Using an RTD to measure temperature in a fluid flow line. When a VFD
gets turned on to start a fan (600V) the temperature reading drops by
~4ฐC. Using National Instruments Fieldpoint RTD module to read
temperature. It is my understanding that noise that causes an offset
like this has a DC component. Is this true? If so, Why (or how)would a
variable frequency drive cause DC level noise? Any ideas on how to
On Thu, 27 Nov 2003 22:20:03 -0500, the renowned Dan Wright
Somewhere there is a DC voltage being produced, that much is true.
Poor electronic design of the signal conditioner is causing
nonlinearities in the circuit to rectify the high frequency noise,
yielding a DC offset.
Hard to do over the internet. Maybe ask National Instruments as it is
their fault. You could also try shielding the VFD wires to the motor,
for example by running them through a conduit. Certainly don't run the
RTD wires too close to (or through the same conduit as) the motor
"it's the network..." "The Journey is the reward"
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If possible and not doing, use differential input.
Are you using twisted shielded pair wiring, all shields grounded to one
point. Is signal wiring and power wiring ran in parallel and unshielded?
Look for ground currents or some sort of RC filter or rectifier being formed
by the hook up and charging up some capacitance.
I have seen this symptom, not only with temperature transmitters, but
pressure and other transmitters, also. It was very scary to see
almost every measured signal in the power house shift somewhat the
first time we turned on the first PWM drive in the old power house.
Apparently the sensitive input circuits act as AM radio receivers and
detect the high frequency noise and add some of that DC to the
We ended up adding quite a bit to the price of the drives by including
both line and motor lead filters (transformers and 5% line chokes on
the input and custom made three phase common mode chokes on the T
leads), in order to eliminate or reduce this distortion to tolerable
levels. But we did experiment with some transmitter filters with fair
success. We tried things like putting a ferrite bead on both the RTD
cable and the 20 ma line, both at the transmitter. These are
available in two piece cores with a plastic snap closure, so they are
easy to try. But wound common mode chokes worked better. However, in
out situation, it was impractical from a time and labor standpoint to
add a pair of common mode chokes to each of hundreds of transmitters.
So we spent money on big hardware to stop the noise at the source.
Ant high gain signal entry path that is also nonlinear is likely to
rectify the high frequency noise into a DC addition to whatever else
it is amplifying. This is analogous to the detector function in an AM
Almost any solid state amplifier will do a little of this
rectification when its inputs are overloaded with high frequency
So my question then becomes, What can I do to eliminate this DC offset,
provided I 1) am shielding my instrumentation cable properly, 2) keeping
the instrumentation wiring as far away as possible from the VFD power
wiring? I can deal with pure AC noise, as this averages out to 0. My
RTD signal is running from the UUT to my NI FieldPoint RTD DAQ module.
Here, I believe the hardware is pulsing a 250 uA signal, and the
'return' voltage is measured across the RTD.
In the relativly short itme I have been in the instrumentation / control
work force (5 years) this is the first time I am totally stumped. How
to deal with a DC offset, when you are measuring a DC signal to begin
with? I honestly value all of this groups input on this matter.
John Popelish wrote:
The spiky input gets into your instrument by capacitive (or direct)
conduction. This can happen because the high frequency signal will
not equalize out to a single voltage at physically separate points,
even if they are all connected together (as building steel is) due ot
the propagation speed of light. So if the RTD is mounted on a piece
of (probably grounded) equipment some distance from the transmitter,
which is some distance from the signal collection hardware (which is
also probably grounded) and all the signal paths are enclosed or near
grounded metal, there is still plenty of opportunities for high
frequency ground noise to get inside the signal paths.
Since it is not usually practical to isolate the entire signal path
from ground, except at a single point (and impossible to isolate it
from capacitive and radiated coupling), the best you can do is try to
reduce the escaping noise at its sources and/or attenuate it at the
points where it is being rectified into an offset.
I am not familiar with the abbreviation UUT. But I am willing to bet
that the DAC is a node that is subject to this rectification effect.
You might try an experiment of putting a common mode choke on the
signal lines as close to the DAC input as possible. This common mode
choke is best placed such that the excitation current for the RTD
dopes not pass through it, because the wiring resistance compensation
will not take its IR drop into account. I am not familiar with the
details of tour digitizing hardware to know if this is possible or
Do you have a way to detail your hardware setup on a web page
(including some photographs and schematics)? The possibilities are so
great it is hard to make useful generalizations.
One more quick question:
If this is DC being added to the signal, why, then, is the rtd reading
dropping? With the increase in signal level caused by this RF
Rectification, I would expect the temperature reading to rise, no??
John Popelish wrote:
I guess the noise source may be too noisy. We have used other VFD's and
I never have seen anything like this. WRT using 4-20 mA xmitters, we
have tried this too, and the same problem is seen, ie the temperature
drops a few degrees. we also use 4-20 mA pressure transducers too.
these show the same symptoms, ie the pressure reading drops by a few
psi. The problem is there whether it is a low level RTD reading, or a
'high' level 4-20 mA signal.
Again, thanks for the insight.
Are your all grounds at the same potential or do you just think they are?
Could very well be a ground loop problem. I really think you have a
grounding problem, maybe in the VFD. VFD's are hi power inverters using
square waves and thus very noisy by nature, over a broad spectrum.
You could use isolated and maybe amplified signal conditioning at the signal
source with differential inputs and a current loop output to your
differential instrumentation inputs.
With a 600 V VFD, this is probably a pretty big motor - 25 - 100+
Hp. With a VFD of that size operating, "ground loops" are unavoidable.
The magnetically induced currents in every conductor (whether wire,
water pipe, structural steel, rebar, air conditioning ducts, etc.)
will be substantial, and will couple to EVERY conductor within reach
of the radiated field. Tight cabling in conduit of the input and
output cables will reduce the field somewhat, but beyond a certain point,
it becomes very difficult to suppress the EMI. At that point, you have
deal with it in the instruments that are affected by it. Supressing the
(high frequency) AC components with filters is almost always successful
when you get a filter configuration that shunts all the induced currents
to a safe grounded node. As long as this RTD conditioner uses DC
excitation to sense a DC signal from the RTD, filtering should be
The most productive step would be to put a low-pass filter at
the input to the RTD conditioner module. (It is also possible that interference
is getting into the current source that powers the RTD sensor and
altering the excitation current, in which case those lines need
filters, too.) Generally, a Pi filter with a modest series inductor,
say 100 uH, and a capacitor to ground from each end of the inductor,
say .1 uF, should take care of the kind of frequencies that would
cause the rectification errors. In worst-case situations, it is
sometimes necessary to match the component values of the L and C
to get the greatest common-mode supression without inducing a possibly
greater differential mode signal.
I had same problem using 4-20 ma speed control signal to the VFD and using
4-20 ma pressure transmitter on the same PLC. I switched the VFD control
signal to 0-10 volts and it fixed the problem. I think that VFD input was
grounded in a way that there was voltage difference between VFD and pressure
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