Final answer:
A tachometer is used instead of armature terminal voltage to measure rotor speed because it directly measures rotational speed and is not affected by variables that could alter voltage readings. It operates on principles similar to galvanometer-based meters in analog voltmeters and ammeters, ensuring accurate current and voltage measurements.
Step-by-step explanation:
We use a tachometer instead of armature terminal voltage to measure the rotor speed because a tachometer directly measures the rotational speed of the rotor. The armature terminal voltage can vary due to several factors such as load changes, armature reaction, and changes in field strength, which do not necessarily correspond directly to changes in rotor speed. A tachometer, on the other hand, is unaffected by these variables and provides a direct and accurate measurement of rotational speed. In galvanometric devices like fuel gauges in cars, the torque on the needle is proportional to the current (I), not the voltage. These meters use magnetic torque on a current-carrying loop where a linear spring serves as a counter-torque to balance the current-produced torque, resulting in the needle deflection being proportional to the current.
The concept of using a tachometer for accurate speed measurements is similar to the principle of galvanometer-based meters used in analog voltmeters and ammeters, which are designed with a large loop area (A), high magnetic field (B), and low-resistance coils to measure small currents accurately. These meters have calibrated readings to account for non-proportionalities between the measured quantity and the needle deflection, ensuring accuracy.