Direct Torque Control

From Wikipedia, the free encyclopedia

Direct torque control is one method used in variable frequency drives to control the torque (and thus finally the speed) of the three-phase AC electric motors. This involves calculating an estimate of the motor's magnetic flux and torque based on the measured voltage and current of the motor.

[edit] Method

The flux and torque are then compared with their reference values. If either the estimated flux or torque deviates from the reference more than allowed tolerance, the transistors of the variable frequency drive are turned on and off in such a way that the flux and torque will return in their tolerance bands as fast as possible.

This control method implies the following properties of the control:

• Torque and flux can be changed very fast by changing the references
• The step response has no overshoot
• The switching frequency of the transistors is not constant. However, by controlling the width of the tolerance bands the average frequency can be kept roughly at its reference value
• Digital control equipment has to be very fast in order to be able to prevent the flux and torque from deviating far from the tolerance bands. Typically the control algorithm has to be performed with 10 - 30 microseconds intervals
• The current and voltage measuring devices have to be high quality ones without noise and filtering, because noise and slow response ruins the control

The direct torque method performs very well even without speed sensors. However, the flux estimation is usually based on the integration of the motor phase voltages. Due to the inevitable errors in the voltage measurement the integrals tend to become erroneous at low speed. Thus it is not possible to control the motor if the output frequency of the variable frequency drive is zero. However, by careful design of the control system it is possible to have the minimum frequency in the range 0.5 Hz to 1 Hz that is enough to make possible to start an induction motor with full torque from a standstill situation.

[edit] History

Direct torque control was patented by Manfred Depenbrock in U.S. Patent 4678248  in 1985. However, Isao Takahashi and Toshihiko Noguchi presented a similar idea only few months later in a Japanese journal. Thus direct torque control is usually credited to all three gentlemen.

The first commercial application was the ACS600 variable speed drive by ABB that saw the daylight in 1995. A good presentation of this product and a full theoretical treatment of direct torque control can be found in the book:

Peter Vas: Sensorless Vector and Direct Torque Control, Oxford University Press, 1998

Since Depenbrock and Takahashi proposed direct torque control (DTC) for induction machines in the mid 1980s, this new torque control scheme has gained much momentum.

From its introduction in 1985, the Direct Torque control or Direct Self Control (DSC) principle was widely used for Induction Motor (IM) drives with fast dynamics. Despite its simplicity, DTC is able to produce very fast torque and flux control and, if the torque and flux are correctly estimated.

In the late 1990s DTC techniques for the Interior Permanent Magnet Synchronous Machine (IPMSM) appeared.