Shifting to VFD from Conventional Starters
I am writing this as a reaction to the article, "Another Look at Optimizing Pump Hydraulics" by Dr. Lev Nelik published in Pump Users Digest, November 17, 2016.
A shift to VFD control from conventional starters driving pumps at fixed speed does not always guarantee savings that will justify the investment.
Take the case of deep well pumps:
Here in the Philippines some water service providers modify their motor control, changing autotransformer starters to VFD to bring down the pump discharge pressure at desired level. This happens most often when pump is over-designed (Excess total dynamic head than what is required at the operating capacity). Naturally, current drawn will be reduced because you reduced speed to reduce output.
But the use of VFD will not guarantee that the shift in operating point will also be at BEP at the new speed.
The new operating point at new speed will be at the intersection of the pump curve at lower speed and the system curve (Total dynamic head and capacity behavior of the water distribution), which I would like to emphasize, may not be the BEP at that speed. In most cases, the BEP at lower speed can be lower than the BEP at pump rated speed. Changing pump speed will not make the pump follow the BEP trend, it will follow the SYSTEM CURVE.
Another influence on the performance of VFD is power factor and motor efficiency.
At reduced speed, PF and efficiency are reduced significantly. So in analysis of VFD driven pumps, pump efficiency, power factor and motor efficiency must be considered.
You may end up using less power at reduced output, but your specific power consumption (kw/hr per gpm) will be higher. That can result in higher pumping cost than what is expected.
Use of VFD in deep well pumps, especially, at deep settings is not recommended. Deep settings are in effect high static head, and you can only vary the discharge pressure (which is only a small percentage of the total dynamic head) resulting in minimal savings that will not justify the cost of VFD.
To optimize water extraction from deepwell, we recommend the tandem of submersible pump and constant pressure booster system.
The deepwell pump can be sized economically to run at fixed speed and BEP at designed TDH and capacity. Water can be stored in cistern tank and distributed using the CPS which is equipped with VFD. This system results in more economical operation. Caveat is you can increase revenue since you can deliver more water by storing during lean times and have more than enough to pump during heavy demand.
In evaluating performance, total system approach not per equipment basis, must be made to come up with a logical conclusion.
As mentioned by Dr. Nelik, life cycle cost analysis of the system, not just the VFD, must be conducted to arrive at an economical justification of making the investment in VFD.
Please refer to this link to learn more about specific power consumption applied to pump system operation.