Tech Topics: Final Installment of Our Look at Variable Frequency Drives
March 1, 2013
This is the third and last article in our series on VFDs. So far, we have seen that a VFD is a solid state inverter that can be used in two basic ways in pumping applications: to increase the speed of a pump motor to increase the performance of a pump, and to vary the rotational speed of a pump’s motor to better match the pumping load, saving energy and money. The drive can be controlled by a pressure reading device to maintain a constant pressure in the system, or by a flow measuring device to maintain a constant flow rate, the former by far the most common.
There are several ways to match a pump’s output to a load, and VFDs are among the most expensive in terms of initial cash outlay. However, they can often pay for themselves through energy savings because of what are known as the Affinity Laws.
• Affinity Law No. 1: As an AC induction motor slows, the flow rate drop-off is proportional to the speed. If the RPM is cut in half, the flow rate drops in half.
• Affinity Law No. 2: As the RPM is reduced, the pressure drops off as the square of the speed. If the RPM is cut in half, the pressure output of the pump drops to one-forth of its full speed capability.
• Affinity Law No. 3: As the RPM is reduced, the horsepower required to operate the pump drops as the cube of the speed. Cutting the RPM in half reduces the horsepower to one-eighth of that required to run the pump at full speed.
From that, you can see how a system with a pump sized for a peak load, but operated at less than peak most of the time, may justify a the use of a VFD-assuming that the head requirements are still met at the slower speed.
VFDs Are Not for Everyone
The primary limitation to the use of VFDs for pumping applications has to do with the second Affinity Law, which, as we said, states that pressure falls off as the square of the speed. If the pressure side of your system curve cannot be satisfied at a slower speed, you are limited as to how slow you can run the pump, and the energy savings will be limited accordingly.
The other area where VFDs may not be right for pumping applications is when there is a long distance between the drive and the pump motor. This can be a particular problem in a submersible application, because the motor is down in the well, and the drive is on the surface. The problem has to do with the type of wave form and voltage spikes created by VFDs. Without getting too detailed, these distorted wave forms and voltage spikes can damage motor windings. This is particularly critical in small motors due to the geometry of the windings, and in motors built with no phase paper between the windings. Luckily, most motors used in pumping applications have phase paper, and most are not small, but there are many well applications where long lead lengths are unavoidable. Consult the motor manufacturer if your motor is more than 200 feet from the drive. They may recommend the use of an output line filter to protect the motor windings.
Configuring VFDs for Larger Well Pumping Applications –
The enclosure: The VFD should be house in an enclosure suitable for the environment to which the VFD will be subjected. VFDs are temperature sensitive, typically having an upper ambient limit of 104-122 degrees Fahrenheit. If they will be installed in an area where the temperature can exceed these numbers, they should be in a fan-cooled, filtered enclosure as a minimum, or in an air conditioned enclosure or building. Additionally, they should be protected from dust and the elements. Ideally, a VFD for a large water well should be in a NEMA 12 enclosure in an air conditioned, weather proof building at the well head.
Bypass: If your VFD pump application is such that it would not be good for the pump to be off line for an extended period of them, an across-the-line bypass should be included, consisting of three contactors: two to isolate the drive, and one to start the pump across the line when the VFD is taken off line for service. The two isolation contactors can be smaller than the across-the-line starter contactor because they do not need to handle the in-rush starting current. Additionally, only the starter contactor needs overload protection, because VFD drives have built-in motor protection.
Interfaces: Pressure transducers are the most common control interfaces used in pumping applications. They allow the pump to maintain a constant system pressure as the load varies. They vary in price from less than $100 to over $1,000. For best results, use a good precision pressure transducer.
Direct or remote reading flow meters are common system interfaces to allow the installer to properly set up the VFD and the operator to monitor the system performance. Adding remote reading pressure and flow meters to the panel front is a nice touch.
Size VFDs for the service factor of the motor: As you know, most pump manufacturers design their pumps to work into the service factor horsepower range of the motor, typically 115 percent of rated horsepower. If your application requires the pump to operate in “service factor,” as it is called, make sure the drive is sized accordingly. Unless ordered otherwise, they will be sized for a service factor of 1.0
Programmability: To make programming easier, an English operator keypad interface is highly recommended. It makes calibration a matter of following real English instructions rather than entering meaningless codes and following complicated instructions in the manual.
Acceleration time: There is a pumping specific program parameter that your drive may need. Motor manufacturers limit the frequency range within which their motors can be operated, and some limit the acceleration time through the lower frequency threshold. This may mean your drive must be capable of accelerating rapidly, say in 1 second, through 30 Hz, and then slowing down for the rest of the acceleration to reduce in-rush current, and to avoid overshooting the pressure set point.
Additional contacts: There may also be a need for an additional set or two of contacts to operate ancillary devices like lubricators, air chargers, etc.
We’ll close this series on VFDs with a schematic for operating several pumps from one VFD. The scenario is a system with a varying load requiring up to three pumps to handle the full load. The idea is to use a single VFD that takes pump No. 1 up to 33 percent of the load, at which point pump No. 1 goes across the line, and the drive takes pump No. 2 to 66 percent, at which point No. 2 goes across the line, and the drive picks up No. 3. There are some tricks that must be employed, but the drive manufacturers know how to do them, so if you have an application with two or more pumps on the same system, ask your drive supplier to quote a multiplex system.
Next month I will review a new product that that takes us into cyberspace. Till then … ND