VFDs, Noise, and RS-485

[Nobody]

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.)

I'm running into some major VFD noise problems, and I don't know how to
solve them.

1. The noise problem is so bad that once the VFD starts running, it kills
the RS-485 signal.

2. The noise problem also kills our ability to use a hall current probe to
sense the current through one of the 3-phase output lines.

I've tried grounding everything. I'm using shielded cables and toroids for
the communication and measurement lines.

On the RS-485 end, we've tried isolated and non-isolated devices (with
proper termination resistors) on the PC.

The noise won't go away! Argh!

Any tips would be appreciated.

- Nobody

 

[Terry Given]

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'm running into some major VFD noise problems, and I don't know how to
solve them.

1. The noise problem is so bad that once the VFD starts running, it kills
the RS-485 signal.

2. The noise problem also kills our ability to use a hall current probe to
sense the current through one of the 3-phase output lines.

I've tried grounding everything. I'm using shielded cables and toroids for
the communication and measurement lines.

On the RS-485 end, we've tried isolated and non-isolated devices (with
proper termination resistors) on the PC.

The noise won't go away! Argh!

Any tips would be appreciated.

- Nobody

Gidday,

Drives are nasty. Firstly, ensure you have some form of a co-axial feed
from the drive to the machine such as neutral screened cable, with the
screen well bonded to the drive *AND* the machine. This is exactly wrong
for an instrumentation cable, but right for the drive. Herewith an
excerpt from a report I wrote for a customer experiencing this sort of
problem in Nov 2003 with a 400kW rock crusher:

"The existing hardware operates in an extremely adverse environment,
both physically and electromagnetically. The physical environmental
hazards are primarily temperature, vibration and fine particles, which
should be assumed to be conductive due to crusher plate abrasion.
Electromagnetic hazards are primarily caused by the motor drives used in
the rock crusher itself; drives generate significant EMI, and are
responsible for the bulk of EMI problems in industry. Ideally drives and
motors should be connected using “screened” cable, typically steel-wire
sheathed. The drive cable screen should be firmly bonded to both the
drive and the motor – note that this is exactly the wrong thing to do
for an instrumentation cable.

Motor drives generate huge high-frequency (HF) current spikes which flow
through the motor winding capacitance to the motor chassis. Both the
drive and the motor chassis are earthed, so these HF currents flow
through the earth connection and back to the drive. The amount of this
HF energy which radiates away is directly proportional to the size of
the physical loop the current flows in. Connecting the cable screen as
above ensures a good high-frequency connection between the drive and
motor – basically it minimises the physical size of the loop the HF
currents flow through, thus greatly reducing the amount of HF energy
which radiates away"


Once you have done this (minimised the source of emissions) you can then
reduce the susceptibility of your setup. Areas to look at are:

- optically isolate the RS485 link. This is necessary to break the earth
loop from PC to drive (which, by Norton, carries some of the HF
currents). A moot point if you are running the PC from batteries, but of
vital importance when mains powered

- keep wiring loops small. twist, twist, twist. ground plane, ground
plane, ground plane. Drives spew out vast quantities of H-field, which
your loops will happily receive. Technically this reduces the coupling,
rather than the susceptibility, but who cares as long as it works right?

- correctly terminate the RS485 cable

- use high permeability common-mode chokes on the various dangly wire
bundles.

As to why your hall effect sensors dont work, I suspect its poor
layout/decoupling, or perhaps ratshit DCCTs. The VFD has 2-3 inside, and
they work fine. The problems I've come across with current transducers are:

- capacitive coupling from the drive output. the fix is simple, a
grounded electrostatic shield around the cable as it passes thru the
DCCT. The ground connection must be low inductance. In practice drive
manufacturers sample in the middle of each pulse, thereby avoiding the
switching edge noise.

- variation of current as a function of cable position, and/or adjacent
metalwork. sign of a crappy DCCT....

Make sure your DCCT supply (supplies) are well decoupled right at the
DCCT. Again, twist the wires (stranded CAT5 cable is great for this sort
of job, esp. STP). Also look at how you measure the DCCT results, and
reduce/remove loop area - if with a scope, use a coax connection to the
scope/probe rather than a 6" ground clip.

Read Linear Technologies AN47.

HTH

Cheers
Terry

 

[Joerg]

Hello,

Can you give us a first name?

1. The noise problem is so bad that once the VFD starts running, it kills
the RS-485 signal.

As Terry said it is probably a PWM signal that actually runs the drive
and not just controls it. The ripple you see would then be the stuff
that didn't get smoothed out in the output filters of the converter
unit. One thing you could look at is filtering some more at the source
but you may no be able or allowed to modify that, let alone get inside
the converter.

2. The noise problem also kills our ability to use a hall current probe to
sense the current through one of the 3-phase output lines.

I usually prefer current transformers. You can filter the output of
these since you are then dealing with isolated and low level signals.
Running the output of the current transformer differentially to the
system where there would be another transformer may also be an option.
But don't ever move the resistor away from the current transformer. Else
havoc can happen should somebody disconnect the cable.

I've tried grounding everything. I'm using shielded cables and toroids for
the communication and measurement lines.

At these power levels you'd have to be a real RF expert to get it
running as is. Or hire one ;-)

On the RS-485 end, we've tried isolated and non-isolated devices (with
proper termination resistors) on the PC.

This situation really calls for transformer isolated transmission, not
just differential receivers/drivers. The required transformers would be
pretty small and in a pinch can be made by hand. Toroids, no pot cores
and the like. Making your own transformers instead of relying on some
"isolation device" or transformer in a can has the advantage that you
know how good and symmetrical it is.

Regards, Joerg

http://www.analogconsultants.com

 

[Nobody]

"Terry Given" <my_name@ieee.org> wrote in message
news:sXVFe.4049$PL5.375853@news.xtra.co.nz...

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'm running into some major VFD noise problems, and I don't know how to
solve them.

1. The noise problem is so bad that once the VFD starts running, it
kills
the RS-485 signal.

2. The noise problem also kills our ability to use a hall current probe
to
sense the current through one of the 3-phase output lines.

I've tried grounding everything. I'm using shielded cables and toroids
for
the communication and measurement lines.

On the RS-485 end, we've tried isolated and non-isolated devices (with
proper termination resistors) on the PC.

The noise won't go away! Argh!

Any tips would be appreciated.

- Nobody

Gidday,

Drives are nasty. Firstly, ensure you have some form of a co-axial feed
from the drive to the machine such as neutral screened cable, with the
screen well bonded to the drive *AND* the machine. This is exactly wrong
for an instrumentation cable, but right for the drive. Herewith an
excerpt from a report I wrote for a customer experiencing this sort of
problem in Nov 2003 with a 400kW rock crusher:

"The existing hardware operates in an extremely adverse environment,
both physically and electromagnetically. The physical environmental
hazards are primarily temperature, vibration and fine particles, which
should be assumed to be conductive due to crusher plate abrasion.
Electromagnetic hazards are primarily caused by the motor drives used in
the rock crusher itself; drives generate significant EMI, and are
responsible for the bulk of EMI problems in industry. Ideally drives and
motors should be connected using “screened” cable, typically steel-wire
sheathed. The drive cable screen should be firmly bonded to both the
drive and the motor – note that this is exactly the wrong thing to do
for an instrumentation cable.

Motor drives generate huge high-frequency (HF) current spikes which flow
through the motor winding capacitance to the motor chassis. Both the
drive and the motor chassis are earthed, so these HF currents flow
through the earth connection and back to the drive. The amount of this
HF energy which radiates away is directly proportional to the size of
the physical loop the current flows in. Connecting the cable screen as
above ensures a good high-frequency connection between the drive and
motor – basically it minimises the physical size of the loop the HF
currents flow through, thus greatly reducing the amount of HF energy
which radiates away"


Once you have done this (minimised the source of emissions) you can then
reduce the susceptibility of your setup. Areas to look at are:

- optically isolate the RS485 link. This is necessary to break the earth
loop from PC to drive (which, by Norton, carries some of the HF
currents). A moot point if you are running the PC from batteries, but of
vital importance when mains powered

- keep wiring loops small. twist, twist, twist. ground plane, ground
plane, ground plane. Drives spew out vast quantities of H-field, which
your loops will happily receive. Technically this reduces the coupling,
rather than the susceptibility, but who cares as long as it works right?

- correctly terminate the RS485 cable

- use high permeability common-mode chokes on the various dangly wire
bundles.

As to why your hall effect sensors dont work, I suspect its poor
layout/decoupling, or perhaps ratshit DCCTs. The VFD has 2-3 inside, and
they work fine. The problems I've come across with current transducers
are:

- capacitive coupling from the drive output. the fix is simple, a
grounded electrostatic shield around the cable as it passes thru the
DCCT. The ground connection must be low inductance. In practice drive
manufacturers sample in the middle of each pulse, thereby avoiding the
switching edge noise.

- variation of current as a function of cable position, and/or adjacent
metalwork. sign of a crappy DCCT....

Make sure your DCCT supply (supplies) are well decoupled right at the
DCCT. Again, twist the wires (stranded CAT5 cable is great for this sort
of job, esp. STP). Also look at how you measure the DCCT results, and
reduce/remove loop area - if with a scope, use a coax connection to the
scope/probe rather than a 6" ground clip.

Read Linear Technologies AN47.

HTH

Cheers
Terry

Nice response, Terry.

I'm using a Delta VFD-A. They no longer have this model on their website.

I don't know how fesable it will be to use screened cable. The problem is
that I'm using the drive to test a motor for production, and I may be stuck
with whatever cables come pre-attached to the motor.

As for the RS-485 isolation, this hasn't helped as much. I was using an
isolated PCI RS-485 with built-in terminator, and I was still getting noise
problems from the VFD when it was running.

Let me see what I can do, and I'll post a follow-up.

- Phillip

(Sorry for the abuse of nettiquite!)

 

[Nobody]

"Joerg" <notthisjoergsch@removethispacbell.net> wrote in message
news:1DWFe.1249$iM7.227@newssvr21.news.prodigy.com...

Hello,

Can you give us a first name?

1. The noise problem is so bad that once the VFD starts running, it
kills
the RS-485 signal.

As Terry said it is probably a PWM signal that actually runs the drive
and not just controls it. The ripple you see would then be the stuff
that didn't get smoothed out in the output filters of the converter
unit. One thing you could look at is filtering some more at the source
but you may no be able or allowed to modify that, let alone get inside
the converter.

2. The noise problem also kills our ability to use a hall current probe
to
sense the current through one of the 3-phase output lines.

I usually prefer current transformers. You can filter the output of
these since you are then dealing with isolated and low level signals.
Running the output of the current transformer differentially to the
system where there would be another transformer may also be an option.
But don't ever move the resistor away from the current transformer. Else
havoc can happen should somebody disconnect the cable.

I've tried grounding everything. I'm using shielded cables and toroids
for
the communication and measurement lines.

At these power levels you'd have to be a real RF expert to get it
running as is. Or hire one ;-)

On the RS-485 end, we've tried isolated and non-isolated devices (with
proper termination resistors) on the PC.

This situation really calls for transformer isolated transmission, not
just differential receivers/drivers. The required transformers would be
pretty small and in a pinch can be made by hand. Toroids, no pot cores
and the like. Making your own transformers instead of relying on some
"isolation device" or transformer in a can has the advantage that you
know how good and symmetrical it is.

Regards, Joerg

http://www.analogconsultants.com

I'm definitely NOT an RF guy. I'm just a poor firmware coder who got
hoodwinked into this VFD fiasco.

When you mention transformer isolation, you are suggesting that use one
transformer at each end, correct? I'll try that tomorrow.

As far as current transformers... I think we tried using a current
transformer and combined it with various types of LPFs. The problem we were
having was that the DMM we were using would pick up the VFD noise. Noise
Noise Noise! Argh!

- Phillip

 

[Terry Given]

Nobody wrote:

"Joerg" <notthisjoergsch@removethispacbell.net> wrote in message
news:1DWFe.1249$iM7.227@newssvr21.news.prodigy.com...

Hello,

Can you give us a first name?


1. The noise problem is so bad that once the VFD starts running, it

kills

the RS-485 signal.

As Terry said it is probably a PWM signal that actually runs the drive
and not just controls it. The ripple you see would then be the stuff
that didn't get smoothed out in the output filters of the converter
unit. One thing you could look at is filtering some more at the source
but you may no be able or allowed to modify that, let alone get inside
the converter.


2. The noise problem also kills our ability to use a hall current probe

to

sense the current through one of the 3-phase output lines.

I usually prefer current transformers. You can filter the output of
these since you are then dealing with isolated and low level signals.
Running the output of the current transformer differentially to the
system where there would be another transformer may also be an option.
But don't ever move the resistor away from the current transformer. Else
havoc can happen should somebody disconnect the cable.


I've tried grounding everything. I'm using shielded cables and toroids

for

the communication and measurement lines.

At these power levels you'd have to be a real RF expert to get it
running as is. Or hire one ;-)


On the RS-485 end, we've tried isolated and non-isolated devices (with
proper termination resistors) on the PC.

This situation really calls for transformer isolated transmission, not
just differential receivers/drivers. The required transformers would be
pretty small and in a pinch can be made by hand. Toroids, no pot cores
and the like. Making your own transformers instead of relying on some
"isolation device" or transformer in a can has the advantage that you
know how good and symmetrical it is.

Regards, Joerg

http://www.analogconsultants.com


I'm definitely NOT an RF guy. I'm just a poor firmware coder who got
hoodwinked into this VFD fiasco.

When you mention transformer isolation, you are suggesting that use one
transformer at each end, correct? I'll try that tomorrow.

As far as current transformers... I think we tried using a current
transformer and combined it with various types of LPFs. The problem we were
having was that the DMM we were using would pick up the VFD noise. Noise
Noise Noise! Argh!

- Phillip

Hi Phillip,

there's a clue in there - DMM. Can you perhaps take a few photos of your
setup, and post them to a.b.s.e ( alt.binaries.schematics.electronic ).

It sounds to me like you have some form of CT who's output is monitored
by a DMM. The great thing about DMMs is that the leads make fantastic
loop antennas, and excel at picking up noise. On several occassions
doing drive EMC tests, I have been bitten by attaching DMMs to the
D.U.T. - in one instance, we gave up on the DMM to measure smps voltage
(which went up 40% when we turned on the 40W transmitter beside it), and
just listened to the fan speed instead! Dis-connecting the DMM actually
made the job easier.

The problem is to do with the size of the physical loop formed by the
leads. This loop is a one-turn inductor, whose inductance is
proportional to the loop area. And inductor converts current into H
field, and vice-versa (terrible description, but good enough here). When
some H field impinges on your loop, the amount of current induced is
again proportional to loop area. The DMM is quite high impedance, so
tiny induced currents can result in large measurement errors
(alternatively, one does not have to supply much current to apply a
voltage across the DMM terminals).

Lets just say, for arguments sake, you have a CT with a 10 Ohm burden
resistor. This will be connected to the DMM leads - my fluke 12 leads
are 1.5m long, enough to make a loop about 1m in diameter, which is
sizeable (0.72m^2 or so). what to do?

well, for a start, cable tie the leads together (say every 6" or so).
Now the loop area drops to a minimum of 1.5m * 2Ti where Ti is the
insulation thickness, say 1.5mm. so the area drops to 0.0045m^2, about
160 times smaller than before. In practice the loop area will be bigger
than this, but such an approach can reduce the loop area by 50 - 100x.

That will help, but is it enough? If not, the next step is to twist the
wires. If the physical dimensions of the twisted "loop" are very much
smaller than the wavelength of the offending H-field (they will be) then
adjacent twists have the same H-field across them. However the twisting
of wires ensures that the current induced in one loop is in the opposite
direction to the current induced in the adjacent loop, so the two
cancel. This in effect reduces the inductance even further (and is why
pretty much all comms cables are twisted, eg ethernet, phone etc).

This "minimise loops" concept applies everywhere in your wiring
(especially PCB layout). Eventually, the loops can be minimised no
further - eg the DMM inputs are a fixed distance apart, and away from
the DMM internal circuitry - aint much you can do about that. So at some
point the unavoidable loops dominate noise pickup, and no further gain
will be had from twisting wires.

once the loops are optimised:

If the CT has a 10 Ohm burden resistor, then paralleling another (say)
1k Ohm across it will have a negligible effect on the measurement. So
slapping a 1k directly across the terminals of the DMM wont bugger up
your measurement, but will make it a lot harder to force a noise voltage
across the DMM terminals.

In practice, I would use a BNC-to-banana converter plugged into the DMM,
with a 50 Ohm coax thru-terminator plugged into that (I have a whole
bunch. I normally get people to use a T and an end-terminator, as its
about 10x cheaper than a thru-terminator, and unless you are at very HF,
its good enough), and a 50 Ohm coax cable to the CT, which has a BNC
socket soldered to the burden resistor, and bugger all loop between the
R and the CT (and the R + BNC). Have a look at a pearson CT...

(and yes, I'd have to re-do my CT output scaling calcs, cos 50 aint >>
10, but the maths is easy)

why dont CM toroids help? the noise picked up by your loop is
differential mode not common mode.

why dont LPFs help? the DMM is picking up noise on its leads, which
effectively gets added to the LPF output.

If it makes you feel any better, everyone else has these problems with
VFDs. An amusing anectode:

A customer of ours built a large computer-controlled sawmill - really
cool, a big bank of paralleled PCs solved a travelling salesman-like
problem to optimise the log cuts to maximise return in real-time based
on market data. Fancy sensors, lasers etc. They had a fancy position
sensor that is basically a plunger with a magnet on the end, in a pipe
fed GHz RF at the other end. Some miraculous electronics & a dead white
guy meant they could have an absolute accuracy of 0.1mm or so over tens
of meters. But it used H field to work.

They had a lot of big drives (>= 250kW), and every time the drives
turned on the whole system went apeshit - rams firing off, 12' radial
arm sawblades whizzing out and back, that kind of thing. Because all the
position sensors started firing off nonsense, and their controller wasnt
very robust (no built-in bullshit detector). The cables were screened,
but the screen was *NOT* connected to the motor, only the drive.

So they got hold of our service dept, who told them to tie the cable
shields to the motor as well. Their engineer was a young (my age)
instrumentation guy, and thought that was wrong, so for 3 months he
didnt do it. Eventually, after repeated calls to the service dept. he
did, and *voila* all the problems went away.

This guys manager wanted to know what was wrong with our drives and why
this fix worked (and why his guy didnt implement it for so long), so I
got sent there (with our service manager), to explain why it worked and
that *all* drives behave the same. In the nicest possible way, I had to
explain to the dozen or so customers why this guy was wrong - actually I
just explained the physics behind the interaction, and the physics
behind this guys erroneous decision, which was right for
instrumentation, but wrong for drives. And man, did his boss rip him a
new one afterwards, it was embarassing to see.

No harm was really done though, because during this 3-month period they
were commissioning the entire plant. It was only when all else worked
and the drives had become a bottleneck that the heat went on this guy,
who finally did what we asked.

Cheers
Terry

 

[Tony Williams]

In article <2OXFe.683049$iM6.433658@fe01.news.easynews.com>,
Nobody <protospherex@gmail_nospam_.com> wrote:

Nice response, Terry.
I'm using a Delta VFD-A. They no longer have this model on their
website.
I don't know how fesable it will be to use screened cable. The
problem is that I'm using the drive to test a motor for
production, and I may be stuck with whatever cables come
pre-attached to the motor.

From bitter experience, (of something similar in aircraft),
I can support Terry's absolute demand that there should be
a low impedance connection between the Earth at the VFD and
the Frame of the Motor. Also the cabling should be screened,
(and twisted if possible).

I don't have much experience of industrial CT's but aircraft
3-phase CT assemblies are normally mounted inside an alloy
screened box, craftily arranged so that the screening extends
down the holes of the CT's without forming shorted turns.
This avoids capacitive coupling between the feeders and the
secondaries of the CT's. The secondary currents are taken
off via screened twisted pairs, with those screens down to
Earth at the destination only.

In one instance I have seen a screened cable jacketed with
a second outer braid. The outer was Earthed at the source
and the inner at the destination.

As for the RS-485 isolation, this hasn't helped as much. I was
using an isolated PCI RS-485 with built-in terminator, and I was
still getting noise problems from the VFD when it was running.

The RS485 isolator should be close to the PC (probably as
above already), cabled as 3 (or 5) wire, so that there is
a direct 0v-0v connection between the isolator and VFD.
This keeps the CMV as low as possible.

Cabling should also be done in screened twisted pair, with
the screen connected to the 0v at the VFD.

120 ohm termination at each end, and each receiver should
have a local differential R-C low pass filter. RS485 comms
can work ok with a surprising amount of low pass slugging.
Waveforms can look awful on the scope, but the receivers
can still sense the correct logic ok. Run the comms at
the lowest speed possible.

BTW: Shielded Cat5e comms cable is good for both comms
and instrumentation.

--
Tony Williams.

 

[Terry Given]

Tony Williams wrote:

In article <2OXFe.683049$iM6.433658@fe01.news.easynews.com>,
Nobody <protospherex@gmail_nospam_.com> wrote:


Nice response, Terry.
I'm using a Delta VFD-A. They no longer have this model on their
website.
I don't know how fesable it will be to use screened cable. The
problem is that I'm using the drive to test a motor for
production, and I may be stuck with whatever cables come
pre-attached to the motor.


From bitter experience, (of something similar in aircraft),
I can support Terry's absolute demand that there should be
a low impedance connection between the Earth at the VFD and
the Frame of the Motor. Also the cabling should be screened,
(and twisted if possible).

I don't have much experience of industrial CT's but aircraft
3-phase CT assemblies are normally mounted inside an alloy
screened box, craftily arranged so that the screening extends
down the holes of the CT's without forming shorted turns.
This avoids capacitive coupling between the feeders and the
secondaries of the CT's. The secondary currents are taken
off via screened twisted pairs, with those screens down to
Earth at the destination only.

Thats the safe way to do it. The twisting on the CT cable takes care of
H-field, and the single-ended shield deals with E-field. I can see why
they wouldnt muck about in aircraft

In one instance I have seen a screened cable jacketed with
a second outer braid. The outer was Earthed at the source
and the inner at the destination.

that doesnt strike me as a good idea for a drive output cable, but I
have seen it in instrumentation. Betcha its a pain to terminate .
Tsaliovich's book has some pretty pictures in it re. cable shielding
effectiveness.

As for the RS-485 isolation, this hasn't helped as much. I was
using an isolated PCI RS-485 with built-in terminator, and I was
still getting noise problems from the VFD when it was running.


The RS485 isolator should be close to the PC (probably as
above already), cabled as 3 (or 5) wire, so that there is
a direct 0v-0v connection between the isolator and VFD.
This keeps the CMV as low as possible.

Cabling should also be done in screened twisted pair, with
the screen connected to the 0v at the VFD.

120 ohm termination at each end, and each receiver should
have a local differential R-C low pass filter. RS485 comms
can work ok with a surprising amount of low pass slugging.
Waveforms can look awful on the scope, but the receivers
can still sense the correct logic ok. Run the comms at
the lowest speed possible.

BTW: Shielded Cat5e comms cable is good for both comms
and instrumentation.

Yep, I'd agree with all of that. To be honest I have never had a problem
with RS485 and drives, but to be fair I started work at a drive
manufacturer, and they already knew all this stuff, which I picked up
very quickly - its nice to not have to make all the mistakes yourself.

So I went and found a whole different bunch of mistakes to make....

Cheers
Terry

 

[Joerg]

Hello Phillip,

I'm definitely NOT an RF guy. I'm just a poor firmware coder who got
hoodwinked into this VFD fiasco.

It's the other way around for me. I am an RF/analog/EMC guy who
sometimes ends up digging through firmware, like when my usual tools
found another bus contention and everybody thought it just can't be so.
Then I probably feel a similar pain.

When you mention transformer isolation, you are suggesting that use one
transformer at each end, correct? I'll try that tomorrow.

Yes, pretty much like Ethernet. It is important to wind them carefully
and bifilar. Bifilar means twisting primary and secondary wires about
two twists per inch and then winding them onto the core together. For
signals in the MHz range I use toroids of #43 ferrite material and for
stuff below a MHz mostly #77 material (Fair-Rite, bought via Amidon).
The required number of turns increases with circuit impedance and with
how low in frequency the signals can be. Use wire with a beefy
insulation for better capacitive isolation. Not quite the stuff
electricians use, maybe half the insulation thickness and much thinner wire.

Good quality LAN transformers can also work. But they would have to be
from a reputable mfg such as Murata.

As far as current transformers... I think we tried using a current
transformer and combined it with various types of LPFs. The problem we were
having was that the DMM we were using would pick up the VFD noise. Noise
Noise Noise! Argh!

As Terry said DMMs are not a good tool here. Their leads pick up noise,
create loops and the internal circuitry of a DMM might not be designed
to handle large electro-magnetic interference loads. If you have the
time try the same with an old-fashioned analog meter. The really old
kind, no battery and certainly no electronics in there.

In fact I had a DMM die on me after an RF susceptibility test and I was
only blasting about 100 watts or so in an RF cell. It was a good name
brand in a fancy holster, not some hobbyist version.

Regards, Joerg

http://www.analogconsultants.com

 

[Joerg]

Hello Phillip,

I don't know how fesable it will be to use screened cable. The problem is
that I'm using the drive to test a motor for production, and I may be stuck
with whatever cables come pre-attached to the motor.

If you really are stuck with regular cable there may be another option.
It's not as good as shielded and twisted cable but at least a little
progress: Run it through metallic conduit. Best would be copper tubing
where the pieces are pre-soldered together and grounded.

I don't know how good the flexible metallic EMT is (the stuff that looks
like a large shower hose). Probably not very good for RF. If it has to
be flexible you can also try thin corrugated copper tubing as is used
for connecting a gas appliance to the gas line (hardware store).

As for the RS-485 isolation, this hasn't helped as much. I was using an
isolated PCI RS-485 with built-in terminator, and I was still getting noise
problems from the VFD when it was running.

Those boards sometimes use isolator chips. While these may isolate well
enough to pass safety muster they may not be that good in isolating RF
loops. Transformers are the thing for data transfer in a noisy environment.

Regards, Joerg

http://www.analogconsultants.com

 

[Joerg]

Hello Tony,

From bitter experience, (of something similar in aircraft),...

Composite structure? I almost pulled my hair out debugging one of those.
Luckily I didn't have much hair left ;-)

120 ohm termination at each end, ...

Or whatever the cable spec says it should have.

BTW: Shielded Cat5e comms cable is good for both comms
and instrumentation.

Just make sure to use the good stuff that doesn't turn into a toxic
plume in a fire. I think that's what killed the people at the
Duesseldorf airport.

Regards, Joerg

http://www.analogconsultants.com

 

[Terry Given]

Hi Joerg,

Joerg wrote:

Hello Phillip,

I'm definitely NOT an RF guy. I'm just a poor firmware coder who got
hoodwinked into this VFD fiasco.


It's the other way around for me. I am an RF/analog/EMC guy who
sometimes ends up digging through firmware, like when my usual tools
found another bus contention and everybody thought it just can't be so.
Then I probably feel a similar pain.

hear hear. I've had some hilarious hardware/software arguments too - me
and the s/w guy basically saying "its your fault". Usually its s/w,
perhaps 75% of the time - I suspect because its so easy to be careless
with s/w, and so hard to spot (not looking is also the most common
technique used for s/w peer reviews and testing).

That we can usually do calculations to prove the efficacy of our designs
is a huge bonus to us, along with spice, design-rule checking etc. A
whole host of reasons not to write firm/software.....

When you mention transformer isolation, you are suggesting that use one
transformer at each end, correct? I'll try that tomorrow.


Yes, pretty much like Ethernet. It is important to wind them carefully
and bifilar. Bifilar means twisting primary and secondary wires about
two twists per inch and then winding them onto the core together. For
signals in the MHz range I use toroids of #43 ferrite material and for
stuff below a MHz mostly #77 material (Fair-Rite, bought via Amidon).
The required number of turns increases with circuit impedance and with
how low in frequency the signals can be. Use wire with a beefy
insulation for better capacitive isolation. Not quite the stuff
electricians use, maybe half the insulation thickness and much thinner
wire.

Good quality LAN transformers can also work. But they would have to be
from a reputable mfg such as Murata.

As far as current transformers... I think we tried using a current
transformer and combined it with various types of LPFs. The problem we
were
having was that the DMM we were using would pick up the VFD noise. Noise
Noise Noise! Argh!


As Terry said DMMs are not a good tool here. Their leads pick up noise,
create loops and the internal circuitry of a DMM might not be designed
to handle large electro-magnetic interference loads. If you have the
time try the same with an old-fashioned analog meter. The really old
kind, no battery and certainly no electronics in there.

In fact I had a DMM die on me after an RF susceptibility test and I was
only blasting about 100 watts or so in an RF cell. It was a good name
brand in a fancy holster, not some hobbyist version.

bugger. I havent killed a DMM yet, but I did have one of the leads on my
fluke 87 break while I was testing a 50kW regenerative rectifier.
Suddnely the sytem went bonkers, it took about half an hour to figure
out my DMM was lying and that it all worked fine. Funny how we always
assume the worst with complex things. Customers are like that too - if
something is wrong it must the the drive, but is usually the wires

a few years back I did my limited electrical registration (allows me to
fiddle with 3-ph fixed wired 400Vac doodads). The instructors were at
pains to recommend test-measure-test with DMMs for exactly that reason,
a strategy I immediately adopted. I fitted a relay to a VFD in a milking
shed yesterday for the local sparky, who laughed at me when I did the
TMT. But I'm still here.....

we had a showering arc generator for EMC susceptibility, and every time
we turned it on the R&D/production door swipe-card died, so all foot
traffic had to go thru the power lab. The mfg eventually fixed it. The
first time the SAG was powered up, the photocopier died too, so we
bunged a line filter in front of it and that kept it alive. Phone
conversations were tricky with the SAG on too

The SAG is a great little toy, easy to make, fun to use and highly
destructive. Rather than using a coupling clamp, we attached a
multimeter probe to the output, and probed directly to the I/O
terminals. We could write our names on the alodised metalwork with the
spark. One product I designed had no clamping on one input, just an 0603
cap. the SAG made the cap flash over, emitting bright flashes - caught a
glimpse, turned the lights out and voila, weakness glows in the dark.
BAV99 to the rescue....

If you're interested, I posted the PDF showing how to make one to abse.

Regards, Joerg

Cheers
Terry

 

[Joerg]

Hello Terry,

we had a showering arc generator for EMC susceptibility, and every time
we turned it on the R&D/production door swipe-card died, so all foot
traffic had to go thru the power lab. The mfg eventually fixed it. The
first time the SAG was powered up, the photocopier died too, so we
bunged a line filter in front of it and that kept it alive. Phone
conversations were tricky with the SAG on too

When I tested one of my designs for defibrillator safety the PBX system
quit and some computers froze up. I got dirty looks despite the fact
that I had placed a huge wallpaper on an easel at the entrance asking
people to back up a lot that morning.

BAV99 to the rescue....

Or BAT54 duals if you need to cap before the substrate diodes wade too
deep into the muck.

If you're interested, I posted the PDF showing how to make one to abse.

Cool. I am surprised they recommend plywood for the applicator side. In
California the fire marshall would have a beef with that. Maybe it isn't
as hot and dry in Australia as it is here. The transformer could
probably be scrapped out of one of those bug zappers. Is "AS" Australian
Standard?

Regards, Joerg

http://www.analogconsultants.com

 

[Terry Given]

Joerg wrote:

Hello Terry,

we had a showering arc generator for EMC susceptibility, and every
time we turned it on the R&D/production door swipe-card died, so all
foot traffic had to go thru the power lab. The mfg eventually fixed
it. The first time the SAG was powered up, the photocopier died too,
so we bunged a line filter in front of it and that kept it alive.
Phone conversations were tricky with the SAG on too


When I tested one of my designs for defibrillator safety the PBX system
quit and some computers froze up. I got dirty looks despite the fact
that I had placed a huge wallpaper on an easel at the entrance asking
people to back up a lot that morning.

to quote JT, BWAHAHAHAHAHA

BAV99 to the rescue....


Or BAT54 duals if you need to cap before the substrate diodes wade too
deep into the muck.

ayup. You've done this before.... I once managed to design a circuit
that occassionally triggered the parasitic supply SCR in a lattice CPLD.
The numbers would burn off the chip, and a little dome would swell up in
the centre of the PLCC. oops.

If you're interested, I posted the PDF showing how to make one to abse.


Cool. I am surprised they recommend plywood for the applicator side. In
California the fire marshall would have a beef with that. Maybe it isn't
as hot and dry in Australia as it is here. The transformer could
probably be scrapped out of one of those bug zappers. Is "AS" Australian
Standard?

I used a neon xfmr fed from a step-down xfmr to get the right voltage
(it was free). When I say "I" I mean my slave ^H^H^H^H^H^H technician.
He did a much nicer job than I would. I think we made the frame from
formica, but your right, plywood might not be such a good idea. Mine is
in a big Al case, so it can catch fire inside if it wants to

Yes, AS/NZS. We often pinch the standard numbers from europe/usa....

Regards, Joerg

Cheers
Terry

 

[Tony Williams]

In article <lA0Ge.4152$PL5.381068@news.xtra.co.nz>,
Terry Given <my_name@ieee.org> wrote:

Yep, I'd agree with all of that. To be honest I have never had a
problem with RS485 and drives, but to be fair I started work at
a drive manufacturer, and they already knew all this stuff,
which I picked up very quickly - its nice to not have to make
all the mistakes yourself.

Flaky comms on site, (in front of the customer), is a
pain in the bum, and the most expensive place have to
sort the problem. It is generally cheaper to do it
properly in the first place, back at base.

So I went and found a whole different bunch of mistakes to
make....

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

--
Tony Williams.

 

[Terry Given]

Hi Tony,

Tony Williams wrote:

In article <lA0Ge.4152$PL5.381068@news.xtra.co.nz>,
Terry Given <my_name@ieee.org> wrote:


Yep, I'd agree with all of that. To be honest I have never had a
problem with RS485 and drives, but to be fair I started work at
a drive manufacturer, and they already knew all this stuff,
which I picked up very quickly - its nice to not have to make
all the mistakes yourself.


Flaky comms on site, (in front of the customer), is a
pain in the bum, and the most expensive place have to
sort the problem. It is generally cheaper to do it
properly in the first place, back at base.

oh yeah, and I've sure seen some terrible comms links. 50m of RS232
cable, both devices earthed with a bitwise protocol sans parity - one
bit = one task = noise sample & hold & act erroneously

So I went and found a whole different bunch of mistakes to
make....


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

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

Cheers
Terry

 

[Charlie Edmondson]

Terry Given wrote:

hear hear. I've had some hilarious hardware/software arguments too - me
and the s/w guy basically saying "its your fault". Usually its s/w,
perhaps 75% of the time - I suspect because its so easy to be careless
with s/w, and so hard to spot (not looking is also the most common
technique used for s/w peer reviews and testing).


Cheers
Terry

I have been in some 'interesting' sw/hw fights myself! I was working
tool roads a few years back, and they had an interesting 'hardware'
problem. Every weekend, around 2-3 in the morning, the system went
crazy. It started a network cascade and every toll system on the road
went down. It was 'obviously' a hardware fault, so they called in some
network experts to solve the problem. They put a network sniffer on the
system, and watched the traffic...

Sure enough, early sunday morning, there was nothing, nothing, nothing,
and then a huge cascade! What was the cascade? I bunch of 'Where are
you?' messages! They programmers had built in a "If I don't hear from a
system for 'x' minutes, check to see if they are still there..."
routine. They had NOT built in the stuff to QUIT asking once it got an
answer...

Charlie

 

[Joerg]

Hello Terry,

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

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

On large systems and even on Phillip's drive system one often has to
accept the rule that there simply is no common potential between
individual modules. From a 50/60Hz point of view, hopefully yes, but for
anything much above that all bets are off.

Regards, Joerg

http://www.analogconsultants.com

 

[Terry Given]

Hi Charlie,

Charlie Edmondson wrote:

Terry Given wrote:



hear hear. I've had some hilarious hardware/software arguments too -
me and the s/w guy basically saying "its your fault". Usually its s/w,
perhaps 75% of the time - I suspect because its so easy to be careless
with s/w, and so hard to spot (not looking is also the most common
technique used for s/w peer reviews and testing).


Cheers
Terry


I have been in some 'interesting' sw/hw fights myself! I was working
tool roads a few years back, and they had an interesting 'hardware'
problem. Every weekend, around 2-3 in the morning, the system went
crazy. It started a network cascade and every toll system on the road
went down. It was 'obviously' a hardware fault, so they called in some
network experts to solve the problem. They put a network sniffer on the
system, and watched the traffic...

Sure enough, early sunday morning, there was nothing, nothing, nothing,
and then a huge cascade! What was the cascade? I bunch of 'Where are
you?' messages! They programmers had built in a "If I don't hear from a
system for 'x' minutes, check to see if they are still there..."
routine. They had NOT built in the stuff to QUIT asking once it got an
answer...

Charlie

Beautiful. And fits nicely with my theory that many programmers never
test anything, and seldom if ever think about failure modes. I bet
you've seen numerous systems hang when no response is received...

the funniest one I have had was a drive we designed using an existing
micro & software, with all new hardware. We re-arranged all the ports,
and vandalised the software to suit. All worked spiffingly well, until
about 3 days after the product launch, when a customer complained to the
service dept his drive kept tripping when he used the 4-20mA input.
After the inevitable argument with Mark, I did some tests and found that
at about 10mA the drive tripped. I configured the input as +/-10V, same
problem at about 0V. It happened when the uC ADC pin reached about 2V or
so, coincidentally the logic threshold. So I looked at the original
product, where that pin was the emergency stop switch input. Armed with
this ammo I re-started and easily won the argument. Mark went and had a
nosey, and came back a few minutes later looking sheepish, with the
problem solved. He'd re-written the I/O code, but had forgotten to
remove the old E-stop code (all the rest was gone). The 80C196 was set
to use that pin as an ADC, but the digital input SFR still worked

(best not to think about the 3 goes I had my tech have at getting a LED
to light up a few weeks later)


the worst one was when a programmer set up a command to shut down the
comms link, and save that state to EEPROM so it wouldnt turn back on
again. The unit is gooped and screwed into an IP68 box with only the
comms link & power coming out, and to get it to talk again the lid had
to be removed (12 screws) so we could access the diagnostic serial port.
That wouldnt be so bad, but another programmer was upgrading a customers
screen (1000 units) and sent out a (untested) broadcast "shut up
forever" command. Luckily the broadcast didnt work so well, and he only
"killed" a few hundred modules. But a guy in a climbing harness had to
undo a thousand screws to remove the crippled modules, then a few
thousand more to revive them on the ground, and of course put it all
back together again.

We made ourselves look like complete idiots to the customer so much for
the "image quality upgrade". We had the screen back and running that
day, which kept him happy, but if it had been a game day we would have
been in the shit, it took hours to fix. When asked why he saved the
state to EEPROM, the programmers reply was he had a choice (to save or
not to save) and "just" chose to save it. The server was updated that
day (*about 15 minutes after we tracked down the root cause) to never
allow a broadcast of that command - until then it was transparent, you
type it, it does it. The firmware was also hurriedly modified to remove
the built-in self-distruct command . And the programmers got an abrupt
lesson in "thou shalt not fuck with the customers equipment"


Cheers
Terry

 

[The Real Andy]

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

<snip>