By Jeremy Corbin
If you’ve considered installing or are using a variable frequency drive (VFD), you know the benefits it brings to any operation: a better, more consistent performance; energy savings; controlled stops and starts; and more.
Getting the most out of a VFD also means understanding how to maximize its operation, taking advantage of all the features a VFD offers, and using those features to troubleshoot any issues quickly.
As a field service engineer for Franklin Electric, VFDs are a big part of my time in the field and the five most common challenges I see are: analog signal loss, digital signal loss, underload/dry well protection fault, overload/e-thermal fault, and VFD sleep issues. Here’s what these can mean in the field.
1) Analog Signal Loss
According to pressure sensor manufacturers, analog signals are continuous. This electrical signal is scaled based on either the voltage or current signal. The most common version of an analog signal in the water systems industry is a pressure transducer. The most common pressure transducer is a 0-100 psi (pounds per square inch) 4-20mA (milliamperes).
In a 0-100 psi 4-20mA pressure transducer, the 4mA represents a 0 psi value and 20mA represents 100 psi. The remaining values are scaled within that range. This scaling needs to be accounted for when conducting readings.
For example, you might get a value of 50 psi on your VFD screen, but a reading of 100 psi on your physical pressure gauge. If the drive reads 50 when the gauge reads 100, then the drive might be programmed for 200 psi max pressure when the actual transducer is a 100 psi.
If there is a scaling issue such as this, the drive will adjust speed and pressure to the incorrect value. In this example, if the drive’s set point pressure is programmed for 60 psi, but the drive is only reading half of the correct value, it will increase pressure to 120 psi.
Checking the HOA (Hand/Off/Auto) switch is a way to troubleshoot digital signal loss issues which impact a variable frequency drive’s performance.
The first—and easiest—way to troubleshoot analog pressure loss is to look at the feedback on the VFD or its companion app if one is available.
Some VFDs have multiple inputs for analog signals. If you look at the feedback values and see that the drive is getting feedback from your transducer, then the issue is likely programming. If you have the transducer wired into the correct terminals and the VFD is showing feedback on the terminals you meant to wire into, you likely have your analog input for your PID (proportional integral derivative) loop set to the incorrect analog input. This is generally an easy programming fix.
The other thing you might find when performing this troubleshooting is that the value is below the expected range, and this is causing a fault. This could be from a faulty transducer or by a vacuum being drawn on the transducer.
If you determine that there is no programming issue and the drive is not getting feedback, check the control voltage. Most VFD control boards have 12 and 24Vdc at the controls. If the control board doesn’t have voltage, this is likely a control board issue. If there is voltage present, check your terminal connections and make sure wire insulation didn’t get trapped in the terminals. Most transducers only have one correct wiring. If the wires are backwards, the unit won’t work.
If the voltage and terminal connections are good, the next step is to check the transducer wiring and connections outside of the controller. If no issues are found and the wiring is good, the transducer likely needs to be replaced.
2) Digital Signal Loss
Another common issue is digital signal loss. This generally won’t lead to a fault, but the drive won’t perform as expected. Digital signals are on/off or open/closed signals to the VFD.
An example is an HOA (Hand/Off/Auto) switch. The switch sends control voltages to the drive, telling it to perform certain functions. In this case, switching to the Auto position will normally result in the drive running in forward on a PID (proportional integral derivative) operation. PID is the loop feedback when the drive is communicating with the transducer and adjusting speed according to that feedback.
You’ll troubleshoot digital signal loss in much the same ways as an analog signal loss. Start by checking the feedback on the drive to see if the VFD is recognizing the terminal closure. Most drives have programmable digital inputs; so if you are getting the correct signal on the correct terminal, then the terminal is likely not programmed correctly. You might also see that the incorrect terminal is getting the closure, and this is typically a simple wiring correction.
If the drive is not recognizing the digital signal, then the next step is to check the control voltage. If this is good, check the wiring. If voltage and wiring are good, check the switch with an ohm meter. You should get continuity or a low ohm resistance when the switch is closed. If not, replace the switch.
3) Underwell/Dry Well Fault
When checking any fault readings, visit the fault log on your unit. This can provide more information about the fault. In addition, VFDs will show you what the amperage was at the time of the fault, and the speed in Hz (hertz). You can look at this information to see if the fault is possibly due to a programming issue or if it is a legitimate fault. To demonstrate what I mean, let’s pick a random motor to use as an example.
The 10 HP 6-inch 230-volt Franklin Electric submersible motor has a full load (FLA) of 28.4A and a max load (SFA) of 32.2A. A good starting point is to set your underload to 70% of FLA for submersible and 50% of FLA for centrifugal pumps. For this motor this would be 19.8A.
Adjusting sleep delays and increasing sleep detection settings is necessary at times to prevent heat issues and high electrical costs.
If we check the fault history on a VFD hooked up to this motor, and see a fault happening at 27.5A, we can assume that the pump is not in an underload and that the protection parameter should be changed closer to 19.8A.
These ratings and suggestions assume a few things. First, our pump and motor are matched HP. Second, our pump and motor are running at full speed. If we check the fault history to find that the fault happened at 19A, but we were only running at 45Hz, we might want to change the protection frequency detection to only look for underload when running at full speed.
If we have determined that the settings are correct and we are still getting an underload fault, it’s helpful to understand why this fault occurred.
Motors are loaded by the pumps, and the more water the pump is moving, the more load there will be on the motor. If you’re moving less water, it can result in an underload. When trying to determine the source of an underload, look for the cause of the flow reduction. This could be due to a water level drop in the well; the pump drawing in air (over-pumped the well); a suction, impeller, or discharge blockage; a stuck valve; or a worn or damaged pump.
4) Motor Overload/E-Thermal Fault
Another common fault is a motor overload. Again, you’ll want to visit your unit’s fault history to determine what the load was at the time of the fault. Was it a programming issue? Or a legitimate problem?
Generally, when programming a VFD, you will set the motor amps and the drive will calculate the overload protection based on what you enter. You can set the motor amps up to the max load or SFA. If you are checking the amps in the fault history and it is tripping below this value, it may need to be adjusted. This is assuming again that we have matched the HP pump and motor and we’re running at full speed.
Just like with an underload fault, you will need to determine what caused the amps to be too high. Was it a high pump flow? If we are moving too much water, this can mean that our water level has risen, there is not enough back pressure on the pump, or that our pump wasn’t sized correctly.
Other things that can cause a high amp overload are:
- Heat (not enough cooling flow passed the length of the motor)
- Improper voltage to the motor (more than 10% of nameplate voltage)
- Current unbalance (Franklin Electric recommends no more than 10% current unbalance when running below FLA and no more that 5% when running at or above FLA)
- Mechanical drag in the pump or motor
- A bad motor.
5) Sleep Issues/Pump Won’t Go to Sleep
Sleep issues generally won’t cause the VFD to fault, but they can lead to heat issues and higher electrical costs. Most sleep issues are due to leaks inside the system (or a check valve) or pressure tank failure. When VFDs are operating on a system that is sealed and doesn’t have leaks, the VFD brings the system up to pressure. It decreases speed to its minimum setting and then goes to sleep.
If the leaks or pressure tank can be fixed, generally the default sleep programs work most of the time. If the leaks cannot be found or repaired, it may be necessary to adjust the VFD programming.
To do this, you can set the minimum frequency to a speed that we know will build pressure and set the sleep delay low so that it will go to sleep before over-pressurizing the system. You can also decrease sleep delays and raise sleep detection settings.
One final way to troubleshoot this issue is to set the proportional gain and integral time to increase the responsiveness of the pump. Take care when adjusting the sleep and other settings to assure that these changes won’t cause other issues—like pressure fluctuations and overpressure.
VFDs offer water system professionals incredible operational benefits, yet—like all system components—they must be serviced quickly. When you understand what to look for on each service call and how the VFD itself can help you with troubleshooting basics, you can keep customers up and operational.
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Disclaimer: VFDs should be installed and serviced by technically qualified personnel who are familiar with the correct selection and use of appropriate tools, equipment, and procedures. Failure to comply with national and local electrical codes and with recommendations, warnings, and instructions contained in any owner’s manual or other document provided with the product may result in electrical shock or fire hazard, unsatisfactory performance, or equipment failure.
As featured in Water Well Journal's article: Quickly Service a VFD