gap has a high magnetic h luctsnce so that aome magnetic I do not cross into the rotor. In a wM gap, more of these magnetic lines fill to connect. Because fewer magr lines enter the secondary the po factor of the motor will decrease. I er power factor means that more rent is required to carry a gtan I resulting tn Increased motor heath^ Before the war, some induction c of European design were found to k^, extremely high power factor- Thei gaps were found to be so small t unusual cere and attention were i qulred In checking the air gap h* time to time to make sure the rotor d not rub on the stator. With a transformer, there I* no n ment of secondary with respect to mary. Consequently, the primary secondary coils can be interconnect with less magnetic leakage ihso U # possible with an Induction motor, a result, we find' that a 100-hp 1 rpm induction motor has a P* ^ whereas o comparable iritnfenijjj might have a pf of 98%. REE (Continued on page 148) >12 (177) Rrticularfy In the Power Plant Field. Plant Outage, for instance, can and usually does strike at any Lime without warning. But bow many or howJew shut-downs you have may depend entirely on the effectiveness of your water conditioning service. 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