Explanation of how kilowatt-hour meters work (electromechanical)----make money online

hey everyone these electromechanical

watt-hour meters are being replaced

nowadays with digital counterparts but

there's actually quite a lot of

interesting stuff going on inside there

to make those work so I thought we'd

take a look today so I have this set up

with a digital watt meter in line and

then I have a heater over here so if I

turn the heater on you can see that the

disc starts spinning in the watt meter

and I've got a digital meter here just

so you can see exactly what it's drawing

in real time ins is about 1200 watts so

let's start taking apart the meter and

see what's inside there the cover is

marked line and load and if we take this

off we can see that there's four

terminals here and I have it set up like

this so that the neutrals are in the

middle and hots are on the outside here

and then this is a commercial power

meter so it actually has a little latch

here and so if you move the latch out we

can unscrew this glass top and have that

come off and you can see that there's

actually six connections inside there

which are connected in such a way that

only four come out and so let's all

unscrew all of this and then take the

actual meter out

so here are the six power terminals that

I mentioned earlier and two of them are

fairly small and go to this coil on the

top and four of them go to these coils

on the bottom so I'm going to take this

off so we can see exactly what's going

on with the coils so imagine you were an

engineer in the late 1800s and your task

was to design a power meter that could

go on the side of someone's house and

measure how much AC current they've

consumed so what we're actually

measuring here are voltage and current

when they are in phase together one idea

of how to do this is use a separate

coils for voltage and current and put

them together in such a way that they

interact and produce a force that is

proportional to the power being consumed

so this this design has been virtually

unchanged for about a hundred years

which is pretty impressive for any sort

of technology so here's the basic design

we've got the voltage coil on the top

and the current coil on the bottom and

this is what it looks like in sort of a

schematic view where we have a voltage

coil with a lot of turns and current

coils with relatively few turns here in

the middle this is the disk of the meter

and what we'll do is connect sort of an

odometer to the disk so we can count how

many times the disk has gone around and

the amount of force generated by this

magnetic arrangement here will be sort

of accumulated over time so that we have

a running total power usage meter so the

question becomes why have three coils in

this arrangement here why not just have

two like this or one offset or something

like that and the answer is that we need

to have some physical offset from here

to actually produce a net torque in the

disk so if you are in the lab just

playing around and you had a coil let's

say we only had a voltage coil to start

and we plug this into the AC line so

that we had a nice changing magnetic

field coming out of that coil and we

brought it near the disk it's true that

that changing magnetic field would cause

any currents to flow in the disk but no

net torque would be produced because

there's no reason for that for those

eddy currents to be pushed so every time

the magnetic field changes changes

polarity it's true that there will be

eddy currents in the disk that are also

changing polarity and opposing it but

the disk still won't really want to turn

so if you were experimenting with this

we thought well maybe what we could do

is put a current coil here and a voltage

coil here and that way the magnetic

field produced by the current will kind

of interact with the voltage field and

that will kind of pull the disc around

because we've got this offset not quite

it seems like that might work but again

if the voltage and current are in phase

there still won't be a net torque

applied to the disk because the magnetic

fields will both be varying exactly at

the same time and so there won't be any

reason for the disc to turn you can

think about this kind of like the legs

of a centipede where if the creature

just picked all its legs up and then put

them all back down at the same time it

really wouldn't go forward at all it

wouldn't even work to pick all of its

legs up move them forward move them back

and then put them back down again

so coordinating all of its legs at the

same time doesn't work and what we need

to introduce is a phase shift so that

you know in the centipede analogy you

would pick up some of its legs move them

forward put them down and then have this

sort of traveling wave that moves

through its legs this coordinated

movement and what we're really talking

about is a phase shift between the legs

moving up and the legs moving forward

and that's exactly what we're talking

what we need to do here so luckily for

us now that we're living in the age of

AC current there's an easy way to

introduce phase shift with a magnetic of

with a coil system like this and what we

do is we just add a copper ring to this

magnetic pole here and that copper ring

is right here in the actual device so

what happens is is if the voltage coil

is connected up to the line and we have

current flowing through the coil there

will be a magnetic field induced by that

current flow now instead of just coming

straight out into the disk and inducing

eddy currents there

the current or the magnetic field has to

go through this copper ring but the

copper is a conductor and it's not

connected to anything it's just an

isolated ring so what happens is that

magnetic field which is changing and

going through that copper ring induces

its own current in the copper ring so

this effectively consumes the change in

magnetic field but the copper ring will

also create a tag netic field so what we

have here is sort of a delay circuit so

the magnetic field can't just shoot

straight through the copper ring it has

to induce a current in the copper first

and then as that current dies down we'll

end up with a magnetic field caused by

those currents induced in the copper

ring so it's really just sort of a delay

device it's a 90 degree delay so then

you might wonder why is this copper coil

introduce a 90 degree phase change why

is it not a hundred and eighty or forty

two or any other number and the answer

is that when the current flowing through

this coil is at its maximum rate of

change then the magnetic field will be

at its maximum value and the reason for

that is so it's a law of nature I mean

it's basically why sines and cosines are

90 degrees out of phase and all kinds of

other good stuff

so basically what we've done with this

90 degree phase shift idea is create a

moving magnetic field just like the legs

of a centipede that sort of pull the

disc around like this so we have these

waves sort of undulating through here

that caused the disc to move it's

similar is if I take this very strong

magnet and just bring it near the disc

like this and move the magnet around you

can see that it pulls the disc around

because the eddy currents are so strong

so this moving magnetic field causes the

disc to turn because it's inducing

current and then the magnet moves and

that current is then pulled toward the

motion that I pull it so it's in sync

with the magnet this arrangement of

coils with the copper ring produces

essentially the same thing it's

analogous to this the same thing as

moving a permanent magnet along like

this this same principle is

used in what are known as shaded pole

motors so this is a cheap motor that

came out of a window fan or something

like that

and if you look at it it's got iron to

channel the magnetics from the coil here

into the rotor but it also has these

copper wires here and as you can see

they're all just connected to each other

they're not connected to anything except

themselves and they're even isolated

from each other this way so what these

loops of wire are do are introduced a 90

degree phase shift and that actually

helps get the motor started in this case

we only need that phase shift to get the

rotor spinning and once it's spinning in

a in one direction then the geometry of

the motor will take over and produce

torque but when you first apply power to

this the motor doesn't know which way to

go because the power is single-phase

so these shaded poles produce a phase

shift which gives it that sort of

centipede leg action to sort of push

this thing around in the right direction

the term shaded pole just means that

we've added a loop of wire to cause this

phase shift to happen so there's a

couple other key bits to this meter now

that we've applied a force to the disc

that's proportional to the amount of

power being consumed we have to have

some way of slowing it down too so in

our current system with just this part

the disc would just spin faster and

faster and faster until it reached its

mechanical friction limit I mean it's we

have nothing to actually oppose the

motion that we're putting into it so the

way that we slow it down in a controlled

way is with permanent magnets so these

these large pieces of iron in the front

here are permanent magnets and they're

set up so that they're the gap is the

the disc is in the gap so as this spins

those eddy currents are induced again

and it will oppose the motion of the of

the spinning so as you can see there's

very little tendency for this to turn

after I let go lucky for us the amount

of braking applied with this system is

proportional to the speed of the disc so

if we put the curve of torque produced

by our power

our meter against the opposing torque

produced by this eddy current break we

have a nice linear system that's very

easily set up to measure power like this

and you can see it's marked F and s for

faster and slower and what this screw

does is it moves the magnet in and out

radially so if it's further away it will

have more effect on the torque of the

disc because it has a bigger lever arm

it's farther away from the rotation axis

and if it's closer and it'll have less

of an effect so the meter has a couple

other interesting features one problem

is that the even though we've done our

best to reduce the mechanical drag in

this system where we have you know a

needle bearing and magnetic suspension

and all that kind of stuff there's still

some magnetic drag that we need to

overcome and since our magnetic setup

only produces torque when we have you

know the magnetic fields delayed when

written when the person is actually

drawing power then we're not actually to

be charging them for the full amount

because we're giving up that mechanical

drag so the motor has this interesting

bit of copper here which is asymmetric

like it's going in one direction on the

disk and I believe this is supposed to

siphon off some of the magnetic field

from the voltage coil and introduce an

additional little phase delay so that

there's always some torque on the motor

essentially what we have here is the

shaded pole motor without any sort of

current sensibility basically just to

produce some torque but then we have the

problem what if a customer notices that

their meter runs a little bit even when

they're drawing zero power that would be

a problem for us to at least legally so

what we do is we put a little hole here

and this is called the anti creep hole

and when that hole comes around into the

area where all these magnetics are going

on there's slightly less disk for it to

operate on since there's a hole there

it's just air and the hole won't turn

through the through the magnetic section

will actually draw out a small amount of

torque and the disk will stop rotating

when that hole comes around so

essentially what we have here is a

system that compensates from mechanical

drag and also doesn't overcharge the

customer or at most it will chart it

will charge them for one rotation and

then stop they aren't using anything and

then of course we have this super high

ratio reduction gear train so that all

the rotations of the disc get divided

down to very very small rotations of the

meters on the dials on the front

I've also often wondered what that

pattern is on the disk there's sort of a

stippling pattern pressed into the

aluminum disk and I always thought oh

that must have something to do with the

eddy currents or whatever but I think

that's really just for to make it easier

to see the disk spinning or to make it

more physically rigid or something also

this particular meter has an unusual

current coil configuration so it has one

coil that is symmetric and then another

current coil that is asymmetric so as

you can see there's kind of only one

turn on this side there's actually a few

turns on this side and you know the line

goes through this one and the year or

the neutral goes through the middle and

the hot goes through the outside but I

have absolutely no idea why you would

want an asymmetric set up quite like

that it might also be something to

produce a very slight amount of torque

in the static case just to overcome

mechanical friction or something but if

you know please let me know because I

can't I haven't figured that out ok see

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