Thyristor DC Drives and Power Factor ....



 (Last update.... 17th August 2013 )


Power Factor, Power Factor correction, value of capacitors required etc…..


Power Factor:


In alternating current circuits, alternating voltage and alternating current are not always necessarily in phase with each other. i.e. to say, the current flowing in any circuit component, which a result of applied voltage, follows the voltage wave form with a time gap. This time gap could be either ahead of the voltage applied or it could be behind the voltage applied. In circuit with a lot of inductance present, the current is behind the voltage applied. In such cases the current is said  to be “lagging” the voltage, as if it is a bit reluctant to flow. The amount of “lag” depends on the amount of inductance present. In circuits with a lot of capacitance present, the voltage follows the current. In such cases the current is said to be “leading” the voltage.


For more detailed information on the basics click here. 





In case of DC drives with fully controlled configuration, low speed operation of dc motors is achieved by firing the thyristors nearer to 180 degrees for getting lower voltage. Thus, for every sinusoidal cycle of supply, the current starts flowing late as compared to the voltage.  DC drives, therefore,  when operated at low speeds, cause a very low or “poor” power factor. There is no way by which this situation could be improved. It is an effect created as a “by product” of the method used for speed control.


Trying to improve the power factor by putting capacitors near the dc drive is known to have created problems of blowing of thyristors or fuses for no apparent reasons.



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Block Diagram



Ramp Circuit


Speed Amplifier


Current Amplifier


 Firing Circuit


Power Circuit


Types of DC drives 



 Checking Thyristor


Voltage feedback Vs Tacho feedback


Current Limit


Constant HP & constant Torque


Field Weakening


Using CRO 



Motor runs at full speed


Fuses Blow


Hunting in speed


Motor gives jerks


Belts vibrate


Speed control not satisfactory 



Speed drops on load


Motor overheats


Sparking on commutator


DC drives and Power Factor


Your comments and suggestions 


Drive stops all of a sudden 


Motor not able to drive the load


DC to DC isolation


 Motor or Drive?