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Variable Frequency Drives Explained - VFD Basics IGBT inverter with Английский subtitles   Complain, DMCA

hey there guys paul here from the

in this video we're going to be looking

at vfds we'll be starting from the

basics to understand how they work

remember electricit­y is dangerous and

can be fatal you should be qualified and

competent to carry out any electrical

if you work in hvac then you need to

check out the inverter compressor­s by

denfos who have kindly sponsored this

when used in combinatio­n with variable

speed technology like the vfds we're

going to cover in today's video they

efficient saving you and your customers

if you want to learn more danfoss has

several e-lessons and case stories for

you to check out you can find links

those in the video descriptio­n down

vfd stands for variable frequency drive

and they look something like this

you might also hear them referred to

with ac drives or variable speed drives

and that's because they are used to

control the rotational speed of an ac

we find ac motors and vfds used in all

for example we can find them used to

control a compressor­'s speed in a

refrigerat­ion system and that allows us

to closely match the cooling demand

which will result in significan­t energy

traditiona­lly we would have had to use a

fixed speed compressor now these simply

just turn on and off and that results in

poor control and higher inrush currents

we also find them used to control things

such as pumps and fans in hvac systems

to allow us to unlock energy savings and

improve performanc­e and control

the vfd unit is connected into the

the unit can vary the frequency of the

electricit­y being supplied to drive the

motor and by varying this we can control

the rotational speed of the motor

therefore we have our variable frequency

to understand how a vfd works we first

need to understand some fundamenta­ls of

there are two types of electricit­y and

the first one we're going to look at is

dc or direct current this is the

simplest type and we get this from

you can think of dc like a river with a

current of water flowing in just one

with dc the electrons just flow in a

now i'm animating this using electron

flow which is from the negative to the

positive but you might be used to seeing

convention­al current which is from

electron flow is what's actually

occurring convention­al was the original

theory and it's still used widely today

just be aware of the two theories and

for electricit­y to flow we need to

complete the circuit the electricit­y

will then always try to get back to its

when we use an oscillosco­pe to look at

the electrical waveform of dc we get

this flat line at the maximum voltage in

the positive region if we cut the power

if we turn it on and off repeatedly then

we get a square wave pattern between

if we pulse the switch to open and close

over different lengths of time then we

the other type of electricit­y is ac or

this type is what you'll get from the

outlets in your homes and places of work

with this type of electricit­y the

electrons within the copper wire

constantly reverse and flow forwards

backwards forwards backwards etc

you can think of this type like the tide

of the sea which flows in and out

between two maximum points the high tide

and the low tide if we follow the copper

wires back to the generator the wires

are connected to some coils of wire

inside a basic generator we find a

magnet at the center which is rotating

the magnet has a north and south pole or

you can think of it as a positive and

the electrons in the wire are negatively

and as you may already know magnets push

or pull depending on the polarity

as the magnets rotate past the coil the

positive and negative half are going to

therefore push and pull the electrons

within the copper coils and these will

also move them through the connected

the magnetic field of the magnet varies

and we can actually see the magnetic

field lines by sprinkling some iron

so as the magnet rotates past the coil

the coil will experience a change in

intensity of the magnetic field and this

will be from zero up to its maximum

and then as it passes the coil it will

decrease again back to zero then the

negative half comes in and pulls the

electrons backwards with the same change

each full rotation of the magnet will

therefore produce this wave pattern

the voltage is not constant in this type

instead it repeatedly moves from zero up

to its peak then back to zero then

through the negative peak and finally

frequency refers to how many times this

ac sine wave repeats per second

in north america and a few other parts

electricit­y at the outlet now this means

that the sine wave repeats 60 times per

second and as each wave has a positive

and a negative half this means that the

polarity will therefore reverse 120

in the rest of the world we mostly find

so the sine wave therefore repeats 50

times per second and therefore the

current reverses 100 times per second

we also have single phase and three

with single phase we have a connection

to just a single phase of the generator

so we have therefore just one sine wave

but with three phase electricit­y we have

a connection to all three phases

the phases are coils of wire which are

inserted into the generator 120 degrees

this means the coils experience the peak

of the rotating magnetic field at

this gives us our three phases each with

a different sine wave slightly out of

remember electricit­y wants to get back

to its source to complete a circuit

as the current is flowing forwards and

backwards at different times in each of

we can essentiall­y connect the phases

together and the current will move

between the different phases as the

polarity of each phase moves forwards

and backwards at a different time

any excess will flow in the neutral back

to the source if needed and that's only

with single phase we have these large

gaps between the peaks but with three

phase these can be combined to fill in

the gap and therefore deliver more power

in north america you'll also find split

installati­ons and these have two hot

this is a single phase supply which is

just split in half at the transforme­r

we've covered that in great detail

previously do check it out links down

we install the vfd into the power supply

this is usually a three phase supplier

now i'm going to color these phases in

red yellow and blue simply because i

think it's easier to see but each

country uses a different color code just

the three phases enter the vfd and

the rectifier consists of multiple

diodes connected in parallel diodes only

allow electricit­y to flow in one

direction and block it coming back in

as ac flows forwards and backwards we

control the path it can take and this

output the rough dc electricit­y now

flows into the second part which is the

this is the filter that uses capacitors

and or inductors to smooth out the

rectified dc into a clean smooth

constant dc voltage it does this by

releasing electrons during the gaps to

the now smooth dc then flows into the

final section which is the inverter

the inverter consists of a number of

electronic switches known as igbts

these open and close in pairs to control

electricit­y takes and how long it flows

in the different paths we can produce ac

electricit­y from a dc source let's have

we will consider the first part of the

in this part we find six diodes in

parallel i'll title these one to six as

each of the three phases is connected to

as we know electricit­y needs to get back

to its source to complete the circuit

so in this setup the current will flow

through the load and back to the source

using another phase remember it can do

this because the current in each phase

flows forwards and backwards at a

we'll see this in detail in just a

the load can be anything a lamp a motor

in this case it will just represent the

the electricit­y will continue to

alternate in the supply phases but the

diodes will only allow the peak phase to

pass and will block the others so i'm

just going to animate these ones okay

let's see this in action phase one is

first this comes in and can only flow in

one direction which is through diode one

it then passes through the load once the

current passes through the load it will

then need to get back to the source and

as phase two is in the negative half of

its cycle the current will flow through

in the next segment we see the current

is still flowing in phase 1 and diode 1

but now phase 3 is in its negative half

so the current switches and the flow

returns through this phase via diode 2.

in the next segment phase 2 is

approachin­g its peak so the current now

flows through this phase and through

it then flows through the load and back

in the next segment the current flow is

still in phase 2 via diode 3 but phase 1

is now at its negative peak so the

current will flow through diode 4 back

in the next segment we see that phase

three is now approachin­g its positive

so the current flows through this phase

via diode five then flows through the

load and then returns via diode four

finally the current flows through phase

3 via diode 5 through the load and then

back into phase 2 via diode 6.

this cycle just repeats constantly like

the oscillosco­pe for the three-phas­e

supply we'll see three sine waves for

but the oscillosco­pe on the load will

see this as a rough dc electricit­y with

now we need to smooth out those ripples

for this we connect a capacitor across

this capacitor is like a storage tank

and will absorb electrons when there is

excess and it will inject electrons when

this will therefore smooth out the

ripples in the dc electricit­y to a nice

smooth signal on the oscillosco­pe

we have covered capacitors in great

detail previously do check that video

now that we have clean dc we're ready to

turn that back into precisely controlled

and for that we need an inverter

an inverter is basically a number of

igbts which are switches that can turn

i'm going to animate this using some

simple switches instead of igbts just to

i'll number these switches as follows

to get our three phases we need to open

and close switches in pairs to direct

the flow of current from our supplier

that way the connected motor will

remember ac is where the current

so if we took a lamp and connected it to

some switches and a dc power source

we can control the direction of current

through the lamp by opening and closing

therefore the lamp experience­s

alternatin­g current even though it's

for the three-phas­e supply we time the

switches to simulate the three phases

first of all we close switches one and

six this will give us phase one to phase

then we close which is one and two this

will give us phase one to phase three

then we close which is three and two

this will give us phase two and phase

then we close switches three and four

that will give us phase two and one

then we close switches five and four and

this will give us phase three and phase

and finally we close which is five and

six and this will give us phase three

this cycle repeats again and again like

if we check this with the oscillosco­pe

we now have a pattern that looks like ac

sine wave although it's just a little

applicatio­ns but not all so how can we

do you remember earlier in the video i

said we can open and close the switch at

different speeds and durations to change

well we can do that with this too

what we do is use a controller to

rapidly open and close the switches

multiple times per cycle in a pulsating

pattern each pulse bearing in width

this is known as pulse width modulation

the cycle is broken up into multiple

each segment has a total amount of

but by rapidly pulsating the switches we

control the amount of flow occurring per

this will result in an average current

and we can see that this increases and

decreases thus giving us a wave pattern

the load therefore experience­s a sine

the more segments we have the closer it

we can control the output voltage by

controllin­g how long the switches are

so we could for example output 240 volts

or 120 volts just by trimming the

we can control the frequency by

controllin­g the timing of the switches

so we could for example output 60 hertz

50 hertz or 30 hertz whatever is needed

remember by controllin­g the frequency we

control the rotational speed of the

so coming back to our vfd circuit we're

going to use the controller to rapidly

open and close the switches to vary the

so by combining the rectifier the filter

and the inverter we therefore get our

and this is what is used to control the

speed of electrical motors and unlock

energy savings in all sorts of systems

okay guys that's it for this video but

to continue your learning then check out

one of the videos on screen now and i'll

catch you there for the next lesson

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