Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

AC current through AC equipment

  1. Jan 27, 2015 #1
    I wonder if AC current follows Sin wave and go through negative and positive cycles alternately, why don't AC equipments, like a bulb, get off and on - off and on....
  2. jcsd
  3. Jan 27, 2015 #2


    User Avatar

    Staff: Mentor

    The filament in incandescent bulbs takes time to cool and warm, the 60 Hz is generally too fast to show as heat variations. But the old fluro tubes flicker 120 times/sec, you can see this if you quickly swipe a pen or ruler through the air in a room lit by one. A lot of AC equipment converts the AC into smooth DC to overcome the bursts of power.
  4. Jan 27, 2015 #3
    Hmm....but whats with waving pen in air...???
  5. Jan 27, 2015 #4
    And what is the point of having AC in the first place? Why dont we use DC for everything...??
  6. Jan 27, 2015 #5
    In principle that could produce a stroboscopic effect.
    Biggest problem with DC (especially in the past) is stepping it up to higher voltage. HV is necessary for a long distance transmission.
  7. Jan 27, 2015 #6


    User Avatar

    Staff: Mentor

    Try it.
  8. Feb 5, 2015 #7
    we can not use DC current because it may be to high for some equipment . and do not forget on AC current can be steped up or steped down
  9. Feb 5, 2015 #8


    User Avatar
    Gold Member

    If we still used DC, your electric bill would be some where around $10,000 per month....instead of the $50 or $100 that it is now with AC.

    History lesson back in late 1800's. Edison loved DC......Tesla invented AC and loved it. These two guys hated eathother.

    In the end, Tesla's AC dominated, and electricity was the greatest invention of its time....still is today.

    AC allows for the use of transformers. Transformers short in DC and essentially useless at steady state. With AC, Transformers allow you to raise or lower voltage, which also lowers or raises amperage accordingly.

    The biggest challenge in transforming electricity is transporting the amps down the power line. Amps are amps....1000 amps requires a certain size wire at a certain cost.
    5,000 amps requires a much larger wire....therefore much higher cost in aluminum, copper or whatever. You will also have higher costs in the towers that hold the heavier wires.

    Since Power = Voltage X Amps...

    From the power plant, they sometimes hike the voltage up to 500,000 volts.....therefore they amps remains somewhat low thre relatively small while delivering the same power thru workable transmission wires.
    By small, I mean around an inch in diameter....or so.

    DC voltage back in the day was set more around the under 500 volt range, hence you needed huge wires and a power plant every mile or whatever. This is huge cost to customer and is essentially useless today.

    However, DC is great in batteries. Like for example, a DC battery starts your car. Computers love it as well!!

    AC motors rule the world now and even in high voltage applications because of the new Variable Frequency Drives. (soft start instead of full load amps at start up)
    Last edited: Feb 5, 2015
  10. Feb 5, 2015 #9
    I recently had a discussion with someone around renewable energy sources and their drawbacks.
    On the topic of solar power, I mentioned the issues around converting the DC solarpower into AC in the grid and started to wonder. Would it be practical to build a house with both AC wiring and a DC wiring.

    That way you have the solar panels supplying the DC lines, with some method to "top up" the power req's from the AC lines if needed.

    Seems like a large portion of consumer electronics switch the AC to DC anyways and if we can skip a conversion step or 2 there might be efficiencies to be had :)
  11. Feb 5, 2015 #10

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    A friend of mine did it. He has a12 VDC light in every room and outlets for RV appliances where practical.
    Swiches for DC lights are mounted high slightly above head level so you dont hit them accidentally at night.
    But if power goes off in the middle of the night and you're groping for a switch, it's the only one up there.
    It's for emergency. Golfcart battery and trickle charger power it, no solar - no way will you beat electric company's cost per kwh .

    Fine for LED lights, radio and TV

    Your big energy users are heating and cooling appliances, like water heater, airconditioner, fridge, clothes dryer, and the "sanitize" cycle on dishwasher.
    I once saw an antique 32 volt toaster made for the old Delco rural windmill electric house system.
    Considering that at 32 volts you need 31 amps per kw which dictates about #8 or #10 wire,,
    and that toasters, coffeepots, microwaves, vacuum sweepers and laundry irons are all about 1 kw loads,
    i just don't see low voltage DC being practical except for DIY'ers like my friend..
    Were i to put a five pound #8 power cord on wife's hair dryer i'd have to hide out for a week.

    If you're living off-grid
    where you can heat and refrigerate with a flame
    and heat water by solar collector
    your proposal makes a lot of sense.

    There may come a day where rooftop solar electric panels can help a suburban homeowner
    but i dont think it's here yet.
    Last edited: Feb 5, 2015
  12. Feb 5, 2015 #11


    User Avatar
    Science Advisor

    High power DC circuits need special switches, fuses and breakers to operate safely. It's possible to have a DC system in a house but once you get above about 50vdc to reduce wiring sizes it becomes very important to engineer it correctly. You can't normally find the correct parts at the local big box stores so it will cost more.

  13. Feb 6, 2015 #12
    I hav a question if we can use a sola panel as a power source is the eny thing that we can use to store the energy from the sun rather than using battry bank's ?
  14. Feb 6, 2015 #13


    User Avatar
    Science Advisor

    You can use the power to heat and/or pump water. It's possible to use a preheat tank as a diversion load for solar electric systems. People take a small AC electric water heater and install a DC heating element to warm water before it goes to the main hot water heater system. It's not as efficient as direct thermal solar heating of water but if you have lots of excess power from panels it's better than nothing.
  15. Feb 6, 2015 #14


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    If only there were a very efficient and cheap way of storing Electrical Energy directly. The only way to make use of the large amounts of relative cheap energy that can be generated off-peak, is to use pump storage (electric pumps to take water into a high reservoir and then hydroelectric generators to provide the electricity when needed). It's inconvenient, not very efficient and you need some mountains with valleys fairly close to your power stations and city.
    One solution to the Energy 'problem' is surely just to use less of it and to make fewer consumer products. That just doesn't suit the modern way of economics, though.
  16. Feb 6, 2015 #15
    Technology of supercapacitors made quite a progress in last 10 years or so. However, it is still quite behind lithium batteries in terms of a specific energy. While molten salt batteries are even better regarding specific energies, their obvious drawback is a demand of a high operating temperature. But I seem to remember there were some reports about progress as concerns later in recent years. Still not enough. Maybe one day.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: AC current through AC equipment
  1. AC Current (Replies: 4)

  2. AC through an inductor (Replies: 21)

  3. AC current (Replies: 8)

  4. AC Current source (Replies: 10)