How do variable-speed 3-phase motor drives actually work?

"Robbie Hatley"

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

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.

"Sylvia Else"

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

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.

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:

Basically uses the inductance of the motor windings and the magnetic reluctance of the core to convert the 400Hz electric pulses into

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:

Very cool chart there showing 8 different valid states that a 6-IGBT bridge can be in (and hinting at the various catastrophic

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.

Apparently so.

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.

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.

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.

"Robbie Hatley"

... we should have him stuffed ........

Apologies to Basil Faulty.

.... Phil

"Robbie Hatley = mad as a Hatter "

But now I see you are actually a full on, raving lunatic

- the concept of relevance is a completely foreign one.

Kindly FOAD.

.... Phil

Mr Hatley... "Phil is ill." If you want to publicly tweak his nose, fine. But don't expect to change him.

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.

Hatters have no more likelyhood of being "mad" (either insane or merely angry) than persons of any other profession.

Furthermore, I am not a hatter. (I'm a computer programmer, electronics technician, and many other things.)

"Drivel" does not mean "new information which does not duplicate things recently said by others". (The phrase for that is "insight", not "drivel".)

"Drivel" means "a series of meaningless and/or false statements", which is not applicable here.

In short, fallacy of equivocation. (Look it up.)

Fallacy of equivocation again ("vague definitions" subvariant).

Also, fallacy of argumentum ad hominem (calling someone names says nothing about the truth or falsehood of their assertions, though it does say something about the maturity level of the name-caller).

No it's not. Not to you, not to me. Fallacy of "argumentum ad nauseum" (attempting to make falsehoods seem true by stating them forcefully or repetitively).

"FOAD" is not a word (nor is it an acronym I recognize).

Please blick at least 7 groaks before the 3rd of Grosvoickair, unless the green grigs outnumber the blue grigs at least 3 to 1. Capornay, proquay? (Yes, I can write gibberish too. Wheeeee! Aint this fun!)

Bottom line: poor post again, Phil. Full of fallacies, falsehoods, and needless insults again. Were you perhaps staying up late again? 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".

Mercury poisoning, from the compounds using in felting.

Exactly :)

Jamie

"William Sommerwerck"

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.

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.

Please do NOT strip the ">". They help others keep track of who said what.

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.

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

metaphor,

emotionally

for

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.

?-)