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CHAPTER 3 PERFORMANCES OF DC GENERATOR,1. OPEN-CIRCUIT (NO-LOAD) CHARACTERISTICS,Conditions:,No-load characteristic :,No-load characteristic is essentially the magnetizing curve,Function - checking rated operating point - knee point,No-load curve primarily depends on the magnetic system and less relates to the excitation modes.,2. EXTERNAL (TERMINAL) CHARACTERISTIC,Conditions:,External characteristic :,1). Separately Excited Generator,Since Ra and the effect of AR are all small, is an approximately constant curve.,2). Shunt Generator,The third reason is a primary reason to make U changed . It could cause the terminal voltage U to decrease greater than separately excited generator.,The third reason is also primary. It could cause the terminal voltage U to increase. So the terminal characteristic of series generator is a rising curve.,3). Series Generator,4). Compound Generator,There are two field windings, shunt winding and series winding, mounded on the stator.,Shunt winding plays main role and series winding provides compensating mmf.,Cumulative compound generator - shunt mmf and series mmf have same direction and aid each other,Differential compound generator - shunt mmf and series mmf have reverse direction and oppose each other,VOLTAGE BUILDUP IN SELF-EXCITED DC GENERATORS,Example: shunt dc generator,Three conditions for buildup:,(1). Residual magnetism exists in stator core.,(2). The mmf from shunt current aids the residual magnetism.,(3). Shunt circuit resistance is less than critical resistanc