Sodium Vapour Lamp Construction and Working Principle


What is Sodium Vapor Lamp?

A lamp in which an electric discharge takes place throughout a metallic vapor is known as a vapor lamp. These lamps are available in two types like sodium vapor lamp and mercury vapor lamp which emit different colors of lights like thick blue and yellow color respectively. These types of lamps are very efficient & provide light with high intensity which is applicable in open areas and also for illuminating large distances.

Principle of sodium vapor lamp

Electrons are emitted from cathode by subjecting the surface of the cathode to very high electric stress so that electrons are forcibly ejected from orbits of atom against nuclear centrifugal force.

Construction and Working :

  • It consists of inner tube of U shape fitted with neon gas and globules of sodium. For good performance 300°C temperature is to be maintained.

  • To protect discharge tube it is contained in evacuated double walled glass jacket. This prevents heat loss.

  • At starting, electric discharge is to be established through neon gas till sodium gas vapourized , so at start for 10-15 min. discharge through lamp will be pink and then as sodium vaporizes , it will turn to yellow.

  • For this voltage of about 410 V is required while normal voltage is about 165 V. Supply is given through a leak transformer which has high leakage reluctance.

  • As soon as discharge through the tube starts , heavy voltage drop takes place in the transformer so that required operating voltage is automatically impressed across cathodes.

  • P.F. of these lamps is 0.3 lagging and therefore condenser can be used for P.F. connection.

Sodium Vapour Lamp Types

There are two types of sodium vapour lamps available in the market like

LPS vapor lamp (Low-Pressure Sodium Vapor Lamp) 

HPS vapor Lamp (High-Pressure Sodium Vapor Lamp)

Advantages :

  1. Yellow light.

  2. Suitable for street light.

  3. More light output for same energy than other sources.

  4. Can be disposed easily.

  5. Effective in rainy and foggy season.

  6. Low operating temperature.

  7. Good for tunnel lighting and avoids stroboscopic effect.

Disadvantages :

  1. Starting time is required.

  2. It requires additional transformer.

  3. Poor power factor.

  4. Cost is more.

  5. Size is bulky.

Precaution :

Care should be taken in handling these lamps, for if inner tube is broken and sodium comes in contact with moisture , fire will result.

Life : 

Normal average life of sodium vapour lamps is 2500 hrs.

Applications : 

  • These lamps are broadly used for airports, goods yards, lighting of roads, etc

  • Sometimes, these lamps are used for advertisement purposes.

  • The most commonly used ratings of lamps are 250 watts and above.

  • HPS lamps are extensively used to provide lighting in industries

  • These lamps are used for outdoor area lighting like roadways, security areas, parking lots, etc.

Sizes available : 

45W, 60W, 85W, 140W.

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