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 motor’s speed regulates the flow. Adjusting the motor’s 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 motor’s 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 motor’s 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 motor
General-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 (<600 volts) are built with an inverter-ready or inverter-capable insulation system and are generally NEMA Design A or B motors that can be started across the line or used with a bypass should the inverter fail. Enclosures for general-purpose motors are usually totally enclosed non-vented (TENV) or totally enclosed fan-cooled (TEFC) with a cooling fan on the motor shaft. Open drip-proof (ODP) motors have an open enclosure and circulate air through the motor for cooling. These motor enclosures work well on variable-torque loads because as the speed decreases, the amount of power the load requires also decreases—as does the amount of cooling the fan can supply. When we talk about the speed range for a motor with a variable-torque load, it is called variable-torque speed range (VTSR) and is usually quite wide.
General-purpose NEMA-premium efficiency TEFC motors can also be used for constant-torque loads, but their speed range may be limited. For example, a constant-torque speed range (CTSR) is expressed as 10:1, or the motor can operate from base speed to 1/10 of base speed (180–1,800 RPM). Generally lower-horsepower general-purpose motors can operate over a wider speed range (20:1) because of their lower temperature rise. Larger motors (100 hp and up) may be limited to 4:1 or 2:1 CTSR due to the effectiveness of TEFC cooling, which is reduced when operated at low speeds.
Small fractional-horsepower ac motors may be limited as to the operating voltage from an inverter. It is not uncommon for these motors to be limited to 230 Vac input from inverter power because it is difficult to machine insert phase paper in these motors. Consequently, they do not hold up well to high-voltage overshoots that are common in the output wave form of most drives.
Only applications requiring a motor to produce constant torque over a wide speed range require a true inverter-duty motor. Such a motor may have a standard premium-efficiency winding (for use with a bypass or line start) or be supplied with a special winding optimized for use with an inverter and may not be capable of starting across the line. In addition to TENV and TEFC enclosures, inverter-duty motors may also have a separately powered constant velocity fan to ensure cooling at low speeds and are totally enclosed blower-cooled (TEBC). These motors usually have a 1,000:1 CTSR and with a flux vector drive can provide full torque at zero speed. Families of vector-duty motors are similar to inverter-duty but usually provided with encoder feedback for more precise speed regulation than can be done with an open-loop vector control. Inverter-duty and vector-duty motors are made in conventional NEMA and IEC frames and may provide increased performance in a drop-in solution.
The style of an inverter-duty motor described above may look like a standard smooth steel band, cast iron, or aluminum finned NEMA or IEC motor, but another type exists that has a frame made of the exposed motor lamination. The motor will be longer, have lower rotor inertia for fast response, and be built in a smaller-diameter frame. These motors typically have higher power densities than the typical cast iron frame NEMA designs. Because of their power density and nonstandard foot mounting dimensions, these exposed-lamination motors are not a drop-in replacement for a conventional NEMA or IEC general-purpose motor.
In the end, the application will dictate the motor used based on a variable-torque load (pump or fan) or a constant-torque load (conveyor or extruder). If it is a variable-torque load, general-purpose NEMA-premium efficiency TEFC or ODP motors should be adequate for the application. If it is a constant-torque load, the speed range and amount of torque needed at low speed will dictate the motor. On many CTSR applications, a general-purpose TEFC motor may be adequate if it provides 4:1 to 10:1 speed range. On applications requiring rated torque at very low speeds (and to zero speed), use of an inverter-duty or vector-duty motor may be required. Read More
