[SI-LIST] Re: threshold and 60 Hz
- From: art_porter@xxxxxxxxxxx
- To: si-list@xxxxxxxxxxxxx
- Date: Mon, 4 Aug 2003 07:42:04 -0600
A couple of interesting, informative, and entertaining books on this topic have
come out recently:
"Empires of Light: Edison, Tesla, Westinghouse, and the Race to Electrify the
World," by Jill Jonnes
"Executioners Current: Thomas Edison, George Westinghouse,and the Invention of
the Electric Chair," by Richard Moran
If you thought advertising, and corporate behavior in general, was egregious
today!
Art Porter
-----Original Message-----
From: Wyland [mailto:dcwyland@xxxxxxxxxxxxx]
Sent: Thursday, July 31, 2003 6:59 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: threshold and 60 Hz
A little more trivia. 60 Hz was chosen mostly for motor reasons, perhaps
the most important use of electricity. .
The 60 Hz number yields an 1800 RPM synchronous speed for a 4 pole AC
motor. (60 Hz * 60 seconds/minute * no of poles/2) Higher frequencies
give higher speeds. This speed (actually 1725 under load) is the
standard speed for almost any AC induction motor you find, such as in
washing machines, pumps, etc. 1800 RPM is a good working speed. Most
useful things seem to happen between 120 and 1800 RPM.
60 Hz also yields nominal transformer sizes, in pounds (and dollars) of
iron per volt-ampere. Higher frequencies use less iron. Europe uses 50
Hz. Their motor speed is 1500 RPM nominal, and their transformers are
20% larger than ours for equivalent rating. Parts Canada (and I think
England) used 25 Hz for a while. However, the transormers were large and
the frequency was low enough that you could see the lights flicker -
annoying.
In WW II, airplanes used 400 Hz as their nomnal AC frequency to reduce
the weight of the iron. This would reduce the weight to 60/400 = 15% of
the 60 Hz value. The down side is that the 4 pole motor speed is now
12,000 RPM! Fortunately, the primary use of the AC was for electrical
and electronic equipment, not motors. The only motor use I am aware of
was for small, high speed cooling fans where it could be made to work.
400 Hz would probably not be good for general purpose use on the ground
because you would need a lot of poles on any motor to get the shaft
speed to any reasonable value for most applications. Many poles = higher
cost because of many independent windings to be tied together. Also,
iron hysteresis losses increase with frequency. This would result in
increased losses due to iron heating on all motors and transformers.
So, 60 Hz is a most useful and optimally painful frequency, as would be
predicted by Murphy's Law.
Dave
Jacobson, Karl wrote:
>60 Hz is a particularly dangerous frequency of AC, which is our standard =
>wall socket value.
>
>Threshold voltages for dry skin vary greatly from wet skin which in turn =
>is much higher than a direct contact under the skin, as with a needle or =
>something like that in contact with interstitial or cellular material. =
>The current that can become dangerous is also varies with voltage, and =
>especially location. =20
>
>Skin effect from a lightning strike has been survived many times, you =
>could say.
>
>Karl
>
>-----Original Message-----
>From: Robert Szalapski [mailto:Rob.Szalapski@xxxxxxxxxxxx]
>Sent: Wednesday, July 30, 2003 6:54 AM
>To: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: threshold
>
>
>The threshold voltage is very different for AC & DC voltages, as was
>demonstrated by Thomas Edison. He electrocuted many dogs in an effort =
>to
>prove that AC was only good for killing; AC currents are far more lethal
>than DC. (Some claim that these experiments gave birth to the idea of =
>the
>electric chair.) I believe that, for AC, the dangerous level is around
>70-80 V, so typical household wall sockets can be dangerous; 110 V is a
>rather odd choice from that perspective.
>
>-- Rob
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx
>[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of hariharan
>Sent: Wednesday, July 30, 2003 8:44 AM
>To: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] threshold
>
>
>what is the threshold Voltage (V) and current (amps) beyond which human
>beings can get hurt or feel it?
>
>any idea...??
>
>
>
>
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