Is direct torque control enough to make you change?

Is direct torque control enough to make you change your VSD brand?

Initially, DC drives were used for variable speed control because they achieved good torque and speed response with high accuracy. DC offers accurate and fast torque control, high dynamic speed response and is simple to control. However, while a DC drive produces an easily controlled torque from zero to base speed and beyond, the motor’s mechanics are more complex and need regular maintenance. Is the answer DTC (direct torque control)?

The main drawback of this technique is the reduced reliability of the DC motor. Firstly. the brushes and commutators wear down and need regular servicing. Secondly, DC motors can be costly to buy as they use encoders for speed and position feedback.

AC variable speed drives evolved to emulate the operation of a DC motor speed control but using inexpensive and rugged AC motors. Frequency control using PWM controls the motor in the form of a pulse train dictating both the voltage and frequency.

Significantly, this method does not use a feedback device, making it too inaccurate for more demanding applications. Without feedback, accurate torque control is not possible. However it offers a low cost and simple solution to controlling economical AC motors such as pumps and fan applications.

The need for feedback led to development of AC drives with flux vector control, emulating the magnetic operating of a DC motor. With flux vector PWM drives, acheiving field orientation is electronic. Also the motor’s electrical characteristics are mathematically modelled with microprocessors. They still need a feedback device to achieve good torque response and speed accuracy. Flux vector drives are complex and expensive and provide indirect torque control for the motor.

Direct torque control DTC

Direct Torque Control is the new way in which the control of torque and speed are directly based on the electromagnetic state of the motor. Achieving this without feedback uses advanced motor theory to calculate the motor torque directly and without using modulation. The result is a drive with a torque response that is 10 times faster than any AC or DC drive.

What are the benefits of DTC?

DTC provides precise torque control without the need for a feedback device. This gives it fast torque response which significantly reduces the speed drop time during a load transient. Moreover it improves process control and delivers consistent product quality.

It also offers torque control at low frequencies, a feature particularly beneficial to cranes or elevators load requiring starting and stopping without jerking. Also with a winder, tension control can is available from zero through to top speed.

Another benefit is torque linearity in precision applications like winders in the paper industry. It delivers accurate and consistent level of winding control. DTC control also offers dynamic speed accuracy so after a sudden load change, the motor can recover.

Conclusion

ABB drives with DTC are suitable for all asynchronous squirrel cage motor. For high performane drives applications they provide faster torque response, improved low speed torque control, torque linearity and dynamic speed accuracy. They are also suited to standard applications, which account for 70% of all variable speed drives installed throughout industry. Common applications are fans and pumps in industries like HVAC, water and food and drinks. In these applications, DTC provides solutions to problems like harmonics and noise. Finally, the drives’ motor flux optimisation feature improves the efficiency of the total drive system improving fan and pump applications.

Reference document: ABB’s Technical guide No. 1 Direct torque control – the world’s most advanced AC drive technology

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