VFDs Drive Energy Efficiency in Motors

The industrial sector consumes approximately one-third of the energy used in the U.S. (see Figure 1). Studies show that up to 80 percent of the energy from the power source to the industrial consumer is lost through the transition of raw material to the point of useful
output—much of that at the point of conversion from electrical to mechanical output (see Figure 2).

Rising energy costs, a sense of environmental responsibility, government regulation, and a need for energy reliability are driving efforts for energy efficiency in manufacturing.

Energy is lost primarily in three areas:

1. Generation
2. Distribution
3. Conversion

The third area, energy conversion—converting energy into useful work via motors, heat exchangers, process heaters, pumps, motors, fans, compressors, and so forth—represents a large opportunity for energy savings in manufacturing.

Industrial electric motor-driven systems represent the largest single category of electricity use in the U.S.—more than 65 percent of power demand in industry. Consequently, motor-driven systems offer the highest energy savings potential in the industrial segment.

Supporting this statistic, studies show that 97 to 99 percent of motor life cycle costs are expended on the energy that the motor uses. This fact alone should be a driving motivation for companies to perform a periodic energy consumption analysis on the motor systems they use in their facilities.

Inefficient and ineffective control methods in two areas waste motor systems’ energy:

1. Mechanical flow control (pumps, fans, compressors)
2. Energy recovery (regeneration of braking energy or inertial energy)

A variable frequency drive (VFD) is an effective tool in conserving and recovering energy in motor systems.

What Is a VFD? Why Use It?

A VFD controls AC motor speed (see Figure 3). The drive converts the fixed supply frequency (60 Hz) to a variable-frequency, variable-voltage output to enable precise motor speed control. Many VFDs even have the potential to return energy to the power grid through their regenerative capability.

A VFD’s precise process and power factor control and energy optimization result in several advantages:

• Lower energy consumption saves money
• Decreased mechanical stress reduces maintenance costs and downtime
• Reduced mechanical wear and precise control produces more accurate products.
• Lower consumption lowers carbon emissions and helps reduce negative impact on the environment.
• Lower consumption qualifies for tax incentives, utility rebates, and, with some companies, energy savings finance programs, which shorten the return on investment (ROI). In many cases, the monthly installment payments on an energy efficiency based loan can be managed via the company’s maintenance budget.

Flow Control Optimization

Figure 4
Pumps, fans, blowers, and compressors using mechanical flow control are excellent applications for VFDs.

Pumps, fans, blowers, and compressors using mechanical flow control are excellent applications for VFDs (see Figure 4). Up to 40 percent of all electricity used in industry today is consumed by pump and fan systems. This fact makes them, as well as other flow control systems,prime targets for energy-efficient VFD installations.

The flow of air, liquid, or gas traditionally is controlled by using valves, throttles, dampers, louvers, vanes, or other mechanical devices. Restricting or reducing the flow with mechanical devices means that the motor supporting the system runs at 100 percent speed even though 100 percent flow is not required. The result is obvious—wasted energy.

What’s more, not only is energy wasted, but mechanical flow control can cause undue stress and high temperatures on system components, resulting in premature system failure.

Laws of Affinity: The Basis of Savings. The affinity laws are used in hydraulics to express the mathematical relationship between the several variables such as head (pressure), volumetric flow rate, speed, and power involved in pump and fan performance.

The affinity laws state:

• Flow is proportional to shaft speed.
• Head (pressure) is proportional to the square of shaft speed.
• Power is proportional to the cube of shaft speed.

Wasted Energy Recovery

Two general areas where VFDs can recover wasted energy:(regenerative applications) are:

• Cyclical applications.
• Braking applications.

Figure 5
A centrifuge is a good example of a cyclical application that generates inertial energy that, normally wasted, can be recovered with a VFD.

Cyclical Applications. In this context, cyclical refers to applications in which the machine comprises of a large inertial load (for example, centrifuges, and punch presses with large flywheels, see Figure 5). These mechanisms are required to coast to a certain speed or return to a set state. While coasting or returning to the set state, a large amount of
inertial energy generated is simply wasted. With a regenerative VFD, this energy can be recovered and returned to the facility power grid for use in other areas.

Braking Applications. Braking here refers to electrical braking, more commonly referred to as dynamic braking. These types of applications are known as holding torque applications. As the word braking implies, the load is either slowed down to maintain a certain speed or it is completely stopped (see Figure 6). For example, in the mining industry, downhill conveyors must have some method of maintaining their speed, for obvious reasons. An escalator is a similar type of application.

As energy is generated from slowing down or stopping the load, it must be dissipated somehow or the VFD will be damaged and all control will be lost. Typically, this is done with a braking chopper (part of the VFD) and a large bank of resistors.

As might be expected, the bank of resistors simply is a way to dissipate the energy but it is wasted in the form of heat. Essentially, the bank of resistors is a huge electric heater.

However, with a regenerative VFD, this energy can be captured and returned back to the facility power grid for use in other areas. In addition, the bank of resistors is no longer needed.

In some cases, the method of braking needs to be cooled as well. This is typically handled via a cooling system using pumps and cooling towers.. Using VFDs completely eliminates the need for a cooling system that would be required in those types of applications.

System Implementation Barriers to Energy Efficiency

Several barriers exist to implementing energy-efficiency programs. These include a reluctance to change a working process, shortage of capital, and the perception that the payback period is too long.

Too, unless all parties in the supply chain are motivated and the program has management support, success can be elusive. Studying the success stories of other manufacturers can help overcome these obstacles. In addition, government incentives, utility rebates, and savings-based financing can help translate to a low upfront investment and ROI in as few as three to six months.