Synopsis of Electrical Engineering

Important Multiple Choice Questions on Electrical Machines 1.        Can a 50Hz transformer be used for a 25Hz if the input voltage is maintained constant at the rated value corresponding to 50Hz? a)       Yes. Since the voltage is constant, current levels will not change. b)       No. Flux will be doubled which will drive the core excessive saturation. c)        No. Owing to decreased reactance of transformer, input current will be doubled at load. d)       Yes. At constant voltage, insulation will not be overstressed. 2.        In a transformer, if the iron losses and copper losses are 40.5 kW and 50 kW respectively, then at what fraction of load will be the efficiency be maximum? a)       0.8 b)       0.57 c)        0.7 d)       0.9 3.        If the frequency of input voltage of the transformer is increased keeping the magnitude of voltage unchanged, then a)       Both the hysteresis and eddy current losses in the core will increase. b)       Hysteresis l

Important Multiple Choice questions on Power Systems 1.        For transmission line which one of the following relations is true? A)      AD-BC=1 B)       –AD-BC=1 C)       AD-BC=-1 D)      AD-BC=0 2.        For a given base voltage and base volt-amp, the per unit impedance value of an element is X. what will be the per-unit impedance value of this element when the voltage and volt-amp bases are both doubled? A)      4X B)       2X C)       X D)      0.5X 3.        A 3-phase transmission line has its conductors at the corners of an equilateral triangle with side 3m. the diameter of each conductor is 1.63 cm. the inductance of the line per phase per km is A)      1.232 mH B)       1.182 mH C)       1.093 mH D)      1.043 mH 4.        A cable has inductance of 0.22mH per km and capacitance of 0.202 micro farads per km. the surge impedance of the cable is A)      28 ohms B)       33 ohms C)       42 ohms D)      50 ohms 5.        The capacitanc

Important Multiple Choice Questions On Power Systems 1.        Plug-setting of a relay can be altered by varying A)      Number of ampere-turns B)       Air gap of magnetic path C)       Adjustable back up stop D)      None of these 2.        If the time of operation of a relay for unity TMS is 10 sec, the time of operation of 0.5 TMS will be A)      20 Sec B)       5 Sec C)       10 Sec D)      None of these 3.        Where the voltages are high and current to be interrupted is low, the breaker proffered is A)      Air blast CB B)       Oil CB C)       Vacuum CB D)      Any of these 4.        Resistance switching is normally employed in A)      All breakers B)       Bulk oil breakers C)       Minimum oil breakers D)      Air blast circuit breakers 5.        For protection of rotating machines against lightning surges a combination of A)      Lighting conductor and capacitors are used B)       Lighting conductor and lighting arrestors

Important multiple choice questions on power systems 1.        Which one of the following statements is true? A)      Steady state stability limit is greater than transient state stability limit B)       Steady state stability limit is equal to transient state stability limit C)       Steady state stability limit is less than the transient stability limit D)      No generalisation can be made regarding the equality of otherwise of the steady state stability limit and transient stability limit 2.        If the integral control of a single area system, when  the system frequency error is reduced to zero A)      The integrator output and the speed changer position attains a constant value B)       The integrator output increases but the speed changer position also comes down C)       The integrator output decreases and the speed changer position also comes down D)      The integrator output decreases but the speed changer position moves up 3.        The steady-st

Important Multiple Choice Questions On Electrical Machines 1. Supply to one terminal of delta-star three-phase core type transformer which is on no-load fails. Assuming magnetic circuit symmetry, voltages on the secondary side will be 230, 230,115 230,115, 115 345, 115, 115 345, 0, 345 2. A 220/440 V, 50Hz, 5kVA single phase transformer operates on 220V, 40 Hz supply with secondary winding. Then The eddy current loss and hysteresis loss of the transformer decrease The eddy current loss and hysteresis loss of the transformer increase The hysteresis loss of the transformer increases while eddy current loss remains the same The hysteresis loss remains the same whereas eddy current loss decreases 3. The voltage regulation of a large transformer is mainly influenced by No-load current and load power factor Winding resistances and load power factor Leakage fluxes and load power factor Winding resistances and core losses 4. If a sinus