Speed Amplifer of a DC drive...






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

This amplifier, in a simple analog dc drive is built around single operatonal 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 220 kilo ohms and 4.7 micro farad. There is also a "gain" potentiometer associated with the speed amplifier. This is set, during commissioning of the drive, to get optimally damped response of the closed loop, i.e. it is set in such a way that the dc motor reaches its set speed as fast as possible but at the same time it has no overshoots or undershoots of speed during acceleration or deceleration. Gain is normally set towards minimum of the range available, i.e. the wiper of the potentiometer is towards the output of the opamp. Output of the speed amplifier acts as the input to the current amplifier through a preset potentiometer called "CURRENT LIMIT". Read more on the current limiting and current amplifier in other links.






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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?