Understanding Shunt DC Generator Load Characteristics

Lecture # 4 Lecture # 4

SHUNT DC GENERATOR o This initial small generated voltage, caused by the residual magnetism, will cause the flow of current out of the armature to the field circuit, which inturns causes flux to be increased. Eg o As the flux goes up, so does Eg. o This voltage build-up process continues until a stable condition is reached which is when there is a sufficient voltage to cause sufficientfield current If. o The field resistance Rf plays a role on reaching the stable condition very rapidly. Shunt DC Generator 22

SHUNT DC GENERATOR o The term critical field resistance Rf crt is the maximum resistance requiredto produce a particular voltage build-up. o The following curves shows the field resistance lines and magnetizationcurve. Rf =Vt1 whereRf Rf _crt Failure of A Generator toBuild-up If1 1. New generator. 2. Leakage in the generator. 3. Flux opposes the residual flux. 4. High resistance in the field. 5. High resistance in the armature circuit. (loose brush or dirty commutator) Field resistance line and magnetization curve 23

CIRCUIT ANALYSIS OF SHUNT GENERATOR o Armature Circuit: Applying KCL: Node 1 Ia =IL+If where Applying KVL: Loop 1 Eg+IaRa+Vt= 0 Vt=Eg IaRa V R V R t f I = = o Field Circuit: f f f Applying KVL: Loop 2 Eg+IaRa+Vf= 0 Vf=Eg IaRa However, Vf=Vt Then Vt=Eg IaRa Field Winding + Armature Winding 1 Load Vf 2 1 - Shunt DC Generator 24

SHUNT GENERATOR LOAD CHARACTERISTICS o As it is loaded, self-excited shunt generator has the samecharacteristics as separately-excited generator. o Self-excited shunt generator has a poor voltage regulation (V.R.) compared to separately-excited generator because it depends on the residual magnetism to set up the magnetic field which produces theinitial voltage. o Other reasons are the decrease of voltage because of the increase in IaRa drop as the load increases. o The decrease of voltage results in the decrease of the field current responsible for the flux generation and so on. Another reason isthe armature reaction. 25

SHUNT GENERATOR LOAD CHARACTERISTICS o As the generator is more loaded above certain limit, the generator comes out of saturation, and a breakdown will be reached atwhich Vt will go to zerovolts. VNL= Eg separately-excited VNL= Eg Ra If = RfIf VFL self-excited %V.R.=VNL VFL Vt(V) x100 VFL RatedCurrent VNL=Eg where Breakdown IL(A) 26

SHUNT GENERATOR LOAD CHARACTERISTICS Example 5: A 5.5-KW, 220-V shunt generator has a field resistance of 140 ,armature resistance of 0.5 and stray power losses of 95 W at ratedconditions. Find at rated load: Sol. a)The field current: [Vf = Vt] If = Vt / Rf = 220/140 = 1.57A b)The load current: [Po= Vt . IL] IL = Po / Vt = (5.5 103)/220 = 25A d) The generated voltage:[KVL] c) The armature current:[KCL] Eg = Ra Ia + Vt Eg = (0.5 26.57) + 220= 233.3V Ia = IL + If = 26.57 A 27

SHUNT GENERATOR LOAD CHARACTERISTICS Example 5: A 5.5-KW, 220-V shunt generator has a field resistance of 140 ,armature resistance of 0.5 and stray power losses of 95 W at ratedconditions. Find at rated load: Po Pi Pconv Sol. Pst Pcu e) The total copperloss: aI2 I2 + R f) The efficiency: Pcu= R Pcu=140 (1.57)2+ 0.5 (26.57)2 Pcu = 698W f f a Pi = Po + Pcu +Pst Pi = 5500+698+95 = 6293W % =Po 100=5500 100=87.4% Pi 6293 28

SERIES DC GENERATOR o In a series dc generator, the field winding is connected in serieswith armature winding. o Because the armature current Ia (i.e. load current IL) is large underload conditions, the series field winding should has the following properties: 1. It is wound with few turns. 2. It is wound with thick wires. o Note in a shunt dc generator, the shunt-field winding is wound with many turn of thin wire since a small amount of current passes through it o The mmf for both type of generators could be the same, because when current lowthe number of turns are high and visa versa, since MMF = I N. Series DC Generator 29

SERIES DC GENERATOR o Under no-load conditions, the residual magnetism causes a small voltage, and the terminal voltage is equal to generated voltage, Vt_NL = Eg since IL =0. ( ) g a a s t V =E I R +R @ Ia = Is = IL=0 V =V =Eg t NL o When there is a load connected, IL 0, the flux will increase whichin turns increases the generated voltage and soon. o A stable condition is reached after saturation at a specific amount ofload. A further increase of the load beyond the stable condition causes a rapid drop of the voltage since the flux isnot increasing (i.e. saturation) as shown. o VNL Eg Load characteristics of a Series DC Generator 30

SERIES DC GENERATOR o A series dc generator has a very poor voltage regulation (V.R.); as aresult, it is not used a constant voltage source; however, it is used as DC voltage booster in a response to a large current. o It also can be used as constant-current generator because of itsrelatively constant current at load variations. o To control the current passing through the series field; thus, the flux,a series-field diverter is connected in parallel with series field. VFL VNL= Eg Load characteristics of a Series DC Generator 31

COMPOUND DC GENERATOR oA diverter is used in parallel with the series field to control the flow of current through the series field to reach the desired types of compounding required. 0 Rd max Rd Rd Shunt Under Flat Over % Compound Short-Shunt Generator with Series-Field Diverter 35