Synchronous Motors

Refer to Figure 60. In Figure 61, the progression ofthe vector can be followed as above, but the stepsnow are at 30º intervals rather than at 60º, as in thesix-step inverter. Another significant advantage isa reduction in harmonics imposed on theincoming line as well as torque harmonics to theload. Figures 62 and 63 further illustrate thedifferences between six and twelve-step inverteractions. The fifth, seventh, seventeenth,nineteenth, and many higher order harmonicscancel in the wye and delta secondaries; they donot appear in the incoming power to the twelvestepconverter. The result is smoother operation,fewer concerns about torsional stresses, quieterrunning, lower harmonics on the incoming line,and a reduced output requirement from eachthyristor bridge.T1T3T21. -T24.T1-T2 T1T2T2-T1 -T12. -T35.T1T1-T3T3-T1 T3 -T13.6.T2-T3 T3-T2T2 -T3 -T2 T3PLAIN WINDINGT1T3PLAIN1. -T2T1T1-T22. -T2T1T1-T23. -T3T1 T1-T3T2PRIME-T2'T1'T1'-T2'-T3T1'-T3'T1'-T3T1'-T3'T1'PRIME WINDINGT1'T2'T3'RESULTANTT1'-T2'T1-T2T1'-T3'T1-T2T1'-T3'T1-T3Figure 59 Single Winding 6 Pulse.Figure 61 Dual Winding 12 Pulse.(Only 3 of the 12 pulses are shown)4 6 2Windings shifted by 30 electrical degrees2 6 4ΥT1T2T31 3 5Winding example of 6 pulse machineT1'T2'T3'5 3 1Both windings used on 12 pulse machine∆Figure 60 Top: 6 Pulse Single Winding.Bottom: 12 Pulse Dual Winding.While two transformers could be used to feedthe twelve-step converter, a three-windingtransformer is frequently used (note in Figure 60).Although this could be done with transformersbetween the inverter and the motor, the motor,rather, is frequently wound with a dual windingto isolate the outputs of the two inverter sections.The two separate windings are shifted 30 electricaldegrees. Each part of the motor stator actuallysees a six-step progression, but the rotor sees thetwelve-step sum of the two inputs to the machine.The twelve step approach is typically used forhorsepowers of 2000 and larger, even up to 50,000.Current-limiting functions are often employedduring starting, keeping currents to valuesconsiderably under normal locked rotor values. Thecurrent limit is adjustable and is set as a function ofthe amount of torque required. Available startingtorque, when operating with an LCI, is more like themaximum operating torque of the motor, since it is12 Pulse Dual Winding 6 Step Single WindingDegreesSCRCurrent InCurrent OutSCR'1 2 3 4 5 6 1 2 3 4 5 6 CommentSingle Winding60 X X T1 T2120 X X T1 T3 SCR’s180 X X T2 T3 Switch240 X X T2 T1 Every300 X X T3 T1 60°360 X X T3 T230 X X T1 T2 X X T1' T2'60 X X T1 T2 X X T1' T3'90 X X T1 T3 X X T1' T3'120 X X T1 T3 X X T2' T3'150 X X T2 T3 X X T2' T3' SCR’s180 X X T2 T3 X X T2' T1' Switch210 X X T2 T1 X X T2' T1' Every240 X X T2 T1 X X T3' T1' 30°270 X X T3 T1 X X T3' T1'300 X X T3 T1 X X T3' T2'330 X X T3 T2 X X T3' T2'360 X X T3 T2 X X T1' T2'Figure 62 SCR firing sequence for 6 pulse and 12 pulse drive.Current InCurrent Out33