DC motor gives jerks while running...






...Read further to know more about the reasons



Jerks observed while running the motor are dangerous to the drive as well as to the motor and connected load. It is harmful to continue running the motor without trying to locate the source.

Jerks, many a times, appear to be arising from the mechanical parts but such conclusions are deceptive. Immediate checking of current waveform will give some clue. Note the difference in the two waveforms. Sharp rise only in one peak is normally not possible due to mechanical parts.

Waveform as shown in the sketch on the left hand side is also possible if a thyristor has collected a lot of dust around gate cathode area. Keeping ceramic insulator around gate clean is one solution to such cases. Thyristors misfire with such dust getting accumulated and can create a big sound from the gears connected to the motor. If one is not aware of this "electrical possibility", it misleads people to think that the gears have developed some problem.

Printed circuit boards also accumulate black dust around the tracks which causes similar symptoms.

When any contactor in the panel drops out, some times, the motor gives jerks. Dropping of a big contactor causes electrical noise to be generated which travels along and is picked up by sensitive “Gate” leads. Such jerks can be reduced by twisting the gate cathode leads with very tight twist. The twisting makes the gate cathode leads immune to picking up of noise. Shorter lengths of these wires also helps. Long leads routed through cable trays only “look” esthetically nice but are bad electrically. Short lengths of gate-cathode leads, running directly in air are good, although they may not look good! R-C circuits across the contactor coils also help in reducing these jerks. R-C circuit eats up the noise generated right at the source.














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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?