As am confused with the comparison of explaination in my notes and textbook I just need a simple explanation on this topics

• In the magnetic drag type of tachometer a transducer produces an analog signal in the form of a continuous drag due to eddy currents induced in an aluminium cup
• The induced current is proportional to speed

Construction And Working

• It consists of a rotor which is coupled to the rotor shaft of the machine whose speed is to be measured
• A permanent magnet is attached to the rotor which rotates within an aluminium cup alongwith the rotor
• A spindle is attached to the aluminium cup to which a pointer is fixed as shown in figure
• A hairspring is fixed to the spindle, which provides necessary controlling torque
• At no revolutions it positions the pointer at zero on the calibrated scale
• As the magnet rotates within the aluminium cup alongwith the shaft of the machine eddy currents are induced in the cup which result in a torque or drag that tends to turn the cup against the spring
• The deflection of the cup is proportional to the induced emf, which in turn is proportional to the speed of the shaft
• The deflection is indicated by the pointer which moves on a calibrated scale
• These type of tachometers are commonly used in automobiles which measure the angular speed of the wheel
• In the case of the car wheel the angular speed is converted into a linear speed kilometres per hour
• In other applications such as for measuring locomotive speed the permanent magnet is kept stationary and a soft-iron rotor produces a revolving magnetic field

Reference: http://studentbank.in/report-explain-edd...z4LMCC3qtF