A VFD swap caused an oven’s burner to stop lighting. Control Systems Inspector traced the wiring and found something unexpected. One of ACES’ clients called with an oven that suddenly wouldn’t light. This is a mission-critical industrial oven in a food processing operation and the company needed it back online, faster than it takes to cook a frozen dinner.


ACES sent a Control Systems Inspector (CSI) to the site, and the first thing he did was interview the machine operators. Turns out the history of the problem was a little more complicated: Two days earlier, during second shift, there was a fault on the variable frequency drive (VFD) for the exhaust blower, which told the operators to power down and reboot, and if that didn’t resolve the issue to replace the drive. When powering down and rebooting didn’t work, the operators replaced the drive with a new one they had on the shelf. The only problem was, it wasn’t programmed.

Now first shift came in and took over. Since the drive wasn’t programmed they decided to reinstall the old drive, to see exactly what it was doing and to capture all the programming parameters.

When they replaced the old drive, the error was gone, and now the burner would light on pilot — and then it would fail. (Originally it wasn’t lighting at all.) When the CSI tested the oven, he found that following purge, the pilot lit as normal. Then the main burner lit for about 10 seconds and dropped out.

The CSI started tracing out the wiring. While tracing out the wiring on a limit string in a crowded cabinet, he pulled on a wire and the ferrule fell off. He replaced the ferrule and re-crimped it thinking this had to be the problem. He retested again, with the same issue.

Next, he jumped out wires to individually test the flame safeguard, valves and igniter. He confirmed that all the field devices were working well on manual.

The CSI hooked his laptop up to the PLC, found the code directing ignition and put in several traps to identify what was dropping out and causing the problem. He narrowed the issue down to a section of code and discovered that it was something to do with the wiring of the limit string that he’d initially been tracing.

The testing uncovered two relays that were at fault: One should have been off and it was on. The other should have been on and it was off.

Finally, this line of inquiry revealed that one of the relays was dropping signal in the pre-purge timing circuit.


The CSI traced that relay back to the exhaust VFD, which needed full speed to purge on the exhaust and half speed for the main burn. He ascertained that these two wires were swapped. During purge the system was looking at the VFD to see if it was running, then when it went to main it checked for half speed — but the contact was reporting back full speed so the system shut itself down.

When the second shift operators had installed the new VFD, they cleared the original fault message — but they also inadvertently flip-flopped two wires. When first shift came on, they reinstalled the original VFD, which would have worked fine, except the operators replicated the faulty wiring, copying it wire for wire.


The CSI switched the two wires back to their proper positions and retested. The pilot light and burners all started up like a charm. He copied the PLC parameters onto a flash drive and gave it to the client for their records.

There are many causes for machine failures that seem mysterious, but upon systematic investigation reveal logical causes and obvious solutions, once the problem is identified. Our ACES technicians are troubleshooters with many years of experience who will have your critical equipment up and running — and keep it that way. Give us a call today for a free estimate.


The Case of the Shy Burner