MGVCL Exam Paper (30-07-2021 Shift 2)
A three-phase 400 V, 6-pole, 50 Hz squirrel-cage induction motor is running at a slip of 5%. The speed of rotor with respect to stator magnetic field and speed of the stator magnetic field with respect to rotor magnetic field are
Slip, s = (Ns - Nr)/Ns Ns = 120f/P 0.05 = (1000 - Nr)/1000 Nr = 950 rpm Speed of rotor with respect to stator magnetic field = 950 - 1000 = -50 rpm
Speed of the stator magnetic field with respect to stator core = 0 Speed of the rotor magnetic field with respect to rotor = sNs Speed of the stator magnetic field with respect to rotor magnetic field = 0 Speed of the rotor magnetic field with respect to stator core = Ns
The neutral point of a generator is usually earthed to facilitate protection of the stator winding and associated system. Earthing also prevents damaging transient overvoltages in the event of an arcing earth fault or ferro resonance. For HV generators, impedance is usually inserted in the stator earthing connection to limit the magnitude of earth fault current.
For lagging power factor = (IR*cosφ + IXsinφ)/V_r For leading power factor = (IR*cosφ - IXsinφ)/V_r Zero voltage regulation occurs on leading power factor only.
EXAMIANS