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

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

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

"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

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

"Jamie" = Maynard A. Philbrook Radio Ham


** **** OFF you PSYCHO HAM RADIO ****

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

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

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

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.

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

"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

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

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/

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

"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

Ham Radio (was: 3-phase motor drives)
 

"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

Ham Radio (was: 3-phase motor drives)
 

"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

Ham Radio
 

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

Ham Radio (was: 3-phase motor drives)
 

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

Ham Radio
 

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/

3 phase motors & PWM
 

"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

Ham Radio
 

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

Ham Radio (was: 3-phase motor drives)
 

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.

--
RH

3 phase motors & PWM
 

"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.

Ham Radio
 

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

3 phase motors & PWM
 

"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.

3 phase motors & PWM
 

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.

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

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.

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

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

Ham Radio (was: 3-phase motor drives)
 

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.

?-)