1. If we use 230v,50hz for a load say 100kw. the current for this supply will be small when compared with 110v,60hz. but the cable size for 110v,60hz is large due to insulation
size requirments. and most interesting thing is that, only the cold countries will use 110v,6ohz supply because the heat dissipated due to these large current will acceptable to their climatical condition but not for india.. so the supply is depended upon the country climatical condition
2. Date: Thu
Apr 26 16:08:13 2001
Posted By: Dave Lawrence, Staff, Microelectronics, Lawrence Consulting
Area of science: Science History
The short (and not so interesting)
answer to your questions is this: the voltage and frequency chosen for
commercial and residential power standardization for a geographic region
(country, continent, etc.) is very distribution are somewhat arbitrary. But
once they're chosen, hand side of the road. The choice doesn't matter very
much, but agreeing on important. It's kind of like having everyone drive on the
right or left the choice matters a lot!
That said, would any frequency and
any voltage be as good as any other? And the answer is no. Let's consider the
reasons for this by looking at what's good
and bad about high and low voltage. From there we can zero in on a
voltage range that's low enough to
avoid the bad things about being high and high enough to avoid the bad things
about being low. Then we'll do the same
thing for frequency. As most everyone knows, really high voltage is
dangerous--deadly dangerous! So for safety reasons in a home, you want voltage
to be as low as possible.Well then why not make it really low, say a volt or
two, like a battery? The reason involves the relationships between electric
voltage, current, resistance and power, so let's review them. When current
flows through a wire, the wire heats up as electrical energy is turned into
heat. This is how electric stoves, toasters, hairdryers etc. work. If enough
current flows, the wire can even glow, giving off energy in the form of light.
That's how light bulbs work. The rate that electrical energy is being turned
into heat and/or light is called power and is given by:
P = I*V
where P (power) is in units of watts,
I (current through the wire) is in amps, and V (voltage difference between one
end of the wire and the other) is in volts. So a watt is equal to a volt*amp. A
100 watt light bulb could be designed to be lit with 50 volts and 2 amps or 110
volts and 0.9 amps or 220 volts and 0.45 amps, and so on. But they would be
different bulbs; a 220 volt bulb would be very dim if connected to a 110 volt
outlet.The equation relating the current flowing through a wire (or any object with
resistance) to the voltage applied across its ends is called Ohm's
law:
V = I*R
where V and I are the same as above
and R is electrical resistance in units of ohms. Ohm's law says that for a wire
of given resistance, the more voltage applied across its ends, the more current
will flow.Now if you substitute Ohm's law into the equation for power you get:
P = (I^2)*R (that's, "p equals i squared times
r" in plain English).
Having eliminated voltage, we can now
see that as current increases in a wire, the power dissipated (that is, the
rate that electrical energy is converted to heat and light) goes up very fast,
namely as the square of the current.
That's all the physics we need for
now; let's see what it's telling us. Suppose in your house there's a 10,000
watt electric stove. The equation for
power, P = I*V, says you'll get just as much heat with high current and low
voltage as with low current and proportionally higher voltage. So the stove
could be designed for, say, 10 volts and 1000 amps or 1000 volts and 10 amps;
it should get just as hot, just as fast either way. But it doesn't; the
1000volt*10amp stove gets a lot hotter a lot faster than the 10volt*1000amp
stove. The reason is that the stove isn't the only thing
heating up; the cord from the stove
that plugs into the wall is getting hot too, and that uses some of the power.
And even a big fat copper (in other words, low resistance) cord will get hot
when a lot of current flows through it. That's what the other equation for
power, P = (I^2)*R is telling us. Even with a small resistance, high current
means lots of heat because the current is getting squared! So to keep from
wasting all your power in the cord you want current to be low. But that means
voltage has to be high, which is dangerous. And that's how residential voltage
standards
were arrived at; 1000 volts is too
dangerous, and 10 volts is too inefficient for high-power appliances. The
balance was struck around 220 volts, low enough to be safe and high enough to be efficient with high-power
appliances (like stoves). Nearly all countries (including the US) use 220 volts
as the basic service into the house. In the US we also use 110 for low power
applications, such as light bulbs and electronic equipment, where current is
low enough for power loss in the cord to be negligible. It turns out that these
products can be made a little cheaper, if designed to use lower voltage. For
example, the filament in a 220V/100W light bulb would have to be thinner and
longer (therefore more expensive to fabricate) than the filament in a 110V/100W
bulb of equal life expectancy. This was a bigger issue in the early days of
light bulb manufacturing than it is now, but Americans are used to 110V
outlets, and changing everything to 220V would (for no good reason) scare the
heck out of us! What about frequency! You'll be glad to know that you've just
learned most of what you need in order to see where the frequency standards
came from. Let's follow our same approach and ask: What's wrong with real low frequency? What's
wrong with real high frequency? Then we'll try to see what a good compromise
would be.The lowest possible frequency is 0Hz or direct current (DC). What's
wrong with that? Why not 110 or 220 volts DC? The answer begins with the same reason
we found for using higher voltage with the stove. You just need to think on a
bigger scale. Imagine a big city. It uses lots of electric power, so it's kind
of like a high-power (VERY high-power) appliance! It gets that electric power
from a huge power plant (usually several huge power plants) located tens or
even hundreds of miles away. The power is delivered through wires from the
plants to the city. So think of the city as a big stove, the power lines as a
cord, and the power plants as the wall socket. Our real stove needed 10,000
watts; a big city might use 10,000 million watts (10,000 megawatts). That's a
million times as much power as the stove. If that much power were delivered at
220volts, the current would be more than 45 million amps (I = P/V), and we know that when electric power is delivered
along a wire, high current means lots of power being wasted in the wire
(because P = I^2*R) Now 45 million squared is over 2000 trillion! And remember,
we're trying to get 10,000 million watts to the city. So even if we could keep
R down to a millionth of an ohm (a micro-ohm) efficiency would be only a
littled over 80%:
efficiency = (power to the
city)/(power to the city + power wasted) = 10,000 megawatts/(12,000 megawatts)
What would a 100 mile long, 1
micro-ohm, copper wire look like? The equation for resistance of a copper wire
is:
R (ohms) = (1.5E-6)*Length
/(pi*Radius^2)
where the length and radius are in
centimeters. I'll let you figure out the radius. But it's way too big to be
practical!
Fortunately there's a better way, and
we know what it is: deliver the power (P=I*V) at high voltage and low current
instead of low voltage and high current. If we increase the voltage by a factor
of a hundred, the current could be reduced by a factor of 100 for the same
10,000 megawatts of generated power. But that reduces the power lost in the
lines by a factor of 10,000 (because power varies with current SQUARED) so most
of the 10,000 megawatts could actually get to the city! That seems too easy! And
you've probably figured out the catch: multiplying 220 volts by 100 means
22,000 volts. We don't want that on the utility pole in front of our house--to
say nothing of letting it inside!So here's how it works. For most of the
distance from the plant to your house, power is delivered at tens of thousands
of volts. That would be dangerous if people could get close to it, so the power
lines are suspended on those huge towers that hold them way up in the air. When
the lines get to your house, the voltage is reduced (stepped down) to 220 and
the current is increased (stepped up) from what ever it was to whatever you
need. Actually it's a little more complicated than that because your house
isn't the only place the power is going. So the voltage gets stepped down a couple
of times, first at a sub station, to around a few thousand volts, then at the
utility pole in front of your house to 220. There's only one cost effective way
to "step down" a voltage without wasting power and that's with a
transformer. These are placed on utility poles close to your house. The input
to the transformer is high voltage/low current (from the power station), and
the output is low voltage/high current (to your house). There's just one last
catch; transformers have no moving parts, and use electromagnetic induction.
Without moving parts, there's no such thing as electromagnetic induction with
constant (direct) current! It has to be AC, which rules out 0Hz!
Ok, how about 1Hz? Just kidding! Actually
transformers do get more efficient with increasing frequency, but around 20Hz
they can be made very efficient. By the way "efficient" here means
that the power out of the transformer (to your house) is very nearly equal to
power in (from the power station). The reason for higher frequency has to do
with lightbulbs. At 20Hz, oscillations
in brightness are noticable (and annoying!) even with incandescent light bulbs.
Flourescent bulbs actually go completely on and off with AC, and at 20Hz this flickering
would be extremely annoying! Flickering goes away around 50Hz, the standard
used in many countries. The 60Hz standard adopted in the US comes from the use
of the periodic voltage as a timing mechanism for electric clocks (60 minutes
in an hour, 60 seconds in a minute, so 60 cycles in a second).There doesn't
seem to be any advantage to frequencies above 60Hz, and there are several
disadvantages. They would require that generators turn faster, or have more
parts. In other words, they'd be more expensive. Also, wires with alternating
current flowing through them emit electromagnetic radiation, and it turns out
that the power radiated away (that is, wasted) through this process increases
with increasing frequency (this is exactly the same physics that makes
transformers only work with AC).
Well, I bet you never thought the
answer to your question could be so long! And since you posted it in the
"Science History" area, I should finish with a reference to the
history of these standards. It's a very interesting story of American industry
involving perhaps two of the greatest inventors of all time, Thomas Edison and
Nicola Tesla.
3. The
supply of electric power to our houses from generating stations ismainly
in the form of alternating current(a.c.). However the lossesexperienced along
the path of travel from the central power grid station to the sub-stations and
then on to the distributors arephenomenal. This loss is dependent on the frequency
of the a.c.supply. Along the path there are transformers, transmission cables
andcores. The loss of energy in these parts depend directly on thefrequency
irrespective of whether the voltage is being stepped down or up.Note: Static
hysteresis loss is proportional to frequency. An equationcalled Steinmetz
equation can be employed to arrive at the fact that 60 Hz supply causes more
dissipation of heat and energy than 50 Hz systems. Hence it is not preferred by
many countries. The losses being proportional to the square of the frequency,
is hence very high for 60 Hz systems.
Now to understand
the geographical areas of usage consider this extract,"North American
110-120 volt electricity is generated at 60 Hz.(Cycles) Alternating Current.
Most foreign 220-240 volt electricity is generated at 50 Hz. cycles)
Alternating Current...tape and CD players, VCR/DVD players, etc. will not be
affected by the difference in cycles. IMPORTANT: Voltage converters and heavy duty
transformers do not convert cycles."
Modifying Foreign electricity:
"What You Should Know About Traveling Overseas With Electrical
Appliances" Copyright © 2001 Hybrinetics, Inc.http://www.voltagevalet.com/foreign.html there are no clear advantages of
60 Hz over 50 Hz
except in a few cases like the one on video graphics I have considered below.
Similarly 50 Hz supply which is used in India also is not of much advantage.
The two systems are now standards and a transformer is requried to step up or
step down from 220 to 110V or vice versa. Sometimes the difference between the
two requencies are not significant.In general, the three phase motor at 50 Hz
can run also at 60 Hz, increasing the voltage of 15%, the motor at 60Hz can run
also at 50 Hz decreasing the voltage of 15%, but without the tolerance ± 10%
expected in the version at 50 Hz.
In a motor, Eddy
current losses can be examined as follows, Pe~f**2, i.e. they will increase by
44% from 50 to 60Hz. note: ~ indicates
'proportionality'
For Hysteresis
losses (C.P. Steinmetz Equation/Law): Ph~f**1.6, i.e. they will increase by
33.9% from 50 to 60Hz. The exponent varies from 1.4 to 1.8; however, it is
generally accepted as 1.6. These losses
described above produce heat that has to be removed. Accordingly one might be
forced to conclude that 60 Hz systems are more lossy. However if you consider
the output power and efficiency, 50 Hz systems are at a very slight
disadvantage. More on this can be found
here, an extract: "Pm(50) is the mech power required at 50Hz. Pm(50)=T*wm(50)
where T is the torque and wm(50) is the angular speed at 50Hz. At 60Hz, wm(60)=
(60/50)*wm(50) = 1.2*wm(50) so the motor runs 1.2 times more than at 50Hz....If
the efficiency is constant, then A(60)= (1.2/0.9) A(50)=1.33 A(50)the apparent
power increases of 33%..." Eng-tips Forums: "50 and/or 60 hz motor
running suitability." Initial post by dubairay. Post by Alex68 on Sept 12,
2002.
An article with
reference to effects of frequency on video display can be found here,
Additional Links:
If you are
mathematically inclined, you can see how the results in the first part of the
answer were reached here:
A good paper which
considers the effect of frequency differences on transformers can be found
here,
Another paper on
low frequency (30-80 Hz) trnsformers can be found here
Search Strategy: 50
Hz 60 Hz difference,50 Hz 60 Hz advantage(s)
4.
Subject: Re:
AC Power Supply: 50 Hz or 60 Hz?
From: neilzero-ga on
19 Nov 2002 17:07 PST
|
A very few people are adversely affected by
floresent tubes powered by 60 hz. The number may double at 50 hz, but I am
speculating. Almost everyone is adversly affected by a single floresent tube
powered by 25 hz or lower frequency as it perceived as a stobe light. Two or
more phase shifted floresent tubes reduces this problem. The construction of
the world's most powerful alternator would be more difficult and costly at 60
hz than at 50 hz, but the cost is essentialy the same for alternators almost
that powerful. On the average, motors and transformers for 50 hz are slightly
heavier and slightly more efficient than the same rating for 60 hz and the
motors typically run at 5/6 the speed of 60 hertz moters. This can be either
a minor advantage or minor disadvantage. The pain of electric shock increases
as the frequecy is lowered, possibly the probability of death. Hum is more
dificult to reduce, but the sensitivety
of the human ear to hum decreases with frequency, so that is about a
tradeoff.
5.
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