3.1. Direct Torque Control Strategy
Induction motors can be driven in many ways. From the simplest controller voltage/frequency approach to complex strategies like DTC or Vector control. The main differences between them are the motor’s performance and the viability and cost of its real implementation. The first method, known as scalar control, applies a constant V/f relation, this approach does not use the rotor speed as feedback variable. Although it is a very simple method, its main disadvantage is the fact that the speed and torque response do not have good precision, because the stator flux and torque are not directly controlled. In contrast, the vector control method introduces both the torque and flux control independently, using vector transformation. By using this method, the control accuracy increases significantly. However, its implementation could require enormous computational capability in the digital processor, and high accuracy in motor parameters estimation. DTC offers a fast response in torque and a higher dynamic performance, because DTC uses a simpler model than vector control.
The main objective of DTC is to calculate the values of flux and torque, in a particular instant of time, by using the stator variables of induction motor. The torque and flux are directly and independently controlled by selecting the optimal inverter commutation state and limiting the torque and flux errors via the torque and flux hysteresis controllers.
The main characteristics are [
1,
2,
3]:
Direct stator flux control and direct torque control
Indirect regulation of stator currents and voltages
Sinusoidal stator fluxes and stator currents approximation
High dynamic performance even at locked rotor
The advantages of this approach are:
Absence of co-ordinate transform
Absence of voltage modulator block
Minimal response time, even better than the vector controllers
However, possible problems could be presented such as starting requirement of flux and torque estimator. Also, a parameters identification is needed to reduce the inherent torque and flux ripples. A block diagram of the DTC is illustrated in
Figure 3. As can be seen, the DTC is comprised by an estimator of the electromechanical torque, the stator flux, sector location, vector
calculator, comparators, hysteresis blocks and a switching table, all of which elements are programmed in a Matlab/Simulink block.
The dynamic behavior of an induction motor is complex due to the coupling between the stator and rotor phases, where the coupling coefficients vary with the rotor position. Therefore, the machine is modeled by a set of differential equations with variable coefficients. A thorough knowledge of the induction motor model is vital to verify the effectiveness of the control algorithm proposed in this paper. The mathematical model of the rotor induction motor short-circuited in dynamic regime, in oriented field, coordinates are represented by a non-linear system of differential equations. To obtain the model, different aspects are considered. Below are the differential equations of the rotor flow belonging to the electric dynamic model of an MI based on the stator currents and the rotor time constant and the speed in the rotor. Since the rotor currents cannot be accessed directly, it is necessary to represent the model by means of the concatenated flow of the rotor from its projections on the d and q axes [
12].
Calculation of the space vector of the stator current (
) can be done by using
,
,
(motor currents) as shown in (5).
where
in d-q system coordinates is like (6)
The stator flux (
), the electromagnetic torque (
) and the sector location of the flux for the induction motor model are estimated in a block designed for that purpose. Equations (7) and (8) define the model used.
The rotor flux and stator currents are used to estimate the electromagnetic torque (
) defined by (9).
where
(rotor constant) is given by:
The stator flux (
) in the stationary frame can be obtained by (11) and (12).
where
is the stator inductance and
is the total link factor, which is determined by (13).
From the
d and
q flux components the flux angle (
) can be calculated as follows:
A basic situation to be considered in the DTC is that the position of the stator flux vector must be known and must be precise, in order to cause the desired effect on the torque and flux [
2]. This could be achieved by dividing the d–q plane into six sectors of 60 degrees each (4), in this way the sector in which the stator flux vector is located is known.
On the other hand, in situations where the speed is very low, vibrations can occur in the motor. This is mainly due to the presence of the increase in torque ripple, this can be resolved by reducing the bandwidth in the hysteresis regulator of the stator flux, thus, the distortion is not increased during the change of sector, this is possible at constant torque [
13]. More generally, the reduction in torque and flux ripple can be resolved when the DTC is performed by the analog method by reducing the hysteresis band of the torque and flux controllers. Since most modern systems use digital control due to its advantages. It should not be forgotten that digital algorithms need a runtime, which results in a delay when two consecutive signal samples are processed. Depending on the time delay, when the torque increases it can exceed the upper limit, defined by Hte+, resulting in an overflow or overreach. In the same way, when the torque is decreasing, it can reach values below the lower limit, defined by HTe-, producing a sub-range. Consequently, a great torque ripple is produced, deteriorating the behavior of the DTC. If the sampling frequency is lower, the ripple will increase [
14].
Also, although the DTC offers good dynamic behavior and fast torque response, the starting current reaches high values. This problem can be solved by adding a closed loop of the magnitude of the stator current vector IS, in that loop the zero vector is applied if the current reaches the maximum limit determined by a current hysteresis controller. When the current exceeds its minimum limit, a vector determined by the switching table of the DTC is chosen [
13]. There are other methods to solve the disadvantages in the DTC, which are applied by modifying the conventional method, such as:
In this method active vectors are applied instead of zero vector to obtain rapid change in the magnitude of the torque. The advantage of this method is to avoid distortion in the flux path of the stator. Although the last two methods are effective in improving DTC behavior, in many cases high switching frequency values are obtained that are not acceptable. Therefore, the realization of the DTC with these methods is more suitable for drives powered by a quasi-resonant inverter due to its availability to work at high switching frequencies without increasing losses through power switches [
14].
3.2. Sector Location
The torque reference regulating the magnitude and rotation of the stator flux is used to adjust the motor torque. Depending on the position of the stator flux space vector, the DTC defines six operating sectors. Once the angle and magnitude of the stator flux space vector are known, the DTC method needs to know the sector where the flux vector is located, which corresponds to one of the six sectors into which the stationary plane
is divided. Any of these six control areas has a width of
, these are given by (15).
where
n is the number of sector, ranging from 1 to 6.
Once
is known the sector can be calculated by using
Figure 4. The sector provides information about the location of the voltage space vector of the inverter to place the voltage space vector of the inverter to control the flux and torque.
The error
is processed by a two levels block of hysteresis, with a lower boundary defined by H
and an upper boundary defined by H
+. The band of the hysteresis controller is limited by two levels as shown in
Figure 5,
. The stator flux is maintained within the hysteresis limits, these limits define the hysteresis band. The narrowness of this band determines how close the stator flux will be to the reference.
The error
, resulting of subtracting the estimated from the reference torque, is managed by a hysteresis block of three-level, with the following error limits:
and
. The loop of hysteresis
is shown in
Figure 6. The purpose of the table for optimal switching is to deliver eight voltage vectors that regulate the amplitude and frequency of the inverter voltages supplying the motor (see
Table 1).
By adjusting the amplitude and frequency, the torque and flux in the motor are controlled. Taking the state of flux and torque as a basis,
Table 2 shows the voltage vectors for each sector.
To control the switching state of the inverter, the DTC uses the eight voltage vectors defined in
Table 1.
The rapid response of the DCT generates high currents that may activate the inverter protections. To cope with this inconvenience the stator currents are monitored to generate a closed-loop which handles the switching table sending a vector 0 whenever the current vector is over the allowed limit.