**7. Conclusion**

96 Induction Motors – Modelling and Control

no-loaded induction motor.

generator are present.

transient in dynamics.

which are several times the normal full load current of the motor. This current causes a significant voltage dip on the isolated electrical grid, the terminal voltage is momentary decreased and after few damped oscillations reached minimal value. During this short period the voltage regulator does not affect jet. The voltage drop increases in case of starting loaded induction motor, the current momentarily reaches higher value of first magnitude

Disturbances in the system are present when the second induction motor is connected to the grid and additionally is growing up at the instant of impact additional load on the previous induction motor shaft. As load on the motor shaft grows up, the current of the motor is higher; oscillations are more damped and longer present in comparison with lightly load or

This current causes the voltage drop at the motor terminals that are connected on the same grid. The mutually effect between source and loads exists. The voltage drop results that the motor speed slows down. Because the motor rotor slows down the higher current is

At the instant of starting of induction motors the locked rotor torque appeared and as the rotor starts to rotate the air-gap torque oscillates about positive average value. Oscillations in transient of electromagnetic torque are damped at the end of start up period and finally the steady state condition is attained without oscillations. The oscillations are longer presented in case of bigger load torque on the motor shaft. Sudden change load on the motor shaft causes changes in air-gap torque. Damped oscillations are longer present in a case of impact additional load than during load disconnected. Both cases influence on the electrical grid and changes in transients of both induction motors as well as in synchronous

The start-up period of the induction motor is longer when the bigger load torque on the motor shaft is applied. Also, decreasing the terminal voltage causes longer duration of speed

Sudden change load on a motor shaft, which occurs during operation period, results in

Direct starting of induction motors on isolated electrical grid, as well as sudden change load, caused voltage dips and also reduces speed of aggregate. The significant voltage dips appear due to faults in power supply, as well as certain faults on loads connected to isolated electrical network. Besides the voltage dips, voltage interruption can also appear. This causes significant disturbances on the grid and affects the operation of other induction motor. If interruption of the supply lasts longer than one voltage period, many AC contactors will switch off the motor. However, in some cases, faulty contactor may produce multiple on and off switching. These interruptions affect the dynamics of both electrical and mechanical variables, which will also cause torsional stresses in the shaft line. The consequences of voltage interruption on the induction motor behavior are current and air-

and oscillates about higher average value than in previous case.

appeared on the grid and voltage are reduced even more.

speed change of the other motors that fed from the same grid.

gap torque peaks that appear at instant of fault and recovery voltage.

The dynamics of induction motors fed directly from the isolated electrical grid is analyzed. In isolated electrical grid, such as for example ship's electrical grid, the main source is a diesel generator and induction motors are the most common loads. Induction machine plays a very important role in that application and a significant number of induction motors are used at critical points of on board processes. The connection of large induction motors (large relative to the generator capacity) to that grid is difficult transient regime for units due to electrical loads and also due to torsional loads on the shaft line. Direct on line starting of induction motors induces high strain on the power system and this strain arises when the next induction motor is connected to the grid. Sudden impact load on the induction motors shaft is an additional strain on the network, especially when impact load on a motor shaft occurs during start up of another one. This in turn affects the quality of electric power system and thus, the dynamic behavior of induction motors. The significant voltage dips appear due to faults in power supply, and also, due to certain faults on loads connected to the isolated electrical grid. These voltage dips cause changes in transients of induction motors, as well as in transients of diesel generator unit. Besides the voltage dips, voltage interruption can also appear, which further effects the operation of the induction motor. Factory production tests demonstrate the capability of the unit to supply defined loads applied in a defined sequence. In order to make changes to the loading of an existing isolated electrical grid, it is necessary to analyze and document the effect of the additional loads on its normal and transient performance. An induction motor starting study may be of use in analyzing the performance of small power systems. Such systems are usually served by limited capacity sources that are subject to severe voltage drop problems on motor starting, especially when large motors are involved. In some cases, specific loads must be accelerated in specially controlled conditions, keeping torque values in defined limits. The results obtained by this analysis can be used as a guideline in choosing as well as setting parameters of the protection devices.
