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Why not 400 Hz AC?

Discussion in 'Electronic Basics' started by Eric R Snow, Apr 30, 2007.

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  1. Eric R Snow

    Eric R Snow Guest

    Reading the post about square wave AC I see that higher frequency AC
    makes for noisy motors and transformers. Is this the only reason not
    to use higher frequency power? It's obvious that smaller motors and
    etc. could save tons of money so it seems that there must be a good
    reason why everybody isn't switching to higher frequency devices.
    Speaking of higher frquency, would it be economical to use inverters
    to raise the frequency at the user's location so that smaller motors
    could be used? I use VFDs on my machine tools but they still drive
    physically large 60 Hz rated motors. I have them mounted in enclosures
    so I don't hear the VFDs singing and the motors don't seem that noisy.
    It is a machine shop though.
    Thanks,
    ERS
     
  2. The losses are higher. 16? Hz power was used for some European rail systems.

    The use of lower frequencies also provided the advantage of lower impedance
    losses, which are proportional to frequency. It was this argument that
    resulted in the use of 25 Hz in the initial generation installations at
    Niagara Falls, in anticipation of long-distance transmission to Toronto.

    http://en.wikipedia.org/wiki/Alternating_current
     

  3. For the same reason 25 Hz was used in mines. Motor speed. Lets see
    you use a VFD on a 400 HZ motor and get any usable torque out of it at
    60 HZ or below.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  4. Ken

    Ken Guest

    Yes, exactly one third of 50Hz we use in scandinavia
    for our rail systems (16000 Volts).

    For long lines low frequency is better. DC is best.
     
  5. Essential for undersea transmission.
     
  6. John

    John Guest

    The optimum frequency depends on the application. Most WWII aircraft
    used a 400Hz power system for the weight savings it gave -
    higher frequency = less iron in generators and power transformers =
    less tare weight = more payload

    John
     
  7. John Larkin

    John Larkin Guest

    Aircraft power systems still run at, or about, 400 Hz.

    John
     
  8. John Larkin

    John Larkin Guest


    At 400 Hz, the speed of a single-pole induction motor would be close
    to 24,000 RPM, which wouldn't be very practical for industrial apps. I
    guess the optimum frequency for generation, distribution, and use must
    be close to 55 Hz, since the US picked 60 and Europe went with 50.

    As noted by others, there were 25 Hz systems, used for big pumping
    stations among others, and 400 Hz for aircraft.

    400 Hz is terrible; it leaks into everything and is very audible.

    We have a VFD drive for a blower on the roof. It's an unfiltered IGBT
    chopper, and radiates immense amounts of emi. We had to hang all sorts
    of toroidal inductors on its outputs to tone it down. The blower on
    the roof still sings audibly.

    John
     
  9. I wound a motor that was originally 4 pole single phase (1720 RPM at 60
    Hz), as a three phase 12 pole motor, which I ran at about 1720 RPM at 180
    Hz. I couldn't make more poles because there were only 36 stator slots. I
    found some three phase 400 Hz induction motors online that were 4 pole and
    ran at 12,000 RPM, which is OK for small motors. I think a 1 HP motor was
    about the size of a one pound can of beans, and not a whole lot heavier.

    It seems that the industry is just resisting what I see as obvious, that
    you can wind a standard motor for a lower voltage at 60 Hz, and use V/F
    drive easily up to 150 Hz or 180 Hz before eddy currents and other losses
    prevail. Better steel and thinner laminations, although more costly, might
    push that to 360 Hz or even maybe 600 Hz. If you can design the rotor and
    stator properly, with something like 72 slots, you could make a 24 pole
    motor that would run nicely at 1800 RPM at 360 Hz. If the losses don't make
    it impractical, you could have a 40 HP automotive motor about the size and
    weight of a lawnmower engine.

    Certainly you could save a lot of weight and volume by using DC to transmit
    power. A 480 VAC motor drive runs on a "link" of about 720 VDC, which is
    just 480 VAC rectified. A drive about the size of a shoebox can produce 3
    phase power of about 5 HP, or 3.75 kW, and you could easily carry two or
    three of them in one hand. An equivalent 50 or 60 Hz transformer would be a
    tough weight for Aarnold to clean and jerk.

    Eventually I would not be surprised to find DC used to power appliances
    directly, with internal 3 phase modules (such as IRAMS from International
    Rectifier) to supply drive power for the motors. Lamps and heaters don't
    care if it's AC or DC, and high end electronics like computers use
    switching supplies which convert incoming AC to DC anyway.

    The problem with DC is that it is harder to switch, and possibly more
    deadly than AC, although 60 Hz is "just perfect" for causing heart
    fibrillation.

    Paul
     
  10. http://en.wikipedia.org/wiki/Utility_frequency

    "Though many theories exist, and quite a few entertaining urban legends,
    there is little certitude in the details of the history of 60 Hz vs. 50 Hz.
    What is known is that Westinghouse in the US decided on 60 Hz and AEG in
    Germany decided on 50 Hz, eventually leading to the world being mostly
    divided into two frequency camps. Frequencies much below 50 Hz gave
    noticeable flicker of arc or incandescent lighting. Westinghouse decided on
    60 Hz before 1892 and AEG decided on 50 Hz by 1899. Tesla is believed to
    have had a key influence in the choice of 60 Hz by Westinghouse. Use of 60
    Hz allowed induction motors to operate at the same speeds as standardized
    steam engines common in the late 19th century.

    However, the first generators at the Niagara Falls project, built by
    Westinghouse, were 25 Hz because the turbine speed had already been selected
    before alternating current power transmission had been definitively
    selected.

    Westinghouse would have selected a low frequency of 30 Hz to drive motor
    loads, but the turbines for the project had already been specified at a
    speed which was incompatible with a generator designed for 30 Hz. Because
    the Niagara project was so influential on electric power systems design, 25
    Hz prevailed as the North American standard for low-frequency AC. A
    Westinghouse study concluded that 40 Hz would have been a good compromise
    between lighting, motor, and transmission needs. Although frequencies near
    40 Hz found some commercial use, this frequency never overcame the
    "installed base" of 25 Hz, 50 Hz and 60 Hz equipment."

    60 Hz is cheaper to use - smaller transformers. 50 Hz is more efficient.

    25 Hz makes lamps flicker - fluorescents are bad - you need two tubes phased
    apart.
     
  11. As do ocean-going vessels. Same benefit from the weight savings, I
    suppose.
     
  12. News to me. In my day they all ran on DC. Now they run on AC and I believe
    many can run 50 or 60 Hz so they can use dockside power in most of the
    world.
     
  13. Well, I was speaking via hearsay. I have been told that the US Navy uses
    400Hz power. Maybe it's because they like to use the same equipment as the
    zoomies. Maybe it's because they have a lot of aircraft and like their
    stuff to be compatible. Maybe I'm up in the night.
     
  14. John Larkin

    John Larkin Guest

    All the commercial ship I've seen were all 60 Hz. Who needs to save
    weight on a ship?

    John
     
  15. msg ID
     
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