1. 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. 2. EXTERNAL (TERMINAL) CHARACTERISTIC Conditions: External characteristic : 1). Separately Excited Generator Since R a and the effect of AR are all small, is an approximately constant curve.

3. 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.

4. 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

5. 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

6. 3.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 resistance.  Process of buildup Two curves: magnetization curve and field resistance line