Given the typical heat-treating production-floor temperatures, electronic power supplies, PLC/PID controllers and data-collection instruments will be subjected to extreme environmental temperature stresses. This is not to say the instrumentation equipment is not up to the challenge; they have been designed to operate in that environment. But all of the equipment’s internal components have a finite usable life span.
An increase in operating temperature will cause resistors’ resistance values to shift, semiconductors (transistor and integrated circuits) to overheat and capacitors’ internal temperature to increase, leading to failures over time. The electrolytic capacitor’s internal temperature can be used to monitor its health and give us an overall reading of the instrumentation equipment condition. That leads us to the topic of capacitor wear-out during its usable life and after reaching the end of endurance time.
Operational Characteristics of Capacitors
How do we determine the usable life of the capacitors in our electronic equipment? The aluminum electrolytic capacitor is the most common type used in industrial control and data-collection equipment. An aluminum electrolytic capacitor’s lifetime is specified in hours per temperature and can be looked up as endurance in the manufacturer’s datasheet.
For our discussion and example, I selected a sample capacitor from one of my company’s PLC systems. We recorded the manufacturer’s name and the series or family for one of the power-supply capacitors. With this information, I located the manufacturer’s datasheet and looked up the endurance specification of 3,000 hours at 105°C.
The 105°C (221°F) is a bit higher than the typical heat-treating production-floor temperature. Components in production-floor equipment are typically in the 45°C (113°F) range. The 45°C temperature is achieved by internal cooling fans, AC cooling units or both. Note: We selected 45°C for this example. In real life, the actual component temperature will be measured during the hottest season for best accuracy.
In order to determine the endurance life of the capacitor at 45°C, we apply the Arrhenius Rule to calculate the new endurance time.
Arrhenius Rule and Capacitor Operating Temperature
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The rule states that for every 10°C drop in the component (capacitor) operating temperature, the endurance time is doubled. The endurance specification for our example capacitor is 3,000 hours at 105°C.
The difference between the 105°C spec and our operating temperature of 45°C is 60°C. Divide the 60 by 10 to yield 6; the endurance life can be doubled six times. The formula is 3,000 x 26 = 192,000 hours. Divide the 192,000 by 8,736 hours/year (based on 168 hours/week × 52 weeks) to convert the endurance life of the capacitor to approximately 22 years.
Wow, 22 years; that’s great. What more can we ask for? Hold that thought. Let’s spend a few minutes running some scenarios using data from figure 1 and see what variables can affect the endurance life of the capacitors as well as the overall life of the electronic control systems used in our heat-treating equipment.
The three ranges of manufacturers endurance life plots (figure 1) are mid-range for this spec parameter. There are temperature ranges on both sides of this group (less than 1,000 hours and greater than 3,000 hours), so the capacitor specifications must be looked up in the capacitor manufacturer’s datasheet for any given piece of electronic control equipment.
Let’s look at the difference between the other two manufacturers endurance life calculations. The 2,000-hour spec at a reduced temperature of 45°C yields 128,000 hours/8,736 hours/year = manufacturers endurance life of 14.6 years. The 1,000-hour spec at a reduced temp of 45°C yields 64,000 hours/8,736 hours/year = manufacturers endurance life of 7.3 years.
The endurance life spec variable is a fixed value that is determined by the component selected when the equipment is manufactured and will be a constant through that equipment’s life.
Example: Calculating Component End of Life
After reviewing figure 1, you will notice how an increase in the equipment operating temperature could have a dramatic impact on the component’s end of life (EOL). Let’s say we lost one of the cooling fans in a system, which resulted in a 20°C increase in the equipment operating temperature. Using the original 3,000 hours at 105°C spec, if undetected, this would result in a change from the original 192,000 hours to 48,000 hours (45°C + 20°C = 65°C, 105 - 65 = 40. Divide the 40 by 10 to yield 4, 3,000 x 24 = 48,000 hours).Divide 48,000 hours by 8,736 hours/year, which equals a new manufacturers endurance life of 5.5 years instead of the 22 years.
That translates into a 75 percent reduction of the manufacturers endurance life and supports the need for a comprehensive equipment preventive-maintenance (PM) program. A PM program is designed to increase equipment uptime and prevent unscheduled equipment downtime due to electronic component failures. Remember, the PM program and endurance life are tools to determine where a capacitor is in its usable lifecycle. As the capacitor ages and the internal operating temperature increases, the liquid electrolyte inside the capacitor will evaporate at an accelerated rate, which increases the internal resistance and feeds a temperature-increase cycle that degrades the capacitor life faster.
If the lower operating temperature is not restored via scheduled PM corrective action, the capacitor will progress on a shorter endurance life. Just monitoring a few key data points (e.g., temperatures and visual inspections) can help the maintenance department manage downtime, increase mean time between failures (MTBF) and extend the useful life of the installed electronic systems.
Taking temperature measurements of an internal electrical component is a very difficult activity. The use of infrared tools can give the technician a higher level of safety by taking a noncontact measurement of the capacitor’s operating temperature. Figure 2 is an infrared picture and a temperature measurement made at the crosshairs of a specific location on the capacitor case. This will establish a baseline temperature for new equipment and track the capacitor’s temperatures over time.
Figure 3 is a standard picture of the same capacitor in case the temperature color in figure 2 is a distraction. Note the information printed on the capacitor.
I hope this has helped clarify how adding a few steps to your existing preventive-maintenance procedure for electronic production equipment will help reduce equipment component failure. The manufacturers endurance life spec is not an exact expiration date for electrolytic capacitors. It is a number generated from data and statistical calculations by the manufacturers to give us an estimated life expectancy that we can use in the care and maintenance of our instrumentation and control equipment.
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