Synchronous Mchine Prmeter Mesurement 1 Synchronous Mchine Prmeter Mesurement Introduction Wound field synchronous mchines re mostly used for power genertion but lso re well suited for motor pplictions where constnt speed nd leding power fctor opertion re desirble. In this experiment, clssicl tests will be performed to determine the synchronous mchine prmeters. Under genertor opertion, the stndrd open-circuit voltge nd short-circuit current chrcteristics re obtined. This llows clcultion of the synchronous rectnce nd Short Circuit Rtio (SCR). The fundmentl principle of the synchronous mchine is tht the electricl frequency (in Hz) is relted to the mechnicl speed (in RPM) through the number of poles by The rdin electricl frequency is given by 120 f n (1) poles ω e 2π f (2) The synchronous genertor stedy-stte per-phse equivlent electric circuit is shown in Figure 1. Typiclly, the rmture voltge is used s reference nd the internl generted voltge (or bck-emf) nd rmture current re defined reltive to this phsor by V ˆ V 0 (3) E ˆ E δ (4) f f I ˆ I θ (5) In this model, the rmture resistnce is neglected nd X s represents the sttor rectnce. All electricl quntities re operting t the frequency ω e with the exception of the DC field voltge nd current.
Synchronous Mchine Prmeter Mesurement 2 A stndrd test tht is performed on the synchronous genertor is to mesure the synchronous rectnce by mesuring the open-circuit rmture voltge nd short-circuit rmture current versus field current while the mchine is driven t constnt mechnicl speed. If the mchine terminls re open-circuited, rmture current will be zero nd thus the mgnitude of the opencircuit voltge will be V,OC ωelf If Ef (6) 2 Since E f is proportionl to field current I f, one would expect the open-circuit voltge to increse linerly with field current. However, s the field current is incresed, the mchine steel sturtes nd the open-circuit chrcteristic exhibits the chrcteristic shown in Figure 2. In Figure 2, the short-circuit current nd sttor rectnce re lso plotted versus field current. From the equivlent circuit, the rectnce cn be defined s
Synchronous Mchine Prmeter Mesurement 3 V,OC X s (7) I,SC It should be noted tht V, OC is line-to-neutrl voltge nd if line-to-line voltge is mesured, the mesured voltge should be divided by 3. It cn be seen from Figure 2, tht the synchronous rectnce chnges s the mchine sturtes. For low vlues of field current, the mchine is unsturted nd the rectnce vlue is referred to s the unsturted rectnce X. The sturted rectnce is defined s the rectnce clculted s, u ccording to (7) t field current which corresponds to rted rmture voltge on the opencircuit curve. If the rmture resistnce be clculted using R is included in the model, n exct vlue for the sttor rectnce cn X,OC 2 s, exct R I,SC 2 V (8) For prcticl mchines less thn 0.05%. R is much less thn X s nd (7) gives vlue of rectnce with n error Another prmeter tht is of interest in synchronous mchines is the short-circuit rtio (SCR). This is defined s the mount of field current tht results in rted rmture voltge AFNL (mperes field no-lod) from the open-circuit test divided by the field current tht results rted rmture current AFSC (mperes field short-circuit) from the short-circuit test. This cn be expressed s AFNL SCR (9) AFSC With this definition, SCR is per-unit quntity. It cn be shown tht the reciprocl of SCR times the rted rmture voltge V over the rted rmture current I is equivlent to the sttor rectnce Eqution (10) is vlid for the sturted rectnce s defined bove. 1 V,rted X s SCR (10) I,rted
Synchronous Mchine Prmeter Mesurement 4 Lbortory Softwre Figure 3 shows screen-shot of the lbortory softwre used in this experiment. For creting the open-circuit voltge nd short-circuit current plots, the primry vrible of interest re the field current If, the rmture voltge VLL(rms), nd the rmture current I(rms). These vribles re logged long with the frequency by clicking the Add button. The field current in the synchronous mchine is controlled by the lbortory softwre in increments of 0.25-A by moving the Commnded Field Current br. After moving the br through ll field currents nd obtining the open-circuit test, the Test Type cn be chnged to Short-Circuit for the shortcircuit test. The exmple below shows on point on the short-circuit test where the field current is set to 2.5-A. Since the terminls re short circuited, the voltge wveform is very smll. The rms vlues show tht the short-circuit rmture current 0.86-A.
Synchronous Mchine Prmeter Mesurement 5 Lbortory Synchronous Mchines Figure 7 shows the digrm of the motor test stnd used for this experiment. The synchronous mchine is ctully wound-rotor induction mchine. It will operte s synchronous mchine when DC current is supplied to the rotor. Besides the rotor windings ccessible form the connector box, the mchine lso hs short-circuited dmper windings on the rotor. Both sides of ech sttor winding (s, bs, cs, n, bn, nd cn terminls) re brought out on the connector box for connection in wye or delt. However, in this experiment, the mchine will be connected in wye. The rotor is wye-connected internlly, nd the three terminls re brought out (r, br, nd cr). Synchroniztion lmps re connected in-between the sttor windings nd the line connection (terminls, b, nd c). A three-phse switch bypsses these lmps when switched on. This setup cn be used for synchronizing the genertor to the line or for synchronous motor strting. The synchronous mchine is rted t 208-V (line-to-line rms), 60-Hz, 200-W. It is 4-pole mchine nd thus hs synchronous speed of 1800-RPM. The dc mchine rmture nd field terminls re vilble for connection (A1, A2, F1, nd F2). The mchine is designed to be shunt-connected (rmture nd field in prllel) nd is used to drive the synchronous mchine s genertor or bsorb mechnicl lod when the synchronous mchine is operting s motor.
Synchronous Mchine Prmeter Mesurement 6 Lbortory Work Connect the DC nd synchronous mchines to the sources s shown in Figure 5. The DC motor will be connected to the dc supply on the source pnel through the meter box so tht the voltge, current nd power cn be displyed. The synchronous mchine field is connected to the Sorenson dc supply through meter box chnnel s shown. The Sorenson supply is locted t the bottom of the power electronics rck (below the source pnel). Connect the synchronous mchine rmture terminls to the first nd second chnnels of the meter box s shown. Switch the meter box power switch to ON. Click Zero DC offset on the lbortory softwre to zero the offset in the DC chnnels. After zeroing these chnnels, remove the short circuit wires. Switch on the Sorenson supply nd test the field current by moving the Commnded Field Current br to 0.25-A, then 0.5-A, then 1.0-A. The mesured field current If should be close to the commnded vlue. After testing the field, set the commnded field current to zero. Switch on the source pnel nd increse the voltge to 20%. At this time, the dc motor should strt. However, the mchine my not strt since the rmture nd filed re being incresed simultneously. If this is the cse, reduce the voltge to zero nd switch off the source pnel circuit breker then connect the motor using the lternte connection shown in Figure 5. This reverses the field flux nd the mchine should strt the next time the source pnel is switched on nd djusted to 15%. If the dc motor is running smoothly, increse the source pnel voltge to 50%. This should correspond to dc voltge of bout 150-V s seen on the computer screen. At this point, the speed will be nerly 1800RPM. Also, the rmture voltge will hve smll vlue even though the synchronous mchine field current is zero. This is becuse the rotor field poles re slightly mgnetized from previous use. This smll voltge cn be used to determine the number of poles. Write down the frequency displyed on the computer screen. Use the hnd-held tchometer t your sttion to mesure the shft speed. From this informtion, the number of poles cn be computed from eqution (1) bove. The number of poles should be n even integer number. Mke sure the Test Type is set to Open-Circuit nd log dt point by clicking Add. Increse the commnded field current to 0.25-A nd log nother dt point. Continue to increse the field current in increments of 0.25-A nd log ech dt point for the entire rnge (up to 2.5-A). You should notice from the wveforms nd rms redings tht the rmture voltge increses nd the rmture current is zero. Decrese the commnded field current to zero. At this point, the mchine should still be running nd the field current should be zero. The rmture voltge should be very smll. Add the short circuit wires (dshed wires in Figure 5). Chnge the Test Type to Short-Circuit. Log dt point by clicking Add. Increse the commnded field current to 0.25-A nd log nother dt point. Continue to increse the field current in increments of 0.25-A nd log ech dt point for the entire rnge (up to 2.5-A). You should notice the rmture voltge is nerly zero nd the rmture current will increse (linerly with field current). Decrese the commnded field current to zero. Turn the source pnel voltge to zero nd switch off the source pnel. Switch off the Sorenson power supply. Click Sve nd sve the test dt to file.
Synchronous Mchine Prmeter Mesurement 7
Synchronous Mchine Prmeter Mesurement 8 Clcultions nd Questions 1. Plot the open-circuit voltge nd short-circuit current versus field current from the test dt. The mesured open-circuit voltge is in line-to-line, so must be converted to line-to-neutrl before plotting. Also plot the ir-gp line V,ir gp s shown in Figure 2. This cn be computed s stright line using some of the points with lower vlues of field current. 2. Use one of the open-circuit dt points in the liner region (low field current) to compute the unsturted synchronous rectnce using (7). The 0.25-A or 0.5-A field current points re probbly best for this. 3. Bsed on the rted rmture voltge of V 120V nd rted rmture current of I 0.7A, determine the AFNL nd AFSC for this mchine. Note tht the mesured dt my not contin these exct vlues nd liner interpoltion should be used for ccurcy. Using AFNL nd AFSC compute the short-circuit rtio using eqution (9). 4. Compute the sturted synchronous rectnce using eqution (7) for the dt which corresponds to rted voltge. Also compute the sturted synchronous rectnce using the short-circuit rtio s in eqution (10). 5. The rmture resistnce for this mchine is given in the tble below. Compute the exct sturted synchronous rectnce using (8) nd compre it to the clcultion using (7). 6. Compute the field resistnce R f t ech dt point by dividing the field voltge Vf by the field current If. Averge the vlues for ll dt points to obtin vlue for field resistnce. 7. Compile the mchine prmeters in tble similr to the one below. Synchronous Mchine Prmeters Rted rmture voltge Rted rmture current Armture resistnce V 120V I 0.7 A R 12. 5 Field resistnce R f Unsturted synchronous rectnce X s,u Sturted synchronous rectnce X s Short-circuit rtio SCR Ω