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Vsd Frequent Bursting Of Semiconductor Fuse


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Dear Friends,



I have a problem with an obsolete Analog Four Quadrant DC Drive (Anti Parallel full wave 3 phase Thyristor Bridge), 90 Kw, 440 Volts, 60Hz.


The drive is employed in Rubber Tired Gantry Crane. The problem is the frequent bursting of 2-355 Amperes of semiconductor fuses.


Bursting of fuses happened intermittently, we decided to replace all the SCR modules as maybe the caused of such problem, but the semiconductor fuses is still blowing after few hours or after a day of operation ( although the cooling system is working fine)


No other option to try since we do not have any related technical documentation available, neither a piece of control cards (regulator, synchro, absorber or firing card).


One thing I noticed is the Incoming voltage supply to Drive, I got a balance of 440 phase to phase voltage and 257 volts potential to ground for each phases which is a normal value.


However, upon powering the drive, it comes out with unbalance ground potential for each phases. Say, R-Ground is 245 volts, S-ground is high as 300 volts! T-ground is 220 volts. Phase to Phase voltage is still balance on this case at 440 volts.


Is the circuit considered unhealthy on this case? Is this unbalance ground potential may cause high current and blows the semiconductor fuses? It seems Phase T leaks to the ground whenever powering up the drive. What is the relevant issue for this scenario?


I am not so familiar with VSD’s, I will appreciate anybody’s view and advice, an apology for this rashly done topic.


Thank you and regards to all viewers.






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Hello Gerry


Welcome to the forum.


It is difficult to give a definite answer to your problem as there are many possible scenarios.


One possibility is the SCRs themselves, but you have eliminated those by changing them.


A problem with the control electronics can cause false trigger information to the SCRs and if two are turned on at the same time, the fuses will fail.


Another possible source of problems, is high voltage or high rate of rise voltage transients on the supply. This can be caused by bouncing contacts, arcing or rapid switching.


I would have a close look at the gate control circuits and ensure that the connections are all very secure. Also ensure that the resistors and diodes in the gate fire circuit are all correct. If the impedance between the gate and the cathode circuit is high, then the SCR is much more sensitive to transients.


Check the snubber circuits across the SCRs. These help to reduce the dv/dt across the SCRs. It in not unknown for the resistors to go open circuit with time.


Finally, I would check the DC supply lines on the control pcb. If the electrolytic capacitors fail or lose capcitance, the supply can become very noisy and this can cause false triggering.


Best of luck,


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Hi Marke,



Your advice is very supportive, I agree with you that false gate triggering had caused this intermittent fault.


Since this drive is employed in Mobile Crane application, vibration is a factor to initiate incorrect firing sequence which maybe due to improper control cable termination.


I closely monitored the Armature current once and I find out that both fuses even fails at low armature current, it maybe the false triggering which turn both the forward and reverse SCR.


I need to pull out then the Drive and do all the necessary checking.


Thank you very much for your sharing knowledge, it is really worthy visiting this site. Keep up the good the work!







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Hi Gerry


Yes, the fuse failure is due to an SCR turning ON when another SCR is already ON and this is creating a phase to phase short and blowing the fuse.


I would have a close look for damaged or corroded connections and cracked joints on the pcbs if the dive is subject to vibrations.


Best regards,

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