Cavitation is destructive to a pump and will destroy the impeller over time, causing small pits in it. Some VFDs automatically detect cavitation and adjust the pressure to the point that the cavitation stops. Once the demand decreases to keep up with the supply, the pressure is returned, preventing damage or nuisance trip offs.
Another problem is running a pump with no flow. This will heat up the pump and damage it within hours. A good no-flow algorithm is not only accurate but adaptable to different head and suction pressures. Since new ideas are proven and tested, there is always room for improvement in a VFD controlled pump.
On-off controlled pumps do not have the ability to change the pump’s response based on advanced algorithms as previously discussed.
Having a VFD should not be complicated. The end user should not be forced to learn drive parameters and constantly program the drive to get satisfactory performance. A good VFD design incorporates no keypad. A simple pressure knob is all that is needed.
Finally, VFDs should be serviceable. An end user should expect a warranty of a minimum of three years, which shows the manufacturer’s confidence.
One component in any pump will eventually fail: the pump seal. How often the seal fails depends on how much the pump runs and how it is controlled. A pump seal has a typical three- to five-year service life. Before choosing a pump, look at how easy the pump seal is to replace and its cost. Some require purchase of a new pump.
The check valve should also be serviceable, with the ability to take it out, take it apart and clean it, replace the seal if necessary and put it back in service. Note if the check valve is internal and sealed inside the pump. External check valves are preferred.
In conclusion, the pump should be totally serviceable providing a lifetime of service without needing to replace the entire unit.
VFD Features to Look For
- They will meet the requirements of IP66, meaning they are 100 percent waterproof and washdown proof, and the inside and outside air do not mix. This prevents contaminants such as moisture, salt, air, dust, insects and other airborne problems from entering the drive.
- The VFD will have conformal coated boards and power components that prevent condensate from damaging the drive’s electronics.
- The drive will keep cool by using proper heat sinks. A drive should be rated to 60 degrees C. An internal fan should come on when the temperature reaches 40 C or above, and it should protect itself from being overheated by direct sunlight.
- Protections from conditions including overcurrent, overvoltage, undervoltage, high heat, low power, low pressure and high pressure should be in place.
- VFDs should have voltage spike resistance. A good VFD will have conservatively designed power components that are rated to withstand 1,200 to 1,800 volts. Metal oxide varistors (MOV) or lightning protection to protect against line surges and properly designed ground planes to prevent against electrical noise are also recommended.