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.

13 thoughts on “How Pulse Width Modulation works in a VFD

  1. 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

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