Constant Current Transformer - Construction & Working

  • The design of constant current transformer is done by General Electrical ( GE ) company for series incandescent lamp for street lighting. 
  • The CCT provides constant current in the variable load conditions. The secondary current becomes constant if highly inductor is connected at the secondary side but it will decrease the power factor. 
  • The inductor should be variable type in order to provide constant current and higher power factor.


  • The core length of the CCT should be long and shell type. 
  • The primary winding of the CCT should be stationary and secondary winding will be movable. 
  • There is an adjustable weight is attached with secondary winding. The weight of secondary winding is high as compared to adjustable weight.


  • The variable reactance of the secondary winding depends upon its vertical movement with respect to primary winding. 
  • When supply is given to primary winding, there is induced emf in the secondary winding.  
  • When secondary winding is closed, the magnetic field of the secondary winding interacts with the primary winding and pushes moveable secondary winding in the upward direction. 
  • The balance point between coil weight and magnetic force must be adjusted to produce desired output current. 
  • The distance between primary winding and secondary winding at full load is approximately 2 inch and the resistance of secondary winding decreases with decrease in load. It will increase the load current. 
  • The leakage reactance becomes minimum when there is minimum distant between primary and secondary winding. 
  • The CCT operates at full load in this condition and produces maximum output. The efficiency and power factor will be best at 100 % full load in the CCT. 
  • The effect of the secondary power factor on the output current of the CCT is shown in the Figure.


  • The rating of the CCT is expressed in the kW rather than kVA. 
  • Its rating is in the range of 5, 7.5, 10, 20, 30, 40 kW.  The secondary current rating is either 6.6 Amp or 20 Amp.


  • The incandescent lamp is connected in series through CCT in the street and highway lighting. 
  • If the load current increases only by 1%, it will reduce life of lamp by 20% to 25%. 
  • The CCT provides good regulation at any load condition. When constant voltage is given to the primary winding, the CCT should not overload in spite of short circuit conditions.
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