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Steven Charlton

Electrical Testing and Measurement LO1 & LO5 Assignment

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1. The Moving Coil instrument uses two permanent magnets to create a fixed magnetic field. These instruments work using the motoring principle. When a coil carrying current is placed in the magnetic field, the coil produces magnetic flux which is proportional to the current, the magnetic field interacts with the magnetic flux which creates torque, this is what moves the pointer. This is known as the D’Arsonval principle.

A diagram of The Moving Coil Instrument is shown below:

The coil is made up of turns of wire, which are suspended so that they are free to rotate.

The torque is controlled by two hairsprings called torsion springs, which can be adjusted to calibrate the instrument.

A spindle carries the pointer which moves across the linear scale. Placing a mirror below the pointer removes the parallax allowing an accurate reading. There is a weight to counterbalance the pointer, this is placed at the bottom of the spindle.

Electrodynamic Watt-meters have two coils which control the pointer. One coil measures the voltage while the other measures current.

A diagram of an Electrodynamic Watt-meter is shown below:

Moving Coil:

This is the coil which measures voltage and moves the pointer. It is connected in parallel to the circuit, this is so the supply voltage is passing through it. Only a small amount of current flows through the moving coil to prevent overheating. This is achieved by connecting a high value resistor in series with the coil. The moving coil can move freely on a pivoted spindle.Fixed Coil:This is the coil that measures current. It is split into two equal parts to allow it to carry more current. It is connected in series with the circuit. The reason for connecting it in series is so the load current will be passing through the coil.

3. Induction KWhr Energy Meters are Induction instruments and operate in AC circuits and they are useful only when the frequency and the supply voltage are approximately constant. The most commonly used technique is the shaded pole induction watt-hour meter, shown below:

The rotating element is an aluminium disc, and the torque is produced by the interaction of eddy currents generated in the disc with the imposed magnetic fields that are produced by the voltage and current coils of the energy meter.