It was one of my earliest commissioning assignments. The job was ideal for a rookie. Just a 45 kW dc drive with very simple functionality. Start, stop and speed set potentiometer. The control panel was built with care and installed properly. All I had to do was to conduct routine checks and start the drive return home.
Cochin Kadalas (Kadalas means paper in Malayalam-Kerala, India) was the factory in which I had to do all this.
Everything went as per expectations. The only hitch was that the place was so hot and humid that, half an hour after I started my work, I looked as if I had fallen into a pool fully clothed and got out. I was sweating so profusely. When it became impossible to bear, I took my shirt off and that is the only time I have conducted official work so dressed, I mean, undressed.
When it was sure that the drive and the paper machine to it was driving were working as expected under no-load, paper pulp was made and conveyed to the machine. As soon as the pulp entered between the rollers the machine stopped. The ammeter indicated full rated current. I opened up the panel and increased the current limit to the rated overload limit. No change. The machine refused to work.
All kinds of investigations followed. But the situation did not change.
The machinery was installed by a man who barely spoke Hindi. All he knew was Bengali. With my broken Hindi and more broken Bengali, I tried to find out more about the machinery, with the hope of finding out the reason for the stalled machine. While we were having this conversation, he commented that he had very recently commissioned an identical machine successfully. I tried to interrogate him to find out if there was even a minor difference between that and this machine. He kept insisting that there were none.
While all this was going on, he let slip that the motor in the earlier case was a 750 rpm machine and that this one was ‘unnecessarily’ 1440 rpm.
Ah ha! The gearbox between the motor and the machine was also exactly as in the previous case.
Now it was clear that the motor was delivering half the power required because it was running at half the speed!
I had a tough time convincing the non-engineers that we either had to change the motor or the gearbox. Finally the latter was done and my one-day-assignment ended after nearly a week.
------ J L Anil Kumar
Friday, May 23, 2008
Saturday, May 10, 2008
Problem ---- watered Down !
My uncle told me this story from his own experience, a long time ago.He was a consultant to a beer manufacturing unit. The unit installed a new bottling plant and tried a dry-run. Everything went well except the bottle capping part. Bottle after bottle passed and the machine refused to cap them. After days of checking everything repeatedly, the man in charge sent for him.After taking a good look at the machine and how it operated, he placed a beer bottle filled with water. Promptly the machine capped it. The machine had weight sensors and was designed not to cap empty bottles !
-- J L Anil Kumar
-- J L Anil Kumar
Friday, February 15, 2008
Monday, November 12, 2007
THE (UN)COMMON SENSE
About 6 years ago I had to set up servers for my colleague at
a laboratory inside ALTTC (Advanced Level Telecom Training Centre),
Ghaziabad,Near Delhi. I hooked up the server, monitor, keyboard,
mouse and the works! I first switched on the monitor and next I
tried switching on the server but it refused to do so.
Based on my years of “on-the-job” experience, the first thing
I suspected was the SMPS. SMPS seemed to be order because the
power ‘ON’ LED on motherboard was on. Next, as a standard procedure, I pulled out all the peripheral cards, disconnected keyboard, mouse and all other
connections except the power cable and tried again.
a laboratory inside ALTTC (Advanced Level Telecom Training Centre),
Ghaziabad,Near Delhi. I hooked up the server, monitor, keyboard,
mouse and the works! I first switched on the monitor and next I
tried switching on the server but it refused to do so.
Based on my years of “on-the-job” experience, the first thing
I suspected was the SMPS. SMPS seemed to be order because the
power ‘ON’ LED on motherboard was on. Next, as a standard procedure, I pulled out all the peripheral cards, disconnected keyboard, mouse and all other
connections except the power cable and tried again.
Still NO SIGN of server switching ON! Due to non-availability of CRO or such other testing equipment,
I could not test the motherboard and called up the supplier and
told all the symptoms, who advised me to bring it to Delhi, as he
did not have service centre at Ghaziabad. Meanwhile, I called up
our Delhi office to borrow another server and after some initial
reservation they agreed but asked me bring along the ‘faulty’ server. We confidently connected power to the new server! But, still no sign
of life! NOW I was really at my wit’s end! My whole confidence was at the
lowest ebb! By now it was late in the evening. Completely exhausted,
on an impulse I checked the supply voltage. It was 110V AC instead of
normal 220V AC, which one would normally expect. (We later found that
a special supply network of 110 V AC had been created for these
equipment supplied by Fujitsu, Japan.) But why did the monitor switch on at 110 V AC? Well it turned out
that the monitor had higher voltage tolerance range. Even today the remark of my boss keeps ringing in my ears “ Gana,
sometimes you need to stop using too much of brain!”
--------- R. GANAPATHI RAO
Research Associate,CENTRE FOR DEVELOPMENT OF TELEMATICS,
ELECTRONICS CITY,BANGALORE.
Saturday, January 06, 2007
Drilling Machine Drills
Drilling machine mystery
Our ½ hp, 3ph, drilling-tapping machine is 10 years old. Recently, the 6A HRC fuse in the R phase blew, followed by tripping of the upstream ELCB.
Could it be an earth fault? Ohm-meter did not indicate any visible earthing. I tried hard but could not trace the fault physically. I just replaced the fuse. The machine operated without problems for few weeks.
Wonderful!
And then the fuse blew again. Well, I reckoned it could be just a transient phenomenon.
Cleaned up the electrical panel. Replaced the fuse. No problem.
After about 10 days the fuse blew again, followed by tripping of the ELCB.
Now I was getting really mad. Opened up the incoming R-phase cable terminals at both ends (about 4 meters). Checked with a “Neon tester’ and it glowed gracefully. Cable must have got shorted inside. Changed the cable. Replaced the fuse. Now the machine was running perfectly. But strangely there was no tell-tell sign of burn mark in the old cable.
After about a week the drama was repeated. This time, without much ado, I replaced the R phase fuse with a 16A HRC fuse.
After about a week, there was a flash at the incoming terminal box of the drilling machine.
The Fuse blew, ELCB tripped.
But I was more than happy. I had found the culprit. The incoming screw holding the Terminal Block had fouled with the terminal of the R phase.
Another lesson. Neon tester had been showing capacitive voltage.
--- P. K. Ray ----
raynar.industries@gmail.com
Wednesday, November 29, 2006
Insulation Value Crisis !
I was looking after the erection and commissioning of electrics at Bhilai Steel Plant (Site : Rajahara Mines) in Central India in the early sixties. Massive construction activity was on. The plant was being set up with the assistance of the erstwhile USSR.
A high-tension slip ring induction motor of 250 KW for driving a cone crusher was to be installed. The standard practice is that the insulation levels are checked and only when the IR (Insulation Resistance) value is more than several Mega ohms, the motor is energised.
On initial checking, the IR value was found to be much less than 1 Mega ohm. This was not considered unusual as the motor was shipped from the USSR long back and during several weeks of transportation and storage, moisture could have seeped in. As a standard remedial measure, the motor was put on heating, initially by external means and later by passing a current in the rotor of value higher than its rated load current. The motor was placed at its final location, which was about 25 meters below the ground level, in a pit. Since it was very cumbersome to get down into the pit every time to measure the IR value, the Meggar was kept at ground level, and a set of cables was sent down into the pit and connected to the motor.
The heating was started and, after every one-hour, the readings were taken. We were surprised to note that the IR value continued to remain low. After several rounds of heating, the result was the same much to the annoyance of the commissioning crew, and adding to the perplexity of the engineers.
Experts from the USSR were also at a loss.
The situation became very critical when it was known that the plant commissioning date was fixed just after 3 days to coincide with the visit of the then minister of Steel and Mines.
After many hours of lost sleep and anxiety of the team, it occurred to me to go down to the pit with the Megger and check the IR value connecting the Megger leads directly to the motor terminals. And lo, the value indicated was more than 100 Mega ohms, which was perfect! The readings were taken many times to ascertain that the values were consistent!
After analysis, it turned out that some one, while searching for a cable of suitable length, found an L.T. 3-core cable lying around, and 2 cores of this cable were used. Since the megger was a 10 kV HT Megger, it was measuring the insulation resistance between two cores of a low-tension cable. Even though the motor at the end of the cable had attained high insulation resistance after heating, the lead insulation itself was much lower and naturally the lower value was being indicated by the Megger. The Megger was measuring the inter-lead insulation and not the motor insulation value.
The team could then energize the electrics successfully and celebrated with a shot of vodka!
-------- Raghavendra Girimaji.
A high-tension slip ring induction motor of 250 KW for driving a cone crusher was to be installed. The standard practice is that the insulation levels are checked and only when the IR (Insulation Resistance) value is more than several Mega ohms, the motor is energised.
On initial checking, the IR value was found to be much less than 1 Mega ohm. This was not considered unusual as the motor was shipped from the USSR long back and during several weeks of transportation and storage, moisture could have seeped in. As a standard remedial measure, the motor was put on heating, initially by external means and later by passing a current in the rotor of value higher than its rated load current. The motor was placed at its final location, which was about 25 meters below the ground level, in a pit. Since it was very cumbersome to get down into the pit every time to measure the IR value, the Meggar was kept at ground level, and a set of cables was sent down into the pit and connected to the motor.
The heating was started and, after every one-hour, the readings were taken. We were surprised to note that the IR value continued to remain low. After several rounds of heating, the result was the same much to the annoyance of the commissioning crew, and adding to the perplexity of the engineers.
Experts from the USSR were also at a loss.
The situation became very critical when it was known that the plant commissioning date was fixed just after 3 days to coincide with the visit of the then minister of Steel and Mines.
After many hours of lost sleep and anxiety of the team, it occurred to me to go down to the pit with the Megger and check the IR value connecting the Megger leads directly to the motor terminals. And lo, the value indicated was more than 100 Mega ohms, which was perfect! The readings were taken many times to ascertain that the values were consistent!
After analysis, it turned out that some one, while searching for a cable of suitable length, found an L.T. 3-core cable lying around, and 2 cores of this cable were used. Since the megger was a 10 kV HT Megger, it was measuring the insulation resistance between two cores of a low-tension cable. Even though the motor at the end of the cable had attained high insulation resistance after heating, the lead insulation itself was much lower and naturally the lower value was being indicated by the Megger. The Megger was measuring the inter-lead insulation and not the motor insulation value.
The team could then energize the electrics successfully and celebrated with a shot of vodka!
-------- Raghavendra Girimaji.
Sunday, July 09, 2006
"Kickbacks" of the Electrical kind
We all know that an “inductor resists change in the current flowing through it”. We also know that when there is a change in the current through an inductor, it gives rise to an inductive kickback. We put freewheeling diodes across dc relay coils or we stick a snubber across ac relay coils and so on.
I realised, with a shock (quite literally), the possible magnitude of this kickback.
In the early part of my career, one of my responsibilities was to test the incoming materials. This included some HT transformers too. One of these had two secondaries, one star connected and the other delta connected. These supplied diode bridges that eventually supplied a twelve-pulse output used as the anode supply in a large RF induction heater.
I was still learning my ropes and I had to test one of these transformers for the first time. I decided to first measure the winding resistances and see I they matched the specifications. I connected the probes of an analog multimeter, set to measure resistance, across the secondary. To make the connections, I held the probes of the multimeter to the terminals by hand. Since the winding inductances were fairly high, the needle started its sluggish motion and I realised that I had set the range wrongly. I removed the probe held in my right hand and I was in for a shock! The small dc current that the meter had already driven into the coil was interrupted and this generated such a kickback that my left hand flew, still holding the probe. That pulled the meter attached to it from its precarious perch on the transformer and sent it crashing to the floor.
So, this is what an inductive kickback “feels” like!
Many would not believe me when I told them this story. However, if I asked them to try it for themselves, there were no takers!
---- JL Anil Kumar.
I realised, with a shock (quite literally), the possible magnitude of this kickback.
In the early part of my career, one of my responsibilities was to test the incoming materials. This included some HT transformers too. One of these had two secondaries, one star connected and the other delta connected. These supplied diode bridges that eventually supplied a twelve-pulse output used as the anode supply in a large RF induction heater.
I was still learning my ropes and I had to test one of these transformers for the first time. I decided to first measure the winding resistances and see I they matched the specifications. I connected the probes of an analog multimeter, set to measure resistance, across the secondary. To make the connections, I held the probes of the multimeter to the terminals by hand. Since the winding inductances were fairly high, the needle started its sluggish motion and I realised that I had set the range wrongly. I removed the probe held in my right hand and I was in for a shock! The small dc current that the meter had already driven into the coil was interrupted and this generated such a kickback that my left hand flew, still holding the probe. That pulled the meter attached to it from its precarious perch on the transformer and sent it crashing to the floor.
So, this is what an inductive kickback “feels” like!
Many would not believe me when I told them this story. However, if I asked them to try it for themselves, there were no takers!
---- JL Anil Kumar.
Thursday, July 06, 2006
A delayed “Blast”
The occasion
Inauguration of the manufacturing facilities for the production of Air Blast Circuit Breaker (ABCB)
Location and time
Central India, Circa 1964
Inauguration by
The then prime minister of India
Background
It was a major event for the plant management. The first ABCB manufactured/assembled at the plant (Technology from UK) was to be inaugurated by pushing a button to actuate the ABCB.
Scenario on the eve
Hectic preparations and trials to ensure that everything goes well without any hitch!
On the D Day
The team of dignitaries was given an overview of the activities. The PM was the requested to push the button; he did so and then ..............................
...Nothing happened!
PM was requested to push it again.....a little harder this time. The result?
Ditto.... nothing happened!
A hush fell over the audience. A red-faced management looked on helplessly not knowing what to do. And then the saving grace! Fortunately the PM was amused instead of getting annoyed.
Everybody heaved a sigh of relief and the event was declared as over. The retinue turned back and started moving towards the exit. AND THEN ..........it happened. There was a BANG and the ABCB operated (apparently) on its own. The team turned back startled to realize that it was indeed the ABCB, which had operated after a delay!
The management felt relieved and that was it.
And now the inside story...
During the last minute checking it had been found that an interlocking relay that was meant to ensure that the air supply to the ABCB was ON prior to the pushing operation, had stopped to function. Therefore, it was arranged to post an employee physically to ensure availability of the air supply. The poor man, who had worked through the numerous trials the previous night, had dozed off at the critical moment and forgot to switch on the air supply. After a late realization that he had goofed up, he acted with great alacrity and switched the supply on which had caused the bang.
( Based on the narration of some of my senior colleagues)
Inauguration of the manufacturing facilities for the production of Air Blast Circuit Breaker (ABCB)
Location and time
Central India, Circa 1964
Inauguration by
The then prime minister of India
Background
It was a major event for the plant management. The first ABCB manufactured/assembled at the plant (Technology from UK) was to be inaugurated by pushing a button to actuate the ABCB.
Scenario on the eve
Hectic preparations and trials to ensure that everything goes well without any hitch!
On the D Day
The team of dignitaries was given an overview of the activities. The PM was the requested to push the button; he did so and then ..............................
...Nothing happened!
PM was requested to push it again.....a little harder this time. The result?
Ditto.... nothing happened!
A hush fell over the audience. A red-faced management looked on helplessly not knowing what to do. And then the saving grace! Fortunately the PM was amused instead of getting annoyed.
Everybody heaved a sigh of relief and the event was declared as over. The retinue turned back and started moving towards the exit. AND THEN ..........it happened. There was a BANG and the ABCB operated (apparently) on its own. The team turned back startled to realize that it was indeed the ABCB, which had operated after a delay!
The management felt relieved and that was it.
And now the inside story...
During the last minute checking it had been found that an interlocking relay that was meant to ensure that the air supply to the ABCB was ON prior to the pushing operation, had stopped to function. Therefore, it was arranged to post an employee physically to ensure availability of the air supply. The poor man, who had worked through the numerous trials the previous night, had dozed off at the critical moment and forgot to switch on the air supply. After a late realization that he had goofed up, he acted with great alacrity and switched the supply on which had caused the bang.
( Based on the narration of some of my senior colleagues)
Friday, June 23, 2006
JLAK’s Law of Troubleshooting
Simply stated, “When a complex system is being troubleshot, an expert in a subsystem always suspects the subsystem he knows least about”.
I arrived at this ‘law’ after repeated observation of this law at play when CNC machine tools were being tested, tuned and qualified. This being a control system with multiple control loops, the effect of one fault would manifest itself in all the variables – jerky motion of the axes, unsteady speed reference to the drive controller, unsteady tacho voltage, unsteady encoder feedback, etc.
This is how the scenario would look:
All the experts have been called because, during the production testing the movement of the axis is found to be uneven and the normal fault tree and troubleshooting chart have not helped.
CNC expert: Have you checked if there is stick-slip? (Being a digital electronics guy he knows a little about the drive electrics. So, he does not question that.)
Drives expert: Are you sure there is no backlash? (Notice that the two suspected causes sound pretty impressive and have a ‘certain something’ (as Asterix would say) about their sound.
The Mechanical expert: Have you tuned the drive?
It would go on and on. Some ‘poor generalist’, who is a down to earth guy, is pottering away at finding the root cause of the problem. Shielding that has come off, a dry solder or whatever would be found and corrected and the system would start behaving itself.
Then all the experts would leave shaking their heads and clucking their tongues muttering “these production guys never do anything right”!
Even in the trivial case of a simple motor controller troubleshooting, the test engineer soldiers on trying to fix the problem all on his own, until he sees a specialist in motors. He immediately starts suspecting all kinds of esoteric problems with the motor. One of the first things the motor man himself would ask is perhaps “is the incoming phase sequence OK?” or “Have you checked the tacho coupling?” – even if the drive is a single phase one or the motor has an integral shaft mounted tacho.-----Contributed by JLAK
I arrived at this ‘law’ after repeated observation of this law at play when CNC machine tools were being tested, tuned and qualified. This being a control system with multiple control loops, the effect of one fault would manifest itself in all the variables – jerky motion of the axes, unsteady speed reference to the drive controller, unsteady tacho voltage, unsteady encoder feedback, etc.
This is how the scenario would look:
All the experts have been called because, during the production testing the movement of the axis is found to be uneven and the normal fault tree and troubleshooting chart have not helped.
CNC expert: Have you checked if there is stick-slip? (Being a digital electronics guy he knows a little about the drive electrics. So, he does not question that.)
Drives expert: Are you sure there is no backlash? (Notice that the two suspected causes sound pretty impressive and have a ‘certain something’ (as Asterix would say) about their sound.
The Mechanical expert: Have you tuned the drive?
It would go on and on. Some ‘poor generalist’, who is a down to earth guy, is pottering away at finding the root cause of the problem. Shielding that has come off, a dry solder or whatever would be found and corrected and the system would start behaving itself.
Then all the experts would leave shaking their heads and clucking their tongues muttering “these production guys never do anything right”!
Even in the trivial case of a simple motor controller troubleshooting, the test engineer soldiers on trying to fix the problem all on his own, until he sees a specialist in motors. He immediately starts suspecting all kinds of esoteric problems with the motor. One of the first things the motor man himself would ask is perhaps “is the incoming phase sequence OK?” or “Have you checked the tacho coupling?” – even if the drive is a single phase one or the motor has an integral shaft mounted tacho.-----Contributed by JLAK
Monday, June 05, 2006
A truly "dynamic" issue
I had been on a servicing trip to a remote part of India. There was this Aluminium rod mill, which had one of our thyristor DC drives. The motor operated at very low speeds most of the time and its speed had started drifting. After some observation, all I had to do was change the operation amplifier on the drive control card and things were back to normal. The customer was very happy. With the traditional Indian hospitality made keener by the remoteness of the place, the customer’s maintenance engineer took me to a nearby dam, one of the largest in the world.
By the time we returned from the trip disaster awaited me. (Incidentally, when we were returning, as we crested a small hillock, the engineer said that there must be problem at the plant as the plant had stopped. I asked how he knew. He said he could not hear the sound of the plant. The sound of the plant? Over the roar of a diesel engine jeep? A kilometre away from the factory? I have never been able to fathom this and the subsequent events did not permit to investigate the issue)
Three semiconductor fuses protecting the thyristors had failed and the mill was at a standstill. The customer was very unhappy and concluded that whatever I had done had caused this disaster. In spite of being tired after the trip to the dam site, I started analysing, testing and troubleshooting late into the evening. After much poring over the drawings, I found that there was a serious flaw in the control logic design of the drive.
The drive had dynamic braking facility and one of its basic requirements had been overlooked. When a motor is running and it has to be stopped quickly, dynamic braking is used. The sequence of events is, open the power contactor connecting the drive to the motor, then and only then close the contactor connecting the motor armature to a bank of high power, low ohmic value resistances. However the designed logic sequence had a flaw and, theoretically, it was possible that once in a while, the dynamic braking contactor could come on before the main contactor had opened. When I discovered this and told the maintenance engineer about it, he would not believe me. His argument was simple. This drive has been working without a problem for the last three years or more. Every shift, we turn this drive off at least twice – once at “lunch time” and once at the end of the shift. The drive works three shifts a day. This means that it is turned off 6 times a day. That means it has been, at the least, turned off two thousand times and this has never happened before.
My arguments about probabilities and the change in the time characteristics of the relays and contacts would not move this man. He did not allow me to change the logic sequence until he was satisfied. I had to set up an experiment to prove that this was happening. I was really worried. What if the rare event does not occur for the next two thousand times?
With no other option, I did set up the experiment and as luck would have it, the fault occurred within a short time and small number of trials. But, you had to carefully observe two sets of lamps. A had to go off before B came on. And the time difference is just a fraction of a second. I saw it and no one else did. After another few trials the maintenance engineer’s assistant saw it but not the big man himself. So it went on and on. And finally, it occurred with such a huge overlap (A and B both on) that the big man himself saw it. And then he granted me the permission to change the logic design. I had to be very careful and finally did the change on paper and executed it in the panel too.
The experiments restarted to make sure that there was no repeat. Thus was solved a tricky problem. It is another matter that I had to wait for days to get the replacement parts as Christmas and New Years day intervened. Finally I got the parts and restored the drive and was allowed to get back home.
--JL Anilkumar
By the time we returned from the trip disaster awaited me. (Incidentally, when we were returning, as we crested a small hillock, the engineer said that there must be problem at the plant as the plant had stopped. I asked how he knew. He said he could not hear the sound of the plant. The sound of the plant? Over the roar of a diesel engine jeep? A kilometre away from the factory? I have never been able to fathom this and the subsequent events did not permit to investigate the issue)
Three semiconductor fuses protecting the thyristors had failed and the mill was at a standstill. The customer was very unhappy and concluded that whatever I had done had caused this disaster. In spite of being tired after the trip to the dam site, I started analysing, testing and troubleshooting late into the evening. After much poring over the drawings, I found that there was a serious flaw in the control logic design of the drive.
The drive had dynamic braking facility and one of its basic requirements had been overlooked. When a motor is running and it has to be stopped quickly, dynamic braking is used. The sequence of events is, open the power contactor connecting the drive to the motor, then and only then close the contactor connecting the motor armature to a bank of high power, low ohmic value resistances. However the designed logic sequence had a flaw and, theoretically, it was possible that once in a while, the dynamic braking contactor could come on before the main contactor had opened. When I discovered this and told the maintenance engineer about it, he would not believe me. His argument was simple. This drive has been working without a problem for the last three years or more. Every shift, we turn this drive off at least twice – once at “lunch time” and once at the end of the shift. The drive works three shifts a day. This means that it is turned off 6 times a day. That means it has been, at the least, turned off two thousand times and this has never happened before.
My arguments about probabilities and the change in the time characteristics of the relays and contacts would not move this man. He did not allow me to change the logic sequence until he was satisfied. I had to set up an experiment to prove that this was happening. I was really worried. What if the rare event does not occur for the next two thousand times?
With no other option, I did set up the experiment and as luck would have it, the fault occurred within a short time and small number of trials. But, you had to carefully observe two sets of lamps. A had to go off before B came on. And the time difference is just a fraction of a second. I saw it and no one else did. After another few trials the maintenance engineer’s assistant saw it but not the big man himself. So it went on and on. And finally, it occurred with such a huge overlap (A and B both on) that the big man himself saw it. And then he granted me the permission to change the logic design. I had to be very careful and finally did the change on paper and executed it in the panel too.
The experiments restarted to make sure that there was no repeat. Thus was solved a tricky problem. It is another matter that I had to wait for days to get the replacement parts as Christmas and New Years day intervened. Finally I got the parts and restored the drive and was allowed to get back home.
--JL Anilkumar
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