Govt. Exams
Entrance Exams
Speed = (V - IaRa)/(kΦ). Decreasing field current reduces flux Φ, increasing motor speed.
Turns ratio k = N2/N1 = 100/1000 = 0.1. Voltage ratio V1/V2 = N1/N2 = 10/1 or 10:1 (step-down).
Damper windings provide asynchronous starting torque to accelerate the rotor near synchronous speed before synchronous operation begins.
Rotor frequency fr = slip × stator frequency = s × fs, where fs = 50 Hz for 50 Hz supply.
Induction motors operate at lagging power factor (0.85-0.95) due to reactive magnetizing current required for flux production.
DC series motors have high starting torque (proportional to load current) and are used in electric vehicles, cranes, and hoists.
Standard starting methods are DOL, star-delta, autotransformer, and rotor resistance. Frequency variation is used for speed control, not starting.
Induction motors draw 4-7 times full-load current during starting because rotor speed is zero and rotor resistance appears as reactance.
Transformer efficiency is maximum when core loss (Pi) equals copper loss (Pcu), i.e., Pi = Pcu.
Flux reduces drastically when field is short-circuited. As back EMF reduces, armature current increases causing motor speed to increase dangerously.
