**1. Introduction**

The circuits used for power conversion applied in drives with induction motor (IM) are classified into two groups: voltage source inverters (VSI) and current source inverters (CSI). The VSI were used more often than the CSI because of their better properties. Nowadays, the development of power electronics devices has enormous influence on applications of systems based on the CSI and creates new possibilities.

In the 1980s the current source inverters were the main commonly used electric machine feeding devices. Characteristic features of those drives were the motor electromagnetic torque pulsations, the voltage and current with large content of higher harmonics. The current source inverter was constructed of a thyristor bridge and large inductance and large commutation capacitors. Serious problems in such drive systems were unavoidable overvoltage cases during the thyristor commutation, as the current source inverter current is supplied in a cycle from a dc-link circuit to the machine phase winding. The thyristor CSI has been replaced by the transistor reverse blocking IGBT devices (RBIGBT), where the diode is series-connected and placed in one casing with transistor. The power transistors like RBIGBT or Silicon Carbide (SiC) used in the modern CSIs guarantee superior static and dynamic drive characteristics.

The electric drive development trends are focused on the high quality system. The use of current sources for the electric machine control ensures better drive properties than in case of voltage sources, where it may be necessary to use an additional passive filter at the inverter output. The Pulse width modulation (PWM) with properly chosen dc-link inductor and input-output capacitors result in sinusoidal inverter output currents and voltages. Methods of calculating proper inductance in dc-link were proposed in [Glab (Morawiec) M. et. al., 2005, Klonne A. & Fuchs W.F., 2003, 2004]. Properties of dc-link circuit of the Current Source Converter (CSC) force the utilization of two fully-controlled inverters to supply

© 2012 Morawiec, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Morawiec, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

system with electric motor. The first of them – CSI - generates the current output vector to supply the induction motor. The second one – Current Source Rectifier (CSR) - generates a DC voltage to supply a dc-link circuit. The strategy for controlling the output current vector of CSI can be realized in two ways [Glab (Morawiec) M. et. al., 2005, Klonne A. & Fuchs W.F., 2003, Kwak S. & Toliyat H.A., 2006]. First of them is based on changes of modulation index while the value of current in dc-link circuit remains constant [Klonne A. & Fuchs W.F., 2003]. The second method is based on changes of dc-link current. In this case the CSI is working with constant, maximum value of PWM modulation index. Control of modulation index in CSI is used in drive systems, where high dynamic of electromagnetic torque should be maintained [Klonne A. & Fuchs W.F., 2003]. High current in dc-link circuit is a reason for high power losses in CSI. The simplified control method is the scalar control: current to slip (I/s). This method is very simple to implement, but the drive system has average performance (only one controller is necessary, the current in dc-link is kept at constant value by PI controller).

Sensorless Control of Induction Motor Supplied by Current Source Inverter 429

**2. Structure of the drive system supplied by CSI or CSC** 

**Figure 1.** The CSI with the chopper

**Figure 2.** The Current source converter

by iteration algorithm.

1 the structure with the chopper as an adjustable voltage source is presented.

The simplified configuration of the drive system with the CSI is presented in the Fig. 1. The integral parts of the system are the inductor in dc-link and the output capacitors. In the Fig.

The chopper with the small inductor Ld (a few mH) forms the large dynamic impedance of the current source. In the proposed system the transistors forms commutator which transforms DC current into AC current with constant modulation index. The current is controlled by voltage source ed in dc-link. In this way the system with CSI remains voltage controlled and the differential equation for dc-link may be integrated with differential equation for the stator. The inductor limits current ripples during commutations of transistors. The transistors used in

In order to avoid resonance problem the CSI or CSC structure parameters (input-output capacitors and inductor) ought to be properly chosen. The transistor CSI or CSC structures should guarantee sinusoidal stator current and voltage of IM if the parameters are selected

this structure are named the reverse blocking IGBT transistors (RBIGBT).

The drive system quality is closely connected with the machine control algorithm. The space vector concept, introduced in 1959 by Kovacs and Racz, opened a new path in the electric machine mathematical modelling field. The international literature on the subject presents drive systems with the CSI feeding an induction motor with the control system based on the coordinate system orientation in relation to the rotor flux vector (FOC – Field Oriented Control). Such control consisted of the dc-link circuit current stabilization [Klonne A. & Fuchs W.F., 2003]. In such control systems the control variables are the inverter output current components. This control method is presented in [Nikolic Aleksandar B. & Jeftenic Borislav I.], where the authors analyze control system based on direct torque control. The control process where the control variable is the inverter output current may be called *current control* of an induction motor supplied by the CSI.

Another control method of a current source inverter fed induction motor is using the link circuit voltage and the motor slip as control variables. That type of control may be called *voltage control* of a CSI fed induction motor, as the dc-link circuit voltage and angular frequency of current vector are the control variables. Proposed control strategy bases on nonlinear multi-scalar control [Glab (Morawiec) M. et. al., 2005, ]. The nonlinear control may result in better properties in case if the IM is fed by CSI. To achieve independent control of flux and rotor speed, new nonlinear control scheme is proposed. In this control method the inverter output currents are not controlled variables. The voltage in dc-link and pulsation of output current vector are the controlled variables which can be obtained by nonlinear transformations and are proposed by authors in [Krzeminski Z., 1987, Glab (Morawiec) M. et. al., 2005, 2007]. The multi-scalar model is named the extended one because the mathematical model contained dc-link current and output capacitors equations. This full mathematical model of induction machine with the CSI is used to derive new multi-scalar model. In proposed method the output current vector coefficients are not controlled variables. The output current vector and the flux vector are used to achieve new multi-scalar variables and new multi-scalar model. The control system structure may be supported on PI controllers and nonlinear decouplings or different controllers e.g. sliding mode controllers, the backstepping control method or fuzzy neural controller.
