EEE 208: Electrical Measurements Second Semester Examination

Electrical measurements paper, sat on Friday the twenty-ninth of January 2021 from eleven to one, question one compulsory at thirty marks and any two of the remaining four at twenty. Question one explains the international, primary, secondary and working classifications of measurement standards, weighs the merits and demerits of the digital measuring system over the analogue one, works the percentage uncertainty expected in power computed from the square of voltage over resistance for a nominal twenty-ohm one per cent resistor when the measured voltage is one hundred and fifty volts at one per cent, describes with clear schematic diagrams how high voltage and high current are measured with instrument transformers, and computes the current through, and the deflection of, a galvanometer of given internal resistance and sensitivity connected across a Wheatstone bridge whose four arms are specified when one arm is changed from three thousand to three thousand and fifty ohms. The other questions explain the scenarios in which measuring instruments need calibration, find the apparent resistance, the actual resistance and the error due to the loading effect of a voltmeter of a thousand ohms per volt reading a hundred volts on its a hundred and fifty volt scale when connected across an unknown resistor in series with a milliammeter reading five milliamperes, and calculate the shunt resistance for a hundred ampere full-scale deflection and the series resistance for a thousand volt full-scale reading of a moving-coil instrument that takes ten milliamperes at a hundred millivolts, with the power dissipated in each case. Others define the electrical transducer and explain a capacitive transducer with a diagram, and derive the balance equations of a Maxwell inductance-capacitance bridge whose four arms are specified before determining the inductance and its equivalent series resistance; explain the elements of a measurement system with a labelled diagram and calculate, for three similar coils of given resistance and inductive reactance connected in star and in delta across a four hundred and fifteen volt three-phase supply, the reading on each of two wattmeters connected by the two-wattmeter method; and take instrumental errors with three of their causes, a current transformer of two primary turns and two hundred and sixty secondary turns feeding an ammeter of two tenths of an ohm through a winding of three tenths of an ohm at a primary current of six hundred and fifty amperes, and the actual power consumed by an eighty-ohm lamp in series with a one hundred and twenty-ohm lamp on a two hundred volt direct supply as read with a voltmeter of a hundred kilo-ohms and an ammeter of a tenth of an ohm in each of the two possible arrangements of the two meters.