27 real Electrical Machines questions from the Electrical Core bank, as asked in Indian campus drives and tech interviews. Every question has a verified answer and an AI-tutor explanation on placd — free to start.
1. What is Transformer emf equation?
Junior
A.motor torque is proportional to slip at low slip and inversely proportional at high slip, peaking where slip equals R2/X2
B.rms induced voltage equals 4.44 f N Φm, so the volts per turn are fixed by supply frequency and peak core flux
C.ratio of energy output to energy input over 24 hours, the correct merit figure for distribution transformers whose load varies
D.reversal of current in an armature coil as it passes under a brush, assisted by interpoles to reduce sparking
A.Transformer emf equation — difference between synchronous speed and rotor speed expressed as a fraction of synchronous speed, typically 2–5% at full load
B.Transformer emf equation — rms induced voltage equals 4.44 f N Φm, so the volts per turn are fixed by supply frequency and peak core flux
C.Transformer emf equation — distortion and weakening of the main field flux by the mmf of the armature current, shifting the magnetic neutral axis
D.Transformer emf equation — plots of armature current against field current at constant load, minimum at unity power factor and leading when over-excited
A.Slip of induction motor — difference between synchronous speed and rotor speed expressed as a fraction of synchronous speed, typically 2–5% at full load
B.Slip of induction motor — oscillation of the rotor about its steady synchronous position after a sudden load change, damped by amortisseur windings
C.Slip of induction motor — distortion and weakening of the main field flux by the mmf of the armature current, shifting the magnetic neutral axis
D.Slip of induction motor — reversal of current in an armature coil as it passes under a brush, assisted by interpoles to reduce sparking
A.Commutation in DC machines — distortion and weakening of the main field flux by the mmf of the armature current, shifting the magnetic neutral axis
B.Commutation in DC machines — reversal of current in an armature coil as it passes under a brush, assisted by interpoles to reduce sparking
C.Commutation in DC machines — motor torque is proportional to slip at low slip and inversely proportional at high slip, peaking where slip equals R2/X2
D.Commutation in DC machines — difference between synchronous speed and rotor speed expressed as a fraction of synchronous speed, typically 2–5% at full load
11. Which term means: "OC test on the LV side gives core loss and the magnetising branch; SC test on the HV side gives copper loss and leakage impedance"?
A.Open-circuit and short-circuit tests — OC test on the LV side gives core loss and the magnetising branch; SC test on the HV side gives copper loss and leakage impedance
B.Open-circuit and short-circuit tests — difference between synchronous speed and rotor speed expressed as a fraction of synchronous speed, typically 2–5% at full load
C.Open-circuit and short-circuit tests — rms induced voltage equals 4.44 f N Φm, so the volts per turn are fixed by supply frequency and peak core flux
D.Open-circuit and short-circuit tests — plots of armature current against field current at constant load, minimum at unity power factor and leading when over-excited
14. Which term means: "ratio of energy output to energy input over 24 hours, the correct merit figure for distribution transformers whose load varies"?
A.Torque-slip characteristic — reversal of current in an armature coil as it passes under a brush, assisted by interpoles to reduce sparking
B.Torque-slip characteristic — plots of armature current against field current at constant load, minimum at unity power factor and leading when over-excited
C.Torque-slip characteristic — distortion and weakening of the main field flux by the mmf of the armature current, shifting the magnetic neutral axis
D.Torque-slip characteristic — motor torque is proportional to slip at low slip and inversely proportional at high slip, peaking where slip equals R2/X2
A.Synchronous motor V-curves — OC test on the LV side gives core loss and the magnetising branch; SC test on the HV side gives copper loss and leakage impedance
B.Synchronous motor V-curves — oscillation of the rotor about its steady synchronous position after a sudden load change, damped by amortisseur windings
C.Synchronous motor V-curves — difference between synchronous speed and rotor speed expressed as a fraction of synchronous speed, typically 2–5% at full load
D.Synchronous motor V-curves — plots of armature current against field current at constant load, minimum at unity power factor and leading when over-excited
A.Hunting of synchronous machine — difference between synchronous speed and rotor speed expressed as a fraction of synchronous speed, typically 2–5% at full load
B.Hunting of synchronous machine — oscillation of the rotor about its steady synchronous position after a sudden load change, damped by amortisseur windings
C.Hunting of synchronous machine — OC test on the LV side gives core loss and the magnetising branch; SC test on the HV side gives copper loss and leakage impedance
D.Hunting of synchronous machine — rms induced voltage equals 4.44 f N Φm, so the volts per turn are fixed by supply frequency and peak core flux
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