Basic Electrical Engineering

Sodium vapour lamp

Sodium vapour lamp

Sodium vapour lamps are some of the most efficient lamps in the world. They have an efficiency of up to 190 lumens per watt compared to an incandescent street lamp which has between 15 and 19 lumens per watt.

This sodium vapour lamps comes in two major groups:

1. High pressure sodium vapour lamps (HPS)

2. Low pressure sodium vapour lamps (LPS)

This lamp consists of discharge tube made from special heat resistance glass, containing a small amount of metallic sodium, neon gas and two electrodes. Neon gas is added to start the discharge and to develop enough heat to vapour sodium. A long tube is required to get more light from this lamp. To reduce overall dimensions of the lamp, the tube is generally bent into U‐shape.

a. Working principle

The construction of sodium vapour lamp is Electric discharge lamps require a high voltage at start and a low voltage during operation. The tungsten-coated electrodes are connected across auto-transformer, having high leakage reactance. The open-circuit voltage of this transformer is about 450 V which is sufficient to initiate a discharge through the neon gas. After 10 to 15 minutes, the voltage falls to 150 V, due to low power factor. A capacitor is connected across the supply to improve the power factor. The colour of light produced is yellowish.

b. Applications

1. LPS lamps are rarely used for indoor lighting and are best suited for outdoor lighting.

2. LPS lamps are used in security lighting as their high efficiency.

3. LPS lamps are also often used in long tunnels.

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