# How Pulse Width Modulation works in a VFD

Hello again from KEB America. Today I

wanted to talk to you about the basics of a VFD and how it uses pulse width

modulation to convert a 480 volt 60 Hertz AC input to a variable voltage

variable frequency AC output to a motor. Alright, so this is kind of a general

overview of a basic VFD system. And to go into a little bit more detail, the first

part, we have here is the converter section. That takes your 480 volt AC 60

Hertz input and converts it into a DC voltage. So your DC voltage will be 480

volts – your input – times the square root of 2 (or 1.41) which

leads to 678 volts DC. And that leads us to the next section which is the DC bus.

That is a capacitor bank that allows you to smooth out your DC voltage and allows

for some voltage storage capacity. So your final section is the

inverter section. That has six insulated gate bipolar transistors or IGBTs. And

that takes your DC bus voltage and converts it back into a variable voltage

variable frequency AC output to the motor. And so now we’ll go to a little

bit more detail on this inverter section. So pulse width modulation uses IGBTs

that are switching on and off up to 16,000 times per second to convert your

DC bus voltage to an AC output at the motor. So a typical configuration is

shown here with six igbts three on the top and three on the bottom. Depending on

which are open affects which phases you go through and in what

direction. So in order for current to flow. one of these transistors from the

top and one from the bottom need to be open. So for example, if we start over

here and run through this transistor… …we can go through the U phase down

through the V phase and back out here. So with these two transistors open we go

through the U phase and then through the V phase and then back out to the DC bus.

Alternatively, to show how we can go through the same U and V phase but in

Reverse from V to U, we would go through here, through this transistor, down here,

and so this time we’re going in the opposite direction, through V to U and

then now we go down and out here. So this is just one quick example to show, depending on if this one or this one is open and this one and this one, we can go

through the U and V phase in either direction. Similar configurations hold

going through U, V, and W in any direction. So next we’ll get into how the width

portion of the pulse width modulation affects your RMS voltage to the motor. So

we covered how the pulse portion of pulse width modulation is the switching

on and off of the IGBTs. Next, we’re gonna dive into the width

portion. So because your DC bus voltage is six hundred seventy eight volts DC,

each pulse of the IGBT produces an amplitude of six hundred seventy eight

volts DC. We can affect the RMS voltage on the output by changing how long the

pulse is on and off. So as you can see here we have a longer on period and a

shorter off period, so that produces an RMS voltage with a higher amplitude. And

that’s opposed to this diagram down here where we have a shorter on period and

longer off period, so our RMS voltage will be much smaller. So even though we

have the same 678 volts pulse. By changing the on and off lengths we can

affect how high the RMS voltage the output is. So that’s the variable voltage

portion of the AC output. So on the variable frequency portion what we do is

we keep these on and off ratios the same, but we squish them all together so

it’s happening much more quickly. So we have the same RMS voltage peak, but it’s just occurring much more often. So by changing this variable voltage and

variable frequency output, we can keep a constant torque at the motor output

while getting more power out of the motor. And so that’s kind of a basic

overview of a VFD system and how it uses pulse width modulation to maintain

constant torque, while increasing the power of a motor.

Great explained and at least a good start for newcomers! Thanks Mike!

that was kind of so easy explanation pf the theory…guys voice was also heavy and clear

Jason Bourne explaining some VFD theory…

Good explanation….concept clearing 👍👍👌

easy and quiet explanation thaanks <3

very good

Nice sir

Thank you for explaining it in an easy fashion.

finally!!!!!!!!

No indian….

hi, excelent video, i have a doubt, in some system like VRF conditioner , it's says that the compressor use DC voltage, but in this video you say that is AC voltage after the inverter , so why i can't test the compressor with AC voltage directly without using the inverter module? Thanks and sorry for the trouble

Very well explained

Why Dc Voltage is more that input AC ?

Good lecture!

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