400 per cent torque with the motor as a sensor

Citation

(1999), "400 per cent torque with the motor as a sensor", Assembly Automation, Vol. 19 No. 3. https://doi.org/10.1108/aa.1999.03319caf.012

400 per cent torque with the motor as a sensor

400 per cent torque with the motor as a sensor

Keywords: Motors, Drives

The latest generation of frequency inverters uses the motor itself as sensor. By measuring the motor current and voltage it is possible to continuously, and with exacting precision, control the motor's torque and speed in real-time. For example, it is possible to achieve four times the nominal motor torque at 0 rpm. Other typical figures are a torque response of less than 1ms and a speed accuracy as good as þ0.1 per cent of rated rpm even though no separate sensor is mounted on the motor. The new inverters are based on direct control of magnetic flux and torque.

Up until now, asynchronous motors used in a wide range of applications have been impossible to control with accuracy and at a reasonable cost. Consequentially, process and machine operators have been forced to use solutions that are either expensive or unreliable.

Last year at the Hannover Fair, the Swedish-Dutch drive manufacturer Emotron presented a new series of frequency inverters, VFB/VFX, with features for high dynamic performance and accurate motor control (Plate 9).

Already in use

The first inverters are already in operation. Swedish-based Windak, which specializes in manufacturing winding machinery for the cable industry, has recently delivered three of its new machines to Nokia in Finland and to another client in the USA. The machines are driven by AC motors and equipped with Emotron's VFB frequency inverters.

President of Windak, Lennart Forsmark, is highly satisfied with the performance of the new winding machines:

It's vital that we're able to control the winding process with an extremely accurate degree of precision. There are enormous forces in motion and it is therefore important that the machine provides full torque even at 0 rpm. The Emotron frequency inverter first provides speed control of the 1.5kW motor which lifts the cable drum into position and then switches to the 3 kW motor which controls the drum rotation.

Plate 9 Illustration of an VFB/VFX high-performance inverter in a cabinet. VFB/VFX high-performance inverters are available in sizes ranging from 1.5 to 710kw. All the units fulfil the EMC directive and the low voltage directive. The VFB is designed to be built into a cabinet while the VFX is available in 1P54 encapsulation

Emotron - the motor is the only sensor needed

In recent years, the development of frequency inverters has moved at a rapid pace. Terms like vector modulation, vector control, field-oriented control, direct torque control, etc. has been used to describe the progress that have been made. Ten years ago, only a handful of drive manufacturers were actively working to develop vector technology for asynchronous motors. Three of these were the former Scandialogic, Eldutronik and Digimoto, which were all later acquired by Emotron. By continuously focusing on vector technology the company has now reached a leading position within this field.

Apart from frequency inverters, Emotron manufactures soft-starters, motor load monitors and complete drive systems. The company is primarily known for its strategy to, as far as possible, use the motor itself as the sensor in a drive or monitoring system thus eliminating the need for expensive sensor installations and cable runs.

A new series of dynamic frequency inverters

Emotron's new frequency inverter range, VFB/VFX comprises units for motor powers ranging from 1.5 to 710 kW. The main features of the new inverters are high motor torque and accurate speed control. The speed estimator has a typical accuracy of þ2 rpm for a 1480 rpm motor. In most applications, this eliminates the need for a separate rotational sensor mounted on the motor. The motor itself is the only sensor required. Superior braking capacity and precise control of acceleration and deceleration are also provided.

The inverters all operate with direct control of the motor's torque and flux, which effectively eliminates false trips caused by shock loads, mains supply disturbances or inaccurately set ramp times. All relevant motor parameters are measured automatically, the load inertia is checked and internal parameters are automatically set accordingly.

The high accuracy of the flux and torque estimator makes it possible to get up to 400 per cent of nominal torque from a motor even at 0 rpm. The torque/current ratio is linear above nominal torque, i.e. 200 per cent current will result in 200 per cent torque compared with approximately 150 per cent obtained with conventional inverters. To achieve such high torque the inverter must match the required peak current.

This text is also available on the Internet on www.emotron.com/news/news.htm

For further information, contact Emotron AB, Mr Per Zellman, Marketing Director, PO Box 222 25, SE-250 24 Helsingborg, Sweden. Tel: +46 42 16 99 00; Fax: +46 42 16 99 49; E-mail: per.zellman@emotron.se; http://www.emotron.com