can anyone explain how this VFD diagram works? especially the IGBT - Electrician Talk - Professional Electrical Contractors Forum
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Old 05-22-2016, 12:22 PM   #1
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Default can anyone explain how this VFD diagram works? especially the IGBT

this VFD turn AC to DC, and then turn DC to AC, it helps to change the frequency of the AC power source

but can anyone explain it step by step how it really works?
especially the IGBT
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can anyone explain how this VFD diagram works? especially the IGBT-qq-20160522111631.jpg  

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Old 05-22-2016, 12:49 PM   #2
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An IGBT is a transistor. A solid state switch. A control voltage turns the switch on and off very rapidly, controlling the frequency of the current going to the motor.

For a more in-depth look at transistor operation, consult a book on solid state physics.
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Old 05-22-2016, 12:51 PM   #3
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http://www.vfds.com/blog/what-is-a-vfd

Basically, it's a rectifier -> filter -> inverter.
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Old 05-22-2016, 01:52 PM   #4
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Quote:
Originally Posted by InPhase277 View Post
An IGBT is a transistor. A solid state switch. A control voltage turns the switch on and off very rapidly, controlling the frequency of the current going to the motor.

For a more in-depth look at transistor operation, consult a book on solid state physics.
I suggest "Competing for the Future". The guy that wrote that book is a physicist that worked for RCA on the first semiconducting lasers.

His book has covered all types of transistors and how they work in chapter 1, down to how they are built and why they are designed that way.
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Old 05-22-2016, 04:52 PM   #5
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IGBT = Insulated Gate Bi-polar Transistor.

These can switch 1000A (DC) at 1000V a million times a second ( or some such ) even though they are not much bigger than your fist. (!!!!)

They can do this with astounding efficiencies -- any other switch would glow and melt down.

Direct current exhibits IMPEDANCE -- just like AC -- except its impedance only exists in the first moments after a switch is opened or closed.

It's the manipulation of these decay curves that permits voltages to be stacked in a logically controlled manner. (Feedback loop controls output.)

Throw in a clock that runs in the gigahertz... and you have a way of building and shaping pulses of DC power that can be added up ( Fourier series ) to create any wave form you might want.

VFD rated motors have robust insulation that can tolerate the quirks of such inverter circuits. ( harmonic peaks & wave distortion )

All of this switching is happening so fast that 60 Hz looks like glacial tempo.

{ 1,000,000,000 Hz versus 60 Hz -- note the difference. }

The symbolic diagram you've posted is a dramatically simplified squib of the actual circuits used. You wouldn't be able to follow them at all. The actual circuits are designed by computer -- not by any man, BTW.

These devices are first designed as virtual devices -- running inside a computer program. Once the values are optimized -- then more software is used to spit out the actual circuit diagrams.
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Old 05-22-2016, 05:33 PM   #6
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First, I never knew there were IGBT that operate at the 1000V+ range. Time to do a little reading on that.

Secondly, I had no idea that DC exhibits impedance so I need to read up on that as well.

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The symbolic diagram you've posted is a dramatically simplified squib of the actual circuits used. You wouldn't be able to follow them at all. The actual circuits are designed by computer -- not by any man, BTW.
He's probably in school; that's the diagram they give you in a basic Motors and Controls class. At least, that'e the exact one that was given to us.
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Old 05-22-2016, 07:12 PM   #7
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Adding to other excellent comments.

There are two functions:

The first is to take the in-coming AC line and convert it to DC.

The second is to chop the DC into a PWM waveform or quasi-sinewave to drive the load.

The first step is required because the output devices (FETs or IGBJTs) need a fairly steady DC supply to work with. These devices, unlike SCRs/TRIACs, cannot handle the sign reversal of the in-coming AC waveform. Most FETs and IGBJTs have body diodes that conduct in the opposite direction of the device in the on state. So you can see that these devices will be on for half the cycle of the in-coming AC no matter what you do!! Even if the body diode was not present, the reverse breakdown of FETs and IGBJTs would not be high enough. SCRs/TRIACs need the sign reversal because they automatically turn off which makes the control easier.

The second step is now commonly implemented with FETs and IGBJTs because they switch much faster than SCRs/TRIACs which gives faster control response and reduced switching losses. Of course the little problem with AC mentioned above needs to be considered.

SCRs/TRIACs aren't going away anytime soon because they fit well in cost sensitive applications because they don't need DC power. If you cruise the IXYS Semiconductor website, you will find FETs up to to 1200V, IGBJTs up to 4000V, and TRIACs up to 2000V.

Neat stuff.

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Old 05-23-2016, 03:43 AM   #8
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IGBT- as the base region is insulated (Not complete insulated, leaky currents are still there ) it can operates at high power. Its frequency response is limited (LF )as the parasitic capacitance limits the operation at high frequencies.
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Old 05-23-2016, 03:41 PM   #9
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So mike883, were you wanting to know how an IGBT itself operates, or were you wanting to know the function of the IGBT in the VFD?
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Old 05-23-2016, 04:02 PM   #10
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Correct me if I'm wrong, but, don't those diodes form a 3phase full wave bridge rectifier?

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Old 05-23-2016, 04:43 PM   #11
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You need no correcting.
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