Posted this on the technical/applications thread also:
Within the last 60 days our cooling tower Lakewood TDS meter's
operation became erratic . We have linked it to the operation of the
Danfoss V.F.D.s. When the drives are bypassed, meter operation is
normal and TDS is right on the money when checked against a handheld.
As the drives ramp down TDS rises, and vice versa. Both drives/towers
exhibit the same effects on the meter. Power was checked with a true
RMS meter and no voltage spikes or harmonics are evident. We also ran
the meter on UPS clean power with no change in results. The meter and
sensor are wall mounted outside in the tower yard approx. 40 ft. from
the drives which are in the chiller mechanical room. The copper
sampling pipe is grounded immediately upstream from the meter to the
tower earth ground arrestor system. At the meter location the TDS can
drift by as much as 1000 in a three hour period. Using a second
Lakewood and sensor placed in the tower basin we have found that the
the effect is still present but to a lesser degree. The closer you
move the second meter toward the first the greater the deviation. No
new equipment has been added and the system has been running trouble
free for over six years. Lakewood pronounced the meter to be in
perfect working order. Has anyone had any similar experiences? Any
input is appreciated.Thought of the RF issue but consider it unlikely
to be radiating from the cabinets. Why would both units develop the
same problem at the same time? Both units individually create the
same issue when the other is off line. What about a high impedance
ground connection? Wouldn't that have the effect of allowing noise
from the drive to radiate from the power leads instead of draining it
away? If so would the connection be local or might it extend back
toward the service entrance through the various MCC's? I'll contact
our electrical contractor for input, but what tests or procedures
should be implimented to track down this problem?
