If your facility employs alternating-current (AC) induction motors that require constant speed but operate for considerable periods at very low load—which means at less than peak efficiency—you may be able to cut your energy costs by installing a device known as a voltage controller.

Voltage controllers are electronic devices that sense the load on a motor and reduce the voltage applied to the motor’s terminals when it’s operating at low load. When motors operate at partial load, they draw excess magnetizing current, creating unnecessary losses in the motor core. Voltage controllers reduce this excess current, thereby reducing core losses. (Figure 1).

These controllers are most likely to be cost-effective in situations where motors operate at constant speed but spend a lot of time at very low loads. Some vendors make excessive claims about voltage controllers’ ability to save energy, but in reality, there are relatively few motor applications in which a voltage controller can save enough energy to pay for itself. Good applications for voltage controllers do exist, but it’s important to understand the conditions under which they can be a viable solution. In a nutshell, a cost-effective application requires a motor that operates for very long hours at very light load. In addition, the motor must not be a candidate for alternative approaches that could save more energy, cost less, or both (such as downsizing the motor to better match the load, turning the motor off when not in use, or using a variable-speed drive). Applications meeting all of these conditions can be hard to find, but where they exist, voltage controllers can save significant amounts of energy. Cost-effective applications often include escalators and elevators, conveyors, crushers, injection molding and vacuum forming machines, lumber saws, sewing and weaving machines, machine tool spindle drives, and washing machines.

Soft-start. Most voltage controllers come with a feature known as a soft-start capability. By gradually ramping up the voltage to a motor, soft-starting cuts the abrupt inrush current to less than half of the current that accompanies full-voltage starting. Benefits include reduced heat in the motor windings and lower mechanical stresses on belts, couplings, and transmissions. In addition, soft-starting can limit the interference with other, voltage-sensitive equipment in a plant. Some products offer an adjustable soft-start capability that can adapt to the operator’s needs—from an instantaneous start to a ramp-up of 20 seconds or more.

Contrary to some claims, soft-starting does not reduce peak demand charge. Soft-starters do not reduce the amount of energy necessary to start a motor; rather, they spread it over a period of seconds rather than fractions of a second. Because utility demand charges are typically calculated based on peak demand within a window of at least 15 minutes, it isn't likely that soft-starting will reduce demand charges.

Motor protection. Some models include circuits that protect motors from overcurrent and phase imbalance.

Motor monitoring. Some voltage controllers provide a digital readout of current, voltage, power factor, and power and energy consumption. The data can be downloaded for analysis of energy use and to assist with predictive maintenance.

Learning capability. A learning capability in some voltage controllers enables them to adapt to the characteristics of the particular motor they’re connected to.

Anticipator circuits. Some controllers have circuits that react quickly enough to load changes to prevent stalling, even with sharp increases in required torque.

Estimate cost-effectiveness. Beware of the misleading marketing claims made by some manufacturers of voltage controllers. At low load, these controllers can reduce losses by an impressive percentage, but saving even a large percentage of the small amount of energy used at low loads may or may not result in a cost-effective application. Pay attention to power and kilowatt-hour savings rather than the percentage by which losses are reduced. For a given application, determine the load profile and estimate the power savings using Figure 2 or Figure 3. For example, a single-phase motor that spends 20 percent of its operating time at full load and 80 percent at one-quarter load might reduce energy use by about 25 percent.

You can use that information to get a rough estimate of a simple payback period for the device (Table 1). The table shows a calculation based on energy savings alone.

Retail prices for voltage controllers depend on the features of the unit and the voltage rating. They range from about $65 to $200 per horsepower (hp) for small motors to about $30 to $60/hp for 10-hp motors to about $20/hp to $25/hp for motors at 200 hp.

Pick a controller that matches the motor. Voltage controllers are available for single-phase and three-phase motors in the full range of NEMA enclosures, with voltage ratings up to 600 volts and in sizes up to 1,000 hp.

Look for devices that produce low total harmonic distortion (THD). The generation of harmonics within an AC induction motor circuit can cause a temperature increase and lead to interference with other electrical equipment. Some voltage controllers increase the harmonic content of the motor they are controlling more than others.