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The AC induction motor drive is the fastest growing segment of the motor control market in the world. There are various reasons for this fast growth of AC induction motor drive. These reasons are presented blow for your reference:

1. Ease of programming
2. Low investment cost for the development
3. Flexibility to add additional features with minimal increase in hardware cost
4. Faster time to market

The first type of AC motors is the synchronous motor, which rotates exactly at the supply frequency or a submultiple of the supply frequency. The magnetic field on the rotor is either generated by current delivered through slip rings or by a permanent magnet.

The second type is the induction motor. The induction motor runs slightly slower than the supply frequency. The magnetic field on the rotor of the induction motor is created by an induced current.

The advantage of Direct Torque Control (DTC) of Frequency Converters (VVVF drives) is the fastest response time, elimination of feedback devices, reduced mechanical failure, performance nearly the same as the direct current (DC) machine without feedback, etc, compare to vector control and scalar control (v/f control). 

The disadvantage of Direct Torque Control (DTC) is due to the inherent hysteresis of the comparator, higher torque and flux ripple exist. Since switching is not done at a very high frequency, the low-order harmonics increases.

Continued from the Direct Torque Control (DTC) of Frequency Converters (ac drives, frequency changers) (part 1)  , below we still introduce this advanced control technique of frequency inverters (ac drives, variable speed drives, VSDs).

The heart of direct torque control (DTC) is its adaptive motor model. This model is based on the mathematical expressions of basic motor theory. The adaptive motor model of DTC requires information about the various motor parameters, like stator resistance, mutual inductance, saturation coefficiency, etc.

The algorithm captures all these details at the start from the motor without rotating the motor. But rotating the motor for a few seconds helps in the tuning of the model. The better the tuning, the higher the accuracy of speed and torque control.

The difference between the traditional vector control (indirect torque control) and the direct torque control (DTC) is that the direct torque control (DTC) has no fixed switching pattern.

The direct torque control (DTC) switches the inverter according to the load needs. Due to elimination of the fixed switching pattern (characteristic of the vector and the scalar control), the direct torque control (DTC) response is extremely fast during the instant load changes. Although the speed accuracy up to 0.5 percent is ensured with this complex technology, DTC eliminates the requirement of any feedback device.

The block diagram of the direct torque control (DTC) implementation for ac drives (variable frequency drives, VFDs) is shown in the figure below.

The main disadvantage of this method is the need of the rotor position information, using the shaft mounted encoder. This means additional wiring and component cost. It increases the size of the motor. When the ac drive (VSD) and the motor are far apart, the additional wiring poses a challenge.

To overcome the sensor/encoder problem, today's main research focus is in the area of a sensorless approach. The advantages of the vector control are to better the torque response, compared to the scalar control (V/F control), full-load torque close to zero speed, accurate speed control and performance approaching direct current (DC) drive, among others.

As the torque producing component in vector control is controlled only after transformation is done, and is not the main input reference, the vector control is known as "indirect torque control".
Vector control variable frequency drives from GreenDriv Tech Limited:

The most challenging and ultimately, the limiting feature of the field orientation, is the method whereby the flux angle is measured or estimated. Depending on the method of measurement, the vector control is divided into two subcategories: direct vector control and indirect vector control. 

In the direct vector control, the flux measurement is done by using the flux sensing coils or the Hall devices. This adds to additional hardware cost and in addition, measurement is not highly accurate. Therefore, the direct vector control is not a very good control technique for frequency changers (variable speed drives, VSDs).

These two decoupled components (d-q) can be independently controlled by passing though separate PI controllers. The outputs of the PI controllers are transformed back to the three-dimensional stationary reference plane using the inverse of the Clarke-Park transformation. The corresponding switching pattern is pulse width modulated (PWM) and implemented using the Space Vector Modulation (SVM). This control simulates a separately exited DC motor model, which provides an excellent torque-speed curve.
Vector control frequency converters from GreenDriv Tech Limited:

The transformation from the stationary reference frame to the rotating reference frame is done and controlled with reference to a specific flux linkage space vector (stator flux linkage, rotor flux linkage or magnetizing flux linkage). Generally there exists 3 possibilities for such selection and hence, three different vector controls. They are:

Stator flux oriented control

Space Vector Modulation PWM (SVMPWM)

Space Vector Modulation PWM (SVMPWM) is based on the fact that three phase voltage vectors of the induction motor can be converted into a single rotating vector. Rotation of this space vector can be implemented by variable frequency drive (frequency converter) to generate 3 phase sine waves.
Scalar Control (V/f Control) Variable Frequency Drive:

Space Vector Modulation PWM (SVMPWM) with overmodulation

Implementation of SVMPWM with overmodulation can generate a fundamental sine wave of amplitude greater than the DC bus level. The disadvantage of Space Vector Modulation PWM (SVMPWM) with overmodulation is complicated calculation, line-to-line waveforms are not "clean" and the THD increases, but still less than the THD of the six-step PWM method.

Sinusoidal Pulse Width Modulation (PWM)

In this method of Sinusoidal Pulse Width Modulation (PWM), the sinusoidal weighted values are stored in the PICmicro microcontroller and are made available at the output port at user defined intervals.

The advantage of Sinusoidal Pulse Width Modulation (PWM) is that very little calculation is required. Only one look-up table of the sine wave is required, as all the motor phases are 120 electrical degrees displaced.

The disadvantage of Sinusoidal Pulse Width Modulation (PWM) is that the magnitude of the fundamental voltage is less than 90%. And the harmonics at Pulse Width Modulation (PWM) switching frequency have significant magnitude.

Six-Step Pulse Width Modulation (PWM)

The inverter of the frequency converter (ac drive) has six distinct switching states. When it is switched in a specific order, the three phase AC induction motor can be rotated.