Application Story Situation: Moley Magnetics, Inc. had been working on quoting a customer on a motor and variable frequency drive that would be used in a kiln application; with extremely high temperatures and 100% humidity.

Spec: The customer was requesting that the motor be inverter duty vs. inverter rated.

Moley Response: Upon further questioning, we were able to explain to the customer that a true inverter duty motor is used in applications where positioning is critical to the application.  It offers a 2000:1 ratio whereas an inverter rated motor offers a 10:1 ratio.  The difference between a true inverter duty motor vs. an inverter rated motor is approximately 50% less in price.

Customer: The customer decided that their application did not require a true inverter duty motor and were able to go with the inverter rated motor thereby; saving money.

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Avoid Over-Specifying Inverter-Duty Motors

“… 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.

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.”

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