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Direct torque control for efficiency and accuracy

Emotron introduced one of the market’s first variable speed drives with direct torque control in 1998. This exact technique offers high efficiency and accuracy in dynamic applications. Full torque at zero speed and precise control of acceleration and deceleration together offer full control of power. And all this using the motor as a sensor.

[19 January 2010]

The Emotron direct torque control is a superior technique for machines requiring accurate control of speed and torque. The figures speak for themselves: data updated 40,000 times a second, 400 per cent of nominal motor torque at 0 rpm, torque response faster than 1 ms and a speed accuracy of ±0.1% of rated rpm. In many cases this is even achieved without a separate speed sensor.

Spectacular introduction in 1998

Tom Jönsson at the Emotron R&D department has been involved in the project at Emotron from the offset. He remembers the spectacular introduction at a trade fair in 1998:

“The exact control we achieve without encoder feedback was demonstrated with a bowling ball thrown in the air and landing softly at the exact right spot, time after time. This attracted a lot of attention and we had a hard time convincing visitors that it was in fact done without sensors.”

40,000 times a second for high accuracy

The direct torque control protects the process from interruptions by reacting extremely quickly to peak loads, abrupt load changes or inaccurately set ramp times. Actual and required torque is compared no less than 40,000 times a second, which is achieved by using a fast DSP (digital signal processor).

This is valuable in, for example, crane control where frequent and critical starts and stops require instant high torque, or in crusher applications where speed quickly needs to be adjusted to type or size of material.

The speed controller of Emotron variable speed drives ensures efficient operation by immediately adapting speed to meet load changes. This is valuable in, for example, crane control where frequent and critical starts and stops require instant high torque, and crusher applications where speed quickly needs to be adjusted to type or size of material.

Peak torque up to 400 per cent

The variable speed drive can provide the highest possible torque per ampere from a normal industrial motor, offering the option of controlling a motor without a speed sensor while still maintaining control of torque and speed.

By measuring the motor current and voltage the motor's torque and speed can be controlled continuously and with exact precision in real time. Emotron variable speed drives are based on direct control of the magnetic flux and the torque of the connected electric motor.

The high accuracy of the flux and torque estimator makes it possible to increase the peak torque of the motor up to 400 per cent of nominal torque, even at 0 rpm. To achieve such high torque the variable speed drive must match the required current.

Current versus torque is 1:1

With direct torque control the torque/current ratio can be considered to be linear above nominal torque, so, for example, 200 per cent current results in 200 per cent torque. Thanks to this, the overload capacity required for the variable speed drive to stay in full control of the motor can be considerably lower than without direct torque control.

High dynamics without encoder feedback

The essential part of the direct torque control is the motor model which provides accurate estimates of the actual flux and torque of the motor. By making a comparison between the calculated actual values and reference values with extremely high calculation frequency, a closed loop of flux and torque control can be achieved.

All relevant motor parameters are measured automatically, the load inertia is checked and internal parameters are automatically set accordingly. This results in a torque response time that can be less than 1 ms and speed accuracy as close as ±0.1% of rated rpm, even without a separate sensor on the motor.

Doubled braking torque

In crane applications, the direct torque control works together with the VSD’s brake choppers to have the braking power go on and off exactly when required. It ensures the motor can deliver the torque needed at the very moment the mechanical brake is released. When braking a heavy load, the required torque is maintained while speed is continuously reduced towards zero. The load is put down safely before the mechanical brake is even activated.

No false over-current trips

Variable speed drives with direct torque control operate with direct control of the motor’s torque and flux. This effectively eliminates false trips caused by shock loads, mains supply disturbances or inaccurately set ramp times. Every switching in the variable speed drive is directly related to the electromagnetic state of the motor, resulting in smooth and accurate control at all times.

User-friendliness and new functionality

The direct torque control is now a mature technology, but the work for the Emotron R&D department continues:

“We focus on further increasing user-friendliness and on implementing new functions and communication options”, says Tom Jönsson. “We aim at continuously improving our customers’ process performance using this technique.”