# 120v AC controller= 120v AC inverter?



## dcb (Dec 5, 2009)

yes and no. It will "essentially" create three phase power from a DC input, but it is an averaged sine wave, created by pulses of varying length using a sine function (smoothed out by motor inductance). And the peak to peak value is approximately the input voltage. So a 120v DC input would make an "equivalent" of 42volts RMS three phase output, if I understand it correctly. So you would need 340v dc to make 120v 3ph.

You could also make 120v 1ph, from 340vDC input by switching one leg at 50% and varying a second leg with a sine function (and ignoring the third leg), that varies from full B+ at the top (90 degrees) to full B- 180 degrees later (270 degrees) (with 50% at 0 degrees and 180 degrees). Hope I got that right, never really thought about it. But I think I finally realize why they say my ac24ls was rewound for 48 volts, when the pack is 156vDC.

Edit, there may be a way to do 1 phase 120v with 170v pack, basically invert the switching pattern between two legs so the swing is 340v peak to peak as seen by the load. Possibly with the three phase too but can't get my head around it at the moment


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## akseminole (Jan 5, 2014)

Neat, thank you for the reply!

I like topics that cause thinking of the unthought by the answerer.
I seem to learn new things that way.

How does a 12v to 120v AC inverter differ from an AC motor controller?
I assume the 12-120 uses a transformer or some other method to boost the voltage? Would it be possible to modify an AC controller(for someone who knew what they were doing, not me...) to produce 120 without the 340v?


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## akseminole (Jan 5, 2014)

The motor is wound for 48v because that is what the RMS output is per phase?


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## dcb (Dec 5, 2009)

re: 48v rms, yah that is my guess. take the pack voltage, divide by 1.414 then by 2. Though looking at the single phase example makes me wonder if there is a way to use the fact that the max difference between leads in three phase is noticeably less than the peak to peak value instead of 2, and pump out a simulated 64v rms or more with fancy switching. Just wondering out loud.

From the little I've seen on single phase inverters, they use a push-pull circuit (which is probably what I described with the 170v 1ph example) fed into a step up transformer. Not very light for the power output.


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## major (Apr 4, 2008)

dcb said:


> And the peak to peak value is approximately the input voltage. So a 120v DC input would make an "equivalent" of 42volts RMS three phase output, if I understand it correctly. So you would need 340v dc to make 120v 3ph.


No, the input voltage is the value of the maximum peak approximately. So an inverter with a 340VDC bus could produce AC up to about 240 Vrms.


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## dcb (Dec 5, 2009)

dcb said:


> Edit, there may be a way to do 1 phase 120v with 170v pack, basically invert the switching pattern between two legs so the swing is 340v peak to peak as seen by the load. Possibly with the three phase too but can't get my head around it at the moment


Yup. And I think you need a pack voltage of 7/8 peak2peak to make a 3 phase inverter.


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## major (Apr 4, 2008)

dcb said:


> Yup. And I think you need a pack voltage of 7/8 peak2peak to make a 3 phase inverter.


What No you don't.


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## akseminole (Jan 5, 2014)

Please continue major?


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## dcb (Dec 5, 2009)

major said:


> What No you don't.


Just curious what your thoughts are. When I looked at it with a "standard" three half-bridge controller that is what I came up with. http://www.diyelectriccar.com/forums/showpost.php?p=515050&postcount=543


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## major (Apr 4, 2008)

akseminole said:


> Please continue major?





major said:


> No, the input voltage is the value of the maximum peak approximately. So an inverter with a 340VDC bus could produce AC up to about 240 Vrms.


True for 3 phase as well as single phase. I've been using VFDs for like 25 years and that's the way it is. Look up the specifications for them. Or google for a tutorial on the subject using something like this _calculate dc bus voltage vfd_


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## Tesseract (Sep 27, 2008)

dcb said:


> Yup. And I think you need a pack voltage of 7/8 peak2peak to make a 3 phase inverter.


This assumes the use of 3rd harmonic injection or specific types of space vector modulation (to shift the apparent neutral point around) and is not something I would generally count on.


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## dcb (Dec 5, 2009)

I'm still confused major (not sure if you saw my last post at the bottom of the previous page). Maybe you have a link? With single phase and two leads, and two h-bridges, I understand how you can reverse the connections to use a DC voltage for the + and - swing and how the peak = Bus.

With one lead per phase, and 3 phases, there isn't anything to "reverse". Though all 6 leads on my ac24ls are easily accessible, so curse you for making me consider bringing all 6 leads out and adding 3 more half bridges to my controller (and getting my 165v pack to act like a 330v pack)  (actually that might be preferable to reconfiguring all the leaf batteries for 15v).

Tess, I was thinking of the homebrew controller (harmonics already being injected), not necessarily prefab, but point taken.


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## major (Apr 4, 2008)

dcb said:


> Maybe you have a link?


First hit when you google my suggestion. http://www.joliettech.com/what_is_a_variable_frequency_drive-how_vfd_works.htm See paragraph 4. And the AC synthesis is just the opposite of the rectification w/r/t DC bus to Vrms or Vpeak.

All VFDs in industry operate this way w/r/t DC bus and the application engineer ends up using the values quite often. Automotive or mobile motor drives operate the same way with the battery linked to the DC bus.


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## major (Apr 4, 2008)

dcb said:


> Just curious what your thoughts are. When I looked at it with a "standard" three half-bridge controller that is what I came up with. http://www.diyelectriccar.com/forums/showpost.php?p=515050&postcount=543


Irrelevant and off-topic


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## dcb (Dec 5, 2009)

major said:


> Irrelevant and off-topic


http://forums.mikeholt.com/showthread.php?t=126538
"If you look at an output terminal relative to the DC rail neutral (which itself is approximately AC source neutral, and approximately ground for a grounded system), you will find a variable duty cycle square wave where the *peak to peak value of the square wave is the internal DC voltage*. Since this internal DC voltage is approximately constant, the amplitude of this square wave is approximately constant."

And apparently with fancy switching, peak to peak of the output can be 8/7ths of the DC voltage as far as anything I can understand goes.


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## Tesseract (Sep 27, 2008)

dcb said:


> ...Tess, I was thinking of the homebrew controller (harmonics already being injected), not necessarily prefab, but point taken.


My point was more or less in agreement with major: the maximum AC output voltage of an inverter is approximately 71% (sqrt(2)/2) of its DC bus voltage. There are overmodulation techniques which can be used to extract an additional ~15% of voltage, but in time-honored fashion There Ain't No Such Thing As A Free Lunch and the price paid is increased distortion which invariably results in more losses in the motor, and, perhaps, even a slight reduction in torque per amp (depends on the specific harmonic number).

My understanding is that the most popular commercial EV inverters - the Curtis 1238/1239 series and the Azure-Dynamics DMOC - use straight carrier-based modulation and therefore deliver a maximum RMS output of 70.7% of the DC input.

Finally, I would not assume that any "homebrew controller" is capable of overmodulation. This is relatively sophisticated stuff that usually requires a lot of processing power to achieve; in othe words, an FPGA or DSP with FPU clocked at 100MHz or higher.


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## major (Apr 4, 2008)

dcb said:


> "If you look at an output terminal relative to the DC rail neutral ...


Of what interest is that measurement to the OP  It is irrelevant and off-topic.


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## dcb (Dec 5, 2009)

from your link, it "seem" obvious that a phase goes from switching b+ to b- to get peak to peak output voltage (the thingie in the lower right). Though that extra inductor might be boosting things a bit.


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## dcb (Dec 5, 2009)

major said:


> Of what interest is that measurement to the OP  It is irrelevant and off-topic.


The premise was how many volts he needs to run an ac-controller as an inverter, thus the title "120v AC controller= 120v AC inverter"? They are usually 3 phase, don't know how many phases he is looking for, and he seemed interested in your thoughts on three phase also.


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## major (Apr 4, 2008)

dcb said:


> The premise was how many volts he needs to run an ac-controller as an inverter, thus the title "120v AC controller= 120v AC inverter"? They are usually 3 phase, don't know how many phases he is looking for, and he seemed interested in your thoughts on three phase also.


I don't see where it is relevant. Afterall, OP indicated:


akseminole said:


> Specifically, can(could) it output 120v 60hz. (If for instance, the inverter could output 120v to a motor..)


I'd say yes but the motor controller would need to have the capability adjusting the voltage and frequency differently than what was programmed for the motor, and to use the inverter output other than on the motor, it may require filtering. Simple as that, isn't it?


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## dcb (Dec 5, 2009)

yes, 'cept battery would be 170v if he is expecting 120rms output, and a cooperative inverter is a good point ( i do take that for granted).

Your technical objections to my conclusions on battery driven 3 phase will have to remain a mystery


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## akseminole (Jan 5, 2014)

I'm not looking for any particular outcome, 1 phase, 3 phase etc. My curiosity is whether it could be done.

I am fine with people going off topic as long as it is part of the thought process.

I figured that people who actually deal with this as part of their business would have better answers for me than my own sitting around day dreaming at guesses.

So I am gratified that tesseract, major and dcb are debating(conversing about?) the subject.



Would a sevcon be more amenable to these programming changes vs a Curtiss or would they be about the same?

Could you build in the (power)inverter capability into a home brew AC controller?


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## akseminole (Jan 5, 2014)

I'm curious what the outcome of the technical objections would be between dcb and major?


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## major (Apr 4, 2008)

dcb said:


> yes, 'cept battery would be 170v if he is expecting 120rms output....


The battery potential would need to be greater than 170VDC. Which is a given because the OP was qualified with this:


akseminole said:


> (If for instance, the inverter could output 120v to a motor..)


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## major (Apr 4, 2008)

akseminole said:


> I'm curious what the outcome of the technical objections would be between dcb and major?


Ehh, I'm lost. I had no technical objection. I just see it being irrelevant to the discussion. If dcb wants to reinvent the inverter, he should start a new thread. As far as existing inverters used in VFDs, it is clear cut and doesn't matter if it is 3 phase or single phase w/r/t DCbus level.


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## dcb (Dec 5, 2009)

You did object to my speculation about needing a battery of 7/8 peak to peak voltage in a 3 phase inverter. I'm not trying to re-invent, just understand better.


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## dcb (Dec 5, 2009)

I mean I "think" I understand it, but it is always troublesome when folks with experience say "no, that isn't how it works". But I do see some bits of supporting evidence for how I think it works, if you assume as I do that peak voltage of a DC source does not imply that there is a peak to peak voltage. You would need two DC sources (+peak and -peak) for that (which is trivial to do on an AC powered VFD).

"In contrast, VFD inverters do not produce sinusoidal output voltage but instead generate a series of pulses that approximate a sine wave. The peak voltage of these pulses at the VFD inverter is equal to the VFD’s DC bus voltage. "

http://www.generalcable.com/NR/rdon...99D4/0/GCOVERVIEWOFVFDCABLES_ARTICLE11_13.PDF


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## dcb (Dec 5, 2009)

@akseminole I *think* I understand some of the confusion (and my own lack of understanding 3 phase).

The full wave rectified voltage is about peak-peak in a 3 phase VFD (I wasn't getting that without a visual), pk*sqrt(3) actually, so it is "trivial" with the inverter topology to switch the high or the low side to approximate the + or - half of the sine wave for each phase. But for a battery DC it means you need pretty much peak to peak voltage as far as I can tell (without relying on boost), but VFDs seem to routinely discard 15% of input voltage anyway (sort of, there is a storage cap), and they *could* reclaim it on output, but it is complicated. The rectified waveform in post 19 is just wrong/misleading. I'm satisfied


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## major (Apr 4, 2008)

dcb said:


> @akseminole I *think* I understand some of the confusion (and my own lack of understanding 3 phase).
> 
> The full wave rectified voltage is about peak-peak in a 3 phase VFD (I wasn't getting that without a visual), pk*sqrt(3) actually, so it is "trivial" with the inverter topology to switch the high or the low side to approximate the + or - half of the sine wave for each phase. But for a battery DC it means you need pretty much peak to peak voltage as far as I can tell (without relying on boost), but VFDs seem to routinely discard 15% of input voltage anyway (sort of, there is a storage cap), and they *could* reclaim it on output, but it is complicated. The rectified waveform in post 19 is just wrong/misleading. I'm satisfied


Those examples of 3 phase full bridge rectification voltage are incorrect.* The diodes in the half bridge are in parallel so the potentials cannot add. As I referred to previously:


major said:


> First hit when you google my suggestion. http://www.joliettech.com/what_is_a_variable_frequency_drive-how_vfd_works.htm See paragraph 4. And the AC synthesis is just the opposite of the rectification w/r/t DC bus to Vrms or Vpeak.


From that: 


> The diodes actually reconstruct the negative halves of the waveform onto the positive half. In a 460V unit, you'd measure an average DC bus voltage of about 650V to 680V. You can calculate this as line voltage times 1.414.


*{edit} Incorrect in this context because the examples apply to a full-wave circuit using a center tapped transformer which is not applicable.


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## dcb (Dec 5, 2009)

Lol, now you are just messing with me  You know that if you put 460rms into that bridge you are gonna get about ~1100 DC out. The author tried to represent DC bus as single phase rectified, and went off the rails from that point.


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## dcb (Dec 5, 2009)

460rms=648peak: I think I broke ltspice so I toned it down to 65 peak input

dc out is 648 * sqrt(3), or 65 * sqrt (3) in this case (113v)

At any point in time, the output voltage is the difference between the lowest of the phase voltages and the highest.


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## major (Apr 4, 2008)

dcb said:


> Lol, now you are just messing with me  You know that if you put 460rms into that bridge you are gonna get about ~1100 DC out. The author tried to represent DC bus as single phase rectified, and went off the rails from that point.





dcb said:


> 460rms=648peak: I think I broke ltspice so I toned it down to 65 peak input
> 
> dc out is 648 * sqrt(3), or 65 * sqrt (3) in this case (113v)
> 
> At any point in time, the output voltage is the difference between the lowest of the phase voltages and the highest.


And the third graph you show in post #29.

All examples make the mistake of using the 3 phase RMS AC voltage as if were one leg off of a center tapped transformer supplying the full wave bridge. This is not the case. The 460 V RMS is the phase voltage, ie. potential between two legs of a balanced 3 wire 3 phase supply.

The rectified DC link voltage after the bridge rectifier with the link filter is 1.414 of the RMS phase voltage from the supply. The synthestised RMS AC phase voltage from the 3 phase full bridge inverter is, at a maximum, 0.707 of the DC bus voltage, or equal to the incoming mains voltage (minus a couple of Volts attributed to device junction drops).


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## PStechPaul (May 1, 2012)

In the simulation, you are showing three AC voltage sources referenced to a neutral (star), and with 65V peak that is 46 VRMS L-N or 79.6 V P-P. A more typical industrial 3 phase source is 480 VAC L-L or 277 VAC L-N, which will give a rectified output of 679 VDC. A typical 480 VAC VFD has a bus link voltage of 400 to 800 VDC.

I have a 12 volt to 120 VAC inverter which generates about 135 VDC and a modified rectangular waveform that provides the equivalent of 120 VAC, and it will run a single phase induction motor. I also have a 24V to 220 VAC inverter which generates about 250 VDC and a similar rectangular waveform.

So you can run an AC motor from a source with peak voltage less than 1.4 times the RMS rating, but it may not be as efficient as a proper VFD which uses PWM at 10-20 kHz and possibly motor lead inductors to smooth out the high voltage transients.

I have even made a three phase drive for a rewound 8 VAC 3 phase motor using just on and off rectangular waveforms at 12 VDC peak with a PIC and some MOSFETs. It eventually blew up but it was able to run the motor at variable speeds and reasonable efficiency. I think it drew about 5-10 amps at 12 VDC no load with a 1/2 or 3/4 HP motor.


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## dcb (Dec 5, 2009)

K, I have to admit that phase 2 phase didn't occur to me, as that isn't how I think about it, but is very handy for a voltmeter so I can see how it came about. ok, 480v it is.

Adjusting the line peak voltages to 392v (480 * 1.414 / 1.732) and the numbers fall into place for dc voltage: peak at 678v.

And the only way I know how to get back to 392 peak line from 678 is with the lower switching pattern here

which I approximated as a boost of 8/7 (in post 7). So 678/2 = 339. 339 * 8/7 = 387, gonna say close enough  (looks more like 2/sqrt(3) than 8/7 now). Except for some terminology, I think I got it. A 678v battery can make a 784pp per line inverter (~7:8), which in the common tongue is a 480v inverter.

Thanks guys.


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## Tesseract (Sep 27, 2008)

dcb said:


> Lol, now you are just messing with me  You know that if you put 460rms into that bridge you are gonna get about ~1100 DC out. The author tried to represent DC bus as single phase rectified, and went off the rails from that point.


No, wrong. If you put 460VAC RMS into a 3ph. full wave rectifier with capacitor filter you get 650VDC out (unloaded). 

The peak value of a sine wave is always 1.414x [sqrt(2)] its RMS value and it is fundamentally impossible for current to flow through a rectifier when the output voltage exceeds the input voltage (otherwise it wouldn't be a rectifier, now would it?).

The previous illustrations showing the DC output voltage being sqrt(3) times the input are wrong; the DC output current is, however, sqrt(3) times the AC current measured in any one phase, assuming all phases contribute to the output equally.


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## dcb (Dec 5, 2009)

because you all are twisted about thinking about it as line to line instead of describing one phase x3  And I already noted that it was wrong because of that terminology, thanks.



dcb said:


> Adjusting the line peak voltages to 392v (480 * 1.414 / 1.732) and the numbers fall into place for dc voltage: peak at 678v.


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## Tesseract (Sep 27, 2008)

dcb said:


> because you all are twisted about thinking about it as line to line instead of describing one phase x3  And I already noted that it was wrong because of that terminology, thanks.


That's because you keep talking about measuring a phase voltage against the negative rail of the DC link, which is not how inverters are generally used. Frankly, you seem to be going to great lengths to prove this 8/7ths ratio which only exists when special inverter switching algorithms are used and those algorithms are only useful for motor loads, anyway (you certainly don't want to use 3rd harmonic injection on a static inverter [e.g. - a UPS] as that just increases the THD).


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## dcb (Dec 5, 2009)

Tesseract said:


> That's because you keep talking about measuring a phase voltage against the negative rail


No, that wouldn't make sense, there are diodes in the way (edit: the only reference I am thinking of is 1/2 PP of one phase, but phase to phase is useful too). I'm not trying to prove anything, just checking my understanding (via cross checking the figures). I appreciate the tip on common carrier in DMOC, that helps interpret their datasheet a bit and set my expectations for the homebrew (will double check SAPWM implementation in the source code).

edit2: the 7/8ths bit was just a guess based on manipulating a graph, the relationship between pack voltage and max peak to peak inverter output for one phase looks more like 2/sqrt(3) and I'm sure some more algebraically motivated person could demonstrate that algebraically.

edit3: also not quite sure why it matters on a static non motor inverter, but I don't have any 3 phase equipment except a motor, but worth noting.


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## major (Apr 4, 2008)

dcb said:


> And the only way I know how to get back to 392 peak line from 678 is with the lower switching pattern here


I see the problem. You don't understand the 3 phase full wave bridge inverter. You previously gave a hint to that fact here:


dcb said:


> With single phase and two leads, and two h-bridges, I understand how you can reverse the connections to use a DC voltage for the + and - swing and how the peak = Bus.
> 
> With one lead per phase, and 3 phases, there isn't anything to "reverse".


I can't begin to teach you how it works, but that is a lesson you need to learn.


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## PStechPaul (May 1, 2012)

One way to envision how a 680 VDC bus can produce an AC output greater than the 480 VAC that was rectified and filtered to produce it, is to compute the RMS voltage of a non-sinusoidal (flattened) waveform with the same peak value. As it approaches a square wave, the RMS value rises from peak/sqrt(2) to the peak value. I know from experience that a motor will run on a rectangular waveform but there will be some issues with harmonics and efficiency. There may be enough inductance in the motor to smooth out the waveform somewhat (to a trapezoidal wave), especially when overclocking, and external inductance (and LC filters) may be able to smooth out the waveform even more. With an LC circuit, there could even be some resonance effects that might boost the peak voltage above the bus voltage. But it might be better to make a boost converter to increase the bus voltage to the true peak value needed.


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## akseminole (Jan 5, 2014)

To clarify for amateurs like me, is the bus voltage the battery DC or is this something else?


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## major (Apr 4, 2008)

akseminole said:


> To clarify for amateurs like me, is the bus voltage the battery DC or is this something else?


Yes, in the EV, the battery connects to the DC bus (or sometimes called the DC link) and the rectifier bridge isn't needed. There is still a capacitor on the DC link but typically no inductor.


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## major (Apr 4, 2008)

PStechPaul said:


> One way to envision how a 680 VDC bus can produce an AC output greater than the 480 VAC ....


Paul,

I know you try to help, but this is off topic and I think confusing to the OP. Nobody here is trying to boost voltage, just invert it to AC.


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## dcb (Dec 5, 2009)

Sorry, you are wrong major, and off topic.

Actually I'm not sorry, you are just wrong and off topic.


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## dcb (Dec 5, 2009)

Tesseract said:


> That's because you keep talking about measuring a phase voltage against the negative rail of the DC link


The thing about looking at the individual voltages on the output is that is precisely what you have to do to get the igbts to do the right thing for their outputs, which is why it is a handy way to look at it also. It hadn't occurred to me to look at the input that way (mainly because I have a battery and there isn't any 480v anywhere around here, dc voltage is/was all that mattered)

Interesting (to me anyway), I decided to look at the input voltages against the negative DC, and it exactly describes a switching pattern required for full voltage output of the igbt array  Learn something new everyday. So either the lower leg igbt is on, or you set the duty cycle of the upper one according to the graph (100% at the top for zero overmodulation), x3 120 degrees out.

(for clarification, the voltage between V1 or V2 or V3 is 480Vrms here)


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