Characteristics of an Induction Motor with Reduced Turns of Terminal Coils KAZUYOSHI HITOSUGI, SHOJI MORIYASU, and NORIYUKI NANBA Kanto Gakuin University, Japan SUMMARY The use of PWM inver ters with high switching char- acteristics for the adjustable speed drive system causes an overvoltage with steep-fronted waveform on the motor terminals. As this steep-fronted transient voltage in inverter drive loads heavily on the terminal coils of motor winding, the insulation failure in the motor winding occurs mostly on the terminal coils. Therefore, it is reasonable to reduce the turns of terminal coils only and to increase the dielectric strength of interturn insulation. This paper presents a comparison of measured char- acteristics of normal induction motors and induction mo- tors with reduced turns of terminal coils and also a comparison of the results measured and calculated by equiva lent circuit. The appropriate rate of reduced turns of terminal coils is about 20%. © 2002 Wiley Periodicals, Inc. Electr Eng Jpn, 142(3): 49–56, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.10093 Key words: induction motor; inverter surge volt- age; equivalent circuit constant; no-load characteristics; speed characteristics; operating characteristics. 1. Introduction Considering electric motors driven by PWM in- verters, when the pulse voltage of the inverter’s output (inverter surge voltage) propagates via cables, the surge reflects repeatedly from the motor and inverter terminals; as a result, an even higher overvoltage occurs at motor terminals, which may lead to coil insulation breakdown [1, 2]. In addition, when the surge voltage is applied to the motor , the voltage propagates via the strands as in a cable; however, a one-turn propagation delay occurs between strands, thus causing a relatively high potential difference. This problem may be solved by enhancing strand insulation in the whole coil; in this case, however, the insulation becomes thicker, and hence a larger core diame- ter is required so long as the core length is unchanged. On the other hand, with the core diameter kept unchanged, the number of conductors must be reduced, resulting in a longer core. Overvoltage due to inverter surges affects primarily the first coil, which is closest to the motor terminals; here such coils are referred to as the terminal coils. Therefore, the number of turns of just these terminal coils may be reduced while enhancing insulation strength. We have already reported on the equivalent circuit and calculation of characteristics for induction motors with reduced turns of terminal coils [3, 4]. In this paper, a motor with a reduced number of turns of the terminal coils ( reduced -turns motor ) is compared to a conventional motor in terms of various characteristics, using a test machine (1.5-kW squirrel-cage induction mo- tor). In addition, the measured results for current and torque are compared to calculations using an equivalent circuit. The motor characteristics when a coil is cut off due to a failure in the stator winding are also compared to those in normal operation. 2. Configurat ion of Motor with Reduced Turns of Terminal Coil, and Calculation of Its Characterist ics The arrangement of windings in the slots is shown in Fig. 1. Figure 1(a) shows a conventional motor with upper and lower coils, having Z n turns each, inserted in the slot. Z T stands for the number of turns in the terminal coil. The proposed design is shown in Fig. 1(b); here, the turns of the terminal coils are reduced by hZ n , that is, Z T = Z n − hZ n ; h is the reduction factor. The strand insula- tion of the terminal coil is made thicker in accordance to the reduction in the number of turns. Z T = 0 ( h = 1) pertains to the case in which the faulted coil is cut off. © 2002 Wiley Periodicals, Inc. Electrical Engineering in Japan, Vol. 142, No. 3, 2003 Translated from Denki Gakkai Ronbunshi, Vol. 122-D, No. 1, January 2002, pp. 22–28 49
