On an estimate of the rotor winding temperature at start-up of high-voltage induction motors

This paper is devoted to research of start-up modes of high-voltage induction motors. To reduce a voltage dips during start of such motors, soft starters based on thyristor voltage converters are widely used. Acceleration of high inertia drives with a long start-up mode can be accompanied by rotor winding overheating. The authors analyze possibilities for estimate of rotor winding overheating during start-up. Analytical expressions for energy dissipated in the rotor winding at start-up are obtained. Having solved the Fredholm integral equation, the authors derive expressions allowing estimate of heat dissipation from the rotor during start-up. Calculation results of analytical expressions are tested with an integrated model that includes all elements of the power network. The adiabatic approach to the assessment of the maximum winding temperature at starting modes excluding heat transfer into the environment is proved to result in a substantial error. Analytical expressions allowing an approximate account of an uneven temperature distribution along the winding rotor are obtained.

I. Introduction

Across-the line starting of high-voltage induction motors (IM) is accompanied by inrush currents up to 6–7 times higher than rated current. Such high currents cause voltage dips more than the permissible value. To a partial solution of these problems are widely used soft starters on the base of the thyristor voltage converters (TVC). However, the use of such soft starters leads to problems with overheating of the machine windings, especially in high-inertial electric drives. According to some sources [1], [2] rotor winding temperature may rise to 600 °C. Severe thermal stresses in the rotor of the IM can cause destruction of the rotor elements and lead to motor failure [3], [4]. The above-mentioned circumstances indicate that, despite the fact that the induction motor rotor winding is not isolated, and in this case there are no problems with the acceleration of the insulation thermal resource expenditure, problems associated with the estimation of the thermal condition of the induction motor rotor in start-up modes are relevant. In particular, it is of interest to obtain simple methods for an approximate estimate of the rotor winding heating at startup modes because it does not always present the sufficient amount of information for a detailed simulation.