Motor gets overheated with no apparent reason....






Motor can run hot for any of the following reasons :

1. cooling inadequate due to air leakages in the blower gasket

2. blower direction wrong

3. bearings faulty

4. gain adjustment of the drive incorrect

5. Four quadrant dc drives which have two sets of thyristors ( Two bridges connected in back to back manner ) can casue overheating of motor if the gain is not adjusted properly. Monitoring Current waveform will show that both the thyristor bridges are firing ( becoming on ) alternately at a fast rate. Torque is thus applied in alternate directions. However, mechanically this is not felt due to inertia. Net effect of torques in both directions is what is rotating the motor in one direction. It is like saying that we get a torque of say 3 units but it is as a result of two opposite torques of unnecesarily high values. ( 23-20 =3, or 56-53=3 !) Torque of +56 and 53 units cause overheating of the winding, but it is 3 units which is driving the load.

5. If motor runs normally at low speeds, reduce the diameter of the pulley on the motor shaft.

This will reduce the current drawn for the same load, thus reduce the heating.





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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?