VFD and Flow Meter Not Fun

P

Thread Starter

pabstpdl

Water Treatment Plant High Service Distribution of potable water.

2 separate insertion McCrometer multi-mag insertion meters model 395X (installed 2015) Lets call them A and B. The meters work fine with any combination of our 4 motors/pumps. Subsequently, we added a 5th motor/pump that has an Eaton VFD Drive (our only VFD Motor). The Motor and VFD is from October 2015.

When this motor is run either alone or in any combination with the 4 other motors/pump the Flow Meter A begins to have highly erratic measurements and B continues to measure accurately.

We have re-grounded the entire MCC panel to 2 earth grounds outside. We have grounded the Flow Meter A multiple ways and triple checked wiring, shielding, etc. We have ensured that the VFD Motor is on a seperate pole than the Flow Meter Transmitters.

We are considering adding EMF Filters before the Flow Meter Transmitters.

What else may we try? Is it a bad Flow Meter? Will filters work? I know there are many many variables.
 
B

Bob Peterson

is this plastic lined pipe by any chance? if so, try adding metallic orifice plates on both sides of the magmeter and tie them to the ground stud on the magmeter.

no amount of grounding will serve any purpose but bonding the meter to the fluid might.
 
A quick look at the McCrometer insertion mag manual shows that its ground electrodes are part of the insertion tube. Being an insertion style, there's no flanged tubes with which to use grounding rings.

Is the problem inside the meter or in the transmission of the signal out of the flow meter? Is the flow rate reading at the converter erratic AND the flow signal out of the converter erratic? Both? If you disconnect the mag's output signal (4-20mA, frequency pulse, whatever), are the readings on the converter display erratic?

One insertion mag works OK, the other doesn't. What's the difference between the two installations?

On the erratic mag, are you absolutely positive that the recommended shielded cable is used for drive-to-motor cabling and connected the shielding exactly by-the-manual, all i's dotted and t's crossed?
 
S
Hi,

Normally VFD's tend to generate harmonics, but not to acceptable levels. However from your message I understand sensitive Instrument is connected to the same power source as the VFD without any power stabilization (ex. UPS etc.) appropriate induction inline filters may help to some extent.
 
You don't mention how the VFD output is wired, but if the VFD is in an MCC, chances are you just have loose single conductors out of the VFD into the MCC wireway, then maybe into conduit out of the MCC structure to the motor.

If that conduit is PVC or some other non-magnetic material, the cumulative effect of all of that exposed output wire creates a relatively powerful FM radio transmission antenna. The output of the VFD should be wired with shielded "VFD cable", with the shields grounded at BOTH ends (unlike signal shielding). If you have steel conduit out of the MCC all the way to the motor, that helps, but the short distance from the VFD terminals to the conduit can still spit out a lot of EMI/RFI that gets picked up on other conductors.

if you did happen to use VFD cable, check to make sure it is grounded on BOTH ends as I said. A very common issue is that electricians who are used to grounding shielded SIGNAL cables on only one end, do so with all shielded cables, so they will not ground the terminus end of the shield. With power cables, you are creating a "farraday cage" around the conductors, and you must give that high frequency EMI an immediate path to ground, otherwise the shield just becomes the antenna.
 
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