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?

 

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