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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 Shenzhen POWTRAN Technology Co., Ltd.: 

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 Shenzhen POWTRAN Technology Co., Ltd.:

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.

In this type of control (scalar control, v/f Control), the motor is fed with variable frequency signals generated by the Pulse Width Modulation (PWM) control from an inverter, using the feature rich PICmicro microcontroller. Here, the V/f ratio is maintained constant in order to get constant torque over the entire operating range. Since only magnitudes of the input variables - frequency and voltage -  are controlled, this is known as "scalar control". Generally, the drives with such a control are without any feedback devices (open-loop control). Hence, a control of this type offers low cost and is an easy-to-implement solution.

In such controls, very little knowledge of the motor is required for frequency control. So, scalar control (v/f Control) is widely used. A disadvantage of scalar control (v/f control) is that the torque developed is load dependent, as it is not controlled directly. Also, the transient response of such a control is not fast due to the predefined switching pattern of the inverter.

A typical name plate on an AC induction motor is shown below. Generally the name tag includes: type, horsepower (H.P.), amps, volts, hertz, rpm...
Typical name plate on AC motor:

Below I explain these parameters of the typical name tag on ac induction motor.

Volts: Rated terminal supply voltage.
Amps: Rated full-load supply current.
H.P.: Rated motor output.

We have introduced 5 design types of the National Electrical Manufacturers Association (NEMA) in our previous articles. Below we explain the motor standard: the International Electrotechnical Commission (IEC).

The International Electrotechnical Commission (IEC) Torque-Speed Design Ratings practically mirror those of NEMA. The IEC Design N motors are similar to the National Electrical Manufacturers Association (NEMA) Design B motors, the most common motors for industrial applications. The International Electrotechnical Commission (IEC) Design H motors are nearly identical to NEMA Design C motors.

The IEC Duty Cycle Ratings are different from those of the National Electrical Manufacturers Association (NEMA)'s. Where NEMA usually specifies continuous, intermittent or special duty (typically expressed in minutes), the International Electrotechnical Commission (IEC) uses nine different duty cycle designations (IEC 34-1).

The standards, shown in the table below, apart from specifying motor operating parameters and duty cycles, also specify temperature rise (insulation class), frame size (physical dimension of the motor), enclosure type, service factor and so on.