A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled electrical conductors. A changing current in the first circuit (the primary) creates a changing magnetic field; in turn, this magnetic field induces a changing voltage in the second circuit (the secondary); this is called mutual induction. By adding a load to the secondary circuit, one can make current flow in the transformer, thus transferring energy from one circuit to the other.
The secondary induced voltage VS, of an ideal transformer, is scaled from the primary VP by a factor equal to the ratio of the number of turns of wire in their respective windings:
By appropriate selection of the numbers of turns, a transformer thus allows an alternating voltage to be stepped up — by making NS more than NP — or stepped down, by making it less.
Transformers are some of the most efficient electrical 'machines',[1] with some large units able to transfer 99.75% of their input power to their output.[2] Transformers come in a range of sizes from a thumbnail-sized coupling transformer hidden inside a stage microphone to huge units weighing hundreds of tons used to interconnect portions of national power grids. All operate with the same basic principles, although the range of designs is wide.
Monday, December 22, 2008
Study: Folic acid, B vitamins offer no cancer protection
In the longest-running trial of its kind, doctors found that folic acid and other B vitamins didn't prevent breast cancer or cancer in general, according to a seven-year study of 5,442 women in today's Journal of the American Medical Association.
BETTER LIFE ON CANCER: 25,000 HPV-related cancers reported
Despite the negative results, some women should still take folic acid, says author Shumin Zhang, an epidemiologist at Boston's Brigham and Women's Hospital and Harvard Medical School. Experts recommend that women who are pregnant or of childbearing age take folic acid because it has been shown to prevent neural-tube defects, such as spina bifida.
But Zhang's paper is the latest in aseries of large, well-designed studies that fail to show that vitamins prevent cancer. These same B vitamins also failed to cut the risk of heart disease, according to a study published in JAMA in May.
Last week, the National Cancer Institute stopped a trial of 35,000 men after finding that the antioxidants vitamin E and selenium didn't prevent prostate cancer but could increase some health risks. And last year, a large analysis in JAMAfound that the risk of premature death increased 7% for people who take beta-carotene, 16% for those who take vitamin A and 4% for vitamin E.
"It would be wonderful if taking a pill could reduce our risk of developing any kind of cancer," says the American Cancer Society's Michael Thun, who wasn't involved in the new study. "People are hoping for an easy solution, and that's difficult to find."
The only trial to really prove that supplements reduce cancer risk was done in malnourished people in China, Thun says. In communities where most people have adequate overall nutrition, vitamins don't seem to have much of an effect. That may be particularly true for health-conscious people with the money to afford supplements, he says: "The people who are most likely to take multivitamin supplements are the least likely to have a vitamin deficiency."
Yet Thun notes that vitamins could have had small benefits that Zhang's study wasn't large enough to detect. And because cancer can take many years to develop, even a seven-year study may not have been long enough to measure the vitamins' effects, authors say.
Zhang says she noticed one intriguing trend: Among those taking the vitamins, women who were 65 and older had a lower risk of breast cancer and cancer in general. That makes sense, she says, because older people need more of these vitamins in their diets, although the finding — based on an analysis that broke the data down according to numerous factors — could have been the result of chance. Scientists will have to do a lot more research to fully understand if older women really benefit, she says.
BETTER LIFE ON CANCER: 25,000 HPV-related cancers reported
Despite the negative results, some women should still take folic acid, says author Shumin Zhang, an epidemiologist at Boston's Brigham and Women's Hospital and Harvard Medical School. Experts recommend that women who are pregnant or of childbearing age take folic acid because it has been shown to prevent neural-tube defects, such as spina bifida.
But Zhang's paper is the latest in aseries of large, well-designed studies that fail to show that vitamins prevent cancer. These same B vitamins also failed to cut the risk of heart disease, according to a study published in JAMA in May.
Last week, the National Cancer Institute stopped a trial of 35,000 men after finding that the antioxidants vitamin E and selenium didn't prevent prostate cancer but could increase some health risks. And last year, a large analysis in JAMAfound that the risk of premature death increased 7% for people who take beta-carotene, 16% for those who take vitamin A and 4% for vitamin E.
"It would be wonderful if taking a pill could reduce our risk of developing any kind of cancer," says the American Cancer Society's Michael Thun, who wasn't involved in the new study. "People are hoping for an easy solution, and that's difficult to find."
The only trial to really prove that supplements reduce cancer risk was done in malnourished people in China, Thun says. In communities where most people have adequate overall nutrition, vitamins don't seem to have much of an effect. That may be particularly true for health-conscious people with the money to afford supplements, he says: "The people who are most likely to take multivitamin supplements are the least likely to have a vitamin deficiency."
Yet Thun notes that vitamins could have had small benefits that Zhang's study wasn't large enough to detect. And because cancer can take many years to develop, even a seven-year study may not have been long enough to measure the vitamins' effects, authors say.
Zhang says she noticed one intriguing trend: Among those taking the vitamins, women who were 65 and older had a lower risk of breast cancer and cancer in general. That makes sense, she says, because older people need more of these vitamins in their diets, although the finding — based on an analysis that broke the data down according to numerous factors — could have been the result of chance. Scientists will have to do a lot more research to fully understand if older women really benefit, she says.
Sunday, December 21, 2008
Internal configuration of programmable controller
The programmable controller is composed of electronic circuit with a micro-computer centered, however, it can be equivalently regarded as an integrated body of ordinary relay, timer, counter, etc.
The input relay (X) built in the programmable controller is driven by the external switch through the input terminal.
The output relay (Y) built in the programmable controller is provided with various internal contacts in addition to the external output contact (1a).
Besides, it is incorporated with various types of elements such as timer (T), counter (C), auxiliary relay (M), state (S), coil (F) for function contact (1a).
In addition, these elements (X, Y, M, T, C, S, F) are provided with many electrically normally-open contact (a-contact) and normally-close contact (b-contact), and can be used optionally within the programmable controller.
Consequently, if the conventional relay panel adopts 2a-pushbutton switch or 2-stage/4-notch selector switch, it will be only necessary to use 1a-pushbutton switch or 1-stage/4-notch selector switch to replace such conventional relay panel with the programmable controller.
Thursday, December 18, 2008
Location and fungtion of programmable controller ( PLC A1 series )
The programmable controller ( PC in short ) is operated by the instruction input from the pushbutton switch, digital switch, etc. provided on the operation panel, or sensor input from the limit switch, proximity switch, photo-electric switch, etc. used to detect the operation condition of the equipment, and serves to control the driving loads such as solenoid valve, motor, electromagnetic clutch, etc. and indication load such as pilot lamp, digital indicator, etc.
The transmission of output signal against these input signals is determined by the content of program to be provided to the programmable controller.
The light loads such as small type solenoid valve, pilot valve, etc. can be directly driven by the programmable controller, however, the heavy loads such as 3 phase motor, large capacity solenoid valve, etc. need to be driven through the contactor or the intermediate relay.
Such contactor, intermediate relay, power breaker, etc. are installed in the control panel together with the programmable controller.
The programmable controller will play the important roles as a small type, high-reliability and flexible brain when designing the automated product machining, assembling, transfer, inspection, packing, etc.
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