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| M.A.H.Y.
Khoory & Co. Trading
> FAQs
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1.1
Electric motors
function on the
principle of magnetism;
where like poles
repel, and unlike
poles attract.
In
a simple motor,
a free-turning
permanent magnet
is mounted between
the prongs of
an electromagnet.
Since magnetic
forces travel
poorly through
air, the electromagnet
has metal shoes
that fit close
to the poles of
the permanent
magnet. This creates
a stronger more
stable magnetic
field. (The electromagnet
functions as the
stator, and the
free-turning magnet
is the rotor.)
Fluctuating polarity
in the electromagnet
causes the free-turning
magnet to rotate.
The poles are
changed by switching
the direction
of current flow
in the electromagnet.
The
direction of current
flow can be changed
in one of two
ways. In a DC
motor, connections
must be interchanged
at the battery.
AC current oscillates
on its own.
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The
stator in an AC motor
is a wire coil, called
a stator winding.
It's built into the
motor. When this coil
is energized by AC
power, a rotating
magnetic field is
produced.
When
a magnetic field
comes close to a
wire, it produces
an electric current
in that wire. This
is called induction.
In induction motors,
the induced magnetic
field of the stator
winding induces
a current in the
rotor. This induced
rotor current produces
a second magnetic
field necessary
for the rotor to
turn.
Induction motors
are equipped with
squirrel rotors,
which resemble the
exercise wheels
often associated
with pet rodents
like gerbils. Several
metal bars are placed
within end rings
in a cylindrical
pattern. Because
the bars are connected
to one another by
these end rings,
a complete circuit
is formed within
the rotor.
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Consider
this close-up of a
2-pole stator and
one of its rotor bars.
Alternating current
flowing in the stator
causes the poles to
change rapidly, from
north to south and
back again. If the
rotor is given a spin,
the bars cut the stator
lines of force. This
causes current flow
in the rotor bar.
This current flow
sets magnetic lines
of force in circular
motion around the
rotor bars. The rotor
lines of force, moving
in the same direction
as those of the stator,
add to the magnetic
field and the rotor
keeps turning.
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1.2
Electric
motors convert from
50 to 95% of their
input energy to
output power, generating
mechanical rotation
much more efficiently
than gasoline engines.
Overall
efficiency, of
course, varies
among electric
motors, depending
on the size and
special characteristics
of each motor.
Motors under one
horsepower typically
fall within a
50 to 75% efficiency
range, whereas
larger gasoline
engines normally
convert only about
25% of the chemical
energy they consume
into useful mechanical
work. Diesel engines,
on the average,
are around 40%
efficient, while
most natural gas
engines approximate
37% efficiency.
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1.3
Motor
bearings should
be lubricated only
according to the
manufacturer's schedule,
except in cases
where the motor
is unusually noisy
or hot. Never over-lubricate
a motor. Excessive
lubrication potentially
does more harm to
the motor than a
lack of lubrication.
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Follow
all instructions supplied
with the motor. Maintenance
schedules normally
appear on the motor
nameplate or terminal
box cover. They may
be included on a separate
instruction sheet.
The type of bearings
installed in the motor
determines the method
and frequency of lubrication.
When a motor's bearings
require frequent service
but the motor is inconveniently
located, proper maintenance
is often neglected.
Therefore, the intended
mounting position
of the motor is an
important factor to
consider when selecting
a bearing type. |
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Most
modern motors have
a tight bearing housing
that stores a generous
volume of lubricant.
Dirt introduced into
the bearings during
the lubrication process
causes more bearing
malfunctions than
the lack of lubrication.
Too much grease packed
into the bearings
causes excessive heating.
Also, surplus lubrication
often seeps into the
motor, collecting
dirt and causing the
insulation to deteriorate.
Motors rated at or
below 10 horsepower
normally have pre-lubricated
bearings designed
to allow long periods
of operation under
normal service conditions
with no bearing maintenance.
The bearings in these
motors should never
be lubricated. |
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1.4
When
a motor is rewound,
the motors efficiency
decreases. A standard
rule of thumb is
to use a reduction
in efficiency of
2% for a rewound
motor. However,
reduction in efficiency
from rewinding motors
can be as high as
20 to 25%. Several
rewind rules of
thumb are typically
used for comparison
of motors:
1.
Never rewind a
motor damaged
by excessive heat.
2.
Replace motors
that are less
than 100 horsepower
and more than
15 years old.
3.
Replace previously
rewound motors.
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If
rewinding costs
more than 65% of
the price of a new
energy efficient
motor, buy the new
motor. This decision
yields increased
reliability and
lower operating
costs. |
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1.5
When
a motor has a significantly
higher horsepower
rating than the
load it is driving,
the efficiency of
the motor is notably
reduced. There are
substantial numbers
of motors presently
operating in this
fashion. There are
several common reasons
given by plant managers
for this:
1.
To ensure against
motor failures
in critical processes.
2.
Plant staff did
not have the correct
size on hand and
used the next
larger size.
3.
Plant staff did
not know the actual
load and selected
one they knew
would do the job.
4.
To ensure capability
to accommodate
future increases
in production.
5.
Original production
requirements were
reduced, leaving
a grossly oversized
motor in place.
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Cost
penalties associated
with using an oversized
motor can be considerable.
These include:
1.
higher purchase
price,
2.
more costly electrical
supply equipment,
3.
increased energy
costs due to lower
efficiency, and
4.
higher costs due
to lower power
factor.
Because
efficiencies of
most motors, especially
smaller horsepower
sizes, decline
rapidly at less
than 50% loading,
it is generally
a good idea to
evaluate downsizing
any motor that
is less than 50
percent loaded.
Replacing grossly
oversized motors
with accurately
sized energy-efficient
motors can yield
considerable energy
cost savings.
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1.6
A motor
has failed. It needs
to be replaced quickly.
Operations are shut
down. Do we simply
order one of the
same, or is this
a good opportunity
to use an energy
efficiency replacement?
And, if so, just
what is the economic
feasibility and
payback? |
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1.7
This
example evaluates
the replacement
of an operable standard
efficiency motor
with an energy efficient
motor. Since the
current motor has
not failed, no action
is required to keep
the operation running.
Any
energy cost savings
from a replacement
motor, then, must
pay back the entire
purchase cost
of the new motor,
minus any salvage
value of the current
motor. From a
simple payback
point of view,
this decision
generally represents
a low economic
opportunity.
However,
there are many
instances where
replacing operable
standard efficiency
motors with energy
efficient motors
can be economically
delightful. Motors
approaching their
useful life, extremely
low efficiency
motors, motors
that are grossly
oversized, and
motors that may
have been rewound
in the past are
all excellent
candidates.
Since
the motor has
yet to fail, another
major advantage
is that replacement
can be scheduled
during non-production
times.
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1.8
All fan
and pump applications
share the characteristic
of fluid flow. The
rate and amount
of flow through
a pipe, duct, damper
or valve, depends
upon an imbalance
of pressure across
the pumping device.
This imbalance is
called pressure
difference.
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For
instance, in order
to force water through
a garden hose, the
pressure from a
water-faucet must
be greater than
the pressure at
the open end of
the hose. Pressure
in a water system
originates at the
pump. When the faucet
valve opens, the
pressure created
by the pump forces
water to move out
of the piping system.
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There
are certain physical
laws that describe
these pressure differences
and flows; they are
called the Affinity
Laws. In short, what
these laws state is
this:
Secondly, the torque
of the pump will vary
as the square of the
change in speed. If
the speed increases
by 25%, the torque
would increase by
56%.
Lastly, and of the
most interest to our
examples, is the law
that states: horsepower
is proportional to
the change in speed
cubed. Considering
a 50% reduction in
required speed, the
laws yield an 87.5%
decrease in horsepower
requirement to meet
the different speed
requirement.
It
is this law that
makes variable speed
drives economical
in applications
where varying flow
rates exist.
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1.9
Electromagnets
are similar to permanent
magnets, but produce
much stronger magnetic
fields. Electric
motors require this
extra capacity.
To
make an electromagnet,
an iron rod is
wrapped with insulated
wire. The rod
is called a "core".
Electric
current flows
through the wire
when it is connected
to a battery.
This current magnetizes
the iron core.
Once magnetized,
the core has both
"N"
and "S"
poles. The poles
of an electromagnet
can be reversed
by changing the
direction of current
flow.
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When
one or both ends
of the wire at the
battery are disconnected,
current flow stops
and the core loses
its magnetism.
Alternating
current changes
directions on
its own, causing
the poles in the
electromagnet
to switch.
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1.10
Three
phase AC power is
comprised of three
independent voltages.
Each phase is displaced
120 degrees from
the others.
When
phase one (A)
is at zero volts,
phase two (B)
is near its maximum
voltage and flowing
in the positive
direction. The
third phase (C)
is near its maximum
voltage as well,
but flows in the
negative direction.
These three phases
will change from
positive to negative
as the AC power
cycles. A rotating
magnetic field
is produced if
each of the three
phases is connected
to an electrically
independent winding
in an AC motor
stator.
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In
this example, using
time 1 as our reference
point, the current
flow in the green
phase A winding
is positive and
pole A1 is north.
The opposite pole,
A2 is magnetically
south. The resultant
magnetic field is
shown moving from
north to south.
The
current flow in
the blue phase
B winding is negative,
so pole B2 is
north and B1 is
south. The resultant
magnetic field
is shown flowing
from B2 to B1.
There is no current
flow in red phase
C, so these poles
are not magnetized.
They are neutral.
The result is
that there is
no magnetic field
being produced
in this winding.
These magnetic
fields produce
a rotating force
in the direction
shown by the arrow.
This arrow represents
the turning of
the rotor.
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Moving
to time 2, the red
phase C current is
negative going, thus
poles C1 and C2 are
south and north respectively.
Their blue phase B
current is positive
going and poles B1
and B2 are north and
south, respectively.
Because the green
Phase A is at zero,
the A poles are neutral.
The arrow represents
rotation in the direction
of the magnetic field.
Finally, at time 3,
we see that the green
Phase A is positive
going and the red
phase C is negative
going. Their respective
poles are energized
with the resultant
magnetic fields producing
a continuation of
the rotating magnetic
field. This force
is what creates the
motion of the rotor.
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AC
power cycles 60 times
per second between
positive and negative.
In a fraction of a
second, the phases
have shifted 60 degrees
causing the relationship
of the north and south
poles to change at
the same rate. Because
the motor has established
an induced magnetic
field, the opposite
fields of the rotor
and stator attract
each other, causing
the rotor to follow
the stator's magnetic
field change. |
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As
the rotor continues
to follow the stators
magnetic field, the
three phases will
shift yet another
60 degrees. It is
this continuous change
in polarity that causes
the rotation of a
motor.
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1.11
Totally
Enclosed Non-Ventilated
(TENV) motors have
no vents or openings.
The enclosure of
this motor limits
the exchange of
air in the motor,
but is not airtight.
TENV motors have
no cooling fan,
and so rely upon
convection of heat
in the motor frame
to surrounding air.
These motors operate
satisfactorily in
dirty, damp, or
oily environments,
but not in hazardous
locations.
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