Film-Tech Forum ARCHIVE: Strong 77 008 Power Supply Ventilation

This is topic Strong 77 008 Power Supply Ventilation in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by David Kilderry (Member # 142) on 04-22-2004, 11:41 PM:

We operate a Strong 77 008 continuous power supply (rectifier) at our Lunar Dandenong Drive-in Theatre here in Melbourne Australia. It is 160 amp maximum current and has the three rotary phase controls on the front.

My question relates to the fan forced ventilation through the unit, at present ours sucks from the grill on the top of the unit and blows the hot air out of the bottom. Coventional wisdom would have this flow going in the other direction i.e. heat rises so pull from the bottom and evacuate from the top. Which way is the fan supposed to spin? Which way does yours go?

The reason I ask is because this unit has activated the heat cut-off on several occasions on hot evenings. It need 2 - 3 minutes to cool and then will operate again. We run 5,000 watt xenon lamps at around 140 amps. We use Strong lamphouses on all three screens but with the other two using Ushio Xebex power supplies. The Strong power supply is the only one to exhibit this problem.

David Kilderry

Posted by John Pytlak (Member # 331) on 04-22-2004, 11:47 PM:

I don't know about the Strong units, but sometimes large enclosed power supplies have their air intake at the top, so as not to suck in dirt and debris from the floor. I agree it's counter-intuitive, as heat rises.

Posted by Daryl C. W. O'Shea (Member # 1303) on 04-22-2004, 11:48 PM:

In most any device you want the fan to be sucking air into the unit, not blowing it out of the unit. So you want the fan turning in the direction that sucks air IN through the surface it is attached to.

Cooling fans placed in the top are usually there to prevent heat build up in the top (caused by rising heat). It's almost always more effective to push air down and out the sides than depending on enough static presure to push the hot air out the top via a fan at the bottom.

Posted by Jack Ondracek (Member # 1466) on 04-23-2004, 12:17 AM:

I wonder if your fan was replaced at some point with a motor that turns the wrong way? I've had several of those power supplies, and they all have pulled the air through the bottom & blown it out the top.

It could also be that your safety switch needs replacing. Those things get touchy after awhile.

Posted by Rick Long (Member # 213) on 04-23-2004, 12:18 AM:

These units have handles on them giving the misleading impression that "handles up" is the way to install them.

The problem with this, as you have noticed, is that the air is taken in at the top of the unit, passed by a hot bridge, transformer, ect. and then exhausted through the bottom outlet, only to rise, as hot air is wont to do, and be sucked in again.

This recycling of hot air results in the bridge-mounted heat sensors cutting out. (Hot-air recycling is not only limited to rectifiers - I have known several service managers who also had this habit).

The cure is to lay the rectifiers flat so that the recylcing condition is eliminated.

Be aware, however, that our experience is that once these heat sensors have been activated more than a couple of times, they become much more sensitive, reacting to normal heat variations as well. Probably best to replace these as well.

Posted by Ken Layton (Member # 133) on 04-23-2004, 12:49 AM:

We had two of those rectifiers at the Sunset Drive-in theater in Tumwater, Wash. I distinctly remember them blowing air out the top as Jack has stated.

Posted by David Kilderry (Member # 142) on 04-23-2004, 02:43 AM:

Excellent advice everyone, thank you. We will look to reversing the air flow and replacing the heat sensitive switch. It has activated several times now and it appears to have become more sensitive.

The unit was installed two years ago after life at another drive-in, that was in an even hotter climate.

David Kilderry