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Accessing more power through 240V receptacles


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#1 Guy Holt

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Posted 10 February 2014 - 06:19 PM

In a post recently underNewbie. Bought 2kW Mole Super Softlite. How to power it?” thread, Marc Roessler posted that operating at 120V through a 240V-to-120V transformer/distro rather than directly into a 240V receptacle with 240V globes does not give any benefit. While Marc makes some very good points, and is even correct in theory on some, in realty things are very different. So different that I do not agree with the conclusions he draws. Where the following discussion pertains to how to access more power through 240V receptacles than it does to the original post, I have started a new thread. I would like to address his points one at a time.

 

Marc Roessler wrote: "Yes, you can compensate for the line loss by choosing different taps at the transformer (like with the old Colortran system). But this will raise power consumption (amperage/current) on the primary 120V side. You don't save any energy. At some point your primary breaker will trip if you keep compensating by upping the secondary voltage by changing the transformer taps." 

 

If we look at the fundamental principles by which transformers operate, we see that while this is true in theory, in practice it is not a concern because the increase in current is negligible. A step-down transformer consists of two sets of coils or windings (a basic two-winding transformer is shown in the Figure below.) Each set of windings is simply an inductor. AC voltage is applied to one of the windings, called the primary winding. The other winding, called the secondary winding, is positioned in close proximity to the primary winding, but is electrically isolated from it.

 

Transformer_Sch.jpg

 

The alternating current that flows through the primary winding establishes a magnetic flux that induces a voltage across the secondary winding. In other words, the secondary winding converts the magnetic field generated by the primary winding into electrical power producing the required output voltage.  Because the same magnetic flux links the turns of both the windings together, the same voltage is induced in each coil turn of both windings.

 

Trans_Flux_SM.jpg

For example, if we have a transformer with a single turn in the primary, and only one turn in the secondary. And, if one volt is applied to the one turn of the primary coil, assuming no losses, enough current will flow and enough magnetic flux will be generated to induce one volt in the single turn of the secondary. That is, each winding supports the same number of volts per turn.  From this example we can see that in an ideal transformer (one with no internal losses), the power available in the secondary winding will be the same as the power in the primary winding. Transformers are then constant wattage devices that do not change the power only the voltage to current ratio. For this reason, a transformer is all about "ratios."

 

The difference in voltage between the primary and the secondary windings is achieved by changing the number of coil turns in the primary winding ( NP ) compared to the number of coil turns on the secondary winding ( NS ). As the transformer is a linear device, a ratio now exists between the number of turns of the primary coil divided by the number of turns of the secondary coil. This ratio is called the "turns ratio" and it’s value determines the corresponding voltage available on the secondary winding. A 240V-to-120V step-down transformer has a turns ratio of 2 to 1.

 

Given how transformers operate, to maintain a desired voltage on the secondary side in the face of a voltage drop from line-loss on the primary side requires increasing the magnetic flux generated by the primary coil.  This can be accomplished by one of two means. As Marc suggests, we can substantially increase the current through the coil which may result in tripping the primary supply overcurrent protection, or we can keep the same current flowing, and instead decrease the number of coil turns of the primary winding. If each turn of the primary coil corresponds to higher voltage, when multiplied by the turns in the secondary coil, the voltage output on the secondary will be boosted. In other words, we can control the voltage output of the secondary by changing the turns ratio slightly.

 

Trans_Boost_Taps_Sm.jpg

 

Since it is quite often desirable to adjust voltage output by means of the turns ratio in this fashion, without substantially increasing the current drawn, a part of the primary winding on the high voltage side of a step-down transformer is tapped out allowing for easy adjustment. The tapping is preferred on the high voltage side as the volts per turn are lower than the low voltage secondary side. Regardless of which side is tapped, controlling the voltage output of the secondary by changing the turns ratio does not appreciably increase the current in the primary coil and so is not likely to lead to tripping the primary supply overcurrent protection as Marc suggests.

 

Guy Holt, Gaffer, ScreenLight & Grip, Lighting Rental & Sales in Boston


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#2 Guy Holt

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Posted 10 February 2014 - 06:31 PM

Marc Roessler wrote:

 

"… the transformer itself has losses. Not too much resistive loss (i.e. heat generated), but it is an inductive load (i.e. reactive current, bad power factor!) so you can't make use of all of that amperage. Depending on the transformer, this means that a 30A/240V dryer receptable will NOT be able to power a full 30*240=7200VA lamp at 120V through a 240-to-120V step-down transformer."

 

Thus far we have assumed an ideal transformer that has no internal losses. But Marc is correct that transformers have losses. However they are not as substantial as Marc implies. A transformer does not require any moving parts to transfer energy. This means that there are no friction or windage losses associated with other electrical machines. The losses transformers do suffer are called "copper losses" and "iron losses" but generally these are quite small.

 

Copper losses, or the resistive losses Marc refers to, is the electrical power which is lost in heat as a result of circulating the currents around the transformer’s copper windings, hence the name. Copper losses represent the greatest loss in the operation of a transformer.

 

Iron losses, also known as hysteresis, is the lagging of the magnetic molecules within the core, in response to the alternating magnetic flux. This lagging (or out-of-phase) condition is due to the fact that it requires power to reverse magnetic molecules; they do not reverse until the flux has attained sufficient force to reverse them. Their reversal results in friction, and friction produces heat in the core which is a form of power loss.

 

The efficiency of a transformer is reflected in this accumulative power (wattage) loss between the primary (input) and secondary (output) windings. An ideal transformer is 100% efficient because it delivers all the energy it receives. Real transformers on the other hand are not 100% efficient and at full load, the efficiency of a transformer is somewhere between 94% and 96% - which is quite good. The efficiency of a transformer like ours, operating with a constant voltage and frequency, can be as high as 98%. So, again Marc is correct in theory, but in practice the losses in a transformer are negligible.

 

In order to demonstrate how little power is lost to transformer inefficiencies, as well as how little effect stepping up voltage by means of taps has on the current drawn by a transformer, I conducted the following test. I used one of our modified Honda EU6500is generators to power a couple of 2ks and a 1k (5000W total load) with one of our 60A Transformer/Distros with the Voltage Select upgrade. The Voltage Select upgrade allows for the switching between taps to boost the voltage output on the secondary side to compensate for line loss. As you can see in the picture of the test set-up below, I then used a Fluke 34B Power Quality Meter to measure the current on one leg of the primary supply (clamping onto one of the hot conductors) and the meter probes to measure the voltage on the secondary side (stabbing into the 60A Gang Box plugged into the Transformer/Distro.) This way the meter screen would have both the secondary voltage (on the upper left) as well as the current drawn on one leg by the primary (half the total load of the transformer primary.)

 

Trans_Eff_Set_Up_Sm.jpg

 

For the first trial I ran the 5000W load (the two 2ks and one 1k) without any voltage boost on the Transformer/Distro and only 50’ of twist-lock extension between the generator and the Transformer/Distro. As you can see in the capture of the meter screen below, at 118.9V our 5000W incandescent load drew 40.32A (20.16A x 2= 40.32)

 

Trans_Eff_50ft_NO_Boost_Sm.jpg

 

For the second trial, I created appreciable voltage drop through line-loss by adding 300 more feet of twist-lock extension between the generator and the Transformer/Distro. Without using the “Voltage Select” capability to boost voltage to compensate for the line-loss, our 5000W incandescent load drew 38.5A (19.25A x 2= 38.5A) at a reduced voltage of 113.9V (see meter screen capture below.)

 

Trans_Eff_350ft_NO_Boost_Sm.jpg

 

For the third trial, I compensated for the appreciable voltage drop over the 350 feet of twist-lock extension between the generator and the Transformer/Distro by using the “Voltage Select” switch to boost voltage on the secondary side by 5% to compensate for the 5V line-loss. Where I have run out of space, I will pick up this post later.

 

Guy Holt, Gaffer, ScreenLight & Grip, Lighting Rental & Sales in Boston

 

 

 


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#3 Guy Holt

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Posted 10 February 2014 - 06:37 PM

Cont. from above:

 

For the third trial, I compensated for the appreciable voltage drop over the 350 feet of twist-lock extension between the generator and the Transformer/Distro by using the “Voltage Select” switch to boost voltage on the secondary side by 5% to compensate for the 5V line-loss. Where I have run out of space, I will pick up this post later.With the voltage boost, our 5000W incandescent load drew 42.5A (21.25A x 2= 42.5A) at a voltage of 119.2 (see meter screen capture below.)

 

Trans_Eff_350_Boost_Sm.jpg

 

For a fourth trial, I created additional voltage drop through line-loss by adding another 200 feet of twist-lock extension between the generator and the Transformer/Distro. Without using the “Voltage Select” capability to boost voltage to compensate for the line-loss, our 5000W incandescent load drew 37.3A (18.65A x 2= 37.3A) at a reduced voltage of 109.7V (see meter screen capture below.)

 

Tans_Eff_550_no_boost_Sm.jpg

 

For a fifth trial, I compensated for the additional voltage drop over the now 550 feet of twist-lock extension between the generator and the Transformer/Distro by using the “Voltage Select” switch to boost voltage on the secondary side by 10% to compensate for the 9.2V line-loss. With the two stage voltage boost, our 5000W incandescent load drew 45.12A (22.56 x 2= 45.12A) at a voltage of 119.4 (see meter screen capture below.)

 

Tans_Eff_550_2xboost_Sm.jpg

 

As you can see by these tests, changing the turns ratio to increase voltage drew only 2.18 Amps more when boosting the voltage by 5%, and only 4.8 Amps when boosting the voltage by 10% - hardly enough to trip primary supply overcurrent protection as Marc suggests.  And, when you compare the 42.5 and 45.12A Amps drawn by our 5000W load to the 41.94 Amps it should draw according to Ohm’s Law (W=VA or 5000W/119.2V = 41.94A), the .56 and 3.18 Amps more current drawn puts the efficiency of our Transformer/Distro at between 92.5 and 98.7% (42.5A - 41.94A = .56A, .56A/41.94A = .013 or 1.3%, 100 – 1.3 = 98.7% efficiency.)

 

So technically, Marc is correct in that a 30A/240V dryer receptacle will NOT be able to power a full 7200VA load  (30A x 240V = 7200VA) at 120V through a 240-to-120V step-down transformer – it will however be able to power only a 7106VA load (7200 x .987 = 7106.4.) If we agree not to quibble over the 93.6 VA difference (7200 – 7106 = 93.6VA), it is fair to say that our 60A Transformer/Distro gives you access to the full power available in the 240V receptacle in a single large 120V circuit that is capable of powering larger lights, or more smaller lights, than you could otherwise.

 

Guy Holt, Gaffer, ScreenLight & Grip, Lighting Rental & Sales in Boston


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#4 Guy Holt

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Posted 10 February 2014 - 06:42 PM

Marc Roessler wrote:

 

It (line-loss) is really no issue with 240V as long as your cables are thick enough, and that's really the only clean solution to the problem. Long runs? - Thicker cables!

 

Again, Marc is correct in theory, but not in practice. In the iRobot Spot, given as an example in the other thread (use this link), half the circuit consists of the house wiring, so it is simply not an option in realty to go to a larger cable size to reduce line-loss. In the case of portable generators, most of the voltage drop is on the generator’s output and not on the cable run (generators will drop nearly 10V under full load). Using a larger cable size will not compensate for voltage drop on a generator, but switching taps on a transformer will. And, as we saw above, the slight increase (2.8A) in the current drawn by a 5000W load by boosting the voltage by means of the transformer’s taps is negligible.

 

You might ask yourself, why not operate at the lower voltage (109.7V) if it means drawing less current (37.3A.) The reason is that low voltage on set can cause problems such as reduced efficiency and excessive heat in equipment, unnecessary additional load on the generator, and a dramatic shift in the color temperature and in the output of lights.

 

For example, the effect of line-loss on tungsten lights can be dramatic because their output falls off geometrically as the voltage decreases. For example a 1k lamp operating at 90% rated voltage (108V) produces about 68% of its normal light output - your 1kw lamp is now a 650W lamp. But, that is not all, as the light intensity decreases, so does the Kelvin color temperature of the emitted light. In the case of fluorescents, HMIs, and LEDs, because their power supplies are typically of a “constant power” type, they will draw more current as the line voltage decreases in order to maintain constant power to the lamp. With an Apparent Power of 2290VA, the Arri 1200 Par that drew 19A at 120V (2290VA/120V = 19.08A) will draw 20.87A amps at 109.7V (2290VA/109.7V = 20.87A.) Since the Arri Ballast has an operating range from 90-125V, it is not likely that the ballast will shut off from under voltage, but it is very likely that the 20A breaker providing over current protection to the Edison U-Ground receptacle it is plugged into will trip and shut the light off.

 

In the case of generator output, voltage loss translates into an exponential loss in power. That is because, if you double the ampere load on the cable, the voltage drop also doubles, but the power loss increases fourfold. What this means is that when a distribution system has a large voltage drop, the performance of the generator (its maximum effective load) is reduced. Given these consequences of voltage drop the negligible increase in current drawn by boosting voltage by means of a transformer’s taps is well worth it IMOH. Use this link for details about other benefits to be gained by powering lights off of 240V with a step-down transformer.

 

Guy Holt, Gaffer, ScreenLight & Grip, Lighting Rental & Sales in Boston

 


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#5 Marc Roessler

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Posted 15 February 2014 - 04:16 PM

Concerning the current increase when choosing different taps at the transformer, it increases linear as you compensate for the drop across the cable. This is a few amperes. But they can make a difference if you are near the circuit's limit and run it for more than a few minutes.

Transformer losses are little, that's true - just what I wrote. But I was talking about cos phi, i.e. power factor of the iron transformer. What's the cos phi of your transformer? Unless it is 1, you can't make full use of the supplied current (VA vs. Watts). For the transformers I commonly encounter, power factor often is way 1.0.

 

I'll not go into the details because I simply don't have the time to do so. For that particular setup that started the conversation (one private owned 2K softlight) I'll stand by my preference for proper cabling and 240V globes. :)

 

Kind regards,

 Marc


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#6 Marc Roessler

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Posted 16 February 2014 - 08:22 AM

"way below 1.0" I meant to say


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