A quiet invasion in motors has finally arrived with a big bang. A whole new category of extruder motors is emerging: ring shaped, high torque, permanent-magnet synchronous motors. Besides being unusually compact and eerily quiet, they use less energy than their predecessors -10% to 20% less than a DC motor and 5% to 10% less than a three-phase AC motor. And their already high torque capability, which permits higher output, is getting higher all the time.

However, visible use of the new motors is very small. Half a dozen European machine builders are developing extruders with these new motors. Over 150 of the new motors that have been delivered to extruder makers are already in use in production at processing plants. Although most of these motors are being used in Europe, a few ring shaped torque motors are also being used by U.S. extrusion machinery manufacturers. Commercial extrusion applications for the motors include tubing, blown film, sheet and continuous extrusion blow molding. Hundreds have also been used on downstream components of an extrusion line like chill rolls, winders and rewinders.

The new motors create virtually no vibration or noise, and need almost no maintenance. They are water cooled and dust free and use less oil - an advantage for makers of medical and food packaging films. The motors are compact - an advantage for co-extrusion. They also deliver constant torque over their entire speed range, starting from zero. Conventional AC and DC motors require high speeds (2000 to 3000 rpm) and gear boxes to generate high torque. Synchronous torque motors have a distinctive hollow cylinder or ring shape, and contain a very high number of magnetic pole pairs, upto 10 times more than other types of electric motors. That's how they generate such high torque (2000 to 11,000 Nm) at low speeds (20 to 500 rpm). At that speed range, the motors can connect directly to the thrust bearing of an extruder without gears (though gears may be used).

Synchronous torque motors are ring shaped and hollow. The outer part of the ring consists of an electrified stator with copper windings. The inner part is a rotor with strips of permanent magnet material (neodymium-iron-boron) mounted lengthwise. No electricity is supplied to the rotor. Torque is generated in the gap between the magnetic pole pairs on the rotor and pole pairs in the windings on the stator. Magnets run lengthwise on the rotor, with alternating positive and negative poles. The number of circuit windings on the stator equals the number of magnets on the rotor.

Each circuit winding on the stator is also either positive or negative. When AC voltage is applied to the motor, the windings alternate their polarity (plus/minus). They change several times a second at a speed that is the same as the motor speed. (That's the origin of the term "synchronous"). The larger the rotor diameter, the greater the number of pole pairs to generate power. A synchronous torque motor may have from 8 to 40 pole pairs, dramatically more than a brushless DC motor-which has two, four, or six pairs or AC motor, which has 4 to 6. Conventional DC and AC motors are magnetized only when electricity is flowing through them. This absorbs power to create the magnetic field and is less efficient than a permanent-magnet motor. A brushless DC servo motor has permanent magnets on the rotor and the windings on the stator, but with a smaller diameter and fewer poles.

Motor builders developed the first synchronous permanent-magnet torque motors in the 1980s. But at that time they were large and prohibitively expensive, so only the military could afford them. They were built first for applications like powering radar or telescopes or tracking missiles, and later were used for elevators and machine tools. When more torque was needed to power machine tools, multiple permanent-magnet motors were lined up in series, which was also expensive. By the mid 1990s, motor manufacturers developed the first standard permanent-magnet models and steadily increased their torque. They targeted injection molding as the biggest plastics market and one for which this type of motor has the advantage of delivering constant torque at all speeds, even when ramping up and down to zero. Injection molding still accounts for three to four times more applications of these motors than extrusion.

Motor builders are constantly raising their torque capacity. Currently, their upper limit is 10,000 to 11,000 Nm, sufficient for extruders up to 100 mm diam, but still not enough for high-output blown and cast film lines, which typically use 5- or 6-in. extruders.

The current size record for permanent-magnet synchronous motors in extrusion was achieved as recently as September. Siemens built a 148-kw motor with 11,000 Nm of torque to drive a 100-mm extruder. Uptil then, its biggest motor of this type for the field of plastics had a torque of 7000 Nm, adequate for an extruder of up to 80 mm diam. The 11,000-Nm permanent-magnet motor was built for the R&D lab of extruder maker. Reifenhauser. Reifenhauser is now making this new concept a standard for their equipment. Builders of synchronous torque motors aren't stopping there. Siemens has a development project underway to reach 32,000 Nm of torque and up to 1000 rpm, enough torque to power screws up to 120 mm for blown film or 150 mm for cast film.

All new plastics machineries will be fitted with the quiet motors in the very near future - a big boon and an answer to the present sound pollution problem.