One unintended consequence of increased use of VFD control is better built motors. In order for motors to survive the voltage spikes and additional heat associated with VFD controls, most motor manufacturers have switched their standard from 600V insulation to 2000V insulation. In order to survive harmonics and vibration from VFD controls, motor windings are packed tighter and supported better to prevent movement. In order to survive EDM currents and other problems of VFD control, motor bearings have also been beefed up considerably. In short, motors are now being built to higher standards, to better handle the abuse from Variable Frequency Drives.
What this means to the end user is, if these new motors are not subjected to the abuse of a VFD control, longer than average life can be expected from the motor. Since we now know that VFD controls do not save energy and even increase energy use, we can find other ways of controlling the system without varying the speed of the motor.
One way to get the same constant pressure performance with variable flow rates as when using a VFD, is to use a Cycle Stop Valve. Cycle Stop Valve will maintain a constant pressure on the system, as the flow rates required vary up and down. What many people do not understand, and VFD manufacturers take advantage of, is that the amp draw of a pump will reduce almost exactly the same when throttled with a valve, as when slowed with a VFD. A 10 HP pump throttled with a valve will also reduce to a 5 HP load, even though the RPM remains constant. When controlled with a Cycle Stop Valve, the motor is running on standard smooth sinusoidal power and running at a constant RPM. This eliminates voltage spikes, harmonics, stray voltage, bearing or EDM currents, and all the other problems associated with VFD controls.
A Cycle Stop Valve is a simple mechanical device that has no electronic parts to fail. No fans needed for cooling that draw in lint, moisture, bugs, etc. No technical programming, or complicated adjustments. The Cycle Stop Valve lets the motor run at full RPM. The motor fan is delivering plenty of cooling, where supplemental cooling is usually required for motors controlled by a VFD. The only environmental concern for a CSV is freezing temperatures.
It is very typical for an electrical engineer to not understand how pumps work, and for a mechanical engineer to not understand how VFD's work. Very few people are versed in both mechanics and electronics, and able to understand the complexities of putting the two together.
Once you realize that VFD's do not save energy, you can take advantage of their unintended consequences. Motors that are made extra strong to handle abuse of VFD's, will last an extra long time when controlled with a CSV.