Govt. Exams
Entrance Exams
Leading power factor means reactive component opposes armature reaction, reducing terminal voltage drop
In an infinite bus system, frequency and terminal voltage are fixed by the bus. Increasing excitation beyond rated value causes the generator to draw leading reactive power (operating as a synchronous condenser).
Synchronous motor speed depends only on frequency and poles. Reducing excitation causes the motor to draw more lagging reactive current to maintain synchronism, operating at lagging power factor.
For induction motor, maximum torque occurs at slip s_max = R2/X2, where R2 and X2 are rotor resistance and reactance. This is derived from dT/ds = 0.
Reluctance torque τrel ∝ sin(2δ), which is maximum when sin(2δ) = 1, i.e., when δ = 45°
Rotor copper loss = s × Pg, where Pg is air gap power. At full load, assuming 96% efficiency, losses ≈ 4.17 kW ≈ 4 kW
Maximum efficiency occurs when variable losses (copper) equal constant losses (friction, windage, and core losses).
Core loss Pc ∝ B² ∝ V². At 1.1V, loss = (1.1)² × rated loss = 1.21 × rated ≈ 21% increase
Pull-out torque τmax = 1.5(PΦIf)/(ωs), depends on pole flux and field current (excitation) and supply voltage.
In equivalent circuit, voltage across R2/s is rotor voltage and current through it gives mechanical power = I2² × (R2/s - R2) = mechanical power developed.
