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Re: Power factor correction for transformers
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Robert B. Bonner
Guys,
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My first job out of my undergrad program was with a building automation (energy) company. We installed PFC caps on buildings all over the place. At that time I wasn't the engineer doing this, but I was around it enough. There's formulas for balancing the act.. How much capacitance to add etc depending on the current PF and how many HP of motor load... PFC is used because Electric motors exhibit a HELACIOUS Inductive load on the mains while converting to work. In an inductive circuit the voltage leads the current by 90 degrees. This inductive load tosses the power company out of SYNC. They are busy trying to match things up at the power plant. They generate 3 phases of power. The load on the generators' phases needs to be balanced. A way off power factor screws the system. If everybody allowed the PF to get away from them you'd have a real mess... The idea is to make the current flow run in sync with the AC sign wave. Commercial power is sold with a base rate for so many KWH, the demand (how fast you use it) and a premium penalty for power factor varying from 100%. I worked with an injection molding plastics factory. The big injection machines had very large motors and that place's PF was way the heck out there. Instead of correcting the building.. We installed smaller correction banks on each machine (about 40 of them at that time) as you can have too many caps installed and shift the PF the other way. We generally don't measure PF in residential as there isn't a huge supply of big motors throwing things off. Some areas with demand limiting of energy conservation do-do demand metering and control. None I've seen here do PF measuring at the residential level. For the Super Big guns that are running 100 AMP primarys with three phase power supplies doing PFC will just make the power to the primary smoother and your power company happier. I don't think it is worth the expense for correction caps. Either way I would install them on the amplifier not the building. BOB DD -----Original Message-----
From: ham_amplifiers@... [mailto:ham_amplifiers@...] On Behalf Of craxd Sent: Friday, November 17, 2006 6:22 PM To: ham_amplifiers@... Subject: [ham_amplifiers] Power factor correction for transformers I looked through about every book I have on transformers and power supplies, and never found anything about using a cap for power factor correction. There's plenty about using a cap with an AC motor, but nothing about transformers. The Standard Handbook For Electrical Engineers only show adding them to motor circuits or circuits feeding motor loads. Nothing under the transformer section. The only power supplies to use a power factor correction cap was a few switching power supplies. It was used after the rectifier and before the choke though. It was never placed in the AC line. I did see some series regulated supplies in transceivers that had a cap across the secondary in a bridge rectifier supply, but it was for a filter. Their values ran from 0.001 uF to 0.01 uF. Nothing of any size. The only way to cut back on magnetizing current is to use more iron in the core lowering its flux density. The more iron for the same amount of turns, the current drops. I researched magnetizing current in C-core Hipersil (or M-6) transformers some time back, and seen they had a good bit more magnetizing current than most EI cores using M-19 steel. The reason being is they run Hipersil from 15 to 17 kilogauss. M-19 and M-22 are ran from 14 to 10 kilogauss. Over 15 kilogauss in Hipersil, the current really shoots up. The way to cure this is to have a transformer wound with the same number of turns, but with a larger core area in either a C-core or an EI core. One would have to tell the winder to use a lower flux density of say 12 to 10 kilogauss using a C-core with Hipersil or M-6 for a low magnetizing current. M-19 for an EI core may be a better choice if available as it will be a good bit cheaper. The core loss isn't much greater than M-6 either. The links below go to several webpages and a couple of PDFs on the subject. power_optimizing_singlestage_power/ Best, Will Yahoo! Groups Links |
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