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
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.
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
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.
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.
"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.
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.
On Wed, 02 May 2007 03:51:38 GMT, in message , "Homer J Simpson" scribed:
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.
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