By Patrick J. Lynch
By Patrick J. Lynch
August 22, 2018 — A very large food manufacturing plant was faced with soaring production downtimes and material scrap costs as they attempted to ramp up production. Electrical problems had been plaguing this facility for over 15 years, with no end in sight. Management used to let these problems slide, but were now forced to take action.
A job that makes lots of dough
The workers at this facility enjoyed a very generous production bonus program while making lots of dough… bread dough, that is.
One of the largest bread-making facilities in the United States was ramping up production, requiring additional workers to oversee the automated bread-making process. Just about every bread variety you can think of was being made here: brown, white, whole grain, and so on. In fact, about 20 different types of bread recipes were being simultaneously produced.
The process begins automatically with all the recipe ingredients added at precisely timed intervals into large, 10-ft. diameter stainless-steel sterilized vats, where they are mixed together. After 30 to 50 minutes (depending on the complexity of the bread recipe), the mixing is complete and the bread dough is ready for the next step. At any given time, you could have up to 10 different recipes in different stages of mixing in their vats.
This fully automated process could fire out thousands of loaves of bread when, all of a sudden, the whole process would come to an abrupt Full Stop, complete with red lights flashing over top of all of the machines and warning horns blasting away.
Workers would scramble as they tried to solve the problem. They would lose that month’s production bonus if they could not get the process up and running again quickly.
The notorious 480V 1600A breaker feeding this production area had electrically tripped out… again. The rest of the plant, however, still had power. Unfortunately, thousands of pounds of partially mixed bread dough now had to be manually scraped out of all of the mixing vats.The vats themselves had to be re-sterilized before the production cycle could restart (24 to 48 hours later).
According to management, this very frustrating, intermittent electrical problem had been going on for the last 15 years. In those years, no one was able to isolate the problem and fix it. Back then, the problem only occurred once every six or nine months but, when the facility started ramping up production, the problem occurred once every three to four weeks!
Production downtime and material scrap costs were quickly soaring out of control and eating into any profits.
Management decided it was finally time to bite the bullet and solve the problem once and for all. They arranged for the original process machine designers and our group to come to the facility and solve this problem onsite as a team. In this case, our combined talents helped us identify the problem in just one day.
What about that notorious breaker?
The circuit breaker’s trip flags indicate it was tripping out each time on instantaneous overcurrent. Could this breaker be defective?
Secondary current injection tests of this breaker revealed it passed with flying colours. During normal production operations, the breaker was only 10% electrically loaded. Physical examination of the electrical switchgear and downstream breakers revealed minor flour dust contamination, but nothing that would contribute to an electrical breaker trip.
The 1600A breaker was then swapped with another fully tested and operational breaker. And, about three weeks later, the replacement breaker also tripped out on instantaneous overcurrent.
So it’s definitely not the breaker. What could the problem be?
A sporadic, long time interval, intermittent electrical breaker trip problem is probably one of the most difficult types of electrical problems to troubleshoot. In addition to any electrical monitoring equipment you may have installed, you also need to be as physically close to the process as possible to use all of your innate monitoring tools (e.g. hearing, seeing, touching, smelling).
High-speed monitoring and capture of the breaker phase current waveform revealed a very brief, high transitory current level (400 to 500 milliseconds in duration) occurred sporadically once every 30 to 50 minutes.
The peak current value captured, however, was still only 10 to 15% of this breaker’s instantaneous trip value, meaning the breaker will not trip at such a low value.
Using those innate senses
We could hear a very faint whirring sound coming from the mixing vats each time this peak current was generated. The machine designers confirmed-according to their process timing schedule—that this was one of their motors starting up. Drilling down electrically through approximately 100 other process loads to monitor that specific motor wiring confirmed that motor start-up was the cause of the momentary large current increase through the breaker.
Observing the overall breaker phase current waveform more closely over a 1-day production period revealed there were random times throughout the day when three different vat motor start-ups would coincide.This increased the breaker trip instantaneous current level to about 30 to 35% which, again, would not trip at this low value, but remember: there are 10 independent process vat motors fed by this 1600A breaker (plus other miscellaneous process loads).
This production area had become a game of Russian roulette. Given enough running time and the right combination of bread recipes, a situation will develop where too high a number of vat motors start-up simultaneously during the same 400 μs to 500 μs time frame, thereby exceeding the 1600A breaker’s instantaneous trip level.
(Note: The instantaneous breaker trip level was already set to its maximum coordinated setting and could not be set any higher.)
The machine process designers were then presented with two solutions: they could either program-in controlled, staggered process operational settings into the 10 separate bread-making operations, or install soft-start drives on the vat motors to reduce the random combined current inrush levels.
Centralized reprogramming of all these mixing operations was considered too complex of an endeavour, so soft-start drive systems were installed by the process machine designers on all of the larger motors.
With that, production is back up and running at 100%; lots of dough is again being made… by both the plant and its workers.
Patrick J. Lynch, P.Eng., has been the president of Power Line Systems Engineering Inc. since 1986. He graduated Electrical Engineering from the University of Waterloo in 1975, and has successfully directed Power Line’s completion of over 1100 complex electrical engineering site disturbance investigations around the globe. Visit www.powerlinesystems.ca.
This article originally appeared in the July/August 2018 issue of Electrical Business Magazine.