978-1-5090-4281-4/17/$31.00 ©2017 IEEE Significance of Rotor Slots Number on Induction Motor Operation under Broken Bars W. Abu Elhaija 1 , V. Ghorbanian 2 , J. Faiz 3 , H. Nejadi-Koti 4 1 Princess Sumaya University for Technology; Electrical Engineering Department, Amman, Jordan 2 McGill University; Department of Electrical and Computer Engineering, Montreal, Canada 3 University of Tehran; School of Electrical and Computer Engineering, College of Engineering, Tehran, Iran 4 Marquette University; Department of Electrical and Computer Engineering, Milwaukee, Wisconsin, U. S. A Abstract— This paper investigates the effect of the rotor slots (bars) number of a three phase squirrel cage induction motor on the motor performance under broken bar fault condition. Two motors have been investigated at full load condition in healthy bars, one broken bar and two broken bars operations. The first motor has 24 stator slots and is investigated with rotor slots 20, 29 and 32 and the second motor has 36 slots and is investigated with rotor slots 28 and 44. The above-mentioned rotor slots numbers have been selected based on manufacturer’s database. The effect of rotor slots numbers on motor performance with broken bars fault has been observed by analyzing the stator current emerging frequency components, steady state copper losses, slip at full load and steady state bar currents. Recommendation for the safest stator and rotor slots combinations has been provided based on simulated key performance indicators. Keywords—Broken bars, rotor slots, frequency components, induction motor. I. INTRODUCTION A. General Overview on Broken Rotor Bars An important failure mode of large electric motors is the break down and subsequent heating and breaking of the rotor bars, especially in motors that experience frequent starts under load. The starting condition places the heaviest stress on the rotor bars because they are carrying the highest current since the rotor is running at much lower than synchronous speed. The high currents cause heating and expansion of the bars relative to the rotor itself, and differences in the electrical resistance of the individual bars result in uneven heating and uneven expansion. This leads to cracking of the joints where the bars are welded to the shorting ring. As soon as a crack develops, the resistance of that bar increases, increasing its heating, and consequently worsening the crack. At the same time, the adjacent rotor bars experience increased currents because of the reduced current in the broken bar. This scenario results in localized heating of the rotor, causing it to warp. In a squirrel cage induction motor with a broken bar rotor, the supply currents contain spectral components at sideband frequencies of e b f ks f ) 2 1 ( ± = , where e f is the supply frequency, s is the operating slip, b f is the detectable broken bar frequencies, and k is an integer odd number, [1-11]. The signature analysis of machine line current (MCSA) investigates the sideband components’ b f around the fundamental for detecting the broken bar fault. While the lower sideband is specifically due to a broken bar, the upper sideband is due to consequent speed oscillation. Torque and speed signals also contain e msf 2 frequency components with broken rotor bars; m is an integer. There has been a large body of work done on the detection of broken rotor bars in induction motors. The pulsation caused by broken bars at twice the slip frequency in the stator current has encouraged authors to use this fact as a means for detecting broken bars in an induction motor [12-20]. B. Rotor slot numbers efficiency as a consequent of broken bar(s) condition Few studies have addressed the impact of rotor slot (bar) numbers on the behavior of induction motors. The effect of six different rotor bar numbers including 24, 28, 30, 40, 41 and 48 have been investigated in [21]. The corresponding results indicate that for the selected number of stator slots and number of magnetic poles, the rotors with the slot number multiple of 3, did not have higher harmonics in the air gap magnetic flux density. It has been concluded that the motor with 48 rotor slots was the best case because it presented good starting behavior along with low air gap magnetic flux density harmonic content and improved characteristics at nominal speed. In [22], the authors presented an analytical vibro-acoustic model of a 700W induction machine, called DIVA to study the effect of slot numbers on noise, independent of the slot geometries. However, their established analytical rule to avoid magnetic noise was difficult to use for practical purposes. The impact of the bars breakage on the harmonic components of the stator currents should be taken into account when calculating copper losses in faulty induction motors. The authors of [23] have proposed a new analytical method for the calculation of copper and core losses in faulty induction motors under broken bars and it was concluded that a broken rotor bar fault in an induction motor increases the copper loss and total loss of the machine. Based on the fact that manufacturers produce motors for long-term survival while operating reliably, this paper recommends a tool to select the optimal rotor: stator slots combination based on the following key performance indicators not only at healthy bars operation but also under at least one broken bar condition: • Free oscillations in the steady state speed,