Even if true, it doesn't invalidate my point.
Motor is pulling twice the nameplate current ?
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JFletcher
Senior Member
- Location
- Williamsburg, VA
Even when it's not...
My favorite was when I installed a VFD for a customer at HIS request for a 100HP pump motor used to pump out a collection tank. I didn't sell him the pump, I just sold him the drive and controls and installation / programming of it all. The tank had to maintain a certain level to avoid sucking air into the line, so I used an ultrasonic level transducer feeding a PID loop in the drive. Worked perfectly in my shop, installed it, tested it, worked perfect in the field under the conditions they gave me to test it. A week later I get a call because the tank is over flowing; the VFD is not keeping up. I go out and the drive is at full speed, but the inflow is greater than the pump can handle. Somehow this is MY fault? I look around more and see an old pump, find out that's what was being replaced. Turned out the old pump was 400HP, the new one was 100HP. Hmmm... why would someone ASSume that a 400HP pump could be replaced with a 100HP pump and do the same job? So this was NOT an "electrical" problem, but I was the only one there, so I was getting the blame. Pump supplier is who told him it would work, not me, but he refused to back down, insisting this was an electrical issue. I had to reconnect the 400HP pump to an old Autotransformer starter they had there to prove it to them... Didn't get paid for that part of it though. I told them the real solution was to have installed a VFD on the 400HP pump they already had and slow it down when not at full capacity, they didn't need a new pump. But they hadn't asked me that. They eventually did abandon the new pump and go with the 400HP drive, but I insisted on getting paid for my "test" so they just went with someone else for the new VFD.
A year later I had to go back out to make that one work right too, so eventually, they ended up paying me what I wanted...
Hahaha, nice. My favorite was from operating something similar; the new outfall at a WWTP. During a hurricane, the outfall couldnt gravity flow, so a 700HP Final Effluent pump, one of three, controlled by an ultrasonic tank level indicator, was supposed to kick on.
I'm monitoring the tank level, which slowly rises to 17', then starts dropping. Pump is running, I think. Go do my round, last part is checking the outfall... get near it, water is cascading over the sides.
hmy: The pump hadnt started, the ultrasonic level transducer was underwater; past 17', where it was mounted, it started reading the water level backward, so that 18' was 16', 19' was 15', etc, until the water poured over the sides of the tank. The pumps had standard H-O-A switches switched to "A", but automatic hadnt been configured yet, so the pumps failed to start. No VFDs either, so we had to run them in manual/hand for most of the storm, which was unfun to say the least.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Classic... :lol:
- Location
- Placerville, CA, USA
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- Retired PV System Designer
The ultrasonic transducer measures distance to the reflection from the air/water interface, and no input was provided to tell it whether the interface was above or below the transducer. Also a classic design error.
If they had put it at the top or the bottom instead of trying to maximize the accuracy by putting it at 17 feet all would have been well.
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If they had put it at the top or the bottom instead of trying to maximize the accuracy by putting it at 17 feet all would have been well.
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FionaZuppa
Senior Member
- Location
- AZ
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- Part Time Electrician (semi retired, old) - EE retired.
The ultrasonic transducer measures distance to the reflection from the air/water interface, and no input was provided to tell it whether the interface was above or below the transducer. Also a classic design error.
If they had put it at the top or the bottom instead of trying to maximize the accuracy by putting it at 17 feet all would have been well.
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yikes, a simple float switch (safety) would have prevented that. always good to have safety backups.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
I don't disagree, yet when you quote adding that back-up safety device, the user often comes back and says it's unnecessary and exercises his line item veto power, or gives the project to the guy who didn't include it... :happysad:
To quote one of my favorite comedians, Ron White:
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