By K. Russ
Back in the mid-80s, I had a job doing control systems for roller-coasters. Our senior engineer dated back to the days of relay logic, and although we had been using PLCs and drives for many years, some of the old relay circuits were still in the system.
I had started up a coaster in Canada, with a DC drive we hadn’t used before. A couple of weeks after commissioning, I got a call that when they pressed the start button, the motor didn’t turn, but then several minutes later, it took off. Fortunately, the mechanic didn’t have his fingers in the machinery, but it was close.
We started out trying to troubleshoot over the phone. I talked to the drive manufacturer, and described the symptoms. We tried a number of tests. Of course the problem only seemed to occur when an electrician wasn’t there to see it. Finally, all the stars aligned, an electrician was there, and I was on the phone. We started checking - drives on, all lights good, etc. Then the drive takes off. Out of that comes a clue. When it took off, it was at the medium speed, rather than the idle speed.
In the old days, the DC drives had one speed input, and the speed was set with a pot. In order to make that a little more automated, our design was to have three pots for the three different speeds, and then switch them in with relays. Apparently, the relays we were using were not passing the speed signal. To test it, we took the relay out for the idle speed. Sure enough, the drive started right up with a zero speed command. When the PLC called for it to go to the second speed, it appeared to start up by itself.
In the post-mortem with the relay and drive manufacturers, it became apparent that this drive pulled very little current through the relay. The relay manufacturer indicated the current was below the rating of the contacts, and therefore the relay wasn’t able to keep its contacts clean. As time went on, this problem would have spread to the other two relays.
The solution for this system was to go to bifurcated relay contacts, but fortunately, the next year the drive manufacturer came out with a system of preset speeds that could be chosen using some logic and three inputs into the drive, and the relay circuit was officially retired.