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.
Synchronous speed Ns = 120f/P. When frequency changes from 50 Hz to 60 Hz with constant slip, new speed will be proportional to frequency. New speed = 960 × (60/50) = 1152 rpm. This is because slip s = (Ns - N)/Ns remains approximately constant for the same load.
If the load has lagging power factor, the secondary current lags the secondary voltage. The phase relationship between primary and secondary depends on the load power factor.
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.
Shaded pole motors use a copper ring or shaded pole around part of the pole face to create a phase shift in the magnetic field, producing starting torque without additional components.
Pitch factor (Kp) accounts for the reduction in EMF when the coil span is less than the pole pitch (full pitch), representing incomplete flux linkage.
In a DC shunt generator, as load increases, armature current increases, causing armature reaction that demagnetizes the field, reducing terminal voltage significantly.
At no-load, the transformer draws mainly reactive (magnetizing) current, which lags the applied voltage by approximately 90°, giving a power factor of about 0.1-0.2.
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.
Back EMF = KΦω. When Φ is halved, back EMF reduces, so armature current (Ia = (V - Eb)/Ra) increases approximately to double for the same load.
