**4. Active front end rectifier**

Various alternative circuits can be used to recover the load energy and return it to power supply. One such scheme is shown in Fig. 8 and presents the most popular topology used in ASD. The diode rectifier is replaced with PWM voltage source rectifier. This is already an industrially implemented technology and known as most successful active front end (AFE) solution in ASD if regenerative operation is needed (e.g. for lowering the load in crane) and therefore was chosen by most global companies: Siemens, ABB, and others.

The term Active Front End Inverter refers to the power converter system consisting of the line-side converter with active switches such as IGBTs, the DC link capacitor bank, and the load-side inverter. The line-side converter normally functions as a rectifier. But, during regeneration it can also be operated as an inverter, feeding power back to the line. The lineside converter is popularly referred to as a PWM rectifier in the literature. This is due to the fact that, with active switches, the rectifier can be switched using a suitable pulse width modulation technique.

change of induction motor load in regenerative regime. Danfoss frequency (series VLT 5000) converter is used in experimental set-up. For the supply voltage of 400 V, DC link voltage is about 540 V. When negative load torque is applied, DC link voltage rises. The chopper transistor voltage control regulates the voltage of the DC bus during regeneration to near 800 V allowing current flow in the resistor. Regenerative energy is then realised into heat. After the end of the regenerative period, DC voltage returns to a value that corresponds to a motor regime. The Fig.7b) shows the line voltage and current at the input of the diode rectifier.

Fig. 7. a) DC voltage and chopper current, b) line voltage and current.

voltage.

**4. Active front end rectifier** 

modulation technique.

A voltage source PWM inverter with diode front-end rectifier is one of the most common power configurations used in modem variable speed AC drives, (Fig. 6). An uncontrolled diode rectifier has the advantage of being simple, robust, and low cost. However, it allows only unidirectional power flow. Therefore, energy returned from the motor must be dissipated on a power resistor controlled by a chopper connected across the dc link. A further restriction is that the maximum motor output voltage is always less than the supply

Various alternative circuits can be used to recover the load energy and return it to power supply. One such scheme is shown in Fig. 8 and presents the most popular topology used in ASD. The diode rectifier is replaced with PWM voltage source rectifier. This is already an industrially implemented technology and known as most successful active front end (AFE) solution in ASD if regenerative operation is needed (e.g. for lowering the load in crane) and

The term Active Front End Inverter refers to the power converter system consisting of the line-side converter with active switches such as IGBTs, the DC link capacitor bank, and the load-side inverter. The line-side converter normally functions as a rectifier. But, during regeneration it can also be operated as an inverter, feeding power back to the line. The lineside converter is popularly referred to as a PWM rectifier in the literature. This is due to the fact that, with active switches, the rectifier can be switched using a suitable pulse width

therefore was chosen by most global companies: Siemens, ABB, and others.

The PWM rectifier basically operates as a boost chopper with AC voltage at the input, but DC voltage at the output. The intermediate DC-link voltage should be higher than the peak of the supply voltage. The required DC-link voltage needs be maintained constant during rectifier as well as inverter operation of the line side converter. The ripple in DC link voltage can be reduced using an appropriately sized capacitor bank. The AFE inverter topology for a motor drive application, as shown in Fig.8, has two three-phase, two-level PWM converters, one on the line side, and another on the load side. The configuration uses 12 controllable switches. The line-side converter is connected to the utility through inductor. The inductor is needed for boost operation of the line-side converter.

Fig. 8. Active front end inverter topology.

For a constant dc-link voltage, the IGBTs in the line-side converter are switched to produce three-phase PWM voltages at *a*, *b*, and *c* input terminals. The line-side PWM voltages, generated in this way, control the line currents to the desired value. When DC link voltage drops below the reference value, the feed-back diodes carry the capacitor charging currents, and bring the DC-link voltage back to reference value.

The steady state characteristics as well as differential equations describing the dynamics of the front-end rectifier can be obtained independent of an inverter and motor load. This is because the DC-link voltage can be viewed as a voltage source, if *Vdc* is maintained constant for the full operating range. The inverter is thus connected to the voltage source, whose terminal voltage *Vdc*, remains unaffected by any normal inverter and motor operation (Jiuhe et al., 2006).

Furthermore, as shown in Fig.8, the rectifier can also be viewed as connected to the voltage source *Vdc*. Thus, the rectifier is able to control magnitude and phase of PWM voltages *Vabc* irrespective of line voltages *E123*.

The dynamic equations for each phase can be written as,

$$
\begin{bmatrix} E\_1 \\ E\_2 \\ E\_3 \end{bmatrix} = L \frac{d}{dt} \begin{bmatrix} i\_1 \\ i\_2 \\ i\_3 \end{bmatrix} + R \begin{bmatrix} i\_1 \\ i\_2 \\ i\_3 \end{bmatrix} + \begin{bmatrix} V\_{a0} \\ V\_{b0} \\ V\_{c0} \end{bmatrix} \tag{7}
$$

In synchronous rotating *d-q* reference frame Equations 8 and 9 represent the dynamic *d-q*  model of an active front end inverter in a reference frame rotating at an angular speed of .

Electrical Drives for Crane Application 143

To get information about the position of the line voltage vector PLL (phase locked loop) is implemented. PI controllers for the *d-q* components of line current are identical and ω*L* terms are included to eliminate the coupling effect among the *d* and *q* components. Outputs of the line current PI controllers present *d* and *q* components of the voltage across the line inductance. Subtracting this voltage from the supply voltage gives the converter voltage from the AC side that is used to get the modulation signal for proper switching of six

The main task of the sinusoidal front end is to operate with the sinusoidal line current; so *d* and *q* components of the line current reference are DC values. Using this approach of control it is possible to control the output voltage of converter as well as the power factor of converter in the same time. To achieve unity power factor the reference of *q* current

Based on analysis, the simulation model of the whole is built using Matlab/Simulink to test the performance of the active front end rectifier. On the load side is the field oriented induction motor drive with topology as shown in Fig.5c). The whole system behavior is

Fig. 9. Decoupled current control of PWM rectifier.

Fig. 10. Simplified block diagram of the AFE.

switching devices.

component need to be set on zero.

$$L\frac{di\_{q\epsilon}}{dt} = E\_{q\epsilon} - \alpha L i\_{de} - R i\_{q\epsilon} - V\_{q\epsilon} \tag{8}$$

$$\ln L \frac{d\dot{\mathbf{i}}\_{de}}{dt} = E\_{de} + \alpha \mathbf{L} \dot{\mathbf{i}}\_{qe} - \mathbf{R} \dot{\mathbf{i}}\_{de} - V\_{de} \tag{9}$$

The differential equation governing DC link voltage also needs to be added to the above set of system equations to completely define system dynamics:

$$\mathbf{C}\frac{dV\_{dc}}{dt} = \mathbf{i}\_{dc} - \mathbf{i}\_{\mathcal{M}}\tag{10}$$

where, *idc* is the total DC link current supplied by the rectifier, while *iM* is the load-side DC current which is the result of induction motor operation.

In Equations 8 and 9, the terms *Eqe* and *Ede* are computed from source voltages, *E*1, *E*2, and *E*3. Since line voltages are known, the angular frequency , can be easily estimated. The PWM voltages *Vqe* and *Vde* are the two inputs to the system which are generated using the sinetriangle PWM controller. *L* and *R* represent series impedance.

Equations (8 and 9) shows that *d-q* current is related with both coupling voltages *Liq* and *Lid,* and main voltage *Ed* nd *Eq*, besides the influence of PWM voltage *Vqe* and *Vde*. Voltage *Vqe* and *Vde* are the inputs, controlled in such a way as to generate desired currents. Now define new variables *V'qe* and *V'de* such that (Hartani & Miloud, 2010):

$$V\_{qe} = -V\_{qe}' - \alpha L \dot{\imath}\_{qe} + E\_{qe} \tag{11}$$

$$V\_{de} = -V\_{de}^{\prime} + o\text{L}\,\text{i}\_{de} + E\_{de} \tag{12}$$

So that the new system dynamic equations become:

$$L\frac{d\dot{l}\_{q\epsilon}}{dt} = -\dot{i}\_{q\epsilon}R + V\,'\_{q\epsilon} \tag{13}$$

$$L\frac{d\dot{l}\_{se}}{dt} = -\dot{l}\_{de}R + V\prime\_{de} \tag{14}$$

We can see from equations that the two axis current are totally decoupled because *V'qe* and *V'de* are only related with *iqe* and *ide* respectively.

The simple proportional-integral (PI) controllers are adopted in the current and voltage regulation, Fig.9. The control scheme of the PWM rectifier is based on a standard cascaded two-loop control scheme implemented in a *d-q* rotating frame: a fast control loop to control the current in the boost inductors and a much slower control loop to maintain constant dclink voltage. The reference angle for the synchronous rotating *d-q* frame , is calculated, based on the three input phase voltages.

For the current control loop *d-q* synchronously rotating reference frame with the fundamental supply voltage frequency is used (Odavic et al., 2005). The line currents (*i*1, *i*2, *i*3) are measured and transformed to the *d-q* reference frame, Fig.10.

*L E Li Ri V dt* 

*di L E Li Ri V*

 

The differential equation governing DC link voltage also needs to be added to the above set

*dc*

Equations (8 and 9) shows that *d-q* current is related with both coupling voltages

' *V V Li E qe qe* 

' *V V Li E de de de de* 

*di*

*dt*

*dt*

*i*3) are measured and transformed to the *d-q* reference frame, Fig.10.

 *qe qe qe*

 *se de de di L iR V*

We can see from equations that the two axis current are totally decoupled because *V'qe* and

The simple proportional-integral (PI) controllers are adopted in the current and voltage regulation, Fig.9. The control scheme of the PWM rectifier is based on a standard cascaded two-loop control scheme implemented in a *d-q* rotating frame: a fast control loop to control the current in the boost inductors and a much slower control loop to maintain constant dclink voltage. The reference angle for the synchronous rotating *d-q* frame , is calculated,

For the current control loop *d-q* synchronously rotating reference frame with the fundamental supply voltage frequency is used (Odavic et al., 2005). The line currents (*i*1, *i*2,

*L iR V*

*dV C ii*

where, *idc* is the total DC link current supplied by the rectifier, while *iM* is the load-side DC

In Equations 8 and 9, the terms *Eqe* and *Ede* are computed from source voltages, *E*1, *E*2, and *E*3. Since line voltages are known, the angular frequency , can be easily estimated. The PWM voltages *Vqe* and *Vde* are the two inputs to the system which are generated using the sine-

*Lid,* and main voltage *Ed* nd *Eq*, besides the influence of PWM voltage *Vqe* and *Vde*. Voltage *Vqe* and *Vde* are the inputs, controlled in such a way as to generate desired currents. Now

'

'

*qe de qe qe*

*de qe de de*

*dc M*

(8)

(9)

*Liq* and

*dt* (10)

*qe qe* (11)

(13)

(14)

(12)

*qe*

*de*

*dt*

of system equations to completely define system dynamics:

current which is the result of induction motor operation.

So that the new system dynamic equations become:

*V'de* are only related with *iqe* and *ide* respectively.

based on the three input phase voltages.

triangle PWM controller. *L* and *R* represent series impedance.

define new variables *V'qe* and *V'de* such that (Hartani & Miloud, 2010):

*di*

Fig. 9. Decoupled current control of PWM rectifier.

Fig. 10. Simplified block diagram of the AFE.

To get information about the position of the line voltage vector PLL (phase locked loop) is implemented. PI controllers for the *d-q* components of line current are identical and ω*L* terms are included to eliminate the coupling effect among the *d* and *q* components. Outputs of the line current PI controllers present *d* and *q* components of the voltage across the line inductance. Subtracting this voltage from the supply voltage gives the converter voltage from the AC side that is used to get the modulation signal for proper switching of six switching devices.

The main task of the sinusoidal front end is to operate with the sinusoidal line current; so *d* and *q* components of the line current reference are DC values. Using this approach of control it is possible to control the output voltage of converter as well as the power factor of converter in the same time. To achieve unity power factor the reference of *q* current component need to be set on zero.

Based on analysis, the simulation model of the whole is built using Matlab/Simulink to test the performance of the active front end rectifier. On the load side is the field oriented induction motor drive with topology as shown in Fig.5c). The whole system behavior is

Electrical Drives for Crane Application 145

The simulation results show that the rectifier has excellent dynamic behavior and following




Fig. 12. Behavior of the rectifier under a step change of the induction motor load

The experimental behavior analysis of some drives is considered in a derrick crane, which serves for load handling in many industry branches. The main task in adjustable speed drives design is a safe, multi-axis movement that allows material handling throughout the working area. The derrick crane with following technical details has been taken for

load such as a crane when the load is being lowered.

with a simple diode rectifier.

(generator operation).

**5. Case study 1: Derrick crane** 

 Main hoist load capacity: 60 t; Auxiliary load capacity: 12.5 t; Main hoist height: 46 m; Auxiliary hoist height: 49 m;

experimentation with adjustable frequency drive:

advantages:

simulated as a discrete control system. The AC source is an ideal balanced three phase voltage source with frequency 50 Hz. The phase to phase voltage is 400 V. capacitor in DC link is 4700 F. The line resistor and line inductance of each phase is 0.1 and 3 mH respectively. The induction motor rated power is 30 kW.

The induction machine is initially running at a constant speed reference (100 rad/s) and under a no load regime. From this situation, we apply a rated load torque during a time interval of 0.5 s and then we remove the load. This case corresponds to a step up and a step down torque perturbation.

The following figures summarize the results of the simulation. Fig. 11 and Fig.12 shows the behavior of the rectifier under a step change of the induction motor load. Fig.11 refers to the motor mode of operation (lifting) and Fig.2 to the generator mode (lowering).

Fig. 11. Behavior of the rectifier under a step change of the induction motor load (motor operation).

Fig. 11b) and Fig. 12b) show the behavior of the DC link voltage of the PWM rectifier in response to a step change in the load. Fig. 11c) and Fig. 12c) presents the active and reactive power on the line side of converter. Since that the q component of the set point current is *Iqref*=0 (Fig.11e and Fig.12e) there is only *d* current component (Fig.11d and Fig.12d), reactive power is zero. Fig.11f) and Fig.12f) show the behavior of steady state voltage and current delivered by the source when the line side converter works in the rectifier and regenerative mode. The input current is highly sinusoidal and keeps in phase with the voltage, reaching a unity power factor.

simulated as a discrete control system. The AC source is an ideal balanced three phase voltage source with frequency 50 Hz. The phase to phase voltage is 400 V. capacitor in DC link is 4700 F. The line resistor and line inductance of each phase is 0.1 and 3 mH

The induction machine is initially running at a constant speed reference (100 rad/s) and under a no load regime. From this situation, we apply a rated load torque during a time interval of 0.5 s and then we remove the load. This case corresponds to a step up and a step

The following figures summarize the results of the simulation. Fig. 11 and Fig.12 shows the behavior of the rectifier under a step change of the induction motor load. Fig.11 refers to the

motor mode of operation (lifting) and Fig.2 to the generator mode (lowering).

Fig. 11. Behavior of the rectifier under a step change of the induction motor load

Fig. 11b) and Fig. 12b) show the behavior of the DC link voltage of the PWM rectifier in response to a step change in the load. Fig. 11c) and Fig. 12c) presents the active and reactive power on the line side of converter. Since that the q component of the set point current is *Iqref*=0 (Fig.11e and Fig.12e) there is only *d* current component (Fig.11d and Fig.12d), reactive power is zero. Fig.11f) and Fig.12f) show the behavior of steady state voltage and current delivered by the source when the line side converter works in the rectifier and regenerative mode. The input current is highly sinusoidal and keeps in phase with the voltage, reaching a

respectively. The induction motor rated power is 30 kW.

down torque perturbation.

(motor operation).

unity power factor.

The simulation results show that the rectifier has excellent dynamic behavior and following advantages:


Fig. 12. Behavior of the rectifier under a step change of the induction motor load (generator operation).
