Film-Tech Forum ARCHIVE: 3-phase power for rectifiers

This is topic 3-phase power for rectifiers in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Matt Hale (Member # 1518) on 04-24-2003, 07:50 PM:

Okay, I've read this thread:

Booth Electrical Loads

and this thread:

3-phase demand question

but theres something I still cant get my head around. I understand how 3-phase 208v becomes 1-phase 120v in a Y configuration, so does that mean that any 3-phase 230v service is automatically Delta - i.e. 115v can be created by using a center-tapped "neutral." Or is 230v service simply another form of Y that is not meant to to have 120v derived from it?

And therefore, if my rectifiers specify 3-phase 230v input, do they need Delta power? And can they be made to run off 208v Y if I cant get delta?

[Confused]

Thanks.

Posted by Paul G. Thompson (Member # 655) on 04-24-2003, 08:00 PM:

They will run under the 208V open delta (Wye configuration) enviroment ok. Closed delta (the 230 volt crap) is obsolete in most places now. That stuff has a wild leg which is vertually useless for load balancing.

In the USA, if you asked the power company for closed delta service, they'll look at you as though you are nuts. Chances are, they will not provide it anyway.

I can't comment on Canada, though. I don't know how backwards or antiquated their power grid is.

Posted by Phil Hill (Member # 371) on 04-24-2003, 08:35 PM:

Most all high-voltage distribution in the US is Delta. The nominal Delta low voltage is 230 or 460.

Many of the larger venues take the High-Voltage Delta feed and have their own transformers to convert it to a Wye configuration of 240/120 or 480/277.

There is also a system that takes 2-legs of a 230-volt Delta system and grounds a center-tap to create a neutral with 115 volts on either side with respect to the CT. That system is found a lot in machine shops that require 230-volt 3-phase for the machines and 115-volt single phase for lighting, etc. That system is usually referred to as a "stinger" system or legs. And has a "Wild Leg" as Paul mentioned.

Most all 230 volt rated equipment will run on 208 volts... usually with less efficiency and, depending if the device is resistive or inductive, with less or more current draw and/or a decrease in performance. The only time I had a problem is when motors were rated for 230 and I ran them on 208 volts... they would overheat due to the increased current draw and trip out the heaters. [Smile]

>>> Phil

Posted by Paul G. Thompson (Member # 655) on 04-24-2003, 09:43 PM:

Same holds true with running 240-volt single phase motors running on 208. Have to be a little careful about that....so as not to overload the motor.

When I got my 4,000-pound razzy dazzy super dooper pooper Army-issue surplus three-phase open delta Diesel-powered generator, I had to make a major modification in my electric furnace which involved making a controller to switch the three elements into a 3-phase configuration so it would not excessively unbalance the electrical load output of the generator. The electric hot water heater worked fine under a 208-volt enviroment, but it took a little longer to get the water hot. As far as the stove was concerned, it really didn't matter. All I used it for was lighting cigarettes. The 240-volt air conditioner does not seem to be overly concerned about running on 208. If power is bumped, it'll go into rotor-lock and blow itself off until the high side pressure equalizes. It'll do that anyway whether or not I am running it on 240 Volts with pink polka-dots or 208 Volts with orange polka dots.

Been running like this for 10 years now with no problems of anything burning up. [beer]

As far as the three-phase rectifiers are concerned - as I stated (and Phil verified) - usually they will run just fine. But I should mention the transformer tap setting may have to be adjusted so that the xenon lamp can be operated within rated parameters. Most rectifiers have sufficient "head room" to accomplish that correctly.

Posted by Matt Hale (Member # 1518) on 04-24-2003, 11:24 PM:

Thanks guys that is exactly what I wanted to know. I fully intend to chuck these old rectifiers anyway as I dont think they'll be powerfull enough for the larger screen, and seeing as the output will be decreased from what it was at the old place that just adds to my argument. So with new gear, such as the Christie Reference Console where it says 208v or 230v operation, these would be the exact same unit, not 2 different models.

Posted by Paul G. Thompson (Member # 655) on 04-25-2003, 12:16 AM:

Yes. The Christie Console in the link you provided is a very good piece of equipment. I think you will be very happy with it.

Posted by John Pytlak (Member # 331) on 04-25-2003, 09:25 AM:

3-Phase tutorials:

http://www.ece.uwaterloo.ca/~ece362/ACrw.pdf

http://www.chipcenter.com/columns/rci/tesla/acbasics.html

http://www.ntu.edu.sg/mpe/ldp/ELS/G260/G260_tut02.pdf

http://www.staff.ncl.ac.uk/p.p.acarnley/4072.pdf

Posted by Steve Guttag (Member # 268) on 04-25-2003, 09:50 PM:

In the US...the 3-phase lines are generally provided to the theatre in a 480/277 WYE configuration...then stepped down to a 208/120 WYE configuration, on site.

In a 3-phase circuit, there is no such thing as a neutral. A typical NEMA spec will be type 15 which is 3-phase with a ground...no neutral.

The fact that most 3-phase has a ground reference...and that it happens to be between each leg is of no consequence...the device is only interested in what the potential difference is between each leg and in the case of items like motors...what the rotation is (phase relationship between the legs).

The 3-phase "high-leg" that is found in many older towns is just a pain in the ass. Since most devices run on single phase, 1/3 of the circuits are thrown away. Most load centers can be quickly identified, as such by noting that every third slot is empty. Behind the cover, the phases are normally taped black, red and Yellow (or Orange or Brown)...yellow denoting the high-leg since that is a "277" color.

Many european products are designed for 230VAC (and possibly 50Hz)...208 is NOT 230VAC. In fact, it is almost 10% less. Another thing to keep in mind is that voltages vary by location and time of day. It isn't too uncommon for a 208VAC line to vary down to near 200 and up over 210 during the couse of a business cycle (particularly in a large city with mood swings based on businesses being open and closed on the 9 to 5 plan).

If you get a product that really wants 230VAC...you can normally supply it. There are items called "Buck-Boost" transformers that can step up your voltage to what you need (or knock it down if you have too much).

In the case of rectifiers for cinema applications...most manufacturers have 230/208 taps to deal with the high/low voltage situation. On top of that, they have taps/adjustments to ensure you have an adequate range.

Steve

Posted by Gordon McLeod (Member # 33) on 04-26-2003, 11:06 AM:

The canadian distribution standard is identical to the US model except normaly the input to a building is 600volts wye or in larger centres higher with your own transformer