Current Amplifer of a DC drive...






Current amplifier in a dc drive does not amplify current ! It derives its name from the simple fact that the feedback signal given to this amplifier is that of current of the dc motor. This amplifier is also called more appropriately as MINOR LOOP. This amplifier forms a minor loop ( inner loop) in the overall closed loop system of a dc drive, the external or MAJOR loop being that of speed with speed feedback.

This amplifier, in a simple analog dc drive is built around single operational amplifier in PI configuration. i.e. it has a resistor and capacitor in the feedback loop deciding PROPORTIONAL GAIN and INTEGRAL TIME of the amplifier. Typical values in an analog dc drives are 10 kilo ohms and 1 micro farad. Some designs may have Current Gain, like that in Speed Amplifier. Output of the Current Amplifier acts as the input to the firing circuit. The output of this amplifier is restricted within two minimum and maximum limits set by two preset potentiometers called minimum and maximum voltage or "conversion" and "inversion" limits. This dc level decides the instance in sinewave at which the thyristors are fired. If we disconnect the output of this amplifier and connect a potentiometer to feed dc level to the firing circuit, we can test any drive in "open" loop manner. This is normally done to diagnose faults in the firing section of the drive. Since there are no safety limits left when the drive is tested is this fashion, it is advisable to take initial tests with Lamp load instead of dc motor.






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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?