Checking a thyristor with a test lamp on table...







Thyristor, when checked on digital multimeter, should show "open circuit" in both directions. Resistance between Anode and cathode is sometimes as low as 2 k but such thyristors also work okay.

Gate and cathode, when checked on "diode test" range , sometimes show voltage drop of approximately 0.5 to 0.7 volts in one direction and "open" in other direction. Some thyristors show "diode" in both directions. But "open" in both directions is not a sign for good thyristor.To check if the thyristor ( rated at 60 amps or above ) is triggering okay with gate pulses, the following set up on table can reveal the result. The switch should be pressed for short time, for a second or so, to avoid damage to the gate junction. If the lamp glows when the switch is closed, the thyristor can be considered to be in good condition. Very small thyristor of ratings less than typically 40 amps may require higher value of resistor to limit the gate current. High values like 47 k can be selected to begin with and reduced if the bulb does not glow. Gate resistor and a switch shown below may have to be shifted before the lamp in some cases. Do not hold the switch down for long time. Just a momentary trigger is enough.








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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 



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Fuses Blow


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Belts vibrate


Speed control not satisfactory 



Speed drops on load


Motor overheats


Sparking on commutator


DC drives and Power Factor


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Drive stops all of a sudden 


Motor not able to drive the load


DC to DC isolation


 Motor or Drive?