VFDs, Noise, and RS-485

[Terry Given]

The Real Andy wrote:

On Thu, 28 Jul 2005 12:48:22 +1200, Terry Given <my_name@ieee.org
wrote:


Nobody wrote:

I have a setup that consists of a variable frequency drive (VFD) and a PC.
The two talk to each other using RS-485. (The VFD output is 3-phase, with a
high frequency square wave riding on a 100 Hz sinusoid carrier wave.)

No its not, its a high-frequency variable duty cycle "square" wave,
whose duty cycle is modulated such that the average output voltage is a
100Hz sinusoid. Who's VFD? (Danfoss is my guess, they like the term VFD)



I have read here before about this business of putting different
signals into the drive. Why is this? I think last time someone
mentioned (sphero perhaps) injecting a 3rd harmonic or something of
the like.

Andy

the idea is to continue increasing the RMS output voltage when the peak
is fixed - clipping aka overmodulation.

many digital algorithms like Space Vector Modulation (SVM) automagically
do this. Many digital algorithms dont, and the 3rd harmonic (and various
other stuff) gets tacked on as an afterthought.

In the case of naturally sampled PWM (3ph LF sinusoidal voltage feeding
3 comparators along with an HF triangular carrier wave), the harmonics
get added in to the voltage reference. Back before micros, that was
really the only way to do it - I have seen a paper describing a direct
field oriented controlledr done entirely in analogue, circa 1974 IIRC.


Cheers
Terry

 

[The Real Andy]

On Fri, 29 Jul 2005 17:43:58 +1200, Terry Given <my_name@ieee.org>
wrote:

The Real Andy wrote:
On Thu, 28 Jul 2005 12:48:22 +1200, Terry Given <my_name@ieee.org
wrote:
snip

I have read here before about this business of putting different
signals into the drive. Why is this? I think last time someone
mentioned (sphero perhaps) injecting a 3rd harmonic or something of
the like.

Andy

the idea is to continue increasing the RMS output voltage when the peak
is fixed - clipping aka overmodulation.

Ahh, interesting. Do you know of any papers on this? One would expect
that 'over modulating' would have on adverse affect on the performance
of the motor, ie heating. I guess this is not the case though if it is
common practice.

many digital algorithms like Space Vector Modulation (SVM) automagically
do this. Many digital algorithms dont, and the 3rd harmonic (and various
other stuff) gets tacked on as an afterthought.

In the case of naturally sampled PWM (3ph LF sinusoidal voltage feeding
3 comparators along with an HF triangular carrier wave), the harmonics
get added in to the voltage reference. Back before micros, that was
really the only way to do it - I have seen a paper describing a direct
field oriented controlledr done entirely in analogue, circa 1974 IIRC.


Cheers
Terry

I am building my first VFD for my father who is a retired engineer. So
far so good. I am running motors, albeit, unloaded atm. Using the new
IR iMotion IGBT module and a pic18f4431. I often wondered about the
third harmonic theory. I guess for driving a 3phase lathe motor from
single phase its probably not worth the effort.

 

[Terry Given]

The Real Andy wrote:

On Fri, 29 Jul 2005 17:43:58 +1200, Terry Given <my_name@ieee.org
wrote:


The Real Andy wrote:

On Thu, 28 Jul 2005 12:48:22 +1200, Terry Given <my_name@ieee.org
wrote:

snip

I have read here before about this business of putting different
signals into the drive. Why is this? I think last time someone
mentioned (sphero perhaps) injecting a 3rd harmonic or something of
the like.

Andy

the idea is to continue increasing the RMS output voltage when the peak
is fixed - clipping aka overmodulation.


Ahh, interesting. Do you know of any papers on this? One would expect
that 'over modulating' would have on adverse affect on the performance
of the motor, ie heating. I guess this is not the case though if it is
common practice.

hundreds, if not thousands.

many digital algorithms like Space Vector Modulation (SVM) automagically
do this. Many digital algorithms dont, and the 3rd harmonic (and various
other stuff) gets tacked on as an afterthought.

In the case of naturally sampled PWM (3ph LF sinusoidal voltage feeding
3 comparators along with an HF triangular carrier wave), the harmonics
get added in to the voltage reference. Back before micros, that was
really the only way to do it - I have seen a paper describing a direct
field oriented controlledr done entirely in analogue, circa 1974 IIRC.


Cheers
Terry


I am building my first VFD for my father who is a retired engineer. So
far so good. I am running motors, albeit, unloaded atm. Using the new
IR iMotion IGBT module and a pic18f4431. I often wondered about the
third harmonic theory. I guess for driving a 3phase lathe motor from
single phase its probably not worth the effort.

Space Vector Modulation is by far the easiest to do digitally, and
automagically takes care of overmodulation. Retter's Matrix and Space
Phasor Theory of Electrical Machines is a darned good reference, and a
lot cheaper than anything by Peter Vas.

The basic idea is to convert the 3-phase rotating vectors into an
equivalent single rotating vector (can do because of fixed 120 degree
relationships). This vector is broken into x- and y- components (called
a,b). The observer then rotates at the same frequency, generating a
stationary (wrt obs) pair of coordinates d,q. Align to one axis, the
other is by def. zero. I'll see if I cant dig up a decent SVM paper...

Cheers
Terry

 

[The Real Andy]

On Sat, 30 Jul 2005 12:55:56 +1200, Terry Given <my_name@ieee.org>
wrote:

The Real Andy wrote:
On Fri, 29 Jul 2005 17:43:58 +1200, Terry Given <my_name@ieee.org
wrote:


The Real Andy wrote:

On Thu, 28 Jul 2005 12:48:22 +1200, Terry Given <my_name@ieee.org
wrote:

snip

I have read here before about this business of putting different
signals into the drive. Why is this? I think last time someone
mentioned (sphero perhaps) injecting a 3rd harmonic or something of
the like.

Andy

the idea is to continue increasing the RMS output voltage when the peak
is fixed - clipping aka overmodulation.


Ahh, interesting. Do you know of any papers on this? One would expect
that 'over modulating' would have on adverse affect on the performance
of the motor, ie heating. I guess this is not the case though if it is
common practice.

hundreds, if not thousands.



many digital algorithms like Space Vector Modulation (SVM) automagically
do this. Many digital algorithms dont, and the 3rd harmonic (and various
other stuff) gets tacked on as an afterthought.

In the case of naturally sampled PWM (3ph LF sinusoidal voltage feeding
3 comparators along with an HF triangular carrier wave), the harmonics
get added in to the voltage reference. Back before micros, that was
really the only way to do it - I have seen a paper describing a direct
field oriented controlledr done entirely in analogue, circa 1974 IIRC.


Cheers
Terry


I am building my first VFD for my father who is a retired engineer. So
far so good. I am running motors, albeit, unloaded atm. Using the new
IR iMotion IGBT module and a pic18f4431. I often wondered about the
third harmonic theory. I guess for driving a 3phase lathe motor from
single phase its probably not worth the effort.

Space Vector Modulation is by far the easiest to do digitally, and
automagically takes care of overmodulation. Retter's Matrix and Space
Phasor Theory of Electrical Machines is a darned good reference, and a
lot cheaper than anything by Peter Vas.

I did do some reading on this concept. There is a good note on
Microchips website.

The basic idea is to convert the 3-phase rotating vectors into an
equivalent single rotating vector (can do because of fixed 120 degree
relationships). This vector is broken into x- and y- components (called
a,b). The observer then rotates at the same frequency, generating a
stationary (wrt obs) pair of coordinates d,q. Align to one axis, the
other is by def. zero. I'll see if I cant dig up a decent SVM paper...

If you can find that paper that would be great.

Cheers
Terry

Thanks:

Andy

 

[Jeff]

"Terry Given" <my_name@ieee.org> wrote in message
news:AeAGe.4566$PL5.413141@news.xtra.co.nz...

The Real Andy wrote:
On Fri, 29 Jul 2005 17:43:58 +1200, Terry Given <my_name@ieee.org
wrote:


The Real Andy wrote:

On Thu, 28 Jul 2005 12:48:22 +1200, Terry Given <my_name@ieee.org
wrote:

snip

I have read here before about this business of putting different
signals into the drive. Why is this? I think last time someone
mentioned (sphero perhaps) injecting a 3rd harmonic or something of
the like.

Andy

the idea is to continue increasing the RMS output voltage when the peak
is fixed - clipping aka overmodulation.


Ahh, interesting. Do you know of any papers on this? One would expect
that 'over modulating' would have on adverse affect on the performance
of the motor, ie heating. I guess this is not the case though if it is
common practice.

hundreds, if not thousands.



many digital algorithms like Space Vector Modulation (SVM) automagically
do this. Many digital algorithms dont, and the 3rd harmonic (and various
other stuff) gets tacked on as an afterthought.

In the case of naturally sampled PWM (3ph LF sinusoidal voltage feeding
3 comparators along with an HF triangular carrier wave), the harmonics
get added in to the voltage reference. Back before micros, that was
really the only way to do it - I have seen a paper describing a direct
field oriented controlledr done entirely in analogue, circa 1974 IIRC.


Cheers
Terry


I am building my first VFD for my father who is a retired engineer. So
far so good. I am running motors, albeit, unloaded atm. Using the new
IR iMotion IGBT module and a pic18f4431. I often wondered about the
third harmonic theory. I guess for driving a 3phase lathe motor from
single phase its probably not worth the effort.

Space Vector Modulation is by far the easiest to do digitally, and
automagically takes care of overmodulation. Retter's Matrix and Space
Phasor Theory of Electrical Machines is a darned good reference, and a
lot cheaper than anything by Peter Vas.

The basic idea is to convert the 3-phase rotating vectors into an
equivalent single rotating vector (can do because of fixed 120 degree
relationships). This vector is broken into x- and y- components (called
a,b). The observer then rotates at the same frequency, generating a
stationary (wrt obs) pair of coordinates d,q. Align to one axis, the
other is by def. zero. I'll see if I cant dig up a decent SVM paper...

Cheers
Terry

I'd also be interested in that paper.
If you wish to email it, levy(underscore)jeff(at)hotmail(dot)com is a
vailid email address, after replacing the obvious in brackets.

Jeffg

 

[Tony Williams]

In article <eR3Ge.4216$PL5.383754@news.xtra.co.nz>,
Terry Given <my_name@ieee.org> wrote:

Do tell.... Earth Management on big systems is a
hard lesson to learn.

Now thats something I'm interested in hearing more about...

You've already brushed on the subject. It seems
to be a question of understanding that all devices
generate earth currents, the obvious and the not-so
obvious, (eg, power transistors on a heatsink, or
a switcher, or the field coils of a motor that you
mentioned). It is important to plot a path to ensure
that all such earth currents are explicitly returned
to where they came from, not allowed to find their
way home via some other device. That is an obvious
thing to say, but sometimes not so easy to see and
implement in practice.

Bitter experience results in automatic design habits
that help to avoid earthing troubles in the completed
system. Stupid little things, such as.......

Make twisted pairs, (triples, quads), your baseline
wiring habit. This forces you to consider and make
the explicit return between source and load, and
twisted pairs reduces differential pickup or radiation.
Keep the twisted pairs going, even through such things
as fuses or CT-assemblies.

Screen each functional set if possible, with the
screen normally Earthed at one end only.

With heavyweight source-loads, Earth at the source,
and have a low impedance Earth-Earth connection between
the source and the load.... already mentioned is the
circumstance where this may be via the screen, although
I would put in a separate Earth wire.

Low level devices which have an 'earthy' 0v and which are
to be connected to an earthed device are always a source
of ambush, because of the possibility of A.N Other's earth
currents coming back through them. This would be things
like scopes, counter-timers, comms from PC's, etc.

Isolation would be nice, but often not practical. The
secondary solution is low-value resistors in the 0v lines,
use differential signals if possible and/or try to have some
sort of CMRR capability.

eg. Use RS485 not RS232 if possible, and try to check the
CMV at the receiver end.

eg. DC to moderate freqency signals can be transmitted as
currents, with a burden resistor at the receiving end.
The c-c source does the CMRR and the transmitted signal
is converted to a voltage at the receiver, with respect
to the receiver's 0v. This can be applied to dc control
signals, RS232 comms (if you are stuck with it) and even
scope or counter-timer pickups.

Note: Run RS232 comms as two single-screened cables.
This avoids Txd/Rxd crosstalk over long distances.

Simple tricks really.................

--
Tony Williams.