Road Test: Strong Technobeam, page 40 - PLSN.com

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Ad info:http:// foh.hotims.com Ad info:http:// foh.hotims.com FOCUS ON FUNDaMENTaLS Me So Stupid A Neutral Fig.1 120V 208V 1500W I1 o C 120V I2 120V 1500W 208V B = A 208V 1500W I1 C I2 B 1500W 208V “ It is our responsibility as scientists, knowing the great progress which comes from a satisfactory philosophy of ignorance, the great progress which is the fruit of freedom of thought, to proclaim the value of this freedom; to teach how doubt is not to be feared but welcomed and discussed; and to demand this freedom as our duty to all coming generations...” — Richard Feynman, “What Do You Care What Other People Think?” PROJECTION LIGHTS & STAGING NEWS Let me put this right up front: I’m not the brightest lamp on the truss. At times, I can make Jessica Simpson look like Marilyn vos Savant, who is listed in the Guinness Book of World Records for five years under “Highest IQ” for both childhood and adult scores. I’m not particularly proud or embarrassed about that, but admitting it has served me well. An empty head can be like being an empty vessel looking for understanding with which to be filled. Last month I was in the Washington D.C. area leading a seminar on electricity for the entertainment electrician. There were a lot of very experienced and very good electricians By Richardcadena The Tao of electrical load calculations fig.1 in attendance. When we started talking about how to calculate the feeder cable current in a three-phase system, the Jessica Simpson in me made her appearance. The classic formula for power in a threephase system is: watts = volts × amps × power factor × 1.73. By manipulating that formula we can come up with the formula for three-phase current, which is: I = W ÷ (V × PF × 1.73). One of the electricians in the class said that he uses a different formula for figuring out the current in a three-phase system: I = (W × 2) ÷ V. Even I could figure out that these two formulas are nothing alike. At this point, I could have invoked the teacher/student clause, which is, “I am the teacher, you are the student; now shut up and listen to me.” But I realized that dismissing his approach would be tantamount to dismissing his 30 years of experience. Admitting you don’t know something, particularly in front of your colleagues, is kind of like when someone is waving to a person behind you, but you think they’re waving at you. So you wave back and then you realize...and you feel…so…stupid. After fumbling for an answer to this conundrum, I punted. I called a break to think about it and when the break was over, I not-so-gracefully moved on, promising myself to address it once and for all in the next day’s class. That night, I couldn’t sleep. I went to bed and for two solid hours, I stared at the ceiling and contemplated the question of why the two approaches yielded different results. My mind went blank. Eventually, the words of Lao-tsu came to my mind: “The usefulness of what is depends on what is not.” In other words, the usefulness of your mind depends not on what you already know, but what you might gain by not knowing, contemplating, and then understanding. The answer suddenly popped into my head. I fell asleep. The next day, I went to the class and here’s what I tried to convey. Suppose we have, for example, a single 1500-watt 208V automated light connected across phase A and phase B of a three-phase wye-connected transformer. With nothing else is connected, the line current will be 1500W ÷ 208V = 7.2 amps (assuming unity power factor). In this case we can use the formula for a single-phase system because we only have one single-phase load connected. If we then connect another automated light across phase B and phase C, then we have a more complex situation. Phase B is now feeding current to both loads. How do we figure out the resulting line current? To simplify it and make it easier to understand, we can redraw it as a delta-configured secondary with the same two loads, as shown in the right half of the illustration in Fig. 1. If we pay close attention to the two drawings we’ll see that they are exactly the same even though they are illustrated differently. Now we can see that the line current in B is being drawn from two different places; phase A and phase B. Does that mean that we can simply add the magnitude of the two currents, I1 and I2, to get the resulting current? Not exactly. Because these two currents are 120 degrees out of phase with each other, the resulting current is something less than twice the current in one of the phases. The mathematical solution to the addition of these two out of phase sinusoidal currents is a little bit complex, but the simple answer is that they add up to 1.73 times the magnitude of any of the phase currents. continued on page 34 60 PLSN March 2008 www.PLSN.com
Entertainment Technician Training Seminar in the Tropics 2008 Presented by ETCP Recognized Trainer Richard Cadena June 2-4, 2008 in San Jose, Costa Rica • Three 8-hour Training Sessions • Eco-Sensitive Travel Excursions The Rainforest or Beach is Optional — The Fun & Learning is NOT Enroll in Richard Cadena’s Seminar TODAY. Visit: http://plsn.com/university 24/7 access to 1,000s of companies The EPD is used year-round by: Event Producers Touring Shows Corporate Producers Rental Companies Production Managers Fairs & Festivals Promoters Production Companies Production personnel Facility Managers Personal Managers Producers Basically, anyone who is involved in live event production will use the EPD. www.EPDweb.com 2008 MARCH PLSN 61
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