Alternating Current and Dynamically Induced Voltage

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Tachogenerator is an electromechanical device which generates an electrical output directly proportional to the speed of the rotating systems. Tachogenerators are basically transducers. This transducer converts the speed of rotation directly into electrical signal. The angular speed is converted into a directly dependent voltage signals. The voltage thus generated is fed to a speedometer for speed measurement and /or used as a feedback signal for controlling the speed of a rotating shaft. An electromechanical generator is a device capable of producing electrical power from mechanical energy, usually the turning of a shaft. When not connected to a load resistance, generators will generate voltage roughly proportional to shaft speed. With precise construction and design, generators can be built to produce very precise voltages for certain ranges of shaft speeds, thus making them well-suited as measurement devices for shaft speed in mechanical equipment. A generator specially designed and constructed for this use is called a tachometer or tachogenerator. Often, the word "tach" (pronounced "tack") is used rather than the whole word. By measuring the voltage produced by a tachogenerator, you can easily determine the rotational speed of whatever it's mechanically attached to. One of the more common voltage signal ranges used with tachogenerators is 0 to 10 volts

Hence tachogeneratros are electrical type of tachometer. Electrical voltages may be statically induced or dynamically induced. Depending on the nature of the voltage induced there are two types of tachogenerators: 1.DC tachogenerator

2.AC tachogenerator
It operates on the principle of Faraday’s Laws of Electromagnetic induction . It states that whenever there is a relative motion between the magnetic field and a conductor , then the voltage is generated in that conductor.

Characteristic requirements of tachogenerator:
Accuracy: The input to the tachogenerator is speed and the output is electrical voltage. The voltage should be measured correctly so that it can indicate the speed accurately Resolution: The analogue voltage measuring device should have division and subdivision of scale small enough, calibrated to indicate the smallest variation of speed. Linearity: If the relationship between voltage and speed is linear the scale will be linear. It may be easy to read in case of fraction of subdivision. Temperature effect: Armature and field winding resistances are affected by temperature depending upon temperature resistance coefficients. The magnetic property of the core is also temperature dependent . If temperature variations are excessive physical expansion coefficient may affect the dimentions. System voltage: If the system voltage use for excitation is fluctuating , the output relation may be affected. Residual error: In case of electromagnetic type field for DC tachogenerator the residual flux may give error. In case of AC tachogenerator statically –induced voltage by transformer action and harmonics may lead to residual error at zero speed. Radio Noise: The moving contacts at brushes, commutators , arcing and ripple create radio frequency disturbing signals . Phase Shift: In case of AC tachogenerator the resistance, reactance and impedence create phase difference. Reliability: Reliability of any system component is essential. Torque : Tachogenerator are like generators running without load but the rotating system has some mass, inertia and friction . The torque should be able to just overcome these components to rotate the moving systems. Waveform: In case of AC tachogenerator the sinusoidal output may be affected by harmonics of higher order and phase angle. DC TACHOGENERATOR

A DC tachogenerator generate electrical voltage corresponding to the speed of the rotating machine. The voltage is dynamically – induced EMF . The requirement of dynamically induce EMF...
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