Instruments

Tachometer Working and Types

October 16, 2020

Tachometer is a device which is used to measure rotational speed of a disk or a shaft of motors or other machines. These devices can either be inbuilt in to some equipment which contains moving parts or it can be a free hand held device which can be used to measure rotational speed in different kinds of equipments. Tachometer is also referred to as revolution counter.

Tachometers which are attached 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 rotational speed of the vane. Tachometers are also used to measure blood flow rates.

The working principle of tachometer is that if there is a relative motion between shaft of a device and 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 speed of the shaft.

Although there are mechanical tachometers also other than electrical tachometers but 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 case of 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. 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. Commutator is used to convert the alternating current of the armature into direct current.
In case of AC tachometer generator, the armature remains stationery but the magnetic field rotates. The rotation of the magnetic field induces 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 amplifed 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 inorder 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 so 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 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 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.

Reference:- circuitglobe, automationforum