Avoid over-specifying inverter-duty motorsWhen combining a motor and inverter, the application should tell you what type of motor fits best. If you follow these guidelines, you can avoid more than is necessary.

Today adjustable-speed drives (ASDs) are commonly used to regulate the flow of air and water from motor-driven fans and pumps. In doing so, old-style dampers and flow valves are eliminated and the motors speed regulates the flow. Adjusting the motors speed saves energy. Other ac motors and drives are used to replace old dc motor technology used on conveyors and extruders. To maximize the life of motors used in this manner, we need to understand the motor features needed to operate with a drive, so we do not specify designs that are more robust and expensive than are really required.Why use an ASD?An ac adjustable-speed drive changes the input voltage and frequency to the motor, which changes the motors speed. There are several types and loads, each with specific load characteristics that affect the motor. The most common type is a variable-torque load where the horsepower required varies by the cube of the speed change. This is called the affinity law. So on a centrifugal pump load (assuming pump efficiency remains constant), this chart illustrates what happens:

Since most equipment is sized for worst-case conditions, it never runs at full capacity. On a variable-torque load such as a pump, the usual running condition may be at 60% speed, which requires only 22% of the motors horsepower. The reduced wattage drops the operating cost significantly. A 100 hp motor operating continuously could cost $27,139 for annual operation at full speed. At 60% speed the operating cost would be reduced to $5,970, a $21,169 annual savings.A second type of load has constant-torque characteristics. The torque requirement remains constant and does not change as speed is adjusted. Such applications are conveyors, extruders, mixers, and positive-displacement pumps. There is a lower energy saving as speed is adjusted on a constant-torque installation. Using a drive on a constant-torque application may save energy through increased productivity and be measured through benchmarking of widgets per kWh.Selecting a motorGeneral-purpose integral horsepower NEMA-premium efficiency motors from most manufacturers can be used for all variable-torque and many constant-torque applications. These 3-phase low-voltage ac squirrel-cage induction motors (