electrical measuring instruments

I. lesson 1

electric current is measures with ammeters which are connected in series with the circuit. Ammeters must be low of resistance, so that they will not alter the current to be measured. For this reason, their coil are wound with wires of sufficiently large diameter to carry the maximum measured current without excessive voltage drop. Ammeters may be of the last two types have their fixed and movable coils connected in parallel.

Voltage is measured with voltmeters which are connected in parallel with or across the circuit, and therefore they must have a high resistance. Owing to this, voltmeter coils are wound with a large numbers of turns of fine wires.

Both voltmeters and ammeters are classified basing on the operation principles of measuring instruments. They can be : moving-iron, induction, electro-dynamic, moving-coil and the other types.

Below we will consider the working principle and structure of a moving-iron instrument:

A moving-iron instrument of attraction types consists of a fixed coil (1), a movable steel vane (2), and aluminum damper sector (3), both are seated on a shaft (4), a pointer (5), and a spiral spring (6), the damper 3 is placed in the field of a permanent damping magnet (7). The shield (8) screens the magnetic field of the damping magnet.

When the current measured is passes through the coil, the coil is magnetized, and attracts the vane against the opposing force of spring, thereby turning the shaft and pointer through an angle. The greater the current passed through the coil, the larger the deflection of the pointer. As it turns, the shaft rotates are induced in the aluminum disc, and interact with the permanent magnet field to produce a braking torque which damps the swings of the pointer. This type of instrument is suitable for measuring in both dc and ac circuits.