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On the use of EC motors in rotational rheometers

Introduction

The main components of a rotational rheometer are the motor with its supporting bearing system and the force measurement. In a typical design, commonly referred as CR (controlled rate or controlled strain), a displacement or speed is applied to the sample by a DC (direct current)-motor and the resulting torque is measured separately by an additional measuring sensor, i.e. a torque transducer. In a so-called CS (controlled stress) rheometer, an electrical current is applied and builds up a magnetic field, which produces an electrical torque. In such a design no separate torque sensor is needed because the torque signal is calculated directly from the motor current. The movement of the motor shaft is measured by an angular displacement sensor. Most CS instruments still employ a drag-cup motor, which is an asynchronous AC (alternating current)-motor, as was the case for the first CS instrument, the Deer Rheometer, build in 1968.

In 1995 a rheometer equipped with an electronically commutated synchronous motor (EC) (also called brushless DC-motor) and Digital Signal Processor (DSP) technology was introduced. A rheometer with an EC-motor combines the advantages of both traditional CR and CS rheometers. It enables fast motor control as in a CR rheometer but, because the torque is gained directly from the electrical current (as in a CS rheometer), it does not need a separate torque transducer.

The EC-motor technology is now over 20 years old and in the fifth generation of instruments commercially available, making the technology, with thousands of installations worldwide, clearly well established. However, there is certainly still a need for a better explanation of this unique rheometer drive technique and its implications. The specific configuration and the performance of the EC-motor and air bearing assembly as it is used in current commercial rheometers are discussed and some typical experiments relevant for modern research on rheological topics are presented.

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