fans what would we do without them even
before we figured out that air is a
fluid we've been pushing it around at
our whimsy
and thanks to electric fans things are
better than ever
they keep us and our things cool by
speeding up the transfer of heat energy
they make your voice all funny when you
talk through them
and if they're old enough they are a
clear and present danger to anything
with fingers
yes the world's a better place thanks to
fans
fans it's what's for dinner but the
common electric fan
has a depending on who you ask mildly
perplexing to
intensely irritating design quirk you
see
most fans run at multiple speeds most
commonly three
and nearly all of them have a speed
selector switch with the order of
off high medium low
why are they in that order it may seem
more logical for the speed to increase
as after all things like volume knobs
and dimmer switches tend to increase
intensity the more you turn it
but fans they're like the opposite
why well there are two basic reasons for
this
the first is that this is just how it is
and we're all used to it by now so why
bother changing it
and the second more real answer is that
this protects the motor inside the fan
and
increases customer satisfaction by
reducing the likelihood of a stall
condition
what's a stall condition i'm glad you
asked
electric motors are weird i mean they're
also amazing and wonderful
but they're also weird at their core
it's really just some sort of magnet
that we figured out how to make go all
spinny
then we put some sort of output shaft on
the spinning magnet and we can make
other things spin
like paddles which push air forward and
thus create a constant stream of moving
air
or the little dish at the bottom of your
microwave or a train or a car or really
anything at all
but getting the magnet to spin isn't as
easy as you might think
obviously we can create a magnetic field
that attracts something to it
coils of wire make excellent
electromagnets but we can't simply
create a magnetic field around a magnet
and get useful motion
sure it will attract the magnet to
itself but once it's done that
well then it won't spin will it it'll
just sort of move a bit and then get
locked in place so we need the magnetic
field
itself to spin then the magnet in the
core of the motor is being attracted to
a continuously moving target and not a
stationary one
there are a whole bunch of ways we can
do this and we don't even need to use
magnets at all in fact we usually don't
instead the spinny bit which i should
probably add is called the
rotor can itself be another
electromagnet or
if we want to get real fancy can be made
of steel and copper and thanks to our
friend electromagnetic induction
fields from the stator induce currents
in it and it becomes its own
electromagnet attracted to the very
fields which created its attraction
in the first place and yes i'm
oversimplifying
so anyway to make a motor spin we need a
rotor which may or may not contain a
permanent magnet somewhere which is
attracted to
a spinning magnetic field induced by the
stator which surrounds it
that's the stationary electromagnetic
bit now this is all well and good except
for one thing
household electricity doesn't spin
it just pulses three-phase electricity
does spin though
grid scale electricity generation and
transmission is done
as three-phase power that's why you
always see transmission lines in
multiples of three
we do this because thanks to math and
stuff it allows us to transmit more
power with a given amount of conductive
material
going from two wires to three allows you
to transmit triple the amount of energy
and that's economical right there the
three phases are staggered 120 degrees
apart from one another
each phase on its own is just pulsing on
and off but in sequence they chase each
other just like the lights on a movie
theater's marquee
if your name is tesla or ironically
ferraris no relation
and you design a motor with a
three-phase stator and ensure they're in
the correct order
you'll get natural rotation of the field
thanks to the chasing effect of the
phases
lots of big industrial motors are built
like this and so are the motors in
modern electric cars
with a motor on the grid it will have a
fixed speed thanks to the fact that the
electric grid
itself has a fixed frequency but in an
electric vehicle the motor is hooked up
to some fancy power electronics which
turn the dc power from the battery
into three-phase ac power at whatever
frequency and voltages you like
this is called a variable frequency
drive and they're not exclusive to
electric vehicles and in fact in evs
they're usually not called that
but that's a story for another time
wasn't this video about
fans ah well i promise we're getting
there
three-phase power is generally not found
in the domestic environment
the closest you'll typically get in the
u.s anyway is in a situation like an
apartment building
where the building has a three-phase
supply and there might be things like
elevators or large hvac units with
three-phase motors in them
but you'll only have two of the three
phases in your personal breaker panel
and large appliances like electric
stoves will be put across two of them
giving you 208 volts rather than the
standard 240.
our electrical system in the u.s is
weird but that's a story for another
time
so anyway with only a single phase you
ain't getting rotation
you're just getting power unless a
magnetic field which pulses on and off
and changes polarity with each cycle now
you can design a motor to work with that
just fine
the universal motor is one such design
here
a commutator and brushes are used to
provide its own
spinning field by switching the polarity
of that field as the rotor which
typically is itself
also an electromagnet spins therefore
always creating attraction in one
rotational direction
they even work on dc power which is why
they're called universal
but the carbon brushes which carry
current through the commutator
wear out over time they're not usually
super efficient they create
a lot of electrical and often audible
noise there's usually some
arcing going on and so they're typically
only used in certain applications which
don't run them for long periods at a
time
like power tools or food processors for
something like a fan which may run for
days at a time you'll want to use an
induction motor these don't have brushes
that wear and will generally last as
long as their mechanical bearings do
and you can make an induction motor work
with a field that isn't rotating
because the rotor itself rotates keep in
mind that from a spinning motor's
perspective
the stator is what's spinning so the
electromagnetic field itself
is two even if it's not rotating from
our global perspective
but of course when you first turn on the
fan
it's not spinning and this is
bad see the amount of torque an
induction motor produces is a function
of its slip slip describes the
difference in speed between the rotating
magnetic field and the spinning rotor
the rotor actually has to be moving
through the field produced in the stator
in order for it to react against it and
produce mechanical force
and this means that single phase
induction motors are not
self-starting if both the field and the
rotor are stationary
no torque is produced in contrast a
stopped
three-phase induction motor will
immediately produce torque when it
receives power
because the field is rotating around the
stationary rotor
and that means that the rotor is
consequently moving through it
even when it's stopped from a standstill
the rotation of the field around the
rotor will induce the necessary currents
in it for it to start moving
and will accelerate until it's almost
but not quite to the same speed as the
rotating field
it never reaches synchronous speed with
the field because then it would not be
slipping at all
and no torque would be produced and this
is exactly the reason a single phase
induction motor cannot start itself
there is no slip when nothing's moving
so now you've got a problem
how do you get the rotor to start
rotating
well you cheat and create your own
rotation somehow
there are a ton of different ways to go
about this you can use capacitors you
can use
shaded poles you can use a separate out
of phase
start winding which gives a temporary
kickstart you could just give it a push
the specifics don't really matter but
what does matter is that you've got to
generate a rotating field at least
temporarily
to produce enough torque to get the
motor going after which point the
rotor's movement relative to the stator
produces the necessary currents within
the rotor's windings to keep itself
going and we're happy
and yes i am still oversimplifying we'll
talk about fleming's rules and lenses
laws when you're older
one of if not the simplest ways to
create rotation with only one phase
is with a shaded pole motor let's look
at one
it's a good thing i still haven't put
the selection accumulator back in the
jukebox
because it's got one of these
now this is a c frame motor these look a
lot like transformers and indeed operate
a lot like transformers but they're
actually decepticons instead of another
coil on the other side of the iron core
for
converting voltages it's got a squirrel
cage
rotor this is the bit that has currents
induced in itself producing
electromagnetic fields which react
against the fields created in the stator
which in this type of motor is the hole
in this side of the core but that's not
what's important right now
instead it's these little bits of copper
these are called shading coils and their
job is to react to the flux
created by the field coil over here and
slightly delay
its phase in these two locations
electromagnetism is weird and
complicated and
i'm getting deeper than i need to really
as always but here's the upshot
these coils break the uniformity of the
magnetic field produced in the stator
causing a very slight delay in flux in
these two locations
which will in effect produce a bit of
rotation in the field overall
thanks to that shifting in time these
shaded poles provide just enough
rotation in the field to get the rotor
spinning from a standstill
and once it has enough speed the
induction currents in the rotor become
quite substantial as it moves through
the bulk of the pulsing field
and now it really takes off so what does
this have to do with fans and their
switches
well everything you see the shading coil
isn't really that great of a way to
start a motor
it works but the amount of starting
torque it creates
is quite small they are after all
completely passive components
sort of stealing a bit of the output
from the stator
that weak saw starting force means that
this style of motor is really only
suitable for loads like
well a fan where the force required to
start it spinning is relatively low
and the load increases as rotational
speed does
to start it really just has to overcome
static friction but
once it's spinning it can really induct
and that's the main reason that high is
typically the first
setting next to off while a shaded coil
motor isn't
super great at producing starting force
it's cheap and
entirely passive you don't need a
component like a capacitor or a
centrifugal switch to disable a start
winding
you just give it power and it goes but
the amount of starting torque you get
depends on what speed the fan is set to
the different speed settings are usually
accomplished with different windings
within the motor that produce varying
amounts of power
and thus torque and since the shaded
pole affects all the windings equally
on lower speeds you get proportionally
less starting torque
making the first selectable speed high
will ensure that the motor will
at least briefly produce the maximum
starting torque it can whenever it's
switched on
that way it's more likely to
successfully start
this hand chopper is nearly a hundred
years old
and it too has its switch in the order
of off high
medium low now it's probably got a
shaded pole
induction motor that design certainly
isn't exclusive to c-frame motors but
this motor is
literally an antique and i would rather
not take it apart to find out for sure
anyway since i can just easily reach out
and
grab the fan blades i can demonstrate
how weak the starting torque is
it's surprisingly easy to hold the blade
back
even set to high this little fan moves a
lot of air on high
and that obviously produces much more
force than it is exerting on my fingers
right now
this is seriously much weaker than you
might expect
now if i dared to reach in and grab the
blade while it was spinning
i would regret it but also i'd find it
would be producing much more torque than
it is
right now because well that's just how
induction motors work
but you don't want to find a motor in
this stalled condition
why well because current is still
flowing through the stator windings when
it's stalled
i mean you can hear it and in fact
there's more current flowing through it
right now than would be if it were
spinning
this fan uses about 42 watts when it's
on high and right now with the motor
stalled it's using more than
70. that's getting dissipated as heat in
the motor's windings
and before too long it could burn itself
up if the fan switch were designed with
low
in the first position you could turn on
the fan and find that the blades didn't
start spinning
this would not only be annoying but
could hurt the motor
and in a fan like this potentially start
a fire
now to be clear modern fans are required
to tolerate being stalled for indefinite
periods of time without burning your
house down
after all you could have left it on low
had a power interruption
and not get the benefit of the starting
kick
new fans will either have some sort of
thermal protection or simply have a
stator design which can't possibly pass
enough current through it to get
dangerously hot
still though you don't want a stalled
motor it's bad
and also just annoying now i should add
that it's not like most
fans can't produce enough starting
torque on low if i have this set to low
and then plug it in
it gets going just fine but it doesn't
hurt to play it safe
and fun fact i have encountered a
ceiling fan
on a fan control which was in fact
stalled on
low i had switched it on and you could
hear it humming
but it wasn't turning i needed to
increase the speed to medium which
produced enough torque to get it
spinning
and then i can move it back to low and
it turns out that these modern
electronical smart fan controls i
installed because i couldn't help myself
are designed to prevent that very thing
from happening
by always initially switching the fan
too high
this here is my biggest fan and i last
had it set to low so when i turn it on
it will go back to that same setting
but for about 2 seconds the fan control
gives it full beams to make sure it
starts spinning
and then it switches to low power
pretty neat huh in fact these fan
controls are themselves pretty neat just
because they're bypassing the fans
internal speed controls and creating its
own and i should also add that ceiling
fans often use capacitors as part of the
speed control schema
really there are just a gazillion types
of electric motor out there and you can
lose your mind trying to keep track of
all of them
and the ways we control them but that's
a story for another time
now it could very easily be argued that
this high comes first positioning isn't
really required anymore
after all how frequently would a fan
motor stall anyway
at least one like this and you could
argue it was never required
anyway i mean this thing's a hundred
years old and it starts fine even on low
and there could very well be fans that
don't use shaded pole motors in fact
there are certainly exceptions out there
and even fans which do in fact put low
first
whether that's because they use a
different motor design or just because
somebody did the math and realize we've
been worried about a very unlikely thing
is unclear but one thing is certain the
fan switch isn't
or at least wasn't designed like this
just to annoy you there is in fact a
method behind this madness
when you take an already not great
method of starting a motor and weaken it
with
lower power settings it doesn't hurt to
give it a kickstart first
maybe at this point it's more tradition
than it is actual need
as a matter of fact i've had this very
fan on a separate switch for years
with it always set to low and it never
once stalled
it clearly produces enough torque on its
own to get going in any speed
and it's almost a century old but the
fact that these modern fan controls
always kick the fan on high from a stop
is i think a sure sign that putting high
first
is very intentional besides even if
stalling weren't
likely well how else are you supposed to
know how many times you're gonna need to
pull the ceiling fan chain to get to the
speed you want
[Music]
there's nothing with the delivery of
that line which i didn't particularly
like
did that say i better record that
because
asked might have sounded a little
different the three phases are st
with a motor on the grid
i should silence that shouldn't i then i
should remember to do that before i
start moving
anyway we're good we'll be back there's
usually some arcing going on and
so they're typically not used in most
only used
not used only used that's fine
that's fine just go ahead and that's
fine
if i have this set to low and plug it in
but i need to turn the quarter in for
design to prevent that very thing
but from happening
[Music]
[Applause]
[Music]
[Applause]
you