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 ratational speed
as a measure of Linear road speed.

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

Electrical Tachometers.

The Electrical Tachometers are prefessed 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 proportational to the product of Flux and Speed.Since the flux of the Permanent
magnet is constant,the voltage generated is proportational 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 Maintainance problems,as their contact resistance may vary and produce apprecible errors.
This is commutator and the brushes requires periodic Maintanance.

02)The input resistance of Meter should be very high as compaired 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.

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