Home |
Search |
Today's Posts |
![]() |
|
Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
Reply |
|
LinkBack | Thread Tools | Display Modes |
#1
![]()
Posted to sci.electronics.repair
|
|||
|
|||
![]()
Greetings, group. Where I work, we repair several different types of variable-speed
3-phase motor drives. All of them I've seen so far have very similar circuitry. The main power-control module generally contains: 6 hi-current hi-voltage silicon diodes in 3-phase bridge (to generate +320VDC) 6 hi-current hi-voltage IGBTs in 3-phase bridge (to power the motor) thermistor (for temp sense) braking IGBT (in some units) The IGBTs are always wired as 3 push-pull pairs, with the 3 center points connected to the 3 phases of the motor. (The down side of this technology being that if both transistors on the same phase turn on at same time, the IGBTs explode with a deafening "BANG!!!" and you're out $65 in 65ns.) I had assumed that the IGBTs would be switched so as to generate 3 AC waveforms, 120deg out-of-phase with each other, and that the frequency would be variable from about 2Hz to about 60Hz, with motor speed proportional to frequency. But no such thing is true!!! When I actually looked at the waveforms at the gates of the IGBTs, with the motor at lowest speed (about 0.5 RPM), the frequencies of all 6 IGBTs are all about 400 Hz, about 50% duty cycle. When I turn increase commanded speed, the frequencies all stay at 400 Hz, but the duty cycles begin to fluctuate, with rate of fluctuation of duty cycle being equal to motor speed in revs/second, and amplitude of fluctuation also increasing with speed. (Eg, if motor is turning at about 10 revs per second, the duty cycles of the signals are fluctuating from about 40% to about 60% at about 10 fluctuation per second.) So how the hell does this bizarre technology work? If the frequency is always stuck at 400Hz, why isn't the motor always turning at exactly 24000 RPM? (My guess: at minimum speed, the three signals are almost perfectly in-phase; but at higher speeds, the phases fluctuate slightly away from 0deg, with the fluctuations "cycling" around the 3 phases at a rate equal to motor revs/sec, but the "carrier" frequency still staying at 400Hz all the while. But I'm just guessing.) So, does anyone here know what this bizarre motor-control technique is called? (Hard to google something one doesn't know the name of.) And, roughly, how it works? -- Curious, Robbie Hatley lonewolf [at] well [dot] com |
#2
![]()
Posted to sci.electronics.repair
|
|||
|
|||
![]() "Robbie Hatley" Greetings, group. Where I work, we repair several different types of variable-speed 3-phase motor drives. All of them I've seen so far have very similar circuitry. The main power-control module generally contains: 6 hi-current hi-voltage silicon diodes in 3-phase bridge (to generate +320VDC) 6 hi-current hi-voltage IGBTs in 3-phase bridge (to power the motor) thermistor (for temp sense) braking IGBT (in some units) The IGBTs are always wired as 3 push-pull pairs, with the 3 center points connected to the 3 phases of the motor. (The down side of this technology being that if both transistors on the same phase turn on at same time, the IGBTs explode with a deafening "BANG!!!" and you're out $65 in 65ns.) I had assumed that the IGBTs would be switched so as to generate 3 AC waveforms, 120deg out-of-phase with each other, and that the frequency would be variable from about 2Hz to about 60Hz, with motor speed proportional to frequency. But no such thing is true!!! When I actually looked at the waveforms at the gates of the IGBTs, with the motor at lowest speed (about 0.5 RPM), the frequencies of all 6 IGBTs are all about 400 Hz, about 50% duty cycle. When I turn increase commanded speed, the frequencies all stay at 400 Hz, but the duty cycles begin to fluctuate, with rate of fluctuation of duty cycle being equal to motor speed in revs/second, and amplitude of fluctuation also increasing with speed. (Eg, if motor is turning at about 10 revs per second, the duty cycles of the signals are fluctuating from about 40% to about 60% at about 10 fluctuation per second.) So how the hell does this bizarre technology work? If the frequency is always stuck at 400Hz, why isn't the motor always turning at exactly 24000 RPM? (My guess: at minimum speed, the three signals are almost perfectly in-phase; but at higher speeds, the phases fluctuate slightly away from 0deg, with the fluctuations "cycling" around the 3 phases at a rate equal to motor revs/sec, but the "carrier" frequency still staying at 400Hz all the while. But I'm just guessing.) So, does anyone here know what this bizarre motor-control technique is called? (Hard to google something one doesn't know the name of.) And, roughly, how it works? ** The technique is called PWM: http://en.wikipedia.org/wiki/Pulse-width_modulation The duty cycle of the 400Hz rectangular wave is slowly modulated to create low frequencies in the 3 phases required. A steady 50% duty cycle wave at 400Hz creates no torque in the motor as the average value is zero and the motor's inductance at that frequency causes little current to flow. Shallow modulation depth creates the low voltages needed for slow running. .... Phil |
#3
![]()
Posted to sci.electronics.repair
|
|||
|
|||
![]() "Jamie" = Maynard A. Philbrook Radio Ham ** **** OFF you PSYCHO HAM RADIO **** |
#4
![]()
Posted to sci.electronics.repair
|
|||
|
|||
![]()
Phil Allison wrote:
"Robbie Hatley" Greetings, group. Where I work, we repair several different types of variable-speed 3-phase motor drives. All of them I've seen so far have very similar circuitry. The main power-control module generally contains: 6 hi-current hi-voltage silicon diodes in 3-phase bridge (to generate +320VDC) 6 hi-current hi-voltage IGBTs in 3-phase bridge (to power the motor) thermistor (for temp sense) braking IGBT (in some units) The IGBTs are always wired as 3 push-pull pairs, with the 3 center points connected to the 3 phases of the motor. (The down side of this technology being that if both transistors on the same phase turn on at same time, the IGBTs explode with a deafening "BANG!!!" and you're out $65 in 65ns.) I had assumed that the IGBTs would be switched so as to generate 3 AC waveforms, 120deg out-of-phase with each other, and that the frequency would be variable from about 2Hz to about 60Hz, with motor speed proportional to frequency. But no such thing is true!!! When I actually looked at the waveforms at the gates of the IGBTs, with the motor at lowest speed (about 0.5 RPM), the frequencies of all 6 IGBTs are all about 400 Hz, about 50% duty cycle. When I turn increase commanded speed, the frequencies all stay at 400 Hz, but the duty cycles begin to fluctuate, with rate of fluctuation of duty cycle being equal to motor speed in revs/second, and amplitude of fluctuation also increasing with speed. (Eg, if motor is turning at about 10 revs per second, the duty cycles of the signals are fluctuating from about 40% to about 60% at about 10 fluctuation per second.) So how the hell does this bizarre technology work? If the frequency is always stuck at 400Hz, why isn't the motor always turning at exactly 24000 RPM? (My guess: at minimum speed, the three signals are almost perfectly in-phase; but at higher speeds, the phases fluctuate slightly away from 0deg, with the fluctuations "cycling" around the 3 phases at a rate equal to motor revs/sec, but the "carrier" frequency still staying at 400Hz all the while. But I'm just guessing.) So, does anyone here know what this bizarre motor-control technique is called? (Hard to google something one doesn't know the name of.) And, roughly, how it works? ** The technique is called PWM: http://en.wikipedia.org/wiki/Pulse-width_modulation The duty cycle of the 400Hz rectangular wave is slowly modulated to create low frequencies in the 3 phases required. A steady 50% duty cycle wave at 400Hz creates no torque in the motor as the average value is zero and the motor's inductance at that frequency causes little current to flow. Shallow modulation depth creates the low voltages needed for slow running. ... Phil Yeah, what ever you say Phil.,. Talk about something you know more of.. From what you just spit out, I can tell this isn't your line of work. Jamie |
#5
![]()
Posted to sci.electronics.repair
|
|||
|
|||
![]()
On 7/03/2012 2:01 PM, Jamie wrote:
Phil Allison wrote: "Robbie Hatley" Greetings, group. Where I work, we repair several different types of variable-speed 3-phase motor drives. All of them I've seen so far have very similar circuitry. The main power-control module generally contains: 6 hi-current hi-voltage silicon diodes in 3-phase bridge (to generate +320VDC) 6 hi-current hi-voltage IGBTs in 3-phase bridge (to power the motor) thermistor (for temp sense) braking IGBT (in some units) The IGBTs are always wired as 3 push-pull pairs, with the 3 center points connected to the 3 phases of the motor. (The down side of this technology being that if both transistors on the same phase turn on at same time, the IGBTs explode with a deafening "BANG!!!" and you're out $65 in 65ns.) I had assumed that the IGBTs would be switched so as to generate 3 AC waveforms, 120deg out-of-phase with each other, and that the frequency would be variable from about 2Hz to about 60Hz, with motor speed proportional to frequency. But no such thing is true!!! When I actually looked at the waveforms at the gates of the IGBTs, with the motor at lowest speed (about 0.5 RPM), the frequencies of all 6 IGBTs are all about 400 Hz, about 50% duty cycle. When I turn increase commanded speed, the frequencies all stay at 400 Hz, but the duty cycles begin to fluctuate, with rate of fluctuation of duty cycle being equal to motor speed in revs/second, and amplitude of fluctuation also increasing with speed. (Eg, if motor is turning at about 10 revs per second, the duty cycles of the signals are fluctuating from about 40% to about 60% at about 10 fluctuation per second.) So how the hell does this bizarre technology work? If the frequency is always stuck at 400Hz, why isn't the motor always turning at exactly 24000 RPM? (My guess: at minimum speed, the three signals are almost perfectly in-phase; but at higher speeds, the phases fluctuate slightly away from 0deg, with the fluctuations "cycling" around the 3 phases at a rate equal to motor revs/sec, but the "carrier" frequency still staying at 400Hz all the while. But I'm just guessing.) So, does anyone here know what this bizarre motor-control technique is called? (Hard to google something one doesn't know the name of.) And, roughly, how it works? ** The technique is called PWM: http://en.wikipedia.org/wiki/Pulse-width_modulation The duty cycle of the 400Hz rectangular wave is slowly modulated to create low frequencies in the 3 phases required. A steady 50% duty cycle wave at 400Hz creates no torque in the motor as the average value is zero and the motor's inductance at that frequency causes little current to flow. Shallow modulation depth creates the low voltages needed for slow running. ... Phil Yeah, what ever you say Phil.,. Talk about something you know more of.. From what you just spit out, I can tell this isn't your line of work. You can? What Phil said sounded perfectly reasonable to me. Now, it's true that motors are indeed not my line of work, but the physics seemed right, and fits with what the OP said. Sylvia. |
#6
![]()
Posted to sci.electronics.repair
|
|||
|
|||
![]() "Sylvia Else" You can? What Phil said sounded perfectly reasonable to me. Now, it's true that motors are indeed not my line of work, but the physics seemed right, and fits with what the OP said. ** One can nit pick anything. The OP needs to read the Wiki and my post and piece them together in his head. Chances are, the 400Hz wave is not phase shifted 120 degrees in each winding so producing zero torque on that basis. Also, at low speeds one needs lower drive voltages in order to to prevent severe saturation of the iron rotor and the resulting large current flows. Speed variation is a result of drive frequency alone. ..... Phil |
#7
![]()
Posted to sci.electronics.repair
|
|||
|
|||
![]() Someone named Phil Allison apparently replied to my message about variable-speed motor drives, but I never got that message direct. (Lost in the ether of Usenet?) I did get someone named "Jamie" replying to it, though, so I can reply. (Lemme strip off one layer of "". Done.) Phil Allison writes: The technique is called PWM While it's true that the pulse widths change, this is kinda far removed from what I normally think of as being PWM. I generally think of PWM as feeding variable-duty-cycle DC pulses into an LC filter so as to get a DC voltage proportional to duty cycle. http://en.wikipedia.org/wiki/Pulse-width_modulation Ok, lemme read and see if there's a variant of PWM that acts like what I'm seeing in these motor drives. ::: read, read, read ::: Yes, you're right. Thanks for the link! First page has a picture that shows why I'm seeing 400Hz where I expected 2Hz: http://upload.wikimedia.org/wikipedi...2C_3-level.svg Basically uses the inductance of the motor windings and the magnetic reluctance of the core to convert the 400Hz electric pulses into 2Hz or 20Hz or 38Hz (or whatever) magnetic sine waves. I also see a link on W'pedia's PWM page to the particular type of PWM used for 3-phase variable-speed AC motors: http://en.wikipedia.org/wiki/Space_vector_modulation Very cool chart there showing 8 different valid states that a 6-IGBT bridge can be in (and hinting at the various catastrophic *unallowable* states as well -- I've experienced some of *those*). Turns out, 6 of the 8 states line up with the axes of the 3 phases, and the remaining 2 (000 and 111) correspond to "off" states. The duty cycle of the 400Hz rectangular wave is slowly modulated to create low frequencies in the 3 phases required. Apparently so. A steady 50% duty cycle wave at 400Hz creates no torque in the motor as the average value is zero Yep, if all 3 phases are in-phase, speed will be 0RPM and current will be very low (10ma? 1ma?) even if the voltages are 320V p-p, because the voltage across each winding is near 0. and the motor's inductance at thatfrequency causes little current to flow. I think any current at 0RPM would have to be due to parasitic capacitance. With 0V across each coil at all times, nowhere else for current to flow to. Shallow modulation depth creates the low voltages needed for slow running. I think it's the brief periods of time that one phase is "Hi" while another phase has switched "Lo" that creates the pulses of differential voltage necessary to ramp up magnetic flux. If these "overlap zones" are then rotated around the 3 phases at 10 rotations per second clockwise, the motor should spin clockwise at 600RPM. I think the overlap durations would need to be proportional to actual power draw, in order to keep the speed from bogging down under load. Perhaps that's why these devices all have current sensors: not so much to protect from overcurrent (though there is that), but mainly to calculate phase overlap durations. Anyway, Phil, thanks for the link; it answers my questions and then some. PS: I'll also reply briefly here to the other 2 responders: Jamie: Actually, Phil's right. Sylvia: Yep, the physics described in W'pedia's PWM and SVM pages does match what I'm seeing on the bench. -- Cheers, Robbie Hatley Santa Ana, CA, USA lonewolf (at) well (dot) com http://www.well.com/user/lonewolf/ |
#8
![]()
Posted to sci.electronics.repair
|
|||
|
|||
![]() "Robbie Hatley" Someone named Phil Allison apparently replied to my message about variable-speed motor drives, but I never got that message direct. (Lost in the ether of Usenet?) I did get someone named "Jamie" replying to it, though, so I can reply. (Lemme strip off one layer of "". Done.) Phil Allison writes: The technique is called PWM While it's true that the pulse widths change, this is kinda far removed from what I normally think of as being PWM. I generally think of PWM as feeding variable-duty-cycle DC pulses into an LC filter so as to get a DC voltage proportional to duty cycle. http://en.wikipedia.org/wiki/Pulse-width_modulation Ok, lemme read and see if there's a variant of PWM that acts like what I'm seeing in these motor drives. ::: read, read, read ::: Yes, you're right. Thanks for the link! First page has a picture that shows why I'm seeing 400Hz where I expected 2Hz: http://upload.wikimedia.org/wikipedi...2C_3-level.svg Basically uses the inductance of the motor windings and the magnetic reluctance of the core to convert the 400Hz electric pulses into 2Hz or 20Hz or 38Hz (or whatever) magnetic sine waves. I also see a link on W'pedia's PWM page to the particular type of PWM used for 3-phase variable-speed AC motors: http://en.wikipedia.org/wiki/Space_vector_modulation Very cool chart there showing 8 different valid states that a 6-IGBT bridge can be in (and hinting at the various catastrophic *unallowable* states as well -- I've experienced some of *those*). Turns out, 6 of the 8 states line up with the axes of the 3 phases, and the remaining 2 (000 and 111) correspond to "off" states. The duty cycle of the 400Hz rectangular wave is slowly modulated to create low frequencies in the 3 phases required. Apparently so. A steady 50% duty cycle wave at 400Hz creates no torque in the motor as the average value is zero Yep, if all 3 phases are in-phase, speed will be 0RPM and current will be very low (10ma? 1ma?) even if the voltages are 320V p-p, because the voltage across each winding is near 0. and the motor's inductance at thatfrequency causes little current to flow. I think any current at 0RPM would have to be due to parasitic capacitance. With 0V across each coil at all times, nowhere else for current to flow to. Shallow modulation depth creates the low voltages needed for slow running. I think it's the brief periods of time that one phase is "Hi" while another phase has switched "Lo" that creates the pulses of differential voltage necessary to ramp up magnetic flux. If these "overlap zones" are then rotated around the 3 phases at 10 rotations per second clockwise, the motor should spin clockwise at 600RPM. I think the overlap durations would need to be proportional to actual power draw, in order to keep the speed from bogging down under load. Perhaps that's why these devices all have current sensors: not so much to protect from overcurrent (though there is that), but mainly to calculate phase overlap durations. Anyway, Phil, thanks for the link; it answers my questions and then some. PS: I'll also reply briefly here to the other 2 responders: Jamie: Actually, Phil's right. Sylvia: Yep, the physics described in W'pedia's PWM and SVM pages does match what I'm seeing on the bench. ** A satisfied customer ..... ... we should have him stuffed ........ Apologies to Basil Faulty. ..... Phil |
#9
![]()
Posted to sci.electronics.repair
|
|||
|
|||
![]() "Phil Allison" wrote in message ... "Jamie" = Maynard A. Philbrook Radio Ham ** **** OFF you PSYCHO HAM RADIO **** For information purposes, not all amateur (ham) radio operators are the same. We come in many different sizes, shapes, colors, creeds, temperaments, etc. Some are suave and polite. (That's not me.) Some are abusive and manipulative. (That's not me either.) Some are authoritarian and legalistic. (Definitely not me.) Some are militantly anti-authoritarian. (Definitely not me either.) Some are blunt and direct but non-ad-hominem. (That's my style.) The only common denominator is a love of electronics and radio. Other than that, hams are an extremely mixed bunch. Judging the group as a whole by the behavior of any one specimen will give a skewed view. -- Cheers, Robbie Hatley KM6HT lonewolf [at] well [dot] com |
#10
![]()
Posted to sci.electronics.repair
|
|||
|
|||
![]()
"Robbie Hatley"
"Phil Allison" "Jamie" = Maynard A. Philbrook Radio Ham ** **** OFF you PSYCHO HAM RADIO **** For information purposes, not all amateur (ham) radio operators are the same. ( snip piles of drivel ) ** FYI pal, I was speaking of only one example = KA1LPA Only a fool thinks that a criticising one example of a group insults the whole group and requires an immediate defence. You are that fool. ..... Phil |
#11
![]()
Posted to sci.electronics.repair
|
|||
|
|||
![]()
Phil Allison wrote:
"Robbie Hatley" "Phil Allison" "Jamie" = Maynard A. Philbrook Radio Ham ** **** OFF you PSYCHO HAM RADIO **** For information purposes, not all amateur (ham) radio operators are the same. ( snip piles of drivel ) ** FYI pal, I was speaking of only one example = KA1LPA Only a fool thinks that a criticising one example of a group insults the whole group and requires an immediate defence. You are that fool. .... Phil Phil, I hate to burst your bubble, but you like every else, including myself does not know everything. And I am sorry to say, you slipped again. I am sorry that you take it so hard when you're wrong. But that is the case. Jamie |
#12
![]()
Posted to sci.electronics.repair
|
|||
|
|||
![]()
On Wed, 7 Mar 2012 13:05:58 -0800, "Robbie Hatley"
wrote: "Phil Allison" wrote in message ... "Jamie" = Maynard A. Philbrook Radio Ham ** **** OFF you PSYCHO HAM RADIO **** For information purposes, not all amateur (ham) radio operators are the same. We come in many different sizes, shapes, colors, creeds, temperaments, etc. Some are suave and polite. (That's not me.) Some are abusive and manipulative. (That's not me either.) Some are authoritarian and legalistic. (Definitely not me.) Some are militantly anti-authoritarian. (Definitely not me either.) Some are blunt and direct but non-ad-hominem. (That's my style.) The only common denominator is a love of electronics and radio. Other than that, hams are an extremely mixed bunch. Judging the group as a whole by the behavior of any one specimen will give a skewed view. Robbie, Phil Allison is sufering from some type of mental disease. He does know some about electronics but most times just posts abusive language. Most folks just killfile him and so they don't see his original posts. Eric |
#13
![]()
Posted to sci.electronics.repair
|
|||
|
|||
![]() Phil Allison wrote: ( snip piles of drivel ) humorWhat kind of scissors do you use for that? I've never tried to snip a pile of drivel before./humor But seriously, there was no "drivel" in what I said. What I said was: For information purposes, not all amateur (ham) radio operators are the same. We come in many different sizes, shapes, colors, creeds, temperaments, etc. Some are suave and polite. (That's not me.) Some are abusive and manipulative. (That's not me either.) Some are authoritarian and legalistic. (Definitely not me.) Some are militantly anti-authoritarian. (Definitely not me either.) Some are blunt and direct but non-ad-hominem. (That's my style.) The only common denominator is a love of electronics and radio. Other than that, hams are an extremely mixed bunch. Judging the group as a whole by the behavior of any one specimen will give a skewed view. All of which are true statements. And your response was: [No response.] I'm so glad you fully agree with everything I said. Yes, that *does* logically follow. You did bother to reply, and yet you did not disagree with a single thing I said. Instead, you went off on a tangent about your (mistaken) ideas regarding the concept of "examples", which has nothing to do with the topic of the message you were purporting to be replying to (namely that amateur radio operators come in all types and descriptions, an assertion which you did not contradict, probably because you know you can't credibly do so). (This is a type of logical fallacy known as "misdirection", more specifically a subvariant of "straw man". I suppose you could call it the "when in doubt, change the subject" fallacy for want of a better name.) FYI pal, Fallacy of self-contradiction. Calling someone a "pal" in one breath and a "fool" in the next is not self-consistent. I was speaking of only one example = KA1LPA Only a fool thinks that a criticizing one example of a group insults the whole group ... You did not mention any call signs. And speaking of an "example" *does* speak of that which it is purported to be an "example" of, yes. That's what "example" means. (Look it up.) Not that you used the word "example" in your post. But yes, you were holding out the person you were talking about as being an "example" of the amateur radio community. Otherwise, you would have no reason to bring up "ham radio" at all, let alone IN ALL CAPITAL LETTERS. TWICE. Furthermore, since the person you're lambasting has been behaving in a substantially more cordial manor than yourself the last few days, you are not in a "high road" position which would allow you to call that person "psycho" or "****" (as you did) without sounding laughable. So don't do that. Duh. Furthermore, your statement is a misquote because I never actually said "criticizing one example of a group insults the whole group". Nor is that quite a true statement. Criticizing an "example" does criticize that which you're asserting it's an "example" of, yes. But criticism is not the same as insults. Also, further fallacy of mis-quoting, because in my post, I did not say that you insulted anyone. Even though, in reality, you did insult "Jamie", I over-looked that insult of yours, because I was more interested in pointing out that amateur radio is much more of a mixed bag than you appear to think it is. ... and requires an immediate defense ... Fallacy of mis-characterization of another's statements. I did not "defend" any one or any thing in the post you're referring to. Indeed, I said, among other things, "Some [hams] are abusive and manipulative". Does that look like an act of blanket "defending" of the amateur radio service? Sure doesn't to me. I was attempting to give a more balanced view of the service, including both its high points and its warts. You are that fool. Fallacy of "argumentum ad hominem": whether or not I am a "fool" (whatever that means to you) in your opinion does not interest me, because it does not bear on the truth or falsehood of my assertions. Bottom line: poor post, Phil. Full of fallacious reasoning, untrue assertions, needless insults, AND UNNECESSARY CAPITAL LETTERS. Were you perhaps staying up late? Better to do Usenet stuff earlier, when you mind is fresher. I know you are capable of much more effective communicating, because I've seen you do it. You're a great person to talk to when you stay on-topic. But when you go off on these emotional tirades, you merely stir up anger, and accomplish nothing good, for yourself or for anyone else. Some people here are kill-filing you. You can change all that by doing just 2 things: 1. Never write in Usenet when tired. Get some rest first. 2. If you write an emotional reply to something, don't immediately hit "send". Save to drafts, sleep on it, then come back the next day and edit-out all the emotional and irrational stuff, and just keep the cordial, logical, truthful, on-topic stuff. *Then* hit "send". -- Cheers, Robbie Hatley Santa Ana, CA, USA lonewolf (at) well (dot) com http://www.well.com/user/lonewolf/ |
#14
![]()
Posted to sci.electronics.repair
|
|||
|
|||
![]() "Robbie Hatley = mad as a Hatter " ( snip piles of drivel ) But seriously, there was no "drivel" in what I said. ** It was drivel because it simply had no relevance to anything in the thread. But now I see you are actually a full on, raving lunatic - the concept of relevance is a completely foreign one. Kindly FOAD. ..... Phil |
#15
![]()
Posted to sci.electronics.repair
|
|||
|
|||
![]()
Mr Hatley... "Phil is ill." If you want to publicly tweak his nose, fine.
But don't expect to change him. |
#16
![]()
Posted to sci.electronics.repair
|
|||
|
|||
![]() |
#17
![]()
Posted to sci.electronics.repair
|
|||
|
|||
![]()
"Robbie Hatley = mad as a Hatter "
Hatters have no more likelyhood of being "mad" (either insane or merely angry) than persons of any other profession. Mercury poisoning, from the compounds using in felting. |
#18
![]()
Posted to sci.electronics.repair
|
|||
|
|||
![]()
William Sommerwerck wrote:
Mr Hatley... "Phil is ill." If you want to publicly tweak his nose, fine. But don't expect to change him. Exactly ![]() Jamie |
#19
![]()
Posted to sci.electronics.repair
|
|||
|
|||
![]() "William Sommerwerck" "Robbie Hatley = mad as a Hatter " Hatters have no more likelyhood of being "mad" (either insane or merely angry) than persons of any other profession. Mercury poisoning, from the compounds using in felting. ** Hatley has something much worse than mere Mercury poisoning. |
#20
![]()
Posted to sci.electronics.repair
|
|||
|
|||
![]()
On Mar 9, 5:01*pm, "William Sommerwerck"
wrote: "Robbie Hatley = mad as a Hatter " Hatters have no more likelyhood of being "mad" (either insane or merely angry) than persons of any other profession. Mercury poisoning, from the compounds using in felting. I recall it's from the mercury fumes. There was a region in south bay, CA called cinnabar, always thought the people form that region were a bit odd, now I know why. Yet, ingesting mercury causes nephritis, as in Madame Bovary, go figure. |
#21
![]()
Posted to sci.electronics.repair
|
|||
|
|||
![]()
On Wed, 07 Mar 2012 02:11:08 -0800, Robbie Hatley
wrote: Yabbut, Phil Allison left a lot out. These are called variable frequency drives. They produce a PWM output that induces a frequency and voltage controlled sinusoidal current in the motor. The typical voltage limit is related to motor rated voltage and for 480 V system rated (460 V nameplate) is about 8 V per Hz at any frequency that produces less than full rated voltage. If you use properly burdened current transformers to measure phase current you will see a much more informative display on your 'scope. That said, 400 Hz switching frequency is uncommonly low and would take excessive load reactors to smooth out. Modern units are a few to several KHz PWM base. IGBTs tend to be rather slow, but they are getting better. Of course they pretty well better, there are super power fets with ratings that would make early IGBTs shudder. Someone named Phil Allison apparently replied to my message about variable-speed motor drives, but I never got that message direct. (Lost in the ether of Usenet?) I did get someone named "Jamie" replying to it, though, so I can reply. (Lemme strip off one layer of "". Done.) Please do NOT strip the "". They help others keep track of who said what. Phil Allison writes: The technique is called PWM While it's true that the pulse widths change, this is kinda far removed from what I normally think of as being PWM. I generally think of PWM as feeding variable-duty-cycle DC pulses into an LC filter so as to get a DC voltage proportional to duty cycle. http://en.wikipedia.org/wiki/Pulse-width_modulation Ok, lemme read and see if there's a variant of PWM that acts like what I'm seeing in these motor drives. ::: read, read, read ::: Yes, you're right. Thanks for the link! First page has a picture that shows why I'm seeing 400Hz where I expected 2Hz: http://upload.wikimedia.org/wikipedi...2C_3-level.svg Basically uses the inductance of the motor windings and the magnetic reluctance of the core to convert the 400Hz electric pulses into 2Hz or 20Hz or 38Hz (or whatever) magnetic sine waves. I also see a link on W'pedia's PWM page to the particular type of PWM used for 3-phase variable-speed AC motors: http://en.wikipedia.org/wiki/Space_vector_modulation Very cool chart there showing 8 different valid states that a 6-IGBT bridge can be in (and hinting at the various catastrophic *unallowable* states as well -- I've experienced some of *those*). Turns out, 6 of the 8 states line up with the axes of the 3 phases, and the remaining 2 (000 and 111) correspond to "off" states. The duty cycle of the 400Hz rectangular wave is slowly modulated to create low frequencies in the 3 phases required. Apparently so. A steady 50% duty cycle wave at 400Hz creates no torque in the motor as the average value is zero Yep, if all 3 phases are in-phase, speed will be 0RPM and current will be very low (10ma? 1ma?) even if the voltages are 320V p-p, because the voltage across each winding is near 0. and the motor's inductance at thatfrequency causes little current to flow. I think any current at 0RPM would have to be due to parasitic capacitance. With 0V across each coil at all times, nowhere else for current to flow to. Shallow modulation depth creates the low voltages needed for slow running. I think it's the brief periods of time that one phase is "Hi" while another phase has switched "Lo" that creates the pulses of differential voltage necessary to ramp up magnetic flux. If these "overlap zones" are then rotated around the 3 phases at 10 rotations per second clockwise, the motor should spin clockwise at 600RPM. I think the overlap durations would need to be proportional to actual power draw, in order to keep the speed from bogging down under load. Perhaps that's why these devices all have current sensors: not so much to protect from overcurrent (though there is that), but mainly to calculate phase overlap durations. Anyway, Phil, thanks for the link; it answers my questions and then some. PS: I'll also reply briefly here to the other 2 responders: Jamie: Actually, Phil's right. Sylvia: Yep, the physics described in W'pedia's PWM and SVM pages does match what I'm seeing on the bench. |
#22
![]()
Posted to sci.electronics.repair
|
|||
|
|||
![]()
josephkk wrote:
On Wed, 07 Mar 2012 02:11:08 -0800, Robbie Hatley wrote: Yabbut, Phil Allison left a lot out. These are called variable frequency drives. They produce a PWM output that induces a frequency and voltage controlled sinusoidal current in the motor. The typical voltage limit is related to motor rated voltage and for 480 V system rated (460 V nameplate) is about 8 V per Hz at any frequency that produces less than full rated voltage. If you use properly burdened current transformers to measure phase current you will see a much more informative display on your 'scope. That said, 400 Hz switching frequency is uncommonly low and would take excessive load reactors to smooth out. Modern units are a few to several KHz PWM base. IGBTs tend to be rather slow, but they are getting better. Of course they pretty well better, there are super power fets with ratings that would make early IGBTs shudder. Someone named Phil Allison apparently replied to my message about variable-speed motor drives, but I never got that message direct. (Lost in the ether of Usenet?) I did get someone named "Jamie" replying to it, though, so I can reply. (Lemme strip off one layer of "". Done.) Please do NOT strip the "". They help others keep track of who said what. Phil Allison writes: The technique is called PWM While it's true that the pulse widths change, this is kinda far removed from what I normally think of as being PWM. I generally think of PWM as feeding variable-duty-cycle DC pulses into an LC filter so as to get a DC voltage proportional to duty cycle. http://en.wikipedia.org/wiki/Pulse-width_modulation Ok, lemme read and see if there's a variant of PWM that acts like what I'm seeing in these motor drives. ::: read, read, read ::: Yes, you're right. Thanks for the link! First page has a picture that shows why I'm seeing 400Hz where I expected 2Hz: http://upload.wikimedia.org/wikipedi...2C_3-level.svg Basically uses the inductance of the motor windings and the magnetic reluctance of the core to convert the 400Hz electric pulses into 2Hz or 20Hz or 38Hz (or whatever) magnetic sine waves. I also see a link on W'pedia's PWM page to the particular type of PWM used for 3-phase variable-speed AC motors: http://en.wikipedia.org/wiki/Space_vector_modulation Very cool chart there showing 8 different valid states that a 6-IGBT bridge can be in (and hinting at the various catastrophic *unallowable* states as well -- I've experienced some of *those*). Turns out, 6 of the 8 states line up with the axes of the 3 phases, and the remaining 2 (000 and 111) correspond to "off" states. The duty cycle of the 400Hz rectangular wave is slowly modulated to create low frequencies in the 3 phases required. Apparently so. A steady 50% duty cycle wave at 400Hz creates no torque in the motor as the average value is zero Yep, if all 3 phases are in-phase, speed will be 0RPM and current will be very low (10ma? 1ma?) even if the voltages are 320V p-p, because the voltage across each winding is near 0. and the motor's inductance at thatfrequency causes little current to flow. I think any current at 0RPM would have to be due to parasitic capacitance. With 0V across each coil at all times, nowhere else for current to flow to. Shallow modulation depth creates the low voltages needed for slow running. I think it's the brief periods of time that one phase is "Hi" while another phase has switched "Lo" that creates the pulses of differential voltage necessary to ramp up magnetic flux. If these "overlap zones" are then rotated around the 3 phases at 10 rotations per second clockwise, the motor should spin clockwise at 600RPM. I think the overlap durations would need to be proportional to actual power draw, in order to keep the speed from bogging down under load. Perhaps that's why these devices all have current sensors: not so much to protect from overcurrent (though there is that), but mainly to calculate phase overlap durations. Anyway, Phil, thanks for the link; it answers my questions and then some. PS: I'll also reply briefly here to the other 2 responders: Jamie: Actually, Phil's right. Sylvia: Yep, the physics described in W'pedia's PWM and SVM pages does match what I'm seeing on the bench. I find it ironic how little knowledge about a subject can make people assume the most off key ideas and then, able to have others believe it. I on the other hand, work with this stuff, A lot.. 400 Hz is not the switching frequency used today. It may work on large motors, it does not work on smaller motors. 400 Hz is the maximum of most inverters on the market that you can generate, that is, if the motor itself can handle that speed. In many cases, 400 hz output inverters are pumped into a transformer where it can then be used to drive other equipment, like induction heaters and like. Btw, We also have several induction heaters that use a inverter drive up to 700 hz output how ever, the PWM is actually around 15kHz. It's unbelievable how people can go running to a web site and grossly miss understand what they're reading. On top of that, how about the articles that are placed on the net by those that have it wrong in the first place. No, I go by what is actually being used out there. 400 Hz is not it, it is how ever, the top end of most inverters that produce a form of switched wave into a motor working with induction of the motor, but that is being generated via a 8khz or more, pwm pulse. If any one ever looks at a vector drive, the magnetic current is normally set at 50% of the motors max current or what ever the suggested max is on the name plate of the motor. That motor will sit there at a stall speed of 0 RPM but, will have at least 8khz or more pulsing in the fields to maintain as near DC perfect current as possible. Of course, the cheaper inverters do not have a high enough resolution output in them and at times, you may see a creep in the shaft that is hard to stop using open loop control at 0 speed. Mechanical load can be factored in to stop that or, just use a feed back system. Jamie |
#23
![]()
Posted to sci.electronics.repair
|
|||
|
|||
![]()
On Fri, 9 Mar 2012 14:30:58 -0800, "Robbie Hatley"
wrote: writes: Robbie, Phil Allison is sufering from some type of mental disease. He does know some about electronics but most times just posts abusive language. Most folks just killfile him and so they don't see his original posts. Eric Well, he's certainly eccentric. But as for "mental disease", that's a metaphor, not an actual thing. I think Phil just has a habit of over-reacting emotionally to things and "going off". If he looses that habit, things will improve for him and those around him. Umm, no. We are talking about medically diagnosed and on prescription medication. When he posts within his competence he is usually spot on, and often not vulgar or abusive. ?-) |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|
![]() |
||||
Thread | Forum | |||
profit-motive question: single phase adapter for 3-phase motor drives | Metalworking | |||
can a three phase motor work on single phase? | Electronics Repair | |||
Variable Speed AC Motor | Electronics Repair | |||
Variable speed motor for my 110V mill/drill? | Metalworking | |||
Variable speed motor for my 110V mill/drill? | Metalworking |