The eddy current tachometer converts the angular speed (ω) of the rotor into pointer deflection ϕ. The diagram of the eddy current type tachometer is shown in figure 1. The eddy current tachometer is also referred as drag-up tachometer.
Working & Construction of Eddy Current Tachometer
The eddy current tachometer consists of a permanent magnet. Here two pole or multiple-pole magnets can be used. This magnet is arranged to a shaft. The shaft is used to make contact with the rotor whose angular velocity is to be measured. An eddy current cup which is nothing but a conducting material is placed near to the magnet. This is carried by a spindle arrangement and restrained by using torsion spring. Mostly aluminium is used to make eddy current cup. When the shaft is in contact with a rotor (which is rotating), the shaft rotates along with magnet.
Due to this, flux passes through the cup. With the change in the rotation of the magnet the direction of induced flux also changes causing the alternate flux density (B) to develop in the cup. So an electric field is produced with the rate of change of flux density B.
\[ \text{Curl E = }\frac{-dB}{dt}\]
This electric field will generate eddy current in the shell of the cup. The generated eddy current density is indicated by J and it will produces a secondary magnetic field of intensity H. The relation between magnetic field of intensity H and eddy current density J is given by,
\[\text{Curl H = J}\]
Due to these two magnetic fields a torque is generated in the eddy current cup, which is proportional to the angular speed. A pointer scale arrangement is provided at the back of the spindle. If the eddy current cup is restrained by torsion spring, an angular deflection of spindle occurs and is indicated over a calibrated scale. If both magnetic torque and spring torque are balanced then it will be indicated by pointer.
Applications of Eddy Current Tachometer
Eddy current tachometers are used,
- In automobile speedometers
- To measure speed in aircraft engines
- To measure locomotive speeds
- In control systems and industrial applications.