What systems would benefit from variable-speed operation? Consider the following factors when evaluating the costs and benefits of potential VFD applications.
Output profile. Any load with throttled output should be considered for a VFD retrofit. To qualify as an economical VFD application, the output from the motor must experience significant hours of operation under lightly loaded conditions, making a motor-loading profile a necessary tool for evaluating any new VFD application.
The load profile describes the output required from the motor system, rather than just motor loading. It involves charting the output from the loaded motor (downstream of any throttling valves) and comparing that profile to the fully loaded rated output. For example, if a 100-gallon-per-minute (gpm) pump is throttled down to an output of 30 gpm, this represents a 30 percent system loading. The motor in this case may actually be operating at about 70 percent of its full load, with the extra energy dissipated across the throttling valve. Adding a VFD so that the pump puts out exactly 30 gpm—with no throttling—will drop the motor’s load from 70 to about 40 percent, providing a dramatic savings in energy for the same pump output.
The average motor-loading fraction at which adding a VFD becomes economical will depend, as it does with most other energy-efficiency investment decisions, on the local cost of electricity, the number of hours the motor system operates, and what the motor output profile looks like. Most loaded motors throttled continuously at 70 percent or less of rated output represent good candidates for VFD control. Pumping systems sized to deliver rated flows for 40 percent of the time or less are also good candidates, particularly if their average throughput over time is below 60 percent.
Duty cycle. In general, the longer a motor operates, the more attractive it becomes as a VFD retrofit candidate. A motor system operating for 6,000 hours per year with throttled output will tend to be about three times more attractive for a VFD retrofit than the same motor operating 2,000 hours per year.
To determine motor duty cycle, the project manager must record the number of hours of motor operation over a regular period of operation and use it to estimate annual operating hours. Some motors have “run meters” that record the total number of hours. However, it is also important to note how the hours relate to the motor’s load. For example, in a pumping application, 130 hours of operation at 70 percent load wastes about the same amount of energy as only 100 hours throttled to 40 percent output. In general, the more throttled the output—and the longer the operation at throttled output—the more attractive the economics of a VFD retrofit.
Motor choice. For an increasing number of applications, alternative variable-speed technologies such as electrically commutated motors may offer benefits over an induction motor with VFD controls. This is especially true for fractional-horsepower motor applications, applications that require very high motor speeds or a wide range of speeds, high torque output at low motor speed, or four-quadrant (motor, brake, and generator) motor performance.
Poorly selected or hastily applied VFD installations can actually increase, rather than decrease, energy bills. To avoid this problem, take these steps when specifying a VFD:
Select premium-efficiency motors. Premium-efficiency motors have emerged as the preferred motor for VFD applications. In fact, most inverter-duty motors (designed especially for service with a VFD) sold today are based on premium-efficiency motor designs. In addition to having better efficiency at all speeds and loads and improved design and construction, premium-efficiency motors offer a number of advantages for VFD-controlled operation, including better thermal management, wider speed ranges, and better motor insulation.
Don’t use a VFD to provide continuous full load. VFDs produce dramatic energy savings by optimizing the motor to match a variable load profile—not by improving the actual efficiency of the motor in isolation, as a premium-efficiency motor replacement does. In fact, an induction motor with VFD controls is about 4 to 6 percent less efficient at full load than an induction motor alone due to the energy required to run the VFD (Figure 1), so a process that requires continuous full-load output from a motor will require more energy with a VFD, not less. However, it takes relatively little operation at reduced load to save more energy than is lost at full load. Average loading as high as 90 percent may still justify VFD retrofit for high-duty, high-utility-rate applications. In addition, the improved power factor provided by a VFD (especially with PWM drives) can reduce any fees your utility charges for low power factor and can free up the capacity of transformers, conductors, and other components of your electric distribution system for more productive uses.
Figure 1: VFD, motor, and system efficiency versus load
Although variable-frequency drives (VFDs) are efficient devices—typically operating at over 94 percent efficiency (energy output divided by energy input) throughout most of their load range—they do consume some energy, so it doesn’t make economic sense to install a VFD in applications where average motor loading is very high. However, in locations with high electric rates, VFDs can make sense even for applications where average motor loading is as high as 90 percent.
Take precautions with low-speed, high-load applications. Most induction motors currently in use can operate with modern VFDs through moderate speed ranges—around 30 to 100 percent of rated speed. But sustained operation at low speeds—and in particular, high load at low speeds—may require a special or larger drive and additional measures to help cool the motor. Induction motors operate hotter with a VFD because of harmonics and other power-quality impacts associated with VFD operation, and problems may result from the slower rotating speed of the motor’s integrated cooling fans. This is usually not an issue at continuous-speed motor operation above around 40 percent of rated loading or during brief periods of low-speed operation. But prolonged operation at or below about 30 percent of rated speed, especially when driving substantial loads, can cause rapid and potentially damaging heat to be generated by the motor when it was not designed to accommodate low-speed, high-torque operating characteristics. Selecting a drive that allows you to set a minimum operating speed can help address this problem, and additional cooling for the motor (such as external fans) may also be required.
Ensure adequate starting torque. In VFD motor systems, starting torque typically is determined not by the motor but by the drive—usually based on how much electrical current the drive can (and should) deliver. In conventional VFD applications, the motor system will experience a peak starting torque of about 130 percent of rated full-load torque—significantly less than what the motor could develop by itself without soft-start control. This level of starting torque is acceptable for most variable-speed loads, but some loads—especially constant-torque loads such as conveyers, escalators, augers, and reciprocating compressors—may require more starting torque. To improve starting torque for these applications, consider applying one of the following variations to a VFD retrofit:
- Specify a VFD for a higher-rated power output. Purchasing a VFD with a hp rating higher than that of the motor (for example, a 100-hp drive to control a 75-hp motor) enables a higher starting torque. This is because starting torque in a VFD system is inherently limited by the current-handling capacity of the drive. VFD oversizing should be considered carefully because larger drives cost more and require additional protection features to prevent the larger VFD from supplying too much current to the motor, leading to excessive energy use and motor degradation.
- Use programmable VFD starting features. High-quality VFDs have programmable features that can improve starting capability for the VFD-motor system. For instance, programming a gentle acceleration to ramp up motor speed can allow a drive to slowly start a motor with a high-inertia load. Dwell—another configurable VFD control parameter—involves initially energizing the motor, then pausing momentarily to allow the magnetic flux induced in the motor’s windings to increase before accelerating the motor to higher speeds. Finally, configuring voltage boost so that the VFD provides higher-than-normal voltage to the motor at low speeds facilitates a more-vigorous response when transitioning from one state of motor operation to another.
Use shorter cabling to prevent motor damage. PWM drives can cause significant damage to motors if the length of the signal-carrying cable that connects the VFD to the motor is greater than about 50 to 100 feet (maximum allowable cable length differs by cable type and manufacturer). Older motors with long cable runs may have shortened lives when controlled by PWM drives. Carefully watch motor lead lengths, and consider buying an inverter-duty motor or select a VFD type and system configuration that specifically guards against this hazard through the use of inductive filters or other safety features.
Use shaft-grounding rings. Though VFDs can save significant amounts of energy when used in the right applications, they also have the potential to induce undesired motor shaft voltages that can wear out bearings and cause motor failure. To prevent this, purchase VFDs that incorporate a shaft-grounding ring. Most motor distributors sell grounding rings that can be installed on new, refurbished, or in-service motors, and service contractors will often install rings when repairing failed motor bearings.
Avoid mechanical resonance frequencies. As with any variable-speed motor application, it is important to consider the mechanical resonance frequencies of the system and program VFDs to avoid steady operation at resonant speeds. Speeds that correspond to resonance frequencies are a common occurrence when operating large fans, gears, and belt-driven motor systems, and such operation can cause significant motor damage from excessive vibration. Identify resonant frequencies by observing as motor speed gradually increases from low to high rpm and monitoring the occurrence of noise and vibration at specific VFD operating frequencies. As part of regular system maintenance, this frequency-inspection technique can reveal worn-out bearings, fan or impeller unbalances, bent shafts, and other problems that might otherwise go unnoticed at constant-speed operation.
Ensure motor-VFD compatibility. To ensure that the VFD and motor are compatible for optimal motor control, it may be desirable to purchase both motor and drive equipment from the same company. Alternatively, VFDs are often designed and tested by the manufacturer to ensure compatibility with motors from other manufacturers. In either case, the facilities manager or project engineer must carefully specify the load requirements, duty cycles, motor speeds, and any other parameters to be controlled by the VFD under safe, efficient operating conditions.
Use appropriate enclosures. VFDs should be housed (sometimes with other electronic equipment) in safety-rated enclosures. The National Electrical Manufacturers Association (NEMA) has published performance specifications for standard enclosures (Table 2), which protect against equipment damage and injury to facilities personnel.
Table 2: NEMA enclosure standards
The most common enclosure types for normal applications include 1 through 4X, which cover the requirements of most general indoor and outdoor variable-frequency drive applications.