Testing interest for a Dryer-Drop Transformer

[J. Bernard Vallon]

I'm planing on building a transformer for three-prong outlets, to convert 220v 30a to 110v 60a with bates paddles such to run production power in residential locations. Nearly every residential location has a three-prong 220 outlet, and using them is a cheep alternative to tie-ins or generators.

The biggest light you could power on these would be a 6k, or several 2.5ks, and as light kits shrink and budgets get choked, thats often the biggest gun on the truck.

Would people me interested in buying these ready made? If so, how much would people be willing to pay?

-J

 

[Zach Gray]

There are typically three types of residential plugs, and the hardware to make a breakout is available at any home improvement store.

I'd be very careful. You never know what kind or condition of wire is going from the appliance plug to the circuit panel. Also, many of those receptacles are just 30a. 6k seems like a bit much to ask of residential wiring.

If you can, get electric to tie into the panel. You can still use your home built breakout, but you'll know what you can safely pull.

 

[Noah Kadner]

My one concern would be safety- as in someone could set this up wrong or in a building with iffy wiring and electrocute themselves or cause a fire... Then guess who gets sued?:emote_couch:


There's good reason to hire a qualified electrician to do heavy duty movie lighting and this is precisely one of them.

Noah

 

[Stephen Pruitt]

We just plug our 6K and 2.5K HMIs directly into the 220 outlets and they work great. I think it's a great idea, but I don't really need one at this point.

Good luck!

Stephen

 

[Mark Allen]

Sadly, I think Noah is right here in that the risk/reward for you would be imbalanced.

You've reminded me though that someone built one of these for me once and I should find that thing.

My one concern would be safety- as in someone could set this up wrong or in a building with iffy wiring and electrocute themselves or cause a fire... Then guess who gets sued?:emote_couch:


There's good reason to hire a qualified electrician to do heavy duty movie lighting and this is precisely one of them.

Noah

 

[Guy Bryan Holt]

the hardware to make a breakout is available at any home improvement store…If you can, get electric to tie into the panel. You can still use your home built breakout, but you'll know what you can safely pull.

Sadly, I think Noah is right here in that the risk/reward for you would be imbalanced.

Using a step-down transformer to convert a 30A 220V circuit to a 60A 120V is a great idea. But, like all great ideas, someone has already thought of it. We build and sell just this for use on dryer/range plugs and on Honda EU6500is generators that we modify to provide 7500W in a single 120V circuit. That aside, it seems to me that Mark and Zach are confusing what Bernard is proposing (which we do already) with what are called “Splitter Boxes.” Splitter Boxes are custom distribution panels, that rental houses used to wire to access more 120V power from 240V circuits and the 240V twist-lock receptacle on generators. Splitter Boxes worked well enough when the lighting load placed on these circuits consisted predominantly of incandescent lights (a linear load.) However, Splitter Boxes are inherently unsuitable to carry a non-linear loads consisting predominantly of non-Power Factor Corrected HMI, Fluorescent, & LED Lights. To understand why this is the case, I would suggest you read the article I wrote on the use of Portable Gas Generators in Motion Picture Production. The article is posted on our website at: http://www.screenlightandgrip.com/html/emailnewsletter_generators.html.

To quickly summarize the article: a "Splitter Box" works around the limitations of a 240V wall receptacle or generator power output panel, and provides additional 120V circuits, by splitting out the two 120V circuits that make up the 240V outlet (see generator wiring schematic below.) For the purpose of this discussion, it is important to understand that Splitter Boxes are wired so that their 120V circuits share in the single ground and neutral of the 240V circuit. Splitter boxes worked well enough back when the load on the 240V circuit consisted of only incandescent lights. As long as you roughly balanced your load between the two legs of the circuit, phase cancellation between the legs resulted in the neutral return being the difference between the legs. The problem is that things get a bit more complicated with inductive (magnetic HMI ballasts) and capacitive (electronic HMI, Kino, & CFL ballasts) non-linear lighting loads.

Generator Wiring Schematic​

Since non-linear loads cause current and voltage to be out of sync, the phase currents no longer entirely cancel when they return on the neutral. In addition to pulling the voltage and current out of phase, the Switch Mode Power Supplies (SMPSs) of electronic lighting ballasts create harmonic currents that stack on top of one another, creating very high currents returning to the power source on the neutral wire. If the lighting package consists entirely of non-linear light sources without power factor correction, as much as 80 percent of the current will not cancel out between legs, resulting in very high current on the neutral return even when the legs are evenly loaded. For this reason, on their website Kino Flo cautions users that some of their lights “will draw double the current on the neutral from what is being drawn on the two hot legs... it may be necessary to double your neutral run so as not to exceed your cable capacity.” ( FAQ “Why is the neutral drawing more than the hot leg”.)

It is important to appreciate the potential hazard that the harmonic currents generated by SMPS stacking on the neutral can pose. The results of a recent study (illustrated below), showed that the combined effect of the phase shift and harmonics generated by substituting incandescent lamps with an equivalent wattage of CFLs in a house resulted in more than a doubling of the current on the system neutral.

Substituting a linear load with a equivalent non-linear load in a small single phase distribution system substantially increases the current on the system neutral.​

Since there is no over-current protection on the neutral of a 240V circuit, these currents can lead to overloading of the neutral and potentially a fire like the one that occurred in Vice President Dick Cheney's suite of offices in the historic Eisenhower Executive Office Building next to the White House on December 19, 2007. The cause of the fire that started in an electrical closet on the building's second floor, was found to be caused by an over-loaded neutral wire that resulted from uninformed staff replacing all the incandescent bulbs in the complex with CFLs.

There is no question that we find ourselves in an analogous situation, since light sources that use Switch Mode Power (HMI, Fluorescent, & LED) have almost entirely replaced incandescent lights as the prevalent lighting source in HD Digital Cinema production packages. To make matters worse, the video cameras, field monitors, hard-drives, lap-tops, and battery chargers that make up the typical location production package also use Switch Mode Power Supplies and hence generate their own harmonics. Where just about every piece of production equipment used on set today generates harmonics, the current returned on the neutral system can be quite high. And, since it is nearly impossible to perfectly balance a load on a splitter box (it requires meticulous attention to loads as you plug in) the return of the neutral is likely to be even higher because of an unbalanced load. Since there is no over-current protection on the neutral of a 240V single phase circuit of a generator (see schematic above) or a house, and the nuetral wire used in a 240V receptacle is only rated for 30A, these currents can likewise lead to overloading of the neutral. The neutral wires of 240V circuits on portable gas generators and in houses are simply not sized to handle the higher return current generated by loads that stack on the neutral rather than cancel out. Regardless, if it’s a 240V circuit in a generator or in a house, splitting a 240V circuit designed to power primarily heating elements and compressors, to instead power field production equipment and motion picture lights is an application for which these circuits were simply not designed.

The only safe way to pull power from 240V wall outlet (whether a three wire or four wire system) is to run your lighting load through a 240v-to-120v step-down transformer. A transformer converts the 240 volts supplied by these industrial and household 240V receptacles back to 120 volts in a single circuit that is the sum of the two legs of the circuit. For instance, a transformer can make a 60A/120v circuit out of a 30A/240v dryer circuit that is capable of powering bigger lights, like a 5k or 4k HMI. What enables a step town transformer to power larger lights on 240V outlets is that the transformer automatically splits the load of whatever you plug into it evenly over the two legs of the 240V circuit so that you have 100 percent phase cancellation on the generator neutral (primary side.)

There are, however, a number of design considerations that go into making step-down transformers safe and effective. For this reason, I have to agree with Noah.

My one concern would be safety- as in someone could set this up wrong or in a building with iffy wiring and electrocute themselves or cause a fire...There's good reason to hire a qualified electrician to do heavy duty movie lighting and this is precisely one of them

Our system is cited in the just released 4th Edition of Harry Box's "Set Lighting Technician's Handbook" and featured on the companion website "Box Book Extras."

What earned Harry’s endorsement was extensive research I have done on the the electrical engineering principles behind these issues and contained in a newsletter article I wrote on the use of portable generators in motions picture production. Of the article Harry Box exclaims:

"Great work!... this is the kind of thing I think very few technician's ever get to see, and as a result many people have absolutely no idea why things stop working."

"Following the prescriptions contained in this article enables the operation of bigger lights, or more smaller lights, on portable generators than has ever been possible before."​

The article is available online at http://www.screenlightandgrip.com/html/emailnewsletter_generators.html.

Guy Holt, Gaffer, ScreenLight & Grip, Lightng & Grip Rental in Boston