Tachometer Working and Types
The tachometer is a device that is used to measure the rotational speed of a disk or a shaft of motors or other machines. These devices can either be inbuilt into some equipment that contains moving parts or they can be a free handheld device that can be used to measure rotational speed in different kinds of equipment. The tachometer is also referred to as a revolution counter.
Tachometers that are attached to the in-vehicle show the rate of rotation of the crankshaft of the engine. Flow rates of the flowing fluids can be measured by introducing vanes in the flowing fluid and measuring the rotational speed of the vane. Tachometers are also used to measure blood flow rates.
The working principle of the tachometer is that if there is a relative motion between the shaft of a device and the magnetic field then an electromotive force is induced in the coil placed inside the constant magnetic field of the permanent magnet. The developed electromotive force is directly proportional to the speed of the shaft. This implies a linear relationship between the electromotive force and the speed of the shaft.
Although there are mechanical tachometers also other than electrical tachometers are widely used all around because they offer high accuracy and precision and can be operated over a wide range.
Types of Tachometer AC or DC type of tachometer generator.
Contact or Non-contact type tachometer.
Time measurement or frequency measurement type tachometer.
AC or DC tachometer generator: In the case of a DC tachometer generator, the machine whose rotational speed is to be measured is coupled with the shaft of the DC tachometer generator. The armature of the device rotates inside the constant magnetic field of the permanent magnet. This rotation induces an electromotive force. The moving coil voltmeter measures the magnitude of the induced electromotive force. Resistance is provided in series in order to control heavy currents generated by the armature. The commutator is used to convert the alternating current of the armature into a direct current.
In the case of an AC tachometer generator, the armature remains stationary but the magnetic field rotates. The rotation of the magnetic field induces an electromotive force in the coil. Either the amplitude or frequency of the electromotive force can be used to measure the rotational speed. The voltage is first amplified and then rectified by passing through a filter.
Contact or Non-Contact tachometer: A contact-type tachometer is brought in physical contact with the device whose rotational speed is to be measured. A magnetic sensor can be attached in order to display the output in terms of revolution per minute. These types of tachometers are generally pre-installed in the equipment.
A non-contact type tachometer is designed in such a way that it need not be brought into contact with the equipment whose rotational speed is to be measured. It consists of a laser mechanism, a beam of laser or infrared radiation projected by the tachometer that provides the measure of speed directly on its LCD display. It can measure very wide ranges of speed at wide angles.
Time or Frequency measurement tachometer: A time measurement tachometer calculates the rotational speed of equipment by measuring the time interval between the incoming pulses. Its reading time is dependent on the rotational speed of the equipment but the resolution is independent of the rotational speed.
A frequency measurement tachometer calculates the rotational speed by measuring the frequency of the pulses. It is capable of measuring very high speeds. Its reading time is independent of the rotational speed of the equipment and resolution is dependent on the rotational speed of the equipment.
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