a) Load current Vs terminal voltage at different power factors.
1. At leading power factor as the load current increases the terminal voltage also increases.
2. At lagging power factor as the load current increases the terminal voltage drops.
3. At unity power factor as the load current increases there is slight drop in terminal voltage.
b) Torque Vs slip characteristics of induction motor.
Torque (T) = φ I2 cosφ2
Where φ - main flux.
I2 – rotor current
Cosφ2 – rotor power factor.
When motor is started from rest the slip is 1 (one) at time of starting and starting torque is 1.5 times of rated torque. As the motor accelerates slip reduces and torque increases. Because the power factor of rotor is improving due to the decrease in rotor frequency and at certain slip the rotor reactance is equal to rotor resistance. At that time torque is maximum (pull out torque). When the motor accelerates to the rated speed the torque comes to the rated value, which is less than the starting torque. Torque is zero when slip is zero, because there will be no relative motion between stator magnetic field and rotor
c) Generator capability curve
This curve gives the operating limits of the turbine generator at different power factor what should the power output of the generator to avoid the heating of generator stator winding, rotor parts and end parts.
