Fring of thyristors in a DC drive...

 

 

 

 

 

In a three phase dc drives, the three line voltages are applied to the thyristors. These are R-Y, Y-B, and B-R. Referring to the sketch below, thyristor 1 will conduct only when its anode is most positive compared to other two i.e. no. 3 and 5, provided, off course, the firing pulse is applied.

Looking at the three phase sinewave diagram, this possibility exists only after point "A", and continues to exist till point "B" which is 180 degrees away from "A". This means that the firing pulse generated by the electronics must be able to swing from A to B.

 

 

 

Pulse is generated using a dc level from current amplifier and a sinewave from the three phase supply. Both are first compared in a comparator. This comparator generates a square wave in which the rising and falling edges occur at the instant when amplitudes of sine wave and that of the dc level are equal. Therefore, one must have a sinewave signal which covers entire 180 degrees between "A" and "B" . This sine wave is normally derived by phase shifting a voltage which has 30 degrees phase shift with respect to Line voltage. Phase shifting capacitors are typically 0.1 mfd to 0.47 mfd. Resistor values are adjusted accordingly and are chosen to have low tolerance of 1 % . Output of these comparators ( 6 for three phase drive ) is passed through capacitors, to generate pulses. These are amplified and applied to gate of the thyristor through a pulse transformer.

 

It must be noted that the point A on the red sine wave above is already 60 degrees behind the zero crossing point of that sine wave. The firing pulse must be generated at point A if we want zero firing angle ( corresponding to maximum voltage ). Out of 60 degrees shift required, 30 degrees is achieved through R-C network and 30 degrees is achieved through Delta / Star configuration of Synchronising transformers.

 

Click here to see more on firing pulse genration...

 

 

 

 

 

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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