Transcript power - Faculty

ENTC 4350
BIOMEDICAL
INSTRUMENTATION I
POWER DISTRIBUTION

The power company, in providing
electricity to the home, hospital, or
laboratory, uses alternating current (AC)
instead of direct current (DC) because of
the ease with which AC can be
transformed from one voltage to another,
• For example, from the high voltage of the
transmission lines to the 115 volts of the
home.

This is done with a device called a
transformer.
• Note that the power company uses high
•
voltages at low currents for the long-distance
transmission of electricity in order to hold
down IR losses.
Whereas lower voltages—115 or sometimes
230 volts—are used for safety reasons in
places where people live and work.

At this point, you are probably dying with
curiosity to know what the differences
are between AC and DC power.

The following voltage-versus-time
signals in are all considered DC or direct
current, because the voltage does not
cross the zero axis.

In general, all signals that do not cross
the zero axis are considered DC,
• even though they might vary somewhat with
•
time.
Where we have a regularly varying AC
voltage superimposed on a DC voltage, we
will call it DC with an AC component.

In an AC or alternating current signal, the
voltage is first positive and then
negative.
• For every positive peak, there is a negative
one of the same size and shape.
• It is the type of signal that is most conveniently
transformed to higher or lower voltages.
Voltage Indicators
Vp
Vrms
Vpp
Vrms = Vp · .707 (Sine wave)

For AC waveforms, we speak of its
frequency, f.
• This frequency is just the number of cycles
the voltage makes each second;
• Each cycle is composed of one complete positive
peak and one complete negative one,
• That is, the voltage must go up, come down, and
come back up to zero again to complete one cycle.
Frequency and Period
Period, T
f1( t )
f = 1/T
w = 2pf

The unit most commonly used for
frequency is the hertz (Hz).
• Named after Heinrich Rudolph Hertz, a 19th
•
century German physicist.
One hertz is simply one cycle per second,
• 60 Hz is 60 cycles per second.

We will spare you any funny stories
about how EEs came to call cycles per
second “hertz,” because it only hertz
when we laugh.

There might be lots of other AC
frequencies about, but 60 Hz is what is
used for power in the United States, and
thus it is the most common frequency.
• It takes exactly 1/60 of a second for one
complete cycle, which is the same as saying
that there are 60 cycles per second.

An apparent problem arises in talking
about AC.
• Namely, what voltage value should we use?
• It is positive at one time, negative at another
time, and zero in between.
• This was a real hassle for Thomas Edison, and he
never quite got used to the idea of AC power.

We now have a simple answer to this
problem, though the mathematics by
which it was obtained are complex.

We begin by noting that it would be convenient
to have our equations such as W = I2R and I =
VR yield the same results with either DC or
AC.
•
•
With DC, there is no question about what values to
use for V or I, because when they are given, they are
either constant or vary so slowly that they may be
assumed constant.
With AC, however, this is not the case, and after some
mathematical manipulations, it was decided that the
so-called root-mean-square or RMS values would be
used.

The RMS value is obtained by
multiplying the peak value of the voltage
or current by 0.707.
• If the peak voltage is 163 volts, for example,
the RMS value is 115 volts

The next question may be, how does the
use of RMS values make our electrical
equations come out all right?
• This RMS business may seem like some kind
of a trick, and it is, but it makes our equations
work.

Consider two 40-watt light bulbs, one
operating on AC and the other on DC.
• The brightness B of each bulb is a function of
the current: B = kI2,
• where k is a constant.


When the bulbs are equally bright, we
can assume that the AC and the DC
currents are equal.
The power output of the DC bulb is:
• W = IV.
• If W = 40 watts and V = 115 volts, I must he
0.348 amp.


If this is the current through the DC bulb,
what is the current through the AC bulb?
Since the bulbs are equal in brightness,
they must have equal effective currents.
• It follows that I(DC) must equal I(AC).

In this case, the value of I(AC) is 0.348 amp
RMS.
•
•
The peak value of the current is about 0.5 amp, but
this is academic.
The RMS value of 0.348 amp works in our equation
because 0.348 amp times 115 volts (RMS) equals 40
watts (of any kind).
• The peak values of 0.5 amp and 163 volts will not work,
because the product of these would give about 82 watts,
but the bulb in fact would burn no brighter.

Wattage or power is the same, then.
whether AC or DC is used, so long as
the RMS values of the AC voltages or
currents are used in its computation.
• Note, too, that the concept of resistance,
when applied to resistors, stays the same in
either AC or DC.
• The point is that the AC resistance of a resistor is
the same as its DC resistance. and there is no
such thing as RRMS.
RMS: Root-Mean-Square
* RMS is a measure of a signal's average power. Instantaneous power delivered to a
resistor is: P= [v(t)] /R. To get average power, integrate and divide by the period:
Pavg= 1
1
R
T
t0+T
[v 2(t)]dt
2
= (Vrms)
2
R
Solving
for Vrms:
Vrms=
1
t0+T
2
[v (t)]dt
T
t0
t0
* An AC voltage with a given RMS value has the same heating (power) effect as
a DC voltage with that same value.
* All the following voltage waveforms have the same RMS value, and should indicate
1.000 VAC on an rms meter:
1.733 v
1.414 v
Waveform
Vpeak
Vrms
1
1v
1v
Sine
1.414
1
Triangle
1.733
1
Square
1
1
DC
1
1
All = 1 WATT
ENTC 4350
POWER DISTRIBUTION
The Generator or Electricity
Pump

The power plant has a big coil of wire built into
a complex device called a generator.
•
When the coil is rotated through a magnetic field, the
electrons in the coil move in response to the magnetic
force.
• We could say that the “electron pressure is increased at
•
one end of the wire and reduced at the other.
If the coil is attached through wires to a light bulb. which
we call a load, the current flows out of one end of the
wire (the high-pressure side) and back to the lowpressure side through the load.

We may note that the same type of
situation exists with the heart.
• The heart does not produce blood:
• The heart just raises the pressure so that the
blood can flow through the body.

Similarly, our friendly electric company
does not produce electricity;
• The copper wires are already full of electrons.

The power company raises the electron
pressure so that the electrons will flow
through the wires to the hospital to
provide
• power,
• heat, and
• light.

Moving the wire through the
magnetic field raises the
electrical pressure (voltage)
at one end of the wire and
reduces it at the other.
•
Notice that all the electricity
always comes back to the
generator.
•
•
The power company sells it at
high pressure and gets it back
at low pressure.
All the coal, oil, or water power
they use is needed just to push
the coil through the magnetic
field.

The power company uses not one, but three
wires or coils that follow each other
through the magnetic field.
• At any given instant, when one coil is at high
•
pressure, the other two coils will be at a lower
pressure.
At every instant of time, then, there will be at
least two coils that are not at zero voltage.

The output from all three coils is carried
through the city as three phase power.
Ground and Neutral

The generator has three output wires
(one from each coil) plus a fourth wire
called ground.

Ground is exactly that.
• It is a big, copper plate buried in the earth at
the power plant.
• One end of each of the three coils is connected to
the copper plate.
• This will insure that all the electricity that the power
company sends out comes back home again.

The points to remember are
• First, the power company distributes
•
electricity by means of this system of three
hot wires plus one ground wire, and
Second, each of the three hot wires can
deliver power directly to a load.

Most homes or hospitals use, the three-wire
system converted into the more familiar hot,
neutral, and ground configuration.

The ground wire is an important part of
the hospital safety program.
• It is the return pathway for any electricity that
might leak out in a defective appliance.

You may be curious as to how the three
hot wires and one ground wire turn into
the one hot, one neutral, and one ground
wire.
• But simply note that
• the neutral is the normal path by which electricity
moves back toward the power plant, and
• the hot and ground wires are effectively the same
as shown before.


We note that eventually all four wires
arrive at the hospital via power poles or
underground cables.
On the power poles, the ground wire is
carried at the top of the pole in the hope
that if lightning strikes, it will hit the
ground wire and go into the earth.

Power is moved across country through
transmission lines at a higher voltage
than that used in the home or hospital,
which is done to save money.
• Delivering electricity at high pressure is
economical because the loss in the wires is
reduced.
• Nature discovered the idea and used it first.

The advantage of a high P and a low Q
lies in the fact that the pressure drop P =
QR.
• Thus, for a given resistance R, we can reduce
•
the pressure drop by using a small value of Q.
This effect appears again in the heat
production equation, W = Q2R.
• Once again, for a given R, we can hold down the
value of W by going to a smaller value of Q.

Nature has taken advantage of the
previous relationship by making the
arteries of a strong, tough material that
can withstand high pressures.
• Because the blood flow in the arteries is quite
fast, only a limited number of arteries


In the vascular system, the number of veins is
far greater than the number of arteries, and the
venous area is greater than the arterial area.
This makes sense if we note that the power
output of the heart is given by the equation
•
•
Wo = PQ.
This means that we can reduce the flow Q as long as
we increase the pressure P to keep the product PQ
the same. Q. are needed.

With the veins, however, it is another
story.
• The pressure is low and the flow is slow, and
• a large venous area is required.
• The veins do not have to be as strong as the
arteries,
• which is why nurses prefer to “stick” a vein instead
of an artery.


The power company, then, transmits
power between cities at high voltage
(300,000 volts) to save money on copper
wire.
At the hospital, these voltages are
reduced to 230 and 115 volts for safer
application.


Electricity comes in through the panel board
(which is usually in the utility closet), then it
goes through fuses or circuit breakers, and it
finally passes through wires behind the walls or
ceilings to the receptacles.
The receptacles are where you actually plug in
to get the electricity.
•
Notice that the power comes in through the hot lead
and goes back to the power company via the neutral
lead.
• Of
course, the electricity only flows if something is
plugged in and turned on.


It may help if you think of the hot wires
as arteries and the neutral wires as the
veins.
Blood flows from the heart to the arteries
and returns to the heart via the veins.
• The situation with electricity is much the
same, only the names are different.

A three-wire, female receptacle is depicted,
into which we are going to plug a two-wire
appliance (a heater).

We can think of the electricity as flowing
out of the hot side of the receptacle,
through the appliance, and back to the
power plant via the neutral lead.
• If there are one or more appliances connected
to the various outlets, there will be a
significant current flow in the neutral wire.


This in turn means that a voltage must exist;
•
no flow occurs without a pressure to push it along.
From this, we may conclude that the neutral
wire is not the same as ground, electrically
speaking.
•
•
Neutral is the line through which the low-pressure
electricity flows back to the power plant.
The ground wire is at the potential of the earth.

In modern buildings,
you will find threewire or grounded
outlets.
•
The openings are
arranged as shown.
• Note the unusual
shape of the ground
connections
•
This enables us to
identify the ground
connection.

The ground wire is what makes the difference
between shock or no shock if the appliance is
defective.
•
If all home or hospital appliances were guaranteed to be
always perfect, no ground wire would be needed.


The defective heater has been plugged
into a grounded receptacle and a person
has one hand on the heater and the
other on a metal sink.
The sink is connected to earth ground by
means of the water pipes.
What happens to the person?

Nothing!
• Some of the electricity flows out of the hot
side of the receptacle, through the
appliance, and back to the power plant via
the neutral wire, but the electricity that
leaks off into the defective appliance goes
back through the ground wire.
• The
nurse is quite safe, but if there were no
ground wire, the result might have been much
different.

The electricity that leaked out through
the defect follows the path of least
resistance to whatever ground it could
find.
• Without an instrument ground, the person with
a hand on the well-grounded sink might
experience the shocking revelation that she
was part of the path—provided she lived to
realize that fact.