Not all diodes use electrons

Sometimes thinking like a geek pays off. Especially when you’re dealing with overheated Judge Judy fanatics.

A recurring problem at the transmitter site of the television station I worked at was the cause of no small amount of labor and expense. Periodically the building’s cooling system failed, which is a bit of an emergency during hot weather, especially as it could cause the transmitter to overheat and shut down. Lots of Judge Judy fans out there wouldn’t be happy. And they could be brutal with their criticism.

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The HVAC system used a two-stage heat exchanger, with three fans. The fans draw air past the hot condenser coils and fins and exhaust the heated air out of the top of the unit. When the cooling load is light, the outer two fans run, and the center fan is turned off. When the load is heavy, or the outside air temperature is high, all three fans run.

Every year or so the center motor would burn out, shorting one phase of 480V to the motor’s case. Fortunately, the transmitter site was wired as 480 volts delta. A delta system can withstand a short of a single phase to ground, something a wye system cannot do. As Figure 1 shows, the main electrical panel at the site had lamps from each phase to ground. If a phase was shorted, its lamp would go out and we’d know we had a problem.

Figure 1 A delta system can withstand a short of a single phase to ground, something a wye system cannot do.

The cause of the burnouts was well-known. When the HVAC system has only two fans running, the two outer fans are pulling air not only through the fins, but down through the center fan as well (Figure 2). This causes the idle motor and fan to run backwards as the air flow is reversed. The fan could get up a pretty good head of steam in a short time.

Figure 2 This HVAC system doesn’t include a damper.

Then, when the center motor is called to start, it has more than its usual start-up inertia to overcome, and in fighting the reversed magnetic field, produces a high back EMF. This increases the voltage stress across the motor’s windings as well as the current through them. After enough of this abuse, one of the windings will give up the ghost and short to the motor’s case.

Over the years we replaced a good number of motors, before and during my time at the station. On one of these occasions, I was up on the roof with our HVAC contractor, discussing the problem (and the Red Sox, as this was Boston and such conversation is mandatory). One thought was to install sheet-metal walls to divide the interior into thirds, isolating the center section, but this was not at all an easy project.

Then my inner geek kicked in. Just as a carpenter might think there’s no problem that can’t be solved with a good sized hammer and nails, or a handyman with duct tape, I tend to think in terms of electronics. So it occurred to me that what we needed was an “air-diode.” Let the air flow up and out, but not down and in.

Believe me, I felt like an idiot a moment later when I realized that such a thing already exists in a very simple device: a damper. It’s just a group of hinged sheet metal plates that, when mounted horizontally, will open when the fan is blowing and allow air to flow, and close via gravity when the fan is not blowing, blocking the air flow (Figure 3). Lucky for me our HVAC man hadn’t thought of it either, so I didn’t feel so bad. And he agreed that it was a good idea. He got a damper of the right size, installed it, and never had to return to replace a burned out motor again.

Figure 3 This HVAC system includes a damper, which can be either open or closed.

Incidentally, I’ve never understood the reasoning behind the wide-open cavity in the fan units. Drawing outside air down through the idle fan means an equivalent amount of air is not being drawn through the fins, which greatly reduces cooling efficiency. I know this from unfortunate experience, when I once accidentally left an access panel open on a heat exchanger (at a different TV station). The result was an overheated and shut down TV transmitter (see earlier mention of Judge Judy fans).

Robert Yankowitz retired as Chief Engineer at a television station in Boston, Massachusetts, where he had worked for 23 years. Prior to that, he worked for 15 years at a station in Providence, Rhode Island.

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