Direct Torque Control in AC Drive Technology
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DRIVES & SWITCHGEAR What direct torque control (DTC) is, why and how it has evolved, the basic theory behind its success and the features and benefits of this new technology. Direct torque control in AC drive technology Variable speed drives (VSDs) control Information from ABB the flow of energy from the mains to the process. Energy is supplied to the process through the motor shaft. Two variables describe the state of the shaft: without the need for sophisticated created by the current through the field torque and speed. We must control electronics. However, the evolution of winding in the stator. This field is always these quantities to control the flow of AC VSD technology was aimed partly at right angles to the field created by the energy. In practice, either one of them at combining advantages of the DC armature winding. This is known as field is controlled and we speak of “torque motor such as its fast torque response orientation and is needed to generate control” or “speed control”. Speed and and speed accuracy with the ruggedness maximum torque. The commutator- torque are determined by load in both and economy of the maintenance-free brush assembly ensures this condition torque and speed control mode. AC motor. is maintained regardless of the rotor DC motors were initially used as VSDs position. DC motor drives as they could achieve the required Once field orientation is achieved, the speed and torque with ease and In a DC motor, the magnetic field is DC motor’s torque is controlled easily Fig. 2: Control loop of an AC drive with Fig. 1: Control loop of a DC motor drive. requency control using PWM. Fig. 3: Control loop of an AC drive with flux vector control using PWM. Fig. 4: Control loop of an AC drive using DTC. July 2013 - Vector - Page 58 Drive Control variables Both voltage and frequency references are fed into a modulator, which simulates Armature current, I DC drives A an AC sine wave and feeds this to the Magnetising cuttent, I M motor’s stator windings. This technique Output volatge, U AC drives (PWM) is called pulse width modulation (PWM) Output frequency, f and uses a diode rectifier towards the Motor torque, T mains (see Fig. 2). The intermediate DC Direct torque control Motor magnetsing flux, voltage is kept constant. The inverter controls the motor in the Table 1: Comparison of control variables. form of a PWM pulse train, dictating both voltage and frequency. Of signifisance is that this method does not use a feedback by varying the armature current and with a commutator-brush assembly. device which takes speed or position by keeping the magnetising current There is, therefore, no need for measurements from the motor’s shaft constant. The advantage of DC drives complex electronic control circuitry and feeds these back into the control is that speed and torque, the two main which would increase the cost of the loop. concerns to the end-user, are controlled motor controller. Such an arrangement, without a directly through armature current: torque Disadvantages is the inner control loop and speed is the feedback device, is called an “open- outer control loop (see Fig. 1). The disadvantages of the DC motor are loop drive”. reduced motor reliability, the need for This controlling principle offers a low- Advantages regular maintenance, its high purchase cost, simple solution to controlling AC DC motor drives provide accurate and price and the fact that it needs an induction motors because there is no fast torque control, high dynamic speed encoder for feedback. feedback device. This type of drive is response and are simple to control. While a DC drive produces an easily- suitable for applications such as pumps They were initially used for variable controlled torque from zero to base and fans which do not require high levels speed control because they could easily speed and beyond, the motor’s of accuracy or precision. and accurately achieve good torque mechanics are more complex and In terms of disadvantages, field and speed response. DC motors can require regular maintenance. orientation of the motor is not used produce a torque that is: AC motor drives with this technique, also known as scalar control. Instead, frequency and voltage Direct: the motor torque is These machines are small and robust, proportional to the armature are the main control variables and are of simple design, compact, require low applied to the stator windings. The status current: the torque can therefore be maintenance and are not expensive to controlled directly and accurately. of the rotor is ignored, meaning that no buy. AC variable speed drive technology speed or position signal is fed back. Rapid: torque control is fast; the was developed to combine advantages drive system can have a very high of the DC drive (such as fast torque Therefore, torque can be controlled with dynamic speed response. Torque can response and speed accuracy) with any degree of accuracy. The technique also uses a modulator which basically be changed instantaneously if the those of the standard AC motor. motor is fed from an ideal current slows down communication between the source. A voltage-fed drive still has Frequency control incoming voltage and frequency signals, a fast response as this is determined using PWM causing the motor to respond to this by the rotor’s electrical time constant The AC drive frequency control technique changing signal. The modulator slows only (i.e. the total inductance and uses as controlling variables parameters down communication between incoming resistance in the armature circuit). generated outside the motor, namely voltage and frequency signals Simple: field orientation is achieved voltage and frequency. AC drives: flux vector control using PWM Evolution of direct torque control The flux-vector drive must know the spatial angular position of the rotor flux inside the AC induction motor to emulate the magnetic operating conditions of a DC motor, i.e. to perform the field orientation process. With flux vector PWM drives, field orientation is achieved by electronic means rather than with the mechanical commutator-brush assembly of the DC motor. Information on the rotor status is obtained by feeding back rotor speed and angular position relative to the stator field by means of a pulse encoder. A drive which uses speed encoders is Fig. 5: DTC comprises two key blocks: speed ontrol and torque control. known as a "closed-loop drive". July 2013 - Vector - Page 60 The motors' electrical characteristics are also modelled mathematically by means of microprocessors. The electronic controller of a flux-vector drive creates electrical quantities such as voltage, current and frequency which are the controlling variables. It feeds these to the AC induction motor through a modulator. Torque, therefore, is controlled indirectly. Advantages include good torque response, accurate speed control and full torque at zero speed. Feedback and modulators are, however, needed and they are costly to buy. A feedback device is required to achieve high torque response and speed accuracy. This can also be costly and adds complexity to the traditional, simple AC induction motor. The modulator slows down communication between incoming voltage and frequency signals, resulting in the motor responding to this changing signal. The motor is mechanically simple but the drive is electrically complex. AC drives: direct torque control Controlling variables DTC technology developed by ABB achieves field orientation without feedback, using advanced motor theory to calculate the motor torque directly, without using modulation. The controlling variables are motor magnetising flux and motor torque. With DTC, there is no modulator and no requirement for a tachometer or position encoder to feed back the speed or position of the motor shaft. DTC uses the fastest digital signal processing hardware available and a more advanced mathematical "understanding" of how motors work. The result is a drive with a torque response typically ten times faster than that of any AC or DC drive. The dynamic speed accuracy of DTC drives will be eight times higher than those of open loop AC drives and will be comparable to DC drives using feedback. DTC produces the first “universal” drive with the capability to perform either as AC or DC drives. VSDs: a comparison When noting the differences between the control blocks (see Figs. 1 – 4), the first observation is the similarity between the control block of the DC drive (Fig. 1) and that of DTC (Fig. 4). Both use motor parameters to control torque directly, but DTC has added benefits. These are the fact that no feedback device or external excitation is required, as well as all the benefits of AC motors. As can be seen from Table 1, both DC and DTC drives use actual motor parameters to control torque and speed. With DTC, no tachometer or encoder is needed to feed back speed or position signals. DTC: basic control theory Fig. 5 shows the complete block diagram for direct torque control. The block diagram shows that DTC has two fundamental sections: the torque control loop and the speed control loop. Torque control loop Step 1: two motor phase currents and the DC bus voltage are simply measured in normal operation, together with the inverter’s switch positions (see Fig. 6). Step 2: the measured information from the motor is fed to the adaptive motor model. The sophistication of this motor model allows precise data about the motor to be calculated. The motor model is fed information July 2013 - Vector - Page 61 Step 4: Both the latest 40 MHz digital signal processor (DSP) and ASIC hardware to determine the switching logic of the inverter are within the optimum pulse selector. All control signals are transmitted via optical links for high-speed data transmission.