DC Drive Thyristor and Fuse Failure Reason

The most common reason a fuse will blow is when excessive current flows through it for too long of a period of time. My WAG why this is happening is that the mechanical load on the motor is larger than the motor can safely handle. Thus the motor is not getting up to speed as quickly as it should.

There is no mention of any cable assessment having taken place in the above description.

Drives need to be set up so that the equipment survives fault conditions without damage. What is being seen is effect of excessive current for longer than it takes to damage the equipment. The fuse is there to protect the cable. However the drive is being asked to supply more current than either it, or the fuse, can withstand with a 50% probability of each becoming damaged. In the absence of any fault indication from the motor, attention must turn to the cable downstream of the drive. Without removing the true cause of the fault, repeated equipment damage is inevitable. Why would the drive be set up to supply more current than the fuse, and it, can withstand (rhetorical question - NNTR)?

A reassessment of the entire installation is overdue and it cannot be seen from here. If in doubt, consult a qualified Electrician.

What did the local electrical contracting company say (that installed the protections, the motor, and the cabling) when you called them?

You must come from a large company environment. Where I work, I was the "electrical contractor" for machinery hook up for 6 years until we got busy enough to put me into engineering full time. Now in the last 15 years we've finally gotten large enough to actually call in a contractor for larger equipment, but the smaller machines are still done by Maintenance. That is quite normal for smaller companies.

I suspect the company that the OP is from is doing their own work at this time, therefore the "call" for help to CR4..

Does the drive have stored event alarm capability? Have you checked these and the manual to see if the events point to a particular failure event?

What is the size of the drive?

Perhaps it is the drive itself that is being run outside its operating range and is failing prematurely (perhaps due to overloading or environmental conditions).

Have you discussed with the drive representative or manufacturer?

A circuit diagram would be invaluable to us with a limited imagination...

A circuit diagram will only show how the circuit is conceived, not how its actually implemented. That difference can easily be the root of the problem.

Hi please Find my reply

1.V11 and V13 would normally never be on together, they are back to back in the R-Y loop. If one shorts, then the next time the other turns on, it will see an R-Y short and blow its fuse.ok This is Clear.

2.Are you saying that one of V11/V13 fails and the other blows its fuse but does not fail? So only one thyristor fails at each failure event? Yes One Thyristor Fails and Other Fuse failed which ever fuse failed its Thyristor is ok.

3.You have not said, but do the fuses of V11/V13 fail when V11/V13 fail? No its Respective Fuse Doesn't fail.

4.Have you changed all the snubbers, not just on V11/V13? yes all the Six Snubber has been Changed.

5.If one thyristor survives a short circuit event, blowing its fuse, it suggests failure is not simply overcurrent or overheating. A fault on the R-Y load side seems possible - are there any signs of a flash-over short or low impedance R-Y? You have not said where you measured impedances and by what means.Yes When the Thyristor failed a Flash mark was seen in Cathode terminal but this was twice but after that no flash mark has been seen in cathode or any terminal.Impedance test was done in transformer.

6.Is this a new drive, or one previously reliable for years?This is a old Drive which has been running for last 17 years.

7.Has motor been insulation resistance checked?Yes Motor Insulation resistance checked has been done found to be ok.

8.What is the fault symptom of thyristor? Short? Gate or anode-cathode?-Anode Cathode is getting short,Previously we have observed flash mark in Cathode but this time leakage current has increased to 160micro Amp and Anode Cathode resistance has also changed as compared to other thyristor.

Well tought 67! You good troubleshooting skills. GA.

A few more questions:

Has any changes been done on this old drive? (re-wiring that would mix phases from the two bridges or voltage/phase sensing board?)

Does it work at all or does it fail as soon as you start it?

Have you ran it with the motor disconnected?

Is there any AC line contactor that could bonce open once in a while?

Two more to add,

Is there a dc link fuse, and is it sized properly?(comm flashover)

That would eliminate external faults, so look for board /infrastructure failure, wire rubbing against other wires in harness, frame.

What is the configuration of the ac supply power, High Res Ground, floating, wye/delta?

(Answers what current flows during a ground fault)

Please find my reply:

"Is there a DC Link fuse, and is it sized properly? (comm flashover)"-No, DC Link only is present in AC Fuses.

"What is the configuration of the AC Supply power, High Res Ground, floating, wye/delta?"-Wye/delta transformer is connected to drive.

*Answers what current flows during a ground fault*-How will I measure ground fault? There is no particular or specific time that the thyristor or fuse will fail so that I can do a bender relay and check the insulation resistance.

Perhaps the characteristic of the driven load (high inertia & lots of gear lash) could create a fault at the commutator. A DC Link fuse is sometimes provided on regenerative drives to prevent the drive SCRS from feeding a bad fault at the commutator.

There is no ground fault current for your configuration except a very small charging current, so phase to phase is what is popping your fuses and limits your search. You can get high transient voltages, though, so maybe two snubbers are conducting, unexpectedly, phase to ground to phase. You can check ground with voltmeter while running; all sides of the circuit should read somewhat close to the same when measured to the ground.

Please find my replies:

"Has any changes been done on this old drive? (re-wiring that would mix phases from the two bridges or voltage/phase sensing board?)"-No such changes have been done; even if any changes are done, why is the motor running for twenty days, one month, ten days, etc., causing the thyristor or fuses to fall?

"Does it work at all or does it fail as soon as you start it?"-No; it works, but failures are uneven. Even in DC drive, we have done block rotor test. After running for twenty days to one month it has, again, failed.

"Have you ran it with the motor disconnected?"-No, motor has been connected; then we have run it, but after some days or even a few hours it has failed.

"Is there any AC line contactor that could bounce open once in a while?"-No: the ine contactor is present directly from transformer cables and is connected to the drive.

This is a Reversing Drive with a DC commutator motor. Being a full converter, it would have 6 thyristors for forward direction and 6 for reverse direction, in parallel pairs, cathode to anode. Questions......

1. It appears, from the description "anti-parallel" used by Khuntia68, that no failures have occurred in the "reverse" thyristors. True?
2. Have failures occurred when changing from forward to reverse or other significant change point in the operating sequence?
3. Relating to last question (2), are there any speed or direction sensors, maybe determining control of direction change, giving an input to the control?
4. Has there been any change in the operating sequence or time length of sequence, say to speed production?
5. Has there been a change of human operator, who might use a more stressfull sequence, more hurried or less skilled?
6. Are there any associations between the two thyristors which fail, like common driver, common heat sink, similar position in airflow?
7. Have the manufacturer's recommendations for cleanliness, heat sink compound and torque when changing thyristors been rigidly followed?

Even for 6 thyristors, forward motor direction, the firing circuits are complex, probably 6, with 6 gate isolating transformers - are these gate transfos part of boards which have been changed, or separate items (which may not have been changed)?

Some firing schemes use gate isolation transfos with two secondaries, each connected to two thyristors which are fired together to connect a transfo phase to the load. While one would expect the primary drive to be adequate for two loads with secondary resistance to ballast each drive, some schemes have each gate driven from two different drivers with diode isolation and each secondary winding drives two thyristor gates via diodes.

It is possible that a diode short circuit would cut the voltage and drive to the associated thyristor.

Manufacturers have generous tolerances on gate drive, but a weak gate drive could lead eventually to failure, with great variation in life before failure.

I suggest that you check that the gate drive to the thyristors which are failing is good.

Since the gate is like a diode, the voltage is not a guide to the current. I would suggest a non-inductive resistive load which gives 3 volts drop at the rated minimum Thyristor gate trigger current, in place of thyristor gate, viewing resistor voltage with oscilloscope.

Comparison with other gate drives than the suspect ones should be made. Typically, minimum 3 volts at twice minimum trigger current with rise time of 1 microsecond is required with 12-20 volts desirable. Minimum trigger currents are about 300mA for 500 amp thyristors. Obviously, you should refer to data sheet for thyristors used.

Hi Please find my reply

1.It appears, from the description "anti-parallel" used by Khuntia68, that no failures have occurred in the "reverse" thyristors. True?yes true no failure had occurred in Reverse Bridges.

2.Have failures occurred when changing from forward to reverse or other significant change point in the operating sequence?It has not fail in changing from forward to reverse Bridges failures has occurred while running idle without ,During running in load,During running in Normal Operation it has failed.

3.Relating to last question (2), are there any speed or direction sensors, maybe determining control of direction change, giving an input to the control?Yes there is sensor for speed and direction determing if this sensor is having any issues drive should tripped then why power parts is failing?

4.Has there been any change in the operating sequence or time length of sequence, say to speed production?No changes has been done it was running normal.

5.Has there been a change of human operator, who might use a more stressfull sequence, more hurried or less skilled?No

6.Are there any associations between the two thyristors which fail, like common driver, common heat sink, similar position in airflow?No associations between thyristor separate heat sink,similar position in airflow.

7.Have the manufacturer's recommendations for cleanliness, heat sink compound and torque when changing thyristors been rigidly followed?yes that have cross checked for heat sink compound used and torque applied for tightening of this thyristor.

Even for 6 thyristors, forward motor direction, the firing circuits are complex, probably 6, with 6 gate isolating transformers - are these gate transfos part of boards which have been changed, or separate items (which may not have been changed)?this pulse transformer board has been changed,even individual transformer given firing for thyristor.

Gate has been checked for all thyristor found to be ok

Kindly suggest if any thing need to be checked or done

Running out of things to check, maybe something in the motor that is mechanically stressed upon forward rotation, loose armature winding at a corner?

(Doesn't explain fails at idle, except some sort of centrifugal & lateral conductor freedom).

Can you run for a while at lower speeds, or loads, to see if any correlation?

One more thing I just remembered, some drives I've worked on need a certain minimum voltage at the line to ensure the gate signal timing is correct. Perhaps looking at the ac voltage signal after the voltage divider with an oscilloscope will show some transient signals that are close to the minimum to fire the gates, or harmonic content. If you have harmonics or notching from other equipment added (One place had switched high voltage power factor capacitors that would knock out an SCR front end, 2 transformations down and a mile away). Nothing else was affected. Another install there were anti-notching reactors added at the ac input to each drive (40 altogether).

If your drive uses burst firing, a marginal timing signal might catch it at a bad time, and jump the gun on the desired phase angle, when running close to full speed/voltage, or running at minimum voltage.

OK - it seems the proper gate drive is OK.

I have realised that, while you have written V11 is connected to R and V13 to Y, it is not clear if their anodes or cathodes are connected to the DC Bus or whether that bus is positive or negative for normal motor direction. Please clarify.

It would help a lot if you could post an image or images of the power circuit diagram for the transfo, thyristors and motors. The voltage/rating of D-Y transfo and motor would also help.

Aside from that, have you checked the auxiliary power supplies for the controller and firing circuits for voltage and ripple?

Faults such as single phase rectification or worn reservoir capacitors could allow noise and misfiring.

It is possible the replacement thyristors have been from a faulty or "fake"batch. Have spares been new and direct from original maker of thyristor used in drive? What type number is used?

You have written that the fuse has blown not on the failed THY but on the other of the two you mentioned. This suggests both fuses carry the same fault current, but the one on the faulty THY does not blow first. Since the fuses are of the same type/rating (one assumes) but have a tolerance, I can only think the failed THY and its fuse were carrying significantly less current, and less hot, before the failure.

It is possible the THY gate is becoming less sensitive, leading to missing "fires" and reduced mean current , till the internal firing is inadequate and sudden failure occurs.

I would try fitting a "splittable" CT on each of the suspect THY anode or cathode leads and on one healthy THY (say B phase) - connecting them to DC moving coil meters (actually moving iron will work). Occurance of unmatched currents would then be measurable and trends evident.