Thyristor DC Drives....

 

 

 (Last update.... 7th June 2013 )

 6, 8, 25, 56, 45, 29

 

Motor not able to drive the load..

 

                                                                                         

Some times, motor draws good amount of armature current, but not able to drive the load connected to it. Common reason for this observation is that the load is too much and is beyond the capacity of motor to produce required torque. The dc drive is said to have gone in “current limit”. This is visible on ammeter connected to the drive. One will find that the motor starts drawing more and more current and finally reaches a point where the ammeter appears to be stuck at one level. If the ammeter is of analog type, the pointer becomes rock steady. If the drive has not reached the current limit level, then the ammeter pointer normally keeps on  moving up and down a bit. In such cases cause for no rotation is not from excessive load.

 

If the load is mechanically not jammed or locked, and the drive goes in current limit, there could be one more probable reason for this. It is possible that the “field voltage” is absent and the mechanism to sense “field failure” condition is bypassed! i.e. out of two magnetic fluxes produced by armature current and field winding, required for production of torque, one is absent.

 

If the filed voltage is also present and okay, there is another possibility, although remote, that the carbon brushes on the commutator are not making contact with commutator. In case of motors with 4 sets of brushes ( located at 90 degrees ) this situation is likely to occur. The armature current drawn by the motor is not at “electrical right angles” with respect to magnetic flux and therefore not able to produce any torque!  

 

 

 

 

 

 

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 Feebback Vs Tacho Feedback

 

Current Limiting

 

Constant HP & Constant Torque

 

Field Weakening

 

Using oscilloscope

 

 

 

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 please

 

 

Drive stops all off a sudden..

 

 

 Motor not able to drive the load

 

DC to DC isolation

 

 

 

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