Working Principle

  • Speedometer is used to measure and display instantaneous speed of the vehicle.
  • Speedometer application uses an Alnico magnet which rotates on an axle in an aluminium cup.
  • As a result of relative motion of the magnet and the cup, eddy currents are induced producing the torque on the cup.
  • Torque is proportional to the speed of rotation of the magnet.
  • Aluminium cup rotates against the force of a spring and a pointer fixed on its shaft indicates the speed of the magnet.
  • The speedometer scale is graduated in accordance with the number of revolutions or, as in motor vehicles, in velocity corresponding to the diameter of the wheels.
  • At constant speed, Pointer remains steady & will show exact speed on the dial.

Schematic for Speedo magnet assembly

Speedometer assembly drawing

Standard sizes of Speedo Magnets in Alnico 2, Alnico 3 alloy

Diameter Thickness I.D. Versions
23 3 13.5 Bare Magnet
23 3.5 6 Bare Magnet
25 5 6 Bare Magnet
25 3.7 6 Bare Magnet
23 4.25 5.25 Molded Magnet