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.

  

  

 

 

 

 

 

 

 

 

 

 

 

Download PDF version of these pages.

 

 

 

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?

 

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