Resistances $${R_1}$$ and $${R_2}$$ have, respectively, nominal values of $$10\Omega $$ and $$5\Omega ,$$ and tolerances of $$ \pm 5\% $$ and $$ \pm 10\% $$. The range of values for the parallel combination of $${R_1}$$ and $${R_2}$$ is

A $$100$$ $$\mu A$$ ammeter has an internal resistance of $$100\,\Omega $$. For extending its range to measure $$500$$ $$\mu A$$, the shunt required is of resistance (in $$\Omega $$)

The minimum number of wattmeter(s) required to measure 3 $$-$$ phase, 3 $$-$$ wire balanced or unbalanced power is

A $$50$$ $$kW$$ synchronous motor is tested by driving it by another motor. When the excitation is not switched on, the driving motor takes $$800$$ $$W.$$ When the armature is short-circuited and the rated armature current of $$10$$ $$A$$ is passed through it, the driving motor requires $$2500$$ $$W.$$ On open-circuiting the armature with rated excitation, the driving motor takes $$1800$$ $$W.$$ Calculate the efficiency of the synchronous motor at $$50\% $$ load. Neglect the losses in the driving motor.