JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III Year INDUSTRIAL INSTRUMENTATION Previous Question Paper (October - 2020 - R16)

An AC tachometer, also known as an AC tachogenerator, is an electromechanical transducer used to measure rotational velocity by converting mechanical rotation into an electrical signal. It essentially functions as a small alternating current generator.

• Construction: The device consists of a rotor, which is coupled to the shaft whose speed is to be measured, and a stationary stator winding. In the most common configuration, the rotor is a permanent magnet, and the stator contains a coil.
• Working Principle: As the rotor rotates, the magnetic field lines from the permanent magnet cut across the stator windings. According to Faraday's Law of Electromagnetic Induction, this changing magnetic flux induces an alternating voltage in the stator coil.
• Signal Characteristics: The frequency of the induced AC voltage is directly proportional to the rotational speed of the shaft (f = PN / 120, where P is the number of poles and N is speed in rpm). The amplitude of the voltage is also proportional to the speed.
• Signal Conditioning: The output AC signal is typically processed using a rectifier and filter circuit to convert it into a DC voltage proportional to the speed, or a frequency-to-voltage converter to provide a precise readout on a digital display.
• Advantages: AC tachometers are highly reliable because they lack brushes or commutators, leading to minimal maintenance and no sparking issues, making them suitable for high-speed applications.

When using a stroboscope to measure the speed of a rotating object, the object appears stationary when the flash frequency is equal to the rotational speed (N) or a sub-harmonic of the speed (N/n). If multiple images are observed, the true speed can be determined by identifying the highest frequency at which a single image is observed.

Given stroboscopic settings: 3500 rpm, 6000 rpm, and 5630 rpm.

• Let the true speed of the turbine be N.
• The observed frequencies are harmonics or sub-harmonics of the true speed.
• The highest observed frequency that shows a single image is typically the true speed of the machine.
• In this scenario, if the turbine speed is N, the stroboscope will show a single image at N, N/2, N/3, etc.
• Comparing the values: 6000 rpm is the highest frequency provided. If 6000 rpm is the true speed, then 3000 rpm would show two images, and 2000 rpm would show three.
• Since 5630 rpm is also a setting where a single image is observed, we must evaluate the relationship. If N = 5630, then 6000 would show a blurred image. If N = 6000, 5630 would show a blurred image.
• Assuming the turbine speed is a common denominator or the highest frequency setting that produces a clear single image, the turbine speed is 6000 rpm.

Note: In practical stroboscopic measurement, the highest frequency at which a single stationary image is observed is taken as the actual speed of the rotating shaft.