Govt. Exams
Entrance Exams
In a DC series motor, when unloaded: (1) Load torque decreases, so armature current reduces significantly since field current equals armature current in series connection. (2) With reduced current, voltage drop (Ia×Ra) decreases. (3) Back EMF increases (Eb = V - Ia×Ra). (4) Since N ∝ Eb/Φ and both Eb increases while Φ decreases (less field current), speed increases substantially. This is why series motors are unsuitable for no-load operation.
At constant V and reduced frequency, the air gap flux increases (Φ = V/f), but the rotor reactance decreases. However, the overall effect results in reduced starting and running torque.
The double-field revolving theory represents a single-phase AC winding's pulsating magnetic field as two counter-rotating fields, used to analyze single-phase induction motor behavior.
Cogging (or coggling) occurs when the number of stator and rotor slots are equal or have common factors, causing reluctance torque variations due to slot permeance interaction.
At no-load with increased excitation, the motor operates at leading power factor. Armature current decreases because the motor absorbs less reactive power from the supply.
Crawling results from 5th, 7th harmonics creating torques at different slip values, causing low-speed oscillations
Over-excitation produces leading current (capacitive), supplying reactive power to the bus
Using slip relationship, Ts/T = (smax/s)² = (0.4/0.1)² = 16 for normal induction motor characteristics
Cogging torque results from interaction between poles and armature teeth, causing reluctance variation
Over-excitation reduces armature current by creating leading reactive component that cancels lagging current
