I manage the new product verification team for a small manufacturer of industrial automation equipment. We sell most of our products through partners who sell them as their own products. About six month ago, we had one of our partners come to us with an intermittent customer issue. The customer had one of our analog output (4-20mA) modules installed next to a relay module (made by our partner).
The analog module controlled the speed of a conveyor through an oven, and the relay module switched a contactor that controlled the heaters in the oven. The customer had installed a number of these systems at various locations with both AC and DC power to the heaters.
After installation, the systems worked very well, but after about a month, only on the AC-powered systems, one channel of the analog output would go to 0 mA, stopping the conveyor and burning a lot of product. After a power cycle, the system would work again, but with a decreasing failure interval. Our module had been redesigned recently and the older version was not showing the problem at all.
Our partner asked us to try to duplicate the problem with our own equipment. They had managed to duplicate it on one system, but could not on another. One of my test engineers worked with the design engineer for two months trying various loads and accelerated switching rates, but he could not recreate the failure. It appeared that either our module was not the source of the problem, or no one understood the conditions of the failure very well.
Our partner came back to us with more information on the system. They told us that they were able to demonstrate the failure regularly on two systems: one with a large contactor as a load, and another with a resistive load. They also had a third system with a resistive load that would not show the failure. While reading the new data, I noticed that the test system they had managed to duplicate it on had a step down transformer between the relay and the resistive load (local power was 220V and the load was designed for 110V). All of the systems that showed the failure were switching a large inductive load with the relay, while those that worked properly were switching a resistive load directly.
I set up a similar system with the relay switching AC to the largest coil I could find (a 10-pound reel of 18-gauge magnet wire, 210mH, 14Ohms) with the relay switching on and off as fast as the system could do it reliably. I figured that if I could grossly exaggerate what I thought was going on, I could duplicate the failure quickly. Within 20 minutes I had demonstrated the reported failure three times and seen two other failure modes as well, one of which was a complete module reset. I called the design engineer, and we started looking for the cause.
John, did you guys compare the old and new analog modules that you had designed? It would be interesting to know what the design engineer changed between the two.
We had changed the output driver from a completely analog design, to one with a digitally controlled output driver. The new driver would reset occasionally with the noise or the communications to it would get interrupted.
On the old module the noise would pass through to the output, but the frequencies were well above the system response time.
It's called surge impedance Zo, which is defined as √(L/C), where L is the inductance in Henrys, and C is the capacitance, which for a coil is the interwinding stray capacitance.
The back EMF V= Zo (δI/δT), and it will have an oscillation frequency 1/(2π√LC). When you open up contacts in an inductive load, δI/δT goes to ∞: You see this as an arc when you unplug an iron; and also when relay & motor starter contacts are switching off an inductive load.
This is also why contactors have serious current deratings when switching off DC: Once the arc is established and current flows through the ionized channel, there is no zero crossing to extinguish the arc, as occurs with AC,
When dealing with AC, you design using the peak (not RMS) value of the load current when calculating, because you don't know where in the AC cycle the contacts will open.
Note: Those of you who are RF jocks will quickly recognize Z(o) = √(L/C) as the equation for the characteristic impedance of a transmission line: Yes, it's the same thing.
Your story demonstrates one of the keys to trouble shooting many problems: Being able to duplicate the failure. So many times there is a problem that appears randomly and until I can find out why, I am never totally satisfied with the fix. Even if it is my own error, just correcting it without knowing the thought process that caused the error to begin with drives me crazy.
From Dell / Intel® New Paradigms in Design Work Scott Hamilton, vertical market strategist for Dell Precision workstations, 5/2/2013 3
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