Nobody said:
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