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.
Sunday, July 09, 2006
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)
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