Variable Speed Pumps Waste Energy
Claims are being made that the variable speed drive is the magic pill that cures excessive energy consumption. As with any magic pill, you should carefully read the fine print. Variable speed drives do not save energy. When compared to using a correctly sized pump, a variable speed drive actually burns or waste energy. Drives also cause many negative side effects compared to running a standard pump at a constant speed.
The following is an actual curve that shows energy used by the same pump when running across the line at full speed or being controlled by variable speed drive.
For this example the system requires 1200 GPM for 12 hours per day and requires 100 GPM for the other 12 hours per day at 10 cents per KWH.
At 1200 GPM and 231' of head, the pump uses 100 HP in energy when running on across the line power at a constant speed.
This equals....
- 12 gallons per horse power or
- 10.3 cents per thousand gallons or
- $89.00 for 864,000 gallons
At 100 GPM and 231' of head, the pump uses 42 HP in energy when running at a constant speed and simply restricted with a control valve.
This equals....
- 2.38 gallons per horse power or
- 52.2 cents per thousand gallons or
- $37.59 for 72,000 gallons
When a pump is running on a variable speed drive, 3% to 5% power is burned due to energy used by the drive itself and the loss of motor efficiency from running on pulsing DC voltage.
At 1200 GPM and 231' of head, the pump uses at least 103 HP in energy when running on a variable speed drive.
This equals....
- 11.65 gallons per horse power or
- 10.66 cents per thousand gallons or
- $92.10 for 864,000 gallons
At 100 GPM and 231' of head, the pump uses 38 HP in energy when using a variable speed drive and decreasing the speed to 3280 RPM. To this you could add back at least 3% loss from the drive which would increase the power required to 39.14 HP.
38 HP equals.....
- 2.63 gallons per horse power or
- 47.22 cents per gallon or
- $34.00 for 72,000 gallons
1200 GPM at 100 HP = 12 gallons per horse power
100 GPM at 38 HP = 2.63 gallons per horse power
12 gallons per horse power divided by 2.63 gallons per horse power = 456%
At 100 GPM using 38 HP, the variable speed drive is burning 456% more energy per gallon than when the pump is running at constant speed at 1200 GPM.
| Variable Speed Control | Constant Speed Control | |
|---|---|---|
| 1200 GPM for 864,000 gallons | $92.10 | $89.00 |
| 100 GPM for 72,000 gallons | $34.00 | $37.59 |
| Energy Cost for 936,000 gallons | $126.10 | $126.59 |
| At 1200 GPM for 936,000 gallons | $96.41 | $96.41 |
| Energy Burned or Wasted | $29.69 | $30.18 |
Whether you are running the pump at full flow or reduced flow, a variable speed drive is burning energycompared to running at the best efficiency point with across the line controls.
A pump control valve will also burn energy when compared to running at the best efficiency point. However, there is very little if any difference between the energy burned by a control valve compared to energy burned by a variable speed drive. Many people falsely believe that restricting the pumps flow with a valve will make the motor work harder. As you can see from the curve, restricting the flow from a pump is "counter intuitive" as it reduces the power consumed making the work easier on the motor. Those who do not understand this simple characteristic of a centrifugal pump will say things such as, "pump control valves are like driving a car with one foot on the gas and one foot on the brake". Those who truly understand pumps will tell you that "a valve does not burn any more energy than a drive".
Most of the articles I have read that claim energy savings when using a VFD control are misleading. If you carefully read these articles you will find that other parameters were changed at the same time as the VFD was installed. Usually the head required was reduced by increasing the pipe size or increasing the NPSHA, the use of a dump valve was discontinued, or a smaller or more efficient pump was installed. Then a VFD is added to the control system and given the full credit for the reduction in energy when it is simply not the case. Other articles have used the Affinity Law to show energy is being saved by the cube of the RPM. However, they neglect to mention that head is lost by the square of the RPM. This limits the minimum RPM possible that will still produce the head required. These articles falsely show incredible reductions in power that go along with reducing the RPM by 50% or more. When in reality the RPM can be reduced by no more than 10% or the head required is no longer attainable. According to the Affinity Law a 10% reduction in RPM produces only a 23% reduction in power consumed, not the 50% or 90% reduction in power that many drive manufacturers would like for you to believe. Only those who lack understanding of how centrifugal pumps really work would claim VFD's save energy.
Not only do VFD's burn energy but, they have a multitude of negative side effects that can create even more problems.
Pulsing DC voltage, EDM currents, critical speed vibration, harmonics, Radio Frequency Interference, and other side effects can cause premature destruction of the pumping equipment and may require considerable technical assistance. Drives are computers with a microprocessor base. Parts needed to repair a fiveyear- old drive are no more available than parts for any other five-year old computer. Many articles state that the expense and side effects or the "myths and legends" of variable speed controls are worth putting up with because of the energy savings. However, when there are NO energy savings with VFD's, all of these side effects can be eliminated by using other simpler and less expensive controls such as a throttling or pump control valve.
It should be made clear that this article is only referring to controlling centrifugal pumps with good brake horsepower characteristics. Variable speed drives can save energy and could be worth the side effects involved when used with positive displacement pumps, conveyor belt systems, fans, compressors, or other similar devices. Drives have a multitude of good applications, but saving energy with centrifugal pumps is not one of them. The only time a drive may decrease energy consumption with a centrifugal pump is on start up. However, with variable speed or valve control, the pump should run continuously producing the varied amount of flow required, so frequent starts should not be a concern. If for some reason frequent starts are a concern, a soft start panel in conjunction with a control valve can eliminate end rush currents the same as a drive.
Many people who have made no other changes to their system than to add a VFD as their pumps control, do not understand why they do not see the energy savings they were promised. Because of the negative side effects of VFD, they also soon learn that the dependability and longevity of their pump system has not increased as they were led to believe. I can understand why the average person does not understand, as this can be a very confusing topic. I do not understand why engineers for pump companies, drive companies, trade magazines, and even the Hydraulic Institute cannot understand this reality. I believe that there has been so much hype published about VFD's that there are very few of us willing to stand up and say "the emperor is not wearing any clothes".