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Topic: 1.5Kw x 2 = No 3 phase ??

Barry Floyd
Phenomenal Film Handler

Posts: 1079
From: Lebanon, Tennessee, USA
Registered: Mar 2000

posted 08-30-2000 07:44 AM

I spoke with an owner of a drive-in in Northern Georgia yesterday, and we were talkin' "nuts & bolts", when he explained he bypassed the need or requirement for 3 phase power by wiring two 1.5kw rectifiers in "parallel" - to run a 3000 watt Xenon lamphouse. He said it's been working fine since the mid 70's. Somebody please explain how this works??

I'm going down there this weekend to take photos - I have to see it for myself!

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Paul Cunningham
Expert Film Handler

Posts: 146
From: Melbourne, Australia
Registered: Jun 2000

posted 08-30-2000 08:07 AM

This is not the same as what you are talking about, but we have a thing called a T-Verter which converts single phase to 3 phase for our projector motor. Dont ask me how though. It also has a variable frequency output so that I could say increase from 50 to maybe 52 hz and speed the movie up slightly.

Paul

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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99

posted 08-30-2000 08:56 AM

The T verter basically rectifies the line to DC and then has 3 synced oscilators driving a very large bank of output transistors or mosfets to produce the 3 phase output

I have seen several parralelled rectifiers but there are two cautionary notes doing it
It is important that they be matched so each one produces the exact same output especially since it is not practicle to use load sharing resistors

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Stefan Scholz
Expert Film Handler

Posts: 223
From: Schoenberg, Germany
Registered: Sep 1999

posted 08-30-2000 11:09 AM

This is standard practise in commercial DC supplies, to increase reliability. In telecom applications we use 2 or 3 identical supplies that are wired in parallel. In case one fails the other rectifier(s) could still keep everything on power. A non urgent alarm notifies the operation and maintainance center guys.
I have seen cine rectifiers which used 3 individual blocks of 1.5kW to feed 3 kW of DC-Xenons. Even if 2 units blew, you would still have enough power to keep the show alive. This was a portable outdoor screening unit.
If they are of identical construction, balancing loads is quite simple.
With modern switch-mode technology, you can easily get 7 - 10 kW out of single phase 220 power, which is normally not done due to local and national electric codes.

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Rory Burke
Expert Film Handler

Posts: 181
From: Burbank, CA, USA
Registered: Jun 2000

posted 08-30-2000 01:41 PM

I can see and understand how it works now, but Gordon brings up a question. If the two rectifiers were NOT ouputting the same then would it run a risk of damaging those finicky bulbs or affect overall operating hours? I thought that was the whole point to what rectifiers today evolved into: to produce a steady constant feed of electricity.

(Ie. Mexico: electricity during the day occilates between 85v to 135v throughout the day. ) Bulbs seem to run fine with no apparent effects due to run-time hours. However, they go through rectifiers more often than their american counterparts. Is it because those rectifiers are just working harder?

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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99

posted 08-30-2000 01:56 PM

If the load isn't shared properly then one rectifier will probably attempt to output more than it is rated for
Also some rectifier designs can have problems with boost supplies when parralleled
It is not a good idea It is also fun when one of the pair losses a diode

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Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99

posted 08-30-2000 04:49 PM

If you put a huge blocking diode(on a large heatsink!) in series with each rectifier then one will not interfer with the other,(s). The blocking diode would keep one supplys output isolated from the others. You will have only a slight voltage drop across the diode but this will not affect anything. Only the effective current capability would be increased. Disconnecting the start circuits in all but one rectifier would also be helpful. This is usually very easy to do. Iwerks builds supplies based on the Strong switchers to run huge xenon lamps. If one supply fails(creating a drop in operating current) all that is needed is to unplug it and replace it with an extra.
The other option is to get the Strong 3KW switcher. It will run fine on single phase power. Irem also made boat anchor 3kw single phase rectifiers that are basically bulit proof. Another good touch is to run high speed intermittants in your X-L's. This will give you 70% light efficiency and the 3kw will look more like a 4to 5 kw on the screen. These are hard to find but with todays polyestar film work very well.
Mark

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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99

posted 08-30-2000 05:09 PM

2 Large diodes Mark will not completely solve the coupling issue
It is very easy for one rectifier to be outputting more current than the other and being over driven
Load sharing resistors would solve that but also cause a heat and power wastage issue
It is normally recomended that each rectifier be oversized such as 2 2K rectifiers for a 3 K lamp so there is some margin of safety
Also on the boost supply issue THe kniesleys seem to work the best slaved
When we slaved to of the XPS rectifeirs we usually disabled one of the boost supplies

I believe Irem has discontinued the 3K single phase unit

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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99

posted 08-30-2000 07:15 PM

Paul come to think of it you don't need a 3phase source for most 3 phase motors
Most can be run with a capacitor between on of the legs and the third phase
The only thing is the amount of capacitance needs to be switched between start and run modes
This is very common on most cinemecanica and prevost machines that they build the only difference between the single and 3 phase version is the starting method and the caps

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Francis Casey
Film Handler

Posts: 44
From: Saint John, NB, Canada
Registered: Jun 2000

posted 08-30-2000 09:03 PM

to Gordon Mcleod FYI
I have 5 vic 8's in our circuit that i have installed "toshiba TOS-VERTERS" in to run the 3phase motor . These are micro processor based variable frequency 3phase in 3 phase out gagets . They also have adjustable ramp up and ramp down speeds .The main reason for installing these was to eliminate the capacitors and two-stage start switches and relays . A very simple installation , 3-phase in , 3-phase out to the motor and a pair of switch leads .They are very reliable and SMALL about 10"x8"x6" and have been working for over 5 years with out any problems .
The down side is that i am told that toshiba no longer makes them (abour $400 each )
regards
Francis.

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Paul Cunningham
Expert Film Handler

Posts: 146
From: Melbourne, Australia
Registered: Jun 2000

posted 08-30-2000 09:58 PM

Francis and Gordon,
ours too are on a Vic 8 but are only about 6x4x3 inches. The main reason we installed it was because on single phase the motor would have a tendency to not turn over on start up, if you weren't there it would continue supplying power, heat up and start smoking! (The smoke only happened once) On 3 phase it starts perfectly and the motor even runs much cooler now. The microprocessor has a whole swag of different functions the only one to interest us was the variable frequency as we were thinking of interlocking sometime in the future. Has anyone out there interlocked with Panalogic automation?
Paul

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John Eickhof
Jedi Master Film Handler

Posts: 588
From: Wendell, ID USA
Registered: Jan 2000

posted 08-30-2000 10:08 PM

I had a drive in that ran a pair of single phase IREM 2KW rectifiers in parallel they ran for over twenty years! 4KW lamp! No problems!
No coupling extras...Remember, You are dealing with brute-force DC supplies, they will not buck each other unless there is excessive ripple
in the output!

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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99

posted 08-30-2000 10:25 PM

Or until the first diode fails

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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99

posted 08-30-2000 10:32 PM

I don't ,have a problem with phase converters other than they are really not cost effective
If you are starting on 3 phase power all that is required is actually 2 40ohm 100 watt resistors in serries with2 legs of the motor
A potter brumfield 10amp time delay relay is wired in parralell to them that will short out the resistors after 2-3seconds
This is how esoldomatic didit with many of the cinemation's
For single phase on a cinemecanica the amount of capacitance can be switched by a similar relay
A prevost does not require any switching as there motors have the same capacitance requirements for both run and start

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Dave Bird
Jedi Master Film Handler

Posts: 777
From: Perth, Ontario, Canada
Registered: Jun 2000

posted 08-31-2000 07:22 AM

Hey, Barry, I like it. I was quoted some rough pricing a couple weeks ago on some rectifiers that will run 3K single-phase. (Like you, that's all I've got). But hey, wait a minute, you can pair these things and
"fake" 3-phase? I can run 4K (or more)? That's very interesting, may have to return to the drawing board. Can we hit SMPTE standard of 22fl outdoors? We'll try!

------------------
Dave Bird

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