Experimental and Theoretical Investigation of Transient Voltage Distribution in an 11 kV Industrial Motor Winding

In large Industrial motors inter turn and turn to ground insulation failures are caused by dielectric stresses. The main cause of dielectric stress in motor winding is steep fronted switching surges. Steep fronted surges cause uneven voltage distribution in the motor coil. In order to evaluate electric stress on inter turn insulation of the line end coil of high power induction motors an Electromagnetic Transient Program (EMTP™) is used. In this paper an 11 kV motor coil having 9 turns is used for experimental and theoretical analysis. For theoretical analysis motor coil having nine turns is represented as an equivalent electrical network with inductance, mutual coupling as well as shunt and series capacitance. Each turn is denoted by the self inductance, mutual coupling to other turns, series capacitance and its capacitance to ground. The network is solved analytically with the help of EMTP™ for various surge rise times. The effect of surge voltage distribution for different rise times is discussed. Comparison of computed and measured results with impulse voltage also discussed.