Synchronous Generator vs Synchronous Motor The main difference between the two is the sign conventions Below are the diagrams for a generator on the left and a motor on the right Each is connected to an infinite bus with voltage Vt voltage and frequency of an infinite bus are fixed regardless of prime mover power and excitation of the machine Note that current references are opposite Current reference is out of the plus terminal of the generator but into the plus terminal of the motor This leads to different diagrams for various DC excitations of the field as shown below Note also that the over excited machine generator or motor is capacitive and will supply vars to the infinite bus system The opposite is true for the under excited machine In the vector diagrams the value of Ra was neglected Can you repeat one of these vector diagrams with the armature resistance included GENERATOR Ra Ia MOTOR XS EG Ra Ia XS Vt Vt Generator EG Vt I a Ra jX a Motor EM Vt Ia Ra jX a Neglecting the armature resistance the diagrams below show the phasor diagrams for voltages and currents for over excited under excited and normally excited unity power factor for these two machines MOTOR GENERATOR EG Ia XSI a Vt Ia Vt X SI a EM Note in each diagram there are three cases shown each in a different color In each case the equations above are satisfied with Ra 0 Since we assume constant V and P the locus of E is a horizontal line and the locus of Ia is a vertical line as seen above Black is for unity power factor Ia and Vt are in phase GENERATOR Red is over excited supplies lagging current to the system infinite bus thus the machine is drawing a capacitive current from the system Like a capacitor it supplies reactive power to the system Blue is under excited supplies leading current or drawing a lagging current Like an inductor it draws reactive power from the system Black is for unity power factor or normally excited generator Note that E G leads Vt in all cases which is typical generator operation MOTOR Red is over excited motor is capacitive and supplies vars to the system Blue is under excited lagging PF motor is inductive and absorbs vars from the system Black is normally excited unity PF Note that in all cases EM lags Vt which is typical motor operation GENERATOR MOTOR EG Ia XSI a Vt Ia Vt X SI a EM In general an over excited synchronous machine motor or generator will supply vars to the system like a capacitor while the under excited synchronous machine will absorb vars from the system like and inductor