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.

 

 

 

 

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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