What is Tachometer Generators?

Before going to Tachometer Generators we will see about Angular Velocity in short.

In many cases the only way to measure Linear Velocity is to convert it into Angular Velocity.For example a Speedometer uses the wheels rotational speed as a measure of Linear road speed.

The measurement of Angular Speed may be made with Tachometers which may be either Mechanical or Electrical type.

Electrical Tachometers.

The Electrical Tachometers are professed over Mechanical Tachometers for all applications because these Tachometers offer all the advantages associated with Electrical Transducers.

Electromagnetic Tachometer Generators.

There are two types of Electromagnetic Tachometer Generators called Tachogenerators.

01)D.C.Tachometer Generators.

02)A.C.Tachometer Generators.

The details of D.C.Tachometer Generators is as below.

01)D.C.Tachometer Generators.

D.C.Tachometer Generators consists of a small Armature which is coupled to the Machine whose Speed is to be measured.This Armature revolves in the field of a Permanent magnet.The Electro Magnetic Force (EMF) generated is proportional to the product of Flux and Speed.Since the flux of the Permanent magnet is constant,the voltage generated is proportional to Speed.

The Polarity of Output Voltage indicates the Direction of Rotation.This Electro Magnetic Force is measured with the help of moving Coil Voltmeter having a uniform scale and calibrated directly in terms of Speed.

Advantages are,

01)The direction of rotation is directly indicated by the Polarity of Output Voltage

02)The Output Voltage is typically 10 mV/rpm and can be measured with conventional type D.C.Voltmeters.

Disadvantages are,

01)Brushes on small Tachometer Generators often produce Maintenance problems,as their contact resistance may vary and produce appreciable errors.This is commutator and the brushes requires periodic Maintenance.

02)The input resistance of Meter should be very high as compared with Output Resistance of Generators.This is required to limit the Armature Current to small value.If the Armature Current is large,the field of the Permanent Magnet is distorted giving rise to non-linearity.