**1. Introduction**

An induction motor is the most common machine used for industrial drives. It is used in variety of drives due to its robust construction, relatively low cost and reliability. In isolated electrical network, such as marine and offshore power systems and emergency generation plant, induction motors are the most power consuming loads and are used for winches, water pumps, compressors, fans and for other on-board applications, in continuous mode or intermittently.

In many applications induction motors are direct-on-line switch-started. Their dynamic characteristics have an obvious influence on the transient process of power system; however, they cause a significant disturbance in transients (significant impacts loads) that can produce disturbances in the isolated electrical network, which in turn affects the quality of electric power system and thus, on the dynamic behavior of induction motors.

Direct-on-line starting represents the simplest and the most economical system to start squirrel-cage induction motor. During starting induction motors draw high starting currents which are several times the normal full load current of the motor. This current causes a significant voltage dip on the isolated electrical grid until the induction motors reach nearly full speed. This voltage drop will cause disturbances in the torque of any other motor running on the isolated electrical grid. Significant disturbances in transients are caused by direct-on-line switch-started induction motors, especially if the load torque on the motor shaft is increased and beside that, also by the sudden change load, such as the impact load on the motor shaft (McElveen, et al., 2001; Cohen, 1995). This situation is particularly difficult because of relatively strong electrical coupling between electrical grid and loads. Besides the voltage dips, an interruption can also appear, which further affects on the fatigue of induction motors connected to the grid. When an interruption of the supply lasts longer than one voltage period, many AC contactors will switch off the motor. In some

© 2012 Mirošević, 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 Mirošević, 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.

cases, faulty contactor may produce multiple switching on and off. However, these interruptions will affect the dynamics of both electrical and mechanical variables. Therefore, it is interesting to analyze dynamics of the induction motor in case when it comes to shortterm interruptions in the motor power supply.

The Dynamics of Induction Motor Fed Directly from the Isolated Electrical Grid 75

 denote voltage, current, resistance and flux respectively. The model of a synchronous generator is given in rotor reference frame ( is rotor electrical speed). The equations of excitation in motor (load) convention system are written. The voltage controller, modeled as PI, is implemented in the model of the synchronous

The model of the prime mover - the diesel engine assumes that the engine torque is directly proportional to the fuel consumption. In order to describe the dynamic behavior of the diesel engine it is necessary to set up a system of differential equations which includes an equation of the engine, the turbocharger, the air collector, the exhaust system, and the speed controller. Taking into account these equations requires the knowledge of characteristics of diesel engines that require complex experimental measurements, according to (Krutov, 1978; Tolšin 1977). The studies carried out in (Erceg & et al. 1996) showed that the mentioned omissions do not affect significantly the results and that the proportionality of torque to the amount of injected fuel can be assumed. This simplification is allowed when it is of interest to observe dynamics of a synchronous generator as well as induction motors. The speed

Sudden impact load on the diesel electrical aggregate is the most difficult transition regime for units due to electricity loads and also due to torsional strains in the shaft lines. A more significant disturbance, which is at the same time very common in practice, is the direct-online starting of induction motors to such a grid. Starting of induction motors will cause voltage dips and will reduce engine speed depending on the time of the starting of each motor. This will also cause torsional stresses in the shaft. Thus, the mechanical coupling of a diesel engine and a synchronous generator is considered to be a rotating system with two concentrated masses. Masses are connected by flexible coupling. The flexible coupling

The variable angle of rotation between these masses occurs during the transient, in period when mechanical balance between diesel engine and electric generator is disturbed., The torque which appears at coupling zone between two concentrated rotational masses allows

Induction motor as an active consumer and its parameters were analyzed in (Maljkovic, 2001; Amezquita-Brook et al., 2009). According to (Jones, 1967; Kraus 1986) three phase squirrel-cage induction motors are represented with stators and rotors voltage equations:

> *dIM sIM dIM qIM <sup>d</sup><sup>ψ</sup> u Ri ω ψ dt*

0 *DIM*

*dIM*

*qIM qIM s qIM dIM <sup>d</sup><sup>ψ</sup> u Ri ω ψ dt*

*r DIM IM QIM <sup>d</sup><sup>ψ</sup> R i ωω ψ dt*

(6)

(7)

(8)

where *u*, *i*, *r,* and

generator.

controller is modeled as PI and implemented in the model.

allows these masses to rotate at a different speed in transients.

thus the analysis of the torsional dynamics in the coupling.

That's why the dynamic behavior of induction motors fed directly from isolated electrical grid, as well as dynamics of aggregate is in focus of researchers. Presently, advanced modeling and digital simulation techniques can be used to analyse the dynamics behavior of electrical as well as mechanical systems.
