Voltage Controllers

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 achieve modest energy savings 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).

Figure 1: Effect of voltage on motor loss
At low loads, motor losses decrease as voltage decreases.

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.

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