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Anonymous Poster #1

Variable Frequency Drive. Energy Consideration

03/15/2017 1:55 PM

I have been having problem understanding About VFD. Do we have to give external supply of energy other than the input supply so as the VFD can work or not? For example if the input is 220V and 60 Hz and output is something, then does VFD has to be supply through an external source of energy which drives the circuit? And also can anybody please explain in layman terms about the working of VFD and what exactly are they and their uses are? Thanks.

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#1

Re: Variable Frequency Drive. energy consideration

03/15/2017 2:00 PM

A1) No.

A2) No.

A3) There is an article in Wikipedia on the topic, and <...their uses...> are to control the speed of electric motors.

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#2
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Re: Variable Frequency Drive. energy consideration

03/15/2017 4:43 PM

I think I smell smoke

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#15
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Re: Variable Frequency Drive. energy consideration

03/16/2017 4:32 AM

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#18
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Re: Variable Frequency Drive. energy consideration

03/16/2017 11:11 AM

VFD is certainly able to release the other required energy in the form of smoke (usually in phase smoke where there should be none).

One can even take a single phase source, bypass the 3 phase live sensing, and use a synthetic three phase PWM output to drive a small 3 phase motor, though there is no 3 phase connection anywhere in sight, or in site.

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#3

Re: Variable Frequency Drive. Energy Consideration

03/15/2017 5:15 PM

Layman's terms, no math.

There are 4 parts to a VFD: Rectifier, DC Bus, Inverter and Control System.

  1. AC is rectified to pulsating (rippled) DC in the rectifier, basic VFDs are all passive in that regard, a simple full wave diode bridge rectifier.
  2. Pulsating raw DC from the rectifier is connected to a DC bus where there are DC chokes and/or capacitors to smooth out the ripple, because the transistors in the next section require very precise turn-on times, and any ripple in the DC affects that. Not all drives use DC chokes, those that don't must then use more capacitors. As a gross general rule, Asian based drive designs use caps only, because the same Asian companies that control the world production of caps are also VFD mfrs. The rest of us use chokes so that we can buy fewer caps from the same people we compete with. We have to buy the transistors from them already, that's bad enough.
  3. The inverter is a set of transistors fed by the (now smooth) DC bus and connected to the output terminals to feed the motor. The inverter transistors fire a series of DC pulses into the motor circuit, first in one direction, then in the other, simulating AC. As the current flows through the inductive circuit of the motor winding coils, in current rise of each pulse is subjected to the "inductive time constant" that says that current cannot change instantaneously in an inductive circuit. So as each pulse is fired, the current rises at a predictable rate and before it gets too high, the pulse is turned off. Then it is turned back on, but the current has not yet begun to fully fall, so it picks up where it left off and starts to slowly* rise again. By manipulating the timing of the pulses and the width of the gaps between them, the inverter is able to control the RMS (average) voltage getting to the motor within each cycle of power. Then because the inverter also controls how often the transistor pairs change from firing on the Positive Rail to the Negative Rail, the inverter can recreate an AC sine wave that is as varying voltage and varying frequency.
  4. The VFD control system takes the user information, such as the commanded speed, and translates that into a PWM (Pulse Width Modulation) algorithm to control the firing of the transistors. it also protects the motor, informs the user of what's going on, control I/O and a host of other details. Some VFDs power the control system from the line terminals with an AC-DC power supply, others power the control system from the DC bus with a DC-DC power supply (there are pros and cons of both).
  5. A motor creates torque based on the ratio of voltage and frequency it was designed for. Changing the frequency alone will change the speed, changing the voltage alone will change the torque it can produce. A soft starter changes the voltage only, so it reduces torque, hence the term "soft". But you cannot do that continuously, because the lack of torque will cause the AC motor to increase slip, pull more current and burn up. So a soft starter can slowly RAMP a motor to FULL speed in something never more than a minute, but cannot have it run at OTHER than full speed because the frequency remains the same, so the motor is not getting it's required V/Hz ratio. The magic of a VFD is that it changes the voltage and frequency TOGETHER to maintain the V/Hz ratio that the motor is designed for, so the motor can produce full rated torque at any speed (up to the design rating of the motor) and do so as long as power is given to the VFD rectifier.
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#4
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Re: Variable Frequency Drive. Energy Consideration

03/15/2017 5:27 PM

Excellent as always JRaef! But, I think you just went 30 miles above OPs head!

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#5
In reply to #4

Re: Variable Frequency Drive. Energy Consideration

03/15/2017 5:48 PM

I teach a class for electricians called "VFD-101" where what I put down here is covered in nice graphic format with animations in a PowerPoint presentation that takes about 2 hours. I accepted the challenge to describe it without pictures. I guess I failed...

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#6
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Re: Variable Frequency Drive. Energy Consideration

03/15/2017 6:23 PM

nope. the page has limitations

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#9
In reply to #5

Re: Variable Frequency Drive. Energy Consideration

03/15/2017 8:24 PM

You didn't fail. An instrument tech I work with has a shirt that reads: "I can explain it to you, but I can't understand it for you".

I took your class a few years ago. Mostly because I wanted the VFD at the end of the class, for a home project I was putting together (1/2 HP 3 phase motor from a 120V supply). I have worked with VFDs for decades, but my responsibility is on the controls side. I still learned a lot, and you dispelled a lot of misunderstandings in that class. Thanks!

Here is a funny story for you: An electrician and I were doing a "bump" test for rotation on a VFD he had installed. I was in the MCC with my laptop to make last minute corrections to the PLC program, while he went in the basement to check the rotation. Called for me to bump the motor, which I did, and he said the rotation was backwards - normal stuff. He came upstairs and swapped 2 phases (I was turned away at the time - busy). Tried again, same result. I didn't think too much about it, sometimes your eyes deceive you right? He changes the phasing again, tries again - same result. By this time he was cussing and sweating (and not just from the stairs). He comes back up, is about to swap phases again, when I stop and watch - you guessed it, he was changing the phases from the MCC feed into the VFD! I had a little educational talk with him.

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#10
In reply to #9

Re: Variable Frequency Drive. Energy Consideration

03/15/2017 9:14 PM

Wow, you took my class? Where? I guess I shouldn't be too shocked, all in all about 3,000 people have taken it now on the West Coast over the last 3 years. When I proposed it, the powers-that-be thought it was a waste my of time because "everyone already knows about VFDs now", but I didn't think so. My point was that lots of people BUY them and USE them, but didn't really understand what was going on in the box.

Thanks for letting me know I got through to someone! Funny story on that phase swap too. That used to be part of my presentation, somehow I think it dropped off in one of the "edit for time allowed" exercises. People paying for the space and time to do them have complained that I talk too much...

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#11
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Re: Variable Frequency Drive. Energy Consideration

03/15/2017 10:53 PM

Vegas (G-M).

"My point was that lots of people BUY them and USE them, but didn't really understand what was going on in the box." Spot on, your absolutely right! Oh, I knew about the incoming conversion to DC, but was misunderstanding that the output to the motor was some form of modified sine wave. In a way it is, but when you explained/showed the PWM waveform - BINGO! I got it! And the realization that the inductance of the motor (and in cases the cabling to the motor), was part of the circuit/calculations in the VFD - WOW, that brought a new understanding. The part about the harmonics, and the importance of the cables to the motor, gave me something to look out for when we have the "weird" problems crop up. Hey, unless you are doing a greenfield start-up, you are going to have degraded connections, insulation...

Alright, one more for you: Same electrician, good tubing bender, light bulb changer - but no real understanding of the equipment, or how to troubleshoot the equipment. Don't get me wrong, ALL jobs must be done, but please give me a "Sparky", once in a while please! We had an old Emerson drive fail (circa 1000 BC). Shut down and locked out the drive, and left it for the next day so it would be dead. Next day, he and I start checking the drive (after insuring the drive is drained of course). I'm checking the fuses, diodes on the incoming, and of course the caps (the usual suspects). I find a suspicious cap, and tell him to check it (forward-reverse) to make sure. He is afraid he is going to get shocked, because I have just "charged the cap", by using the fluke multimeter on the thing. Another talk about how the 9 volt battery in the fluke, couldn't possibly, be magically boosted to a bazillion volts on a dead drive!

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#12
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Re: Variable Frequency Drive. Energy Consideration

03/16/2017 4:00 AM

Thanks for your answer sir. Thank you very much @ jRaef

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#20
In reply to #4

Re: Variable Frequency Drive. Energy Consideration

03/16/2017 11:14 AM

I think he brought reality down from the heights on stone tablets to us all. Kudos JRaef!

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#13
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Re: Variable Frequency Drive. Energy Consideration

03/16/2017 4:05 AM

The answer written by some other members have told that sometimes you are required to supply additional energy . Now I am a little confused. Please help me solve this doubt.

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#14
In reply to #13

Re: Variable Frequency Drive. Energy Consideration

03/16/2017 4:28 AM

Then look for technical-sales information about real VFDs from the many manufacturers on the 'net.

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#16
In reply to #14

Re: Variable Frequency Drive. Energy Consideration

03/16/2017 9:08 AM

Yes but that is what I am asking sir. Why do we need to supply to certain vfd external source of energy and to some no external source of energy ? Both work on same principle then why the difference ? Forgive my ignorance as I am new to vfd .

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#17
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Re: Variable Frequency Drive. Energy Consideration

03/16/2017 11:08 AM

It's not an "additional source", it's the same source either way. If power for the controls is derived with a separate AC-DC power supply, that is going to be an internal part of the drive system. The user would not need to provide that separate source (with the exception of MV drives as pointed out).

Why a LV drive mfr would choose to do it that way as opposed to tapping off of the DC bus is often simply an economic consideration. A DC-DC (chopper) power supply can be more expensive to make than an AC-DC (SMPS) power supply depending on the amount of power required. Also, if the drive is designed primarily to be low cost, the power components are selected to be the bare minimum necessary to function at the given specs, meaning no "fudge factor" or room for error. The parasitic load of the control power supply being tapped off of the DC bus may end up pushing lower cost components too far.

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#21
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Re: Variable Frequency Drive. Energy Consideration

03/16/2017 11:16 AM

..."to de choppas!" Oh SNAP! That is a different movie.

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#19
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Re: Variable Frequency Drive. Energy Consideration

03/16/2017 11:14 AM

Then it depends upon the specific equipment, the application for which has been withheld from the forum.

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#7

Re: Variable Frequency Drive. Energy Consideration

03/15/2017 7:06 PM

The manual will tell you if you need to supply another power source other than the motive power. Most smaller VFD's will convert the motive power into voltages required for the control circuits.

A Rockwell PowerFlex 7000 Medium Voltage AC Drive requires both 115 VAC and 24 V(AC or DC) power for the control circuits.

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#8

Re: Variable Frequency Drive. Energy Consideration

03/15/2017 7:15 PM

"VFD is a power electronics based device which converts a basic fixed frequency, fixed voltage sine wave power (line power) to a variable frequency, variable output voltage used to control speed of induction motor(s). "

https://www.electrical4u.com/variable-frequency-drive/

"The speed of an induction motor can be easily controlled by varying the frequency of the 3-phase supply; however, to maintain a constant (rated) flux density, the applied voltage must also be changed in the same proportion as the frequency (as dictated by Faraday’s law).This speed control method is known as Volts per Hz. Above rated speed, the applied voltage is usually kept constant at rated value; this operation is referred to as constant HP. At low frequencies (i.e. speeds), the voltage must be boosted in order to compensate for the effects of the stator resistance."

http://people.ece.umn.edu/users/riaz/animations/imvfmovie.html

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