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Old December 6th, 2017, 05:02 PM   #16
lesmar96
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That is a very good point which I hadn't thought of. The drive was not originial and when the drive was installed, they used the same CT. I cannot say at the moment, but I am quite sure, though, that the CT is on the line side of the drive. That would make some difference, right, or would noise, etc. still mess it up?
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Old December 6th, 2017, 05:49 PM   #17
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Re-tighten all terminal screw.
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Old December 6th, 2017, 06:09 PM   #18
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If the CT is on the line side of the drive, you're unlikely to get any meaningful data at all. If your low limit hasn't been changed since the drive went in, you'll absolutely have problems.

I once saw a VSD reporting 75A, but the power meter on the main incomer was only reading 40A. That's the sort of efficiency gains you can get out of a VSD. A CT on the line side of the drive won't really tell you anything about what's going on on the load side.
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Old December 6th, 2017, 06:28 PM   #19
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Quote:
Originally Posted by ASF View Post
If the CT is on the line side of the drive, you're unlikely to get any meaningful data at all. If your low limit hasn't been changed since the drive went in, you'll absolutely have problems.

I once saw a VSD reporting 75A, but the power meter on the main incomer was only reading 40A. That's the sort of efficiency gains you can get out of a VSD. A CT on the line side of the drive won't really tell you anything about what's going on on the load side.
I disagree. You won't get an accurate motor amperage reading from a CT on the line side of the drive, but the signal that you do receive will tend to represent power in vs. power out.
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Old December 6th, 2017, 08:13 PM   #20
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$20 says it's marginal in the summer, and below the threshold in the winter..

The new motor is more efficient than the old, so they had to slow it down. It now draws less current (line side), thus the trip. In the winter the motor and the drive are cooler and slightly more efficient. Therefore, it
s now under the trip point.

My guess anyway.

Just check the normal running current and readjust the sensor for the new load. Then go have a beer
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Old December 6th, 2017, 10:37 PM   #21
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Quote:
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I disagree. You won't get an accurate motor amperage reading from a CT on the line side of the drive, but the signal that you do receive will tend to represent power in vs. power out.
Well, true, you'll get something loosely proportional to the output power from the VSD to the motor. But in this application, it's not really of any use.
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Old December 6th, 2017, 11:53 PM   #22
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The thing about CT's is they are designed for 60 hz AC when you start getting down to say 40hz the coupling will act more and more like DC the same with an increase in hz say about 70 or so. DC is not coupled through a transformer so you will show low current.

Most CT's are mounted on the load side of a contactor, it's jut easer that way the wire are readily available and you have more room in panel. So mu guess is that they are mounted o load side of the VFD in the new installation witch would mean that the reading you get from are next to worthless.
Just connect the input to the PLC to the analog output of the VFD ( set you for load) and I would bet you will see a big difference.
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Old December 7th, 2017, 05:34 AM   #23
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Is this a line shaft turbine or a submersible pump. Is the pump and motor inside or outside. If it is a line shaft turbine I would check the packing box to make sure there is enough grease and it has no water in it.
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Old December 7th, 2017, 09:49 AM   #24
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Where is the one way valve, when it is cold it can leak, thus the water pressure is low, and the pump will be low on power.
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Old December 7th, 2017, 10:51 AM   #25
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Water temperature at the pump itself won't change much winter to summer at 100 ft + deep.
My guess is that the foot valve (check valve) is holding water in the discharge pipe, and somewhere in the discharge piping there is a location that is freezing.

Most VFDs will let you set the analog output to give an amp signal. I would use that instead of a C/T for current indication.
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Old December 8th, 2017, 10:16 AM   #26
lesmar96
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Thanks for all your suggestions. Sorry for my absence and not answering your questions.

I definitly and going to make a couple changes. I can easily see why a CT could not read accurately on the line or load side of a drive. I will switch to the analog output of the VFD. But I don't understand why that works all summer and has a fit in the winter! Some things are hard to explain, I guess.

I also plan to add more of a delay for low current before it actually shuts the pump off. I think it is a pretty short time right now.

I don't really think freezing is a problem, but I will have to go some details about their valves etc with the operators to see what they say.
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