Mighty motors

Industrial applications requiring high torque at low speeds typically use synchronous motors with reducers. Induction motors generate plenty of torque, but not at low speeds. However, new synchronous motor designs using permanent magnets make it possible to have low speed and high torque in one package, eliminating gearboxes and other mechanical components.

Traditional synchronous motors rely on rotor windings and brushes for excitation. Newer designs instead use permanent magnets to create a constant flux in the air gap, resulting in synchronous performance with the robust design of an asynchronous induction motor that eliminates the need for the rotor windings and brushes normally used for excitation in motors.

The permanent magnets are made from neodymium iron boron (NdFeB). NdFeB is the most powerful magnetic material at room temperature, with high flux density at high magnetization. It also resists demagnetization and is less costly and brittle than samarium cobalt, another widely used rare-earth material.

The motors also deliver more power in a smaller size. For instance, to power the in-line drives of a paper machine directly at 220 to 600 rpm with a conventional asynchronous motor would require a motor frame substantially larger than that of a 1,800-rpm motor. The new permanent-magnet motor is, in most cases, the same size or smaller than an existing induction motor.

Standard induction motors, normally designed to run at base speeds between 850 to 3,500 rpm, are not particularly suited for low-speed operation because efficiency drops with reduced speed. They also may be unable to deliver smooth torque at low speeds. A gearbox is the traditional mechanical solution for this challenge. However, gearboxes take up space, reduce efficiency, and need both maintenance and constant lubrication.

Eliminating the gearbox saves space and installation costs, energy, and maintenance, and provides more flexibility in production line and facility design. The motor also delivers high torque at low speed, a benefit traditionally associated with dc motors.

For example, low-base-speed permanent-magnet ac motors are 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, based on the ACS 600 or ACS 800 ac drive and connected directly to a motor/load without gearboxes or pulse encoders.

With a favorable weight-to-performance ratio, the new Direct Drive permanent-magnet motor from ABB provides high accuracy and reliability for industrial applications that require high torque at low speed without gearboxes and encoders.

ABB's Drive IT Direct Drive also incorporates the company's exclusive Direct Torque Control (DTC) technology, which optimizes motor control and enables the drive to provide speed feedback without an encoder.

"The DTC algorithm lets each motor drive calculate the state (torque and flux) of the motor 40,000 times/sec, which makes the drive virtually tripless," notes Chuck Hollis, manager of ABB's ACS 600 drives line. Eliminating the encoder further reduces maintenance, decreases downtime/stoppage, and increases a production facility's uptime and throughput. Optimal motor operation, based on load conditions, also saves energy.

Sensorless vector control (SVC) evolved from what is called full field-oriented control (FOC). So most of the SVC algorithms use the FOC control architecture as the starting point, and then try to estimate the speed using motor current and voltage information instead of the encoder. In SVC, the flux and torque-producing currents (two separate current regulators) produced in the machine control torque and flux in the machine. In DTC, flux and torque of the machine are estimated and two (hysteretic) loops are closed without the intermediate torque and flux producing current loops, which run at 25 &micor;sec.

Their small size and high accuracy have, in the past, made permanent-magnet motors preferred for use in servo applications and computer hard drives. But now, large permanent-magnet motors weigh in at up to 7 tons.

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 with voltages ranging from 380 to 690 Vac. The line is available in standard IEC-frame sizes from 280 to 400. All of the usual options and modifications available for traditional cast-iron induction motor frames are also available. Standard modifications include fan-cooled (TEFC), separate-cooled (TEAO), and water-cooled (TEWC) units.

Because there are virtually no rotor slip losses, the rotor of the permanent-magnet motor stays cool, boosting power density. Water cooling the motor stator can increase power density even more. The use of permanent-magnet rotors in standard ac motor frames can double the torque produced.

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. ABB's Direct Torque Control algorithm offers this capability. Compact ac drive cabinets in various enclosure protection classes ease installation of the system in a wide variety of production environments.