Adjustable-speed drives (ASDs)—also called variable-frequency or variable-speed drives—allow induction-motor-driven loads such as fans and pumps to operate in speed ranges as wide as 10 percent to 300 percent of nameplate speed. By controlling motor speed so that it finely corresponds to varying load requirements, ASD installations can increase energy efficiency (in some cases energy savings can exceed 50 percent), improve power factor and process precision, and afford other performance benefits such as soft starting and overspeed capability. They also can eliminate the need for expensive and energy-wasting throttling mechanisms such as control valves and outlet dampers.

Loads ideal for ASD application: The large majority of ASDs are installed on loads where torque increases with speed, including centrifugal pumps, fans, blowers, and most kinds of compressors.

Loads requiring careful ASD application: Constant-torque loads require the same torque regardless of speed. Examples are reciprocating compressors, positive-displacement pumps, conveyers, center winders, and drilling/milling machines. Although constant-torque loads are suitable for ASDs, operation of these loads at low rpm will be limited, and the ASD must be carefully sized to ensure adequate starting torque.

Loads difficult for ASD application: Loads in which torque decreases with speed usually involve very high inertia loads such as vehicular (for example, railway traction) drives or flywheel-loaded applications; it takes less torque to keep these loads turning than to accelerate them. Loads of this type are difficult, but not impossible, for ASD application. Custom-engineered solutions are often required to handle the extra heat generated in starting and stopping such loads.

Back to top  

There are several types of ASD, each with its own benefits and drawbacks. Many hybrid forms combine characteristics of two or more of the basic ASD types listed below:

Historically, the six-step voltage source drive was the industry workhorse. However, mass production and pricing pressure have enabled the pulse-width modulated drive to become increasingly dominant, particularly for ASDs under 200 horsepower (hp).

Present-day economics favor pulse-width modulated drives for applications under 200 hp, and, in most cases, these drives will provide excellent service. In both retrofit and new applications, the user should consider heating and cabling distance to make sure the supplier will guarantee long-term performance. Some applications may also use the older, but very reliable, six-step voltage-source inverter technology. Many thousands of these drives are in service, and many companies still design and manufacture them for applications under 200 hp.

Larger systems may use current source, load-commutated, or cycloconverter type drives. Specifying and implementing larger systems usually will involve more detailed design, close technical support, and, in some cases, custom engineering.

Back to top  

What motor/load systems would benefit from adjustable-speed operation? Consider the following factors when evaluating potential ASD applications:

Output profile of the application. In general, all loads with throttled output should be evaluated for ASD retrofit. To qualify as an economical ASD application, the output from a motor/load system must have significant operation at less than rated output, thus an output profile is a necessary tool for evaluation.

The output profile documents the output required from the system, rather than motor loading. It involves charting the output from the motor/load system (downstream of any throttling valves) versus rated full-load output. If a 100-gallon-per-minute (gpm) pump is throttled to 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 an ASD so that the pump puts out exactly 30 gpm with no throttling will drop the motor’s load from 70 to about 4 percent, providing a dramatic savings in energy for exactly the same output.

The average loading at which adding an ASD becomes economical will depend, as with other energy efficiency investment decisions, on the local cost of electricity, on how many hours the motor and driven equipment operate, and at what output. Most loads throttled continuously at 70 percent or less of rated output are good candidates for adjustable speed. Motor and load systems that deliver rated flow less than 40 percent of the time are also good variable-speed prospects, particularly if their average throughput over time is below 60 percent.

Duty cycle of the motor. In general, the longer a motor operates, the more attractive it becomes as an ASD retrofit candidate. A motor/load system operating for 6,000 hours per year with throttled output will be three times more attractive for ASD retrofit than the same motor operating 2,000 hours per year.

To determine the duty cycle, the user must record how many hours the motor operates for a set time and then estimate the yearly operating hours. Some motors have “run meters” that will 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 at 40 percent output. In general, the more throttled the output—and the longer the operation at throttled output—the more attractive the economics of an ASD retrofit.

Motor choice. For an increasing number of applications, alternative variable-speed technologies such as electrically-commutated motors (ECMs) may offer benefits over an induction motor/ASD system. This is especially true for applications that require very high speeds or a large range of speeds, high torque at low speed, or four-quadrant (motor, brake, and generator) performance.

Poorly selected or applied ASDs can increase, rather than decrease, energy bills. To avoid this problem, consider the following when specifying an ASD:

Which motor to use: standard or premium efficiency? Premium-efficiency motors have emerged as the preferred motor for ASD applications. In fact, most inverter-duty motors (designed especially for service with an ASD) 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 ASD service, including better thermal management, wider speed ranges, and better insulation systems.

No savings at full load. ASDs provide dramatic energy savings by optimizing the motor/load system—not by improving the actual efficiency of the motor in isolation, as a premium-efficiency motor retrofit would. In fact, an induction motor/ASD system is about 4 to 6 percent less efficient at full load than an induction motor alone (Figure 1). A process that requires continuous full-load output from a motor/load system will require more energy with an ASD, 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 can justify ASD retrofit for high-duty, high-utility-rate applications. In addition, the improved power factor provided by an ASD (especially pulse-width modulation [PWM] ASDs) 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 use.

Figure 1: ASD, motor, and system efficiency versus load

Although adjustable-speed drives (ASDs) 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 an ASD in applications where average motor loading is quite high. However, in locations with high electric rates, ASDs can make sense even for applications where average motor loading is as high as 90 percent.

Problems at low speeds and high loads. Most induction motors currently in use can operate with modern ASDs through moderate speed ranges (around 30 to 100 percent speed). Sustained operation at low speeds and, in particular, high load at low speeds may require a special or larger drive and special measures to cool the motor. Induction motors will operate hotter with an ASD because of harmonics and other impurities in the electric power they provide to the motor and also as a result of the slower rotating speed of the motor’s integral cooling fans. This is usually not a problem at continuous speeds above about 40 percent or for brief periods of slow-speed operation. Prolonged operation at or below about 30 percent speed, especially when driving significant loads, can cause rapid and potentially damaging heat in motors not designed to accommodate this service. Selecting a drive that allows the user to set a minimum operating speed can deal with this problem. Additional cooling for the motor (such as external fans) may also be appropriate.

ASDs decrease starting torque. In ASD/motor systems, starting torque typically is determined not by the motor but by the drive—particularly how much electrical current it can deliver. For conventional ASD applications, the ASD/motor system will have a peak starting torque of about 130 percent of rated full-load torque—significantly less than what the motor could develop by itself. This level of starting torque is acceptable for most variable-speed loads, but some loads (especially constant-torque loads such as conveyers, escalators, augers, or reciprocating compressors) may require greater starting torque. To improve starting torque for these applications:

Motor damage from ASDs located too far from motor. Pulse-width modulated (PWM) drives can cause significant damage to motors if the length of cable between the ASD and the motor exceeds 50 to 100 feet. (The number seems to differ by manufacturer.) Older motors with long cable runs may have shortened lives using PWM ASDs. Carefully watch motor lead lengths, consider buying an inverter-duty motor, or select an ASD system that will specifically guard against this hazard with inductive filters or other methods.

Mechanical resonance frequencies. As with any variable-speed system, it is important to determine any mechanical resonance frequencies and to program the ASD to avoid steady operation at those speeds. These resonance frequencies, common in large fans, gears, and belt-driven systems, can cause significant damage through vibration. Users can identify these frequencies by monitoring noise and vibration while instructing the ASD to gradually increase speed from low to high speed. When used as part of regular system maintenance, this technique can reveal weaknesses in bearings, fan or impeller unbalances, bent shafts, and other problems that may escape notice at constant speed.

Motor-ASD compatibility. To ensure that the ASD and motor are compatible, both motor and drive can be purchased from the same company, or the ASD may be designed and tested by the manufacturer for compatibility with another company’s line of motors. In either case, the user must carefully specify load, duty, and other critical system requirements.

Back to top  

Look for continuing decreases in first cost for ASD units and ever-improving reliability. Manufacturers are working on integrating ASD controls into other control systems, such as heating, cooling, and ventilation systems.

Back to top