Direct torque control

Maximum power under full control

In 1977, Emotron was the first company to introduce an AC drive based on Pulse Width Modulation (PWM). Further developments led to one of the first market launches of AC drives with direct torque control in 1998.

The Emotron technology has an extremely fast response time since actual and required torque is compared 40,000 times per second. The AC drives can provide the highest possible torque per ampere from a normal industrial motor, offering the option of controlling a motor without a rotor sensor while still maintaining control of torque and speed.

Direct torque control eliminates disturbances

The direct torque control method, invented by Depenbrock and Takahashi, offers highly efficient control of dynamic and demanding applications. The Emotron technique reacts quickly to peak loads, abrupt load changes or inaccurately set ramp times. This is invaluable in, for example, crane control where frequent and critical starts and stops demand instant high torque or in crusher applications where speed quickly needs to be adjusted to the type or size of material.

Peak torque up to 400 %

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 AC 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 to up to 400% of nominal torque, even at 0 rpm. To achieve such high torque, the AC drive must match the required current.

1:1 ratio of current versus torque

With direct torque control the torque/current ratio is linear above nominal torque, i.e. 200% current results in 200% torque, compared to approximately 150% torque at 200% current without direct torque control. Therefore, the overload capacity required for the Emotron AC drive to stay in full control of the motor can be considerably lower than when using control methods without direct torque control.

Built-in braking functionality

The direct torque control works with the AC drive’s integrated vector brake to ensure rapid and protective braking. The braking energy is dissipated through the motor itself, which helps avoid interruptions due to excessive brake voltage. The available braking torque is doubled compared to conventional braking methods. In most cases, no mechanical brakes are required – brake choppers and brake resistors are only needed for an extremely short braking time.

Eliminates false overcurrent trips

AC drives with direct torque control operate with control of the motor’s torque, and flux effectively eliminates false trips caused by shock loads, mains supply disturbances or inaccurately set ramp times. This is due to the quick response and accurate control of the motor flux based on the motor model used in the software. Each and every switching in the AC drive is directly related to the electromagnetic state of the motor, resulting in smooth and precise control at all times.

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 comparing the calculated actual values with reference values at an 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. Together with superior braking capacity and precise control of acceleration and deceleration, this results in a torque response time that can be less than 1 ms and a speed accuracy as good as ±0.1% of rated rpm, even without a separate sensor on the motor. Typical values with conventional sensorless vector control are 50-100 ms torque response and ±1-2% speed accuracy.