VFDs: Beyond Fans and Pumps

When discussing energy savings and variable frequency drives (VFDs), the attention often focuses on centrifugal fan or pump applications. However, you should not overlook other applications which also have large potential energy savings and energy recovery. Applications such as regeneration, power factor correction, common bus applications or a combination of the three can also quickly achieve a significant reduction in energy use.

The historical trend has been in the HVAC industry and putting drives into buildings for fans to save energy. Now, that trend is transitioning into the pump market because it has the same affinity curve and the same potential to save energy. Pump applications with VFDs are generating a lot of interest and are moving in that direction.

But there are also other applications which, in certain situations, have the ability to recover even more energy. We classify these are as "energy saved" or "energy recovered" applications.  

Big Savings with Regeneration

An ac motor may act either as a motor that turns electrical power into mechanical power or as a generator that converts mechanical power into electricity. It all depends on whether the motor is turning a machine that requires power to turn the load, or if the load will at times overhaul the motor. This overhauling condition may exist in several types of applications.

Periodic Deceleration:  One type of application is when a load is stopped quickly and the inertia of the load wants to keep turning, such as a large drum. In this case, the cycle time or number of times the load is stopped over time, as well as the magnitude of the stopping power required, determines how much energy can be saved.

Continuous Deceleration:  When a load such as a decline or "downhill" conveyor is operating under the influence of gravity, it will overhaul the motor's speed. The drive is used to regulate the speed in a slower, more controlled fashion than what the natural physics of the application would produce. The same is true for crane and hoist applications.

Click here for a larger version

System Tension/Holding Torque:  Another type of application is when two sections of a machine are used to create tension on the material between them such as on the metal strip in a strip mill. The two sections may be running at the same speed, but the process may require a certain amount of tension on the strip to run properly. This means the lead section will run in the forward direction and pull the strip, and the following section will also run in the forward direction and at the same time provide the needed torque in the reverse direction of the strip thus creating the proper tension.

In each of these examples, the motor and drive combination has the ability to "recover" the electrical power produced by the motor that is acting as a generator and sent by the drive to the utility company. How much energy is saved depends on the application, but it can be significant.

One such application where significant savings can be recovered is a gearbox test stand. When the gearbox is tested, a single drive and motor combination is used to turn the gearbox while another drive/motor is used on the other end of the gearbox to simulate the load. Done correctly, this application will operate using a very low amount of total energy because the amount of energy used to turn the gearbox is the same amount of energy that is recovered from the simulated load on the gearbox, minus the losses in the system. The one critical item to determine if the application is regenerative is whether the load is trying to turn the motor (regenerative recovery) at any time or if the motor is being used to turn the load.

With applications where regeneration occurs, the choice is often to either recover the energy or use a dynamic brake. Using a dynamic brake creates a heat source which is lost energy. When you have regeneration there's a big emphasis on recovering the energy you are expending. With deceleration conveyors, cranes, fast stopping/starting and/or high inertia loads, there's an opportunity to recover a significant amount of energy.  

Power Factor Correction
Another opportunity for energy savings is tied to power factor correction. Most people don't realize it but motors typically have a power factor rating. A standard motor power factor will be from 80 to 94 percent at the very best, and a typical drive will be 95 to 96 percent. Basically, the closer you are to unity power factor (1.0) the more efficient the application will be 100 percent of the time.

With most utilities (though every utility may charge a penalty in a different manner and will be slightly different), if you have a power factor that is less than .9 or .95 they typically start charging a penalty. Usually it's about 1 percent of your bill, per percent from unity. So if the power factor changes from .90 to .89, it would be 1 percent of your bill as a penalty besides the energy you're losing because you're not at unity. If you use a motor with a .85 power factor and it's running across the line, then you'll have a 5 to 10 percent penalty on the energy you are using, plus you're paying for the extra energy that is needed because it's not being used efficiently.

The main issue with power factor is that ac power has two basic components: voltage and current. When these two components are not in sync (called Power Factor Displacement) ac power is wasted through inefficiency. Moreover, when the ac power has a high level of harmonic content called Power Factor Distortion, the displacement and distortion are multiplied by each other, which further decreases efficiency. (Total PF = Distortion PF x Displacement PF)  Therefore, getting the Power Factor Displacement close to unity is very important.

While power factor correction devices such as capacitors and filters exist on the market today, there is an often overlooked method for correcting power factor displacement. AC variable frequency drives with an Active Front End (AFE) may have the ability to adjust its power factor operating point as well as produce harmonics at less than four percent. A standard six pulse ac drive with a diode rectifier converts input ac voltage to dc bus voltage, with a typical harmonics level of 30 to 40 percent. There is at least one AFE ac drive available in the market today that has the ability to adjust its power factor from 0.8 leading to 0.8 lagging and which meets IEEE 519 harmonic standards which also results in a low Power Factor Distortion.

Common Bus Applications
One final area to explore for energy savings is common
bus applications.

When there are multiple drives in one location, the common bus system is usually the most efficient way to operate and can incorporate the energy savings and recover concepts that have been previously discussed.

However, if there is a regenerative drive and motor section in the system, it is ideally suited for maximizing energy recovery and cost savings. The reason is that losses are generated when power is converted from the ac supply to the dc Bus or from the dc bus to the ac supply. When you have multiple stand-alone drives the power must go through two or more ac to dc conversions and two dc to ac conversions.

In a common bus configuration, power only goes through one ac to dc conversion in the motoring direction. When an inverter section of the drive regenerates power to the dc bus, the power goes straight to another inverter, via the common dc bus link, which is motoring and does not have to travel through a converter at all. This method eliminates two conversion points where energy would be lost which increases efficiency by two to four percent for each regenerative section.

The more sections that are regenerative, the more energy savings are accumulated. In addition to the savings of a common bus solution, an Active Front End will have the ability to do Power Factor Correction, therefore increasing the savings of a common bus system. The gearbox test stand is a great example of a common bus solution. There is one forward motoring drive motor section and one regenerative drive motor section.

In this specific case, the two drive and motor sections were rated at 1,000A at 690V ac each. Yet the incoming ac line and input modules were able to be sized at less than 1,000A at 690V ac. The reason this was possible is that one of the two sections required 1,000A in the motoring or torque producing direction, while the other section that provided the load was able to recover through regeneration close to 1,000A, less the losses in the system. Therefore, the amps generated from the recovery section almost canceled out the 1,000A from the section providing torque to turn the gearbox and the input ac could be sized at slightly larger than the losses of the system, which in this case was roughly 200A at 690V ac. This resulted in a lower installation cost due to the smaller ac to dc section and the application recovered $75,000 per year in energy costs which translates to a four-year payback.

Beyond Fans and Pumps
The perception in the field is that energy savings means a focus on centrifugal fans and pumps. Many companies don't think a lot about the possibilities beyond those applications. With specific industries such as HVAC, energy savings has been a priority for a long time but with pumps the industry is transitioning to this mindset.

Most companies want to quantify the results to provide the before and after measurements, even though the application can be evaluated using theoretical calculations. Suppliers can also come in and quantify the different energy levels used in a facility based on a customer's request. Another option is "try before you buy" which allows you to prove out the technology and savings in an application before purchasing the equipment.

Engineers generally need to see the calculations, the logic and the curves to convince them these applications are the right thing to do. The potential is to move into these additional areas but the general thinking is not moving into those areas yet.

Click here for more information.