Using oscilloscope on Thyristor DC Drives....















Electronic components in Thyristor drives have voltages of the order of 400 volts as well as dc +/- 5 volts, 12 volts etc. High voltages present around thryistors are normally derived directly from three phase supply which, many a times, has a neutral connection also. This means that there are dangerous voltages present around thyristors with respect to neutral. Oscilloscopes operating from mains supply voltages have invariably an “Earth” connection in the mains cord. Internally, this is connected to the “zero” or “common” of electronic circuitry of oscilloscope. The BNC connectors where we connect the CRO probes, thus, have a zero volt point getting connected all the way upto the measuring tip of the probe.


C A U T I O N :


Here comes the major risk. When the probe is connected to, say Gate-Cathode of a Thyristor, to check firing pulses, a dead short circuit occurs between a phase voltage and the earth,  causing a big blast in the probe shield !


To avoid such a situation from arising, “earth” pin of the CRO must be  REMOVED when using on DC drives. This sounds against the basics of handling electrical equipment where Earthing is a must! However in this situation it must be done with due care in handling. It is further important to keep in mind that there are dangerous voltages present on CRO metal parts which must not be allowed to come in contact with human body or other metal parts connected to ground.






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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?