Saving Energy and Water with VFDs

02/01/2006


In today's world, with power transmission overloads in the Northeast and blackouts in the West, the U.S. Dept. of Energy (DOE) is concentrating its efforts on reducing energy consumption. According to DOE, approximately 63% of industrial electrical power consumed in the U.S. is used to operate electric motors.

Water pumping is one of those applications that uses electric motors. Every municipality deals with getting water to and from every home and business. For each of these communities, water usage varies throughout the day. When demand decreases, control valves are adjusted, yet the motor stays running full speed. This is similar to when you approach a red traffic light in your car and instead of reducing engine speed, you simply apply the brakes. Most people would say that's an inefficient way to operate your car. The same is true for electric motors.

There's a better method of reducing the flow or pressure when the demand doesn't call for it: using an adjustable speed drive (ASD). This technology is also known by many other names: variable-frequency drive (VFD), variable voltage/variable frequency (VVVF), variable-speed drive (VSD) or just plain "inverters."

Here, I'll refer to them as ASDs, because that is what they truly are—adjustable. ASDs adjust the speed of AC induction-type motors from zero to three or four times their normal across-the-line speed, or base speed, by changing the voltage and frequency supplied to the motor. This adjustment can come from several different sources, such as the operator or by intelligence built into controls. Which adjustment source used is entirely based upon the initial set up of the control.


Figure 1 - Input Power Necessary

 

Let's look at an example to see how adjusting speed can benefit an application. In this example, a 100-hp pump is required.

First, there are three important facts to consider about variable-torque type pumps. These are also known as the affinity laws:

  1. The flow produced is proportional to the motor speed.

  2. Pressure produced is proportional to the motor speed squared.

  3. Horsepower required is proportional to the motor speed cubed.

Let's put all this in perspective by looking at our 100-hp pump. If the flow needed is only one-half of rated, then the motor could be operated at half speed and the pressure would become (0.5)2= 25% of rated. The horsepower needed to operate the pump would be (0.5)3= 12.5 hp.

We plot the input kW needed to operate the 100-hp pump using 40%%%MDASSML%%100% and also using both throttle valves and bypass valves. Let's compare an adjustable speed drive as well. The kW curves are plotted on Figure 1, p. 57, which uses a 92% efficiency standard based on DOE energy average effiency. Note that bypass valves are not the best choice, as the motor is still pumping the same amount of water and using the same amount of energy at full load or 40% load. A throttle valve looks somewhat better at a 67% use of energy at 40% load. However, the adjustable speed drive looks best at 40% load with a power consumption of only 13% in relation to bypass valves.

The bottom line

Let's look at the bottom line and compare how much money each type uses. The pump is run at various loads throughout the day as follows:

Load %

% of Day

50

10

60

15

70

25

80

20

90

15

100

15

If the power rate is $0.10 per kilowatt hour, the resulting plot for each technology is shown in Figure 2. Totaling the graph, total annual dollars would be:

Bypass valve $71,032

Throttle valve $61,727

ASD $39,565

As can be seen from the graph, payback is much greater as the pump speed is reduced, which correlates with the affinity laws mentioned above. At 100% load the ASD approach doesn't reduce kW used, as the drive itself is about 96% efficient. This builds the case that if the pump is ran at variable speeds throughout the year, it will soon pay for itself.

Figure 2 - Annual Dollars Used at $0.10 per kWh

Other ASD benefits

There are a number of additional benefits to specifying ASDs, including the following:

Soft-starting. Inherent to all ASDs is soft-starting and soft-stopping. For applications involved in water pumping, the soft-start feature alone can prevent water hammer from occurring. Since the control changes frequency and voltage to the motor, it demonstrates improved smooth starting over its predecessor, the reduced-voltage starter. In the range of low-hp motors, the ASD is very competitive in comparison with reduced voltage starters and can prove to be the better choice.

Leak reduction. Running at reduced speed lessens the chances of pipe failures associated with high pressures, when demand doesn't actually call for it. Also any water leakages are more pronounced when high pressures exist, leading to more wasted water.

PID mode. All manufacturers of ASDs now have some type of process control inherent in their drives. Process control allows the drive to maintain a particular water pressure or flow based on a transducer feedback. These are usually in the form of zero to 10 volts DC or 4 to 20 mA signal. As the demand increases or decreases, the drive automatically throttles up or down to maintain the set point. This is typically less than 1 psi fluctuation. Taking it one step further, there is the issue of what drive manufacturers typically refer to as "sleep mode."

Sleep mode is the newest form of pump control in ASDs. Suppose the drive slows to a preset speed. (Usually this minimum speed is set where the pump is no longer pumping water, but simply heating it.) The drive enters a sleep mode and stops the motor. Then, after the pressure drops below set point, the control "wakes up" and begins pumping water again.

Obviously to avoid multiple starts and stops within a few minutes, usually some type of hysteresis loop must be programmed into the drive allowing some deadband. Clearly, The sleep mode can add to the savings benefits already noted.

Additional cost savings. Premium efficiency motor can save additional money. These motors work well with ASDs for many reasons. When running a conventional motor with an ASD, its temperature can be as much as 10°C higher. Premium efficiency motors run cooler, with better steel, and thus, the result of heating is about the same with a regular motor across the line vs. an ASD with premium efficiency motor.

Increased reliability. Premium efficiency motors use higher rated windings with an H-type insulation class and phase paper to separate the windings. This prevents the voltage ring-up failures that are sometimes associated with ASDs. Today, premium efficiency motors are inverter-ready as defined by NEMA MG 1 Part 31.4.4.2.

Application notes

The electrical system designer must keep in mind that the output of an ASD is not a perfect sine wave, and the effects of voltage spikes on the motor are possible, especially when lead lengths approach 100ft. To prevent early motor winding failures, use load reactors or low pass filters added onto the output of the control. (See "Putting a Stop to Drive-Related Motor Damage," next page, for new technology innovations helping to resolve this problem).

Although adding an ASD will improve displacement power factor, it can add harmonics to the line, sometimes as high as 80% total harmonic distortion. The remedies include the following:

  1. Using a line reactor (choke) will reduce this value down to 40% or less.

  2. If fewer harmonics are desired, a 12-pulse input or 18-pulse input can be used. These are designed with a special input transformer that phase shifts the voltage so several diode bridge inputs can be used and keep the line in conduction.

  3. Even further are inverse line harmonic filters, which inject negative harmonics into the line to cancel the harmonic, created by the drive.

  4. And lastly, there is the option of an active front-end type inverter, where the input behaves much like a resistive load and results in harmonics less than 5% distortion.

Seek assistance

Beyond these technical traps, the cost of microprocessors continue to come down, meaning manufacturers are prone to add more features to their controls. In fact, many manufacturers offer application-specific type programs built into the control-all this without adding to the cost.

Don't be afraid to find out exactly what these new control offerings are. Furthermore, manufacturers are also available to help you better understand whether it's more economical to add an ASD or combine it with an ASD/premium motor. Software is also available from the DOE and motor manufacturers that allows consultants/operators to input motor ratings and duty cycle. The software, in turn, calculates the total savings—or losses—that are occurring in that pumping application.

Proper balance

There are several methods used to reduce flow or pressure on a water system. Each of these does its job rather well, but they all have one common problem: wasted energy. The electric motor operating the pumps continues to run at full speed. Using adjustable speed drives in an effort to reduce energy consumption outweighs that of its counterparts.

 



Putting a Stop to Drive-Related Motor Damage

Often, we must accept a little bad with the good. The advent of variable-frequency drives paved the way for great energy savings. On the other hand, the increased use of adjustable drives sometimes resulted in damage to the motors they regulated, notably the pitting of motor bearings.

"This is no new phenomenon, but it got a lot worse with VFDs," said William Oh, a mechanical engineer with Electrostatic Technology, Mechanic Falls, Maine.

Specifically, because of the high switch rates of VFDs, Oh explained, radiation and voltage spikes are often produced. These spikes, added Adam Willwerth, the company's sales and marketing manager, hit the bearings as they try to go to ground.

In fact, the problem came to their attention from a consulting engineer who was experiencing problems with drive-assisted motor failure. In developing a solution, Oh invented an independent shaft grounding ring. The device, made up of approximately 10,000 microfibers, discharges the current as these spikes happen, passing them to ground.

Pitting problems can be dealt with by applying a spring-loaded brush, but the problem with that solution, according to the manufacturer, is that the brush will eventually glaze over.

Electrostatic feels the microfiber ring is a long-term solution. But the proof is in the pudding. Willwerth added that this particular consultant was so pleased with the solution that his firm is now making it part of their drives specification package.



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