Speed goes up and down with a rhythm...






This phenomenon of motor speed going up and down in a noticeable rhythm is called “hunting”.

Hunting, or speed of motor going up and down in rhythm, can be caused because of following reasons :

- Load on the motor too small and the adjustments done at normal load do not hold good for light loads.

- Current feedback absent. Check the signal on Burden Resistor of Current Transformers, with an oscilloscope.

- Gain of the speed amplifier does not match with the mechanical inertia. Normally too much gain causes this type of hunting.

- Too much of Integral time.

Irregular speed variation can be caused by loose coupling of tachogenerator, unstable output of RAMP generator, dry solder in feedback path of tachogenerator signal etc.





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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?