34 APRIL 2024 WorldWide Drilling Resource® It Works, but It Doesn’t Work Very Well by Britt Storkson Owner, P2FlowLLC I’m amazed at the number of purchasing agents who get “upgrades” or “updates” for their computer controls equipment when the task(s) their computer is expected to do doesn’t change. Is this like wanting to have the latest and greatest car model even though the latest and greatest car model doesn’t do anything provably better or cheaper than the old one? Or does one need the “update” because the existing machine isn’t doing something it should be doing or is doing something it shouldn’t be doing? If this is the case, why isn’t the vendor compelled to provide the “fix” at no charge? There is also such a thing as doing something, but not doing it very well. I’ve been asked to evaluate a water pumping system that will be powered by a single-phase-output generator. This generator runs single-phase power into a 60-horsepower (hp) variable frequency drive (VFD), which then creates a three-phase power feed to run the 30-hp submersible pump motor. It’s fine because this eliminates the need for a phase converter as long as the VFD is “oversized” by a factor of two. As near as I can tell from discussions with the equipment owner/user, everything is fine except the software controlling the pump motor speed is flawed. The problem is, it periodically “spikes” the motor speed, meaning the motor abruptly speeds up, resulting in a greater than normal amperage draw, which blows the 135-amp circuit breaker on the generator, which shuts everything off, making this equipment useless for all practical purposes. Yes, it works all right, but it doesn’t work very well. In this case, it’s not reliable. Because of this, it’s useless to the owner. All because of, it appears, a single software “glitch.” And the vendor of this equipment is clueless. They would have had it fixed by now if they knew how. We also have to start the variable speed motor quickly to protect the motor thrust bearing, which uses the water rushing through the motor casing for lubrication. The quick start causes a temporary high amperage condition. One way to deal with the high amperage draw on motor start-up is to slow the ramp-up speed, which reduces the amperage going to the motor on start-up. We have some practical limitations there as well. Regarding the ramp-up speed, the Franklin (submersible) motor specification states: “[Variable speed motor drives] must be adjusted so the motor is at full voltage within THREE SECONDS MAXIMUM to prevent excessive radial and thrust bearing wear.” So what I’m going to do first is bypass the pump motor speed controls which are already there and use the pump controls we make which do not spike the motor speed . . . something we have confirmed by testing with other systems. Eliminating this rpm (revolutions per minute) spike in and of itself may likely solve the problem. If that doesn’t work, I’ll monitor the VFD amperage output and program the software in our control unit to adjust the motor speed so it will never exceed the 135-amp threshold. Most VFDs have an output that represents the three-phase amperage output of the motor. Depending on the brand and features of the VFD, the output may be a voltage like 0-10 volts with 0 volts representing 0 amps and 10 volts representing the full-scale amperage. So if the full-scale amperage is 100 amps, then measuring 5 volts on that output would indicate the VFD is outputting 50 amps at the time. The same basic principle is used for control current outputs like 4-20 mA (milliamps) where 4 mA = 0 output amps and 20 mA = full-scale amperage output. There is also a PWM (Pulse Width Modulation) output on some VFDs which is often used because it’s cheaper to implement. This technique uses the timing of different digital voltage levels to represent a variable voltage. This output can be smoothed out (filtered) using an external resistor and capacitor to produce this variable voltage. To do this for a 10-second period, if one outputs +10 volts for 1 second and then output 0 volts for 9 seconds, one will read 9 volts at this output which, in turn, represents the motor amperage output at that instant. This routine typically repeats itself much faster than every 10 seconds (more like several hundred times a second) so the filtered voltage is consistent and well-controlled. Another example of a product that worked, but didn’t work very well: A variable speed water pump and motor was vibrating when it reached a certain rpm, but the vibration stopped when the pump/motor speed went slightly higher. Of course, the operators were once again clueless as to how to deal with this condition. It’s really simple. First, determine at what rpm “vibration band” this occurs. Let’s say it occurred during acceleration between 2000 and 2200 rpm. Those speeds have a corresponding control voltage. Just program the control unit to when it hits 2000 rpm to immediately skip to 2200 rpm and avoid the problem band entirely. This can be implemented with a simple adjustment so it can be fine-tuned under actual working conditions if necessary. This modification requires very little in the way of software and can be enabled or disabled if desired. Computer vendors like to sell extra components and complexity because it increases costs and decreases reliability, but they’re clueless when it comes to anything outside of that box. Oftentimes, the problem can be boiled down to a few components that simply need to be adjusted or otherwise modified, and the problem can be solved quickly and inexpensively. Sometimes it is not physically possible to fix the problem. You have to understand the physical and computing limitations of the equipment and act accordingly. Britt Britt Storkson may be contacted via e-mail to michele@worldwidedrillingresource.com ENV
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