Thyristor fuses blow too often....


               ..A symptom which needs immediate attention.




If main thyristor fuses blow on applying the mains power to the drive even when "enable" command has been removed or disconnected, most likely cause is "defective thyristors". Checking with multimeter, on diode range, between each AC leg and +ve ( and -ve later on ) one by one will indicate if the thyristor is faulty. Reading as low as 2k Ohm on resistance range are sometimes acceptable and thyristors will work in normal way. However, most of the good thyristor will show "open" or a few hundreds of kiloOhms. Some button type thyristors, require pressure to be applied before the outer metallic part gets connected to the thyristor wafer inside. Therefore, even a dead short thyristor will escape the test if tested without pressing the probes hard. If the fuse is bowing with enable command given, the following steps will lead to some clue.

1. Disconnect the motor armature. Field can be left untouched.

2. connect two filament lamps of same wattage in series with each other, across the drive + and - terminals.

3. Firing circuit normally gets a DC voltage from "current amplifier" . The levels are either 0 to +15 volts or -15 to +15 volts or +15 to 0 volts. Looking at the circuit diagram, determine which range of voltage the drive normally works with. Accordingly, connect a 5 or 10 kiloOhm potentiometer to a suitable dc supply, preferably from the drive itself, and connect the wiper of the potentiometer to the input of the Firing Circuit. Take care to remove the normal interconnection between firing circuit and the previous stage i.e. current amplifier.



4. Apply power to the drive and give Enable command, i.e. start the drive through normal control available.

5. Vary the potentiometer from one end to the other end and you should see a very gradual and flicker free variation in the intensity of the lamp, much like a light dimmer at home.

6. If the above is observed, then the reason for the fuse blowing is not in the firing section of the electronic cards. It could be in the motor, the load, or closed loop part of the circuit etc.

7. If the variation in the intensity of the lamp is erratic, or you see no control over intensity, then the firing circuit is defective and this could be the cause for fuses to blow. Here we assume that no one has played with the phase sequence of the supply to the drive.

8. Defects in the firing circuit could be faulty synchronizing transformers, faulty phase shift capacitors etc.

9. Phase shift capacitors, after long use for years, may become open. You will find 6 of them in three phase drives. Replace all.

10. If the intensity of the lamp varies smoothly from zero to full, check the maximum voltage available across drive +/- terminals. For a three phase 415 volts system, it should be as high as 560 volts dc. If not, check if the firing circuit has a potentiometer limiting the maximum firing angle. Verify that you have control over the maximum voltage available. If the voltage is not rising more than 500 volts, connect oscilloscope across one of the lamps and check the waveform. One of the thyristors not firing, can lead to this symptom of low voltage. The wave forms with all 6 thyristors firing properly and with only 4 thyristors firing, are shown below.













A bad tachogenerator can also sometimes, depending on the severity of damage, cause fuses to blow. Putting the drive in current limit and then checking the signal of tachogenerator on Oscilloscope will reveal if the tachogenerator is bad or not. Putting the drive in current limit ensures that the closed loop corrective action is disabled and dignosis is correctly made. See the signal from a bad tachogenerator which can blow the fuses occasionally. If it is not possible to put the drive in current limit, one can run the mtor in “Open Loop” as mentioned above and check the tachogenerator signal.





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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?