SLOVAK UNIVERSITY OF TECHNOLOGY Fulty of Mteril Siene nd Tehnology in Trnv ELECTRICAL ENGINEERING AND ELECTRONICS Lbortory exerises Róbert Riedlmjer TRNAVA 00
ELECTRICAL ENGINEERING AND ELECTRONICS Lbortory exerise DC Motor Introdution A DC motor is simple eletro-mehnil system. Figure shows simple model of interior of brush-type permnent mgnet DC motor. Figure : DC motor model Figure : DC motor rmture As urrent psses through wire in the presene of mgneti field, fore norml to the wire nd the field is reted tht is proportionte to the urrent, wire length, nd mgneti field strength. This fore retes torque bout the enter of the motor, using it to rotte. In tul DC motors, the wire is oiled mny times round ore lled rotor nd the ommuttor hs mny elements. This oil ssembly is lled the rmture. The rmture is tthed to the motor shft to trnsmit the torque. Figure shows DC motor rmture from n utomobile strter motor. DC Mhine Armture Winding The DC mhine rmture windings re lwys of the losed ontinuous type of doublelyer lp or wve winding. For smll mhines, the oils re diretly wound into the rmture slots using utomti winders. In lrge mhines, the oils re preformed nd then inserted into the rmture slots. Eh oil onsists of number of turns of wire, eh turn tped nd insulted from the other turns nd form the rotor slot. Eh side of the turn is lled ondutor. The number of ondutors on mhines rmture is given by Z = CN () Where Z = number of ondutors on rotor C = number of oils on rotor N = number of turns per oil Sine the voltge generted in the ondutor under the South Pole opposite the voltge generted in the ondutor under the North Pole, the oil spn is mde equl to 0 eletril degrees, i.e., one pole pith. In -pole mhine 0 eletril degrees is equl to 0 mehnil degrees, wheres in -pole mhine 0 eletril degrees is
equl to 90 mehnil degrees. In generl, the reltionship between the eletril ngle θ e nd mehnil ngle θ m is given by P θe = θ m () Where P is the number of poles? The most ommonly used winding is two-lyer winding. The number of oils for two-lyer winding is equl to the number of rmture slots. Thus, eh rmture slot hs two sides of two different oils. In the preformed oils, one side of oil is pled t the bottom hlf of slot nd the other side t the top hlf. If S is the number of slots in the rotor, the oil pith is designted by y is given by S y = Integer vlue of ) P ( Tht is if we ple one side of the oil in slot m, the other side must be inserted in slot m+ y. The mnner in whih the oils re onneted together will form the type of rmture winding. Let E nd I re the voltge nd urrent per oil. If is the number of prllel pths between brushes, then the number of series ondutor per prllel pth between brushes is Z /. Hene the verge generted emf nd the rmture urrent re given by Z E = E () I = I Simplex Lp Winding In one type known s the simplex lp winding the end of one oil is onneted to the beginning of the next oil with the two ends of eh oil oming out t djent ommuttor segments. For progressive lp winding the ommuttor pith y =. A typil oil of turns for simplex lp winding is shown in Figure. N In the simplex lp winding the number of prllel pth between brushes is equl to the number of poles, i.e., P =. This type of winding is used for low-voltge, high-urrent pplitions.
N turns insulted from eh other y oil spn pole pith l = length of o y Commuttor segments Commuttor pith y = for simplex lp winding Figure : A typil oil for lp winding with y =. Let us onsider four-pole, two-lyer, simplex lp winding with slots. If number of turns per oil N =, the totl number of ondutors is Z = ()() =. From () the oil pith is y = = slots Figure () shows the winding onnetion where the two ends of oil re onneted to djent ommuttor segments. The oils in eh pth re onneted in series nd the winding lose upon itself. The number of prllel pth between the brushes nd the winding rrngement n be seen more lerly by the developed digrm of the rmture s shown in Figure (b). In the developed view, the winding is ut long the xil length nd is lid flt on the pper. Simplex wve winding As we hve seen in the lp winding, the two ends of oil re onneted to djent ommuttor segments ( y = ). In the wve winding, the two ends of oil re onneted to the ommuttor segments tht re pproximtely 0 eletril degrees prt (i.e., -pole pith). This wy ll the oils rrying urrent in the sme diretion re onneted in series. Therefore, there re only two prllel pths between the brushes, i.e., = independent of the number of poles. This type of winding is used for lowurrent, high-voltge pplitions.
S S Figure () A polr-winding digrm of -pole, -oil, lp wound d mhine b b Figure (b) Developed digrm of -pole, -oil, lp wound d mhine N N N N S S
Wiring types The dynmi behviour of the DC mhine is minly determined by the type of the onnetion between the exittion winding nd the rmture winding inluding the ommuttion nd ompenstion winding:. Seprtely exited DC mhine: exittion nd rmture winding supplied t seprte voltges. Shunt DC mhine: exittion nd rmture winding re onneted in prllel (i.e. fed by the sme soure). Series-wound mhine: the exittion nd the rmture winding onneted in series; if the sttor is lminted, series-wound mhines n operte t AC urrent. Compound mhine: This is ombintion of nd (both shunt nd series winding re vilble). In speil ses shunt-, series-, nd seprte exittion n be ombined. Ativity Self exittion of the shunt DC mhine The equivlent iruit for shunt wound d motor is shown in Fig.. I I I f R R f V E N f turns rmture winding field winding The remnent voltge enbles opertion of the DC mhine s genertor, with no dditionl voltge soure. When the mhine is onneted in shunt onnetion, i.e. the exittion iruit with field wekening resistor Rfv shunt to the rmture iruit, nd mehnilly driven to ertin rottionl speed, smll urrent flows through the exittion winding, on the ount of the remnent voltge. If the winding is onneted
in suh wy tht this urrent mplifes the residul effet, the voltge for the exittion iruit n be generted on its own. This is lled self exittion wiring. The opposite se where the urrent used by the remnent indution wekens the field is lled suiidl wiring.. The vlue of the indued voltge n be djusted by field wekening resistor Rfv nd determined t the intersetion point of the resistne line (Rf + Rfv). If with the no-lod hrteristi. (Fig. ). If Rfv nd onsequently the ngle ρ in Fig. re inresed, smller voltge is obtined until the ritil vlue of the ngle is exeeded. At the ngle ρritil there is no definite intersetion point between the nolod hrteristi nd the resistne line, so tht the resulting terminl voltge is strongly lod dependent; t nolod onditions the upper nd under lod the lower mutul point of the grphs re the points of opertion. Experimentl set-up In this experiment the lod hrteristi of the DC shunt motor should be determined. Connet the shunt mhine with the ommutting nd ompenstion windings. R fv R fv ma Q ~ 0 V 0 Hz ~ / = ma D D V R f Q Fig. :