Film-Tech Forum ARCHIVE: New booth build-out: triple or single phase

This is topic New booth build-out: triple or single phase in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Dan Chilton (Member # 2288) on 03-12-2008, 04:35 PM:

We're moving our theater to a new building which will allow us to double our capacity. We're currently in the stage where the engineers are asking us a million questions about electrical loads, booth equipment, etc.. When I asked the electrical engineer if triple phase was available for the booth, he said it was, but he argued it would be better to stick with single phase and use a rotary phase converter, since our city charges $25 per month on top of your regular bill to use triple phase.

I've heard that triple phase is both less expensive to install and run, so which side should I take? Our current booth uses single phase with a rotary converter, only because that's what was available when we moved in. What are the benefits of having triple phase, in your opinion? Is it really less expensive? Is it more stable and easy to work with than using single phase with a converter? If you were in this situation, what would you do?

Thanks!

Posted by Mark Gulbrandsen (Member # 72) on 03-12-2008, 06:28 PM:

Three phase is the least expensive IF you use enough of it. Assuming that all your air conditioning stuff will also be three phase then you should be ok there. Three phase is not mandatory in the booth any longer with switching supplies and modern digital projectors... But we need to know your image size and what gear you'll be running in the booth to give you an idea of the load.

Mark

Posted by Dan Chilton (Member # 2288) on 03-12-2008, 06:35 PM:

Well, I know for sure that the projector for the small auditorium will be our current 2k system. The larger auditorium will probably be a 3k console... and that's it! I'll check on the HVAC equipment to see if it's available in three-phase as well.

Posted by Gordon McLeod (Member # 33) on 03-12-2008, 07:48 PM:

Go with 3 phase there is no way that the energy cost of running a rotary converter will be less than $25.00 per month
remember that there is energy losses when ever you add a conversion of electrical to mechanical and then back to electrical

Posted by Gordon Bachlund (Member # 124) on 03-12-2008, 07:58 PM:

The booth loads you cite don’t really justify three-phase service, but, as Mark notes, your HVAC equipment may drive your selection.

A rotary converter is an inefficient device at best as Gordon notes, and electricity costs will go only one way – up – in coming years.

Your best bet will probably be a three-phase four-wire service that can feed your HVAC equipment, which will probably be three-phase anyway, and your other loads irrespective of phase.

Posted by Jack Ondracek (Member # 1466) on 03-13-2008, 07:19 AM:

I've run a couple of radio stations on rotary converters, with very good results. However, it's extra mechanical equipment to buy, install and and keep an eye on. If one motor fails, everything that converter feeds will go down. I only used them because I couldn't get 3 phase service where I needed it.

Rotary converters do not like widely varying loads. Switching lamps and HVAC equipment on & off WILL be noticed on operating screens, unless you install individual converters for each load ($$$$.$$).

Unless you have VERY few individual units that need 3 phase, getting the power company to provide it is probably the best idea. In the end, the extra $300 you'll pay them is probably a better way to go.

Posted by Mark Gulbrandsen (Member # 72) on 03-13-2008, 08:37 AM:

I have to admit that I find it amazing that any electrical contractor/enngineer would even bring up using a rotary convertor unless you're in a very rural/residential area that still has no three phase... yes, there are some areas still like this and many an area where the third leg is a high leg... which is also not too preferable. Switching rectifiers can runn on single phase and you can parallel more units if you need to operate a higher wattage lamp.

Three phase will cost a little bit more to install... you're pulling 4 wires instead of three and the panels have to accomodate it too. The price of copper is through the roof these days.

Mark

Posted by Louis Bornwasser (Member # 3063) on 03-13-2008, 01:35 PM:

Mark; on the other hand 4 small wires rather than 2 very large wires. That was the selling point for 3 phase from the beginning.

I find it rather odd, that while 3 phase was commonplace in rural Kentucky & Indiana in the 40s & 50s; it is now hard to find in today's drive in locations.

I lived my life in broadcasting & cinemas for 57 years without having to use a rotary converter even on mountain tops. Within the last 2 years, I had had to do so twice.

Switchers are not an option; I won't do THAT to a customer! Louis

Posted by Gordon McLeod (Member # 33) on 03-13-2008, 01:37 PM:

But also the copper size for the same KVA load is smaller so there is a saving Also 3phase large motors are cheaper

Posted by Gordon Bachlund (Member # 124) on 03-13-2008, 02:51 PM:

Arguably, local conditions vary. However, several things do not…..
• Three-phase motors are less expensive and more efficient than single-phase
• Depending on summer temperatures, air conditioning may be the largest component of a theatre’s electrical load

Posted by Bill Enos (Member # 440) on 03-13-2008, 03:31 PM:

Three phase motors are also smaller for a given power and run cooler. Chillers of very large capacity may be difficult to find in single phase.

Posted by Mark Gulbrandsen (Member # 72) on 03-13-2008, 03:59 PM:

quote: Louis Bornwasser
Mark; on the other hand 4 small wires rather than 2 very large wires.

Louis,
I always pull the next larger guage for feeds, especially to rectifiers and large motors!

Switchers are no trouble at all if you install them correctly... they ABSOLUTELY have to have a supressor on the feed side. I haven't had a switcher fail that we installed in I can't renmember how long... we do see several a month form other customers that unfortunately didn't add surge supression to theor booths. At least they are simple to fix once you get past the 4 pounds of hardware holding them together. They are also incredibly easy to parallel... sort of made for it as you can link the control boards together for this purpose and control all of them from just one switcher.

Mark

Posted by Jim Cassedy (Member # 4115) on 03-16-2008, 11:59 AM:

The theater near my house was built around 1918. It still has single phase power. They've modified their 3ph rectifiers to run single phase. They make an unbearably annoying loud buzzing sound when they're operating, but the owner tells me they've been running OK for over 20 years.

> I'm not in any way suggesting that anyone try such a modification- I'm just thowing this in as as an anecdote to the topic <

Just for fun- here's a picture of their main electrical
service switch. I'm pretty sure this is as old as the building.