3 phase PWM motor controller filtering

[colin]

Hi,
Whats the best way to arrange filtering between mosfets and motor ?

ie. is there anything to be gained here by using center aligned as opposed
to edge aligned PWM?
so can common mode chokes be used ?

With no filtering the mosfets get very hot becuase the motor has very little
inductance,
but then the inductors I have atm just a few turns of 1mm wire on a iron
powder torriod get very hot indeed
when the motor is running, but at no voltage out, ie when all op are at 50%
duty, theres no problem.

its a model motor, quite smal but rated 400w,
im running it at 10krpm , possibly overunning it to 30k,
only about 2 turns on each of the 12 poles,
but with the very strong magnets gives 1000rpm/volt.
with no load I dont expect the current to be that high but with 7 electrical
cycles per rev
its going quite a bit to get 10krpm or more,
still it takes 2 amps before the thing will first move,
and im stil debugging the mcu code thats driving it.

Also do they make level shifting high AND low side drivers ?
If I use it for higher voltage motor I might want my output to be centered
closer to ground but dont want my micro sitting at
-v, I gues I could opto couple ofc.

Colin =^.^=

 

[colin]

"colin" <no.spam.for.me@ntlworld.com> wrote in message
news:%44th.73354$z01.6871@newsfe3-gui.ntli.net...

Hi,
Whats the best way to arrange filtering between mosfets and motor ?

ie. is there anything to be gained here by using center aligned as opposed
to edge aligned PWM?
so can common mode chokes be used ?

With no filtering the mosfets get very hot becuase the motor has very
little
inductance,
but then the inductors I have atm just a few turns of 1mm wire on a iron
powder torriod get very hot indeed
when the motor is running, but at no voltage out, ie when all op are at
50%
duty, theres no problem.

its a model motor, quite smal but rated 400w,
im running it at 10krpm , possibly overunning it to 30k,
only about 2 turns on each of the 12 poles,
but with the very strong magnets gives 1000rpm/volt.
with no load I dont expect the current to be that high but with 7
electrical
cycles per rev
its going quite a bit to get 10krpm or more,
still it takes 2 amps before the thing will first move,
and im stil debugging the mcu code thats driving it.

Also do they make level shifting high AND low side drivers ?
If I use it for higher voltage motor I might want my output to be centered
closer to ground but dont want my micro sitting at
-v, I gues I could opto couple ofc.

oops sorry wrong group, moving to sci.electronics.design

Colin =^.^=

 

[Jon Elson]

colin wrote:

Hi,
Whats the best way to arrange filtering between mosfets and motor ?

ie. is there anything to be gained here by using center aligned as opposed
to edge aligned PWM?
so can common mode chokes be used ?

With no filtering the mosfets get very hot becuase the motor has very little
inductance,
but then the inductors I have atm just a few turns of 1mm wire on a iron
powder torriod get very hot indeed
when the motor is running, but at no voltage out, ie when all op are at 50%
duty, theres no problem.

This is a real problem. I haven't come up with any way to build

a good output filter for traditional PWM schemes like synchronous
antiphase, in which either the high-side or low-side transistor
are always on. I have a DC motor drive that uses sign-magnitude
instead, and it works MUCH better. I haven't extended this to
3-phase yet, but am working on it. It seems that if the two
poles that are to be driven according to the commutation signals
are shorted except when the PWM says to apply power, then the
circulating currents in the motor and filter will be much less.
(This requires enough series inductance in the filter so the
decelerating current never gets very large in a single cycle.)

its a model motor, quite smal but rated 400w,
im running it at 10krpm , possibly overunning it to 30k,
only about 2 turns on each of the 12 poles,
but with the very strong magnets gives 1000rpm/volt.
Oh, my, that is a real problem! There is no way you can run

such a motor without adding inductance externally. You either
have to drastically limit the number of turns/core or use a
modulation scheme that cuts down on the current ripple. A
higher PWM frequency can help, up to a point.

with no load I dont expect the current to be that high but with 7 electrical
cycles per rev
its going quite a bit to get 10krpm or more,
still it takes 2 amps before the thing will first move,
and im stil debugging the mcu code thats driving it.

Also do they make level shifting high AND low side drivers ?
Look at the IR 2113

Jon

 

[colin]

"Jon Elson" <elson@pico-systems.com> wrote in message
news:KNednYBbqKlBZSnYnZ2dnUVZ_vipnZ2d@giganews.com...

colin wrote:
Hi,
Whats the best way to arrange filtering between mosfets and motor ?

ie. is there anything to be gained here by using center aligned as
opposed
to edge aligned PWM?
so can common mode chokes be used ?

With no filtering the mosfets get very hot becuase the motor has very
little
inductance,
but then the inductors I have atm just a few turns of 1mm wire on a iron
powder torriod get very hot indeed
when the motor is running, but at no voltage out, ie when all op are at
50%
duty, theres no problem.

This is a real problem. I haven't come up with any way to build
a good output filter for traditional PWM schemes like synchronous
antiphase, in which either the high-side or low-side transistor
are always on. I have a DC motor drive that uses sign-magnitude

instead, and it works MUCH better.

Thanks, Im not familiar with those terms, but all my 3 outputs are always
switching,
thus at idle each is at 50% pwm.

I haven't extended this to
3-phase yet, but am working on it. It seems that if the two
poles that are to be driven according to the commutation signals
are shorted except when the PWM says to apply power, then the
circulating currents in the motor and filter will be much less.
(This requires enough series inductance in the filter so the
decelerating current never gets very large in a single cycle.)

with synchronous 50% pwm the phases are also effectivly always shorted
together,
with one phase at 51% pwm the phases are shorted 99% of the time.
with the pwm applying power between phases for 1%.
although the CM signal is large this isnt a power consuming issue.

I tried making one output always low but this transfered most of the
dissipation in the top FET to the lower one at low duty cycle.

its a model motor, quite smal but rated 400w,
im running it at 10krpm , possibly overunning it to 30k,
only about 2 turns on each of the 12 poles,
but with the very strong magnets gives 1000rpm/volt.

Oh, my, that is a real problem! There is no way you can run
such a motor without adding inductance externally. You either
have to drastically limit the number of turns/core or use a
modulation scheme that cuts down on the current ripple. A
higher PWM frequency can help, up to a point.

yes it does seem a bit of a problem, ofc too much inductance will limit my
top speed.
Im wondering if I should feed the 3ph bridge with an adjustable voltage from
a switching regulator to
reduce the pk-pk HF content.

with no load I dont expect the current to be that high but with 7
electrical
cycles per rev
its going quite a bit to get 10krpm or more,
still it takes 2 amps before the thing will first move,
and im stil debugging the mcu code thats driving it.

Also do they make level shifting high AND low side drivers ?
Look at the IR 2113

the low side level shifting only tolerates 5v, I was after 100-600v for both
hi and low,
so the logic input gnd is totaly isolated from the motor supply gnd.

Colin =^.^=

 

[Jon Elson]

colin wrote:

"Jon Elson" <elson@pico-systems.com> wrote in message
news:KNednYBbqKlBZSnYnZ2dnUVZ_vipnZ2d@giganews.com...


This is a real problem. I haven't come up with any way to build
a good output filter for traditional PWM schemes like synchronous
antiphase, in which either the high-side or low-side transistor
are always on. I have a DC motor drive that uses sign-magnitude


instead, and it works MUCH better.



Thanks, Im not familiar with those terms, but all my 3 outputs are always
switching,
thus at idle each is at 50% pwm.

This is the same as what would be called synchronous-antiphase on an

H-bridge, like for a DC brush motor. The continuous triangle-wave current
is the problem. If the inductance is small, then the peak-peak current is
large. without changing the PWM modulation scheme, the only fix is to
add inductance, and then you move the heating problem from the motor and
transistors to the inductor core. That is not a great improvement, in most
cases.

I haven't extended this to
3-phase yet, but am working on it. It seems that if the two
poles that are to be driven according to the commutation signals
are shorted except when the PWM says to apply power, then the
circulating currents in the motor and filter will be much less.
(This requires enough series inductance in the filter so the
decelerating current never gets very large in a single cycle.)



with synchronous 50% pwm the phases are also effectivly always shorted
together,
with one phase at 51% pwm the phases are shorted 99% of the time.
with the pwm applying power between phases for 1%.
although the CM signal is large this isnt a power consuming issue.

I tried making one output always low but this transfered most of the
dissipation in the top FET to the lower one at low duty cycle.

Now, you are on the right track! Just add series inductance. The change

in modulation from full voltage one way or the other across the inductance
all the time, to full voltage only 1% of the time is an ENORMOUS
improvement! You would need huge cores with the 50% scheme, but a
MUCH smaller core will do for the 1% scheme. Your test didn't work
out because you were shorting the motor - clearly a lot of loss there. With
enough inductance in series, then the circulating currents both at the PWM
frequency as well as the motor's synchronous frequency will be reduced.

yes it does seem a bit of a problem, ofc too much inductance will limit my
top speed.
Im wondering if I should feed the 3ph bridge with an adjustable voltage from
a switching regulator to
reduce the pk-pk HF content.

They used to do this, but it is not necessary. And, you don't need much

inductance,
just enough to cut the P-P currents at the PWM frequency, not the
motor's synch
freq.

Look at the IR 2113



the low side level shifting only tolerates 5v, I was after 100-600v for both
hi and low,
so the logic input gnd is totaly isolated from the motor supply gnd.



Opto-isolate then. I use the TLP2200 with the logic running at 12 V for

noise immunity. The same 12 V is the FET gate bias supply, too.

Jon

 

[colin]

"Jon Elson" <jmelson@artsci.wustl.edu> wrote in message
news:45B7C3B6.5040909@artsci.wustl.edu...

colin wrote:

"Jon Elson" <elson@pico-systems.com> wrote in message
news:KNednYBbqKlBZSnYnZ2dnUVZ_vipnZ2d@giganews.com...


This is a real problem. I haven't come up with any way to build
a good output filter for traditional PWM schemes like synchronous
antiphase, in which either the high-side or low-side transistor
are always on. I have a DC motor drive that uses sign-magnitude


instead, and it works MUCH better.



Thanks, Im not familiar with those terms, but all my 3 outputs are always
switching,
thus at idle each is at 50% pwm.

This is the same as what would be called synchronous-antiphase on an
H-bridge, like for a DC brush motor. The continuous triangle-wave current
is the problem. If the inductance is small, then the peak-peak current is
large. without changing the PWM modulation scheme, the only fix is to
add inductance, and then you move the heating problem from the motor and
transistors to the inductor core. That is not a great improvement, in
most
cases.

but my 3ph pwm outputs are not antiphase.
whatever scheme is used there must be enough inductance to limit the rise of
the current during the on period.
otherwise the FETs realy will get very hot indeed wich was my first problem.

The motor has much less than 10uh (limit of my measurer)
initialy I tried winding 200uh with one of my cores from a big bag ive got,
but this got way to hot so i had to use thicker wire with less turns.

I would agree that antiphase is as if its trying to deliberatly do the worst
thing lol
and anything would be better.

I haven't extended this to
3-phase yet, but am working on it. It seems that if the two
poles that are to be driven according to the commutation signals
are shorted except when the PWM says to apply power, then the
circulating currents in the motor and filter will be much less.
(This requires enough series inductance in the filter so the
decelerating current never gets very large in a single cycle.)



with synchronous 50% pwm the phases are also effectivly always shorted
together,
with one phase at 51% pwm the phases are shorted 99% of the time.
with the pwm applying power between phases for 1%.
although the CM signal is large this isnt a power consuming issue.

I tried making one output always low but this transfered most of the
dissipation in the top FET to the lower one at low duty cycle.

Now, you are on the right track! Just add series inductance. The change
in modulation from full voltage one way or the other across the inductance
all the time, to full voltage only 1% of the time is an ENORMOUS
improvement! You would need huge cores with the 50% scheme, but a
MUCH smaller core will do for the 1% scheme.

but with my in phase pwm outputs there is also definatly no reverse voltage
across the inductors, as they are accross the outputs.
there is only a forward voltage difference when the falling edges are skewed
by different duty cycles wich wld be 1% at low power.

Ive also decided to switch to center aligned PWM in my dsPIC wich means
power pulses between phases on both edges of the PWM output.

Incidently I used another method where I only used 2 PWM modules for 3ph.
one PWM controlled the first phase completly.
one of the other phases was either high or low as appropriate,
and the other was pwm driven.

Its also neat to keep the motor center volts close to ground.

Your test didn't work
out because you were shorting the motor - clearly a lot of loss there.
With
enough inductance in series, then the circulating currents both at the PWM
frequency as well as the motor's synchronous frequency will be reduced.

yes you need enough total series inductance to flywheel the current so that
the motor sees just an average voltage.
otherwise the current might also reverse wich will just multiply all losses.

another big reason why it was getting so hot was software, with such a
incredibly low motor resistance not just inductance a huge current flows if
the op voltage does not match the back emf from the motor, the motor speed
is of course determined by the comutation frequency. took me a while to get
this right, what with problems with hall sensors not working too well etc.
and the smoke was kinda putting me off tbh.

Opto-isolate then. I use the TLP2200 with the logic running at 12 V for
noise immunity. The same 12 V is the FET gate bias supply, too.

yes I had considered that if I couldnt find what I wanted.
also gate drive transformers driven from a HF chopped signal and rectified,
as some mcu have PWM outputs wich can do this directly.

thanks for all the help and ideas, its clearer now.

Colin =^.^=

 

[joseph2k]

colin wrote:

"Jon Elson" <jmelson@artsci.wustl.edu> wrote in message
news:45B7C3B6.5040909@artsci.wustl.edu...
colin wrote:

"Jon Elson" <elson@pico-systems.com> wrote in message
news:KNednYBbqKlBZSnYnZ2dnUVZ_vipnZ2d@giganews.com...


This is a real problem. I haven't come up with any way to build
a good output filter for traditional PWM schemes like synchronous
antiphase, in which either the high-side or low-side transistor
are always on. I have a DC motor drive that uses sign-magnitude


instead, and it works MUCH better.



Thanks, Im not familiar with those terms, but all my 3 outputs are
always switching,
thus at idle each is at 50% pwm.

<Big Snip>

yes I had considered that if I couldnt find what I wanted.
also gate drive transformers driven from a HF chopped signal and
rectified, as some mcu have PWM outputs wich can do this directly.

thanks for all the help and ideas, its clearer now.

Colin =^.^=

Please do not shoot the messenger, but i do not think that hobby 3-phase
motor controllers do not do any PWM at all. Except at stop, to prevent
burning the motor up. You can get one at any RC hobby shop, or on the
Internet.

I think that running the waveforms look like this:

A. |-----_ |-----_ |-----_ |-----_
_____- _____- _____-

B. ---_ |-----_ |-----_ |-----_
_____- _____- _____- ___

C. -_ |-----_ |-----_ |-----_
_____- _____- _____- _____


This is intended to represent "square waves" with a little dead time.

--
JosephKK
Gegen dummheit kampfen die Gotter Selbst, vergebens.  
--Schiller