AC Motor-Based Drive System Functions Without Gearboxes or Pulse Encoders

ABB Inc., New Berlin, Wisconsin

Standard induction motors, normally designed to run at base speeds between 850 to 3500 rpm, are not particularly well suited for low-speed operation, as their efficiency drops with the reduction in speed. They may also be unable to deliver sufficient smooth torque at low speeds. Use of a gearbox is the traditional mechanical solution for this challenge. However, the gearbox is a complicated piece of machinery that takes up space, reduces efficiency, and needs both maintenance and significant quantities of oil. Elimination of the gearbox via these new permanent magnet motors/drive configurations saves space and installation costs, energy and maintenance, and provides more flexibility in production line and facility design.

A unique line of low-base-speed permanent magnet AC motors is the heart of a system known as the DriveIT Direct Drive Solution. This consists of a DriveIT permanent magnet synchronous AC motor, controlled by a DriveIT low-voltage AC drive, which is connected directly to a motor/load without gearboxes or pulse encoders.

The permanent, NdFeB magnets in the AC motor simplify traditional synchronous motor construction by creating a constant flux in the air gap that eliminates the need for the rotor windings and brushes normally used for excitation in synchronous motors. The result is a motor that combines the high-quality performance of the synchronous motor with the robust design of the asynchronous induction motor. The motor is energized directly on the stator by the variable speed drive. In addition, the elimination of slip losses in the rotor of the motor increases the efficiency of the motor from 1% to 3%.

The actual motor design is a radial-flux construction, air- or water-cooled, with a permanent magnet rotor. Powers range from 22 to 670 HP (17 to 500 kW) and include base speeds from 220 to 600 RPM; motor voltages range from 380 to 690 VAC. Standard IEC-frame sizes range from 280 to 400. Standard modifications include fan cooled (TEFC), separate cooled (TEAO), and water-cooled (TEWC) units.

The permanent magnet AC motor is designed for variable-speed operation only and must be controlled by an AC drive specifically developed for permanent magnet flux control. An exclusive Direct Torque Control (DTC) algorithm has been enhanced to achieve this.

The DTC algorithm optimizes motor control and enables the drive to run without an encoder to provide speed feedback. The DTC algorithm enables each machine's motor drive to calculate the state (torque and flux) of the motor 40,000 times per second. Elimination of the encoder further reduces maintenance and dramatically decreases downtime/stoppage, increasing a production facility's uptime and throughput. Optimal motor operation, based on load conditions, also saves energy consumption, yielding energy savings.

The DriveIT Direct Drive Solution is being used in and targeted for a host of industries, including pulp and paper, marine propulsion (Azipod), ski lifts, elevators, wind-turbine generators, mine conveyors, rotary kilns (cement and iron ore production), and paper, film and foil converting machines.

Drive configuration with (a) conventional induction motor drive, gearbox and jackshaft; and (b) the drive IT Direct Drive Solution

This work was done by Ken Graber for ABB Inc. For more information on AC drives contact Chuck Hollis at (262) 785-3505 or chuck.hollis@us.abb.com. For information on electrical machines, contact Lars-Erik Thand at (262) 785-8678 or lars-erik.thand@us.abb.com. Visit ABB online at www.abb.com/motors&drives.