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!  







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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