Vibration Analysis of Induction Motors with Unbalanced Loads Selahattin GÜÇLÜ 1 , Abdurrahman ÜNSAL 1 and Mehmet Ali EBEOĞLU 1 1 Dumlupinar University, Department of Electrical Engineering, Tavşanlı Yolu, 10. km. Kutahya/TURKEY unsal@dpu.edu.tr, selahattin.guclu@dpu.edu.tr, mebeoglu@dpu.edu.tr Abstract One of the factors that cause the failure of induction motors which are used intensively in industry is the unbalanced loading of the motor. In this paper, a system to simulate the unbalanced loading condition of a three-phase squirrel cage induction motor was designed. The stator current and vibration signals of the motor were recorded. The vibration signals under balanced and unbalanced loading conditions were recorded with an accelerometer which was mounted on the motor housing. The stator current and vibrations signals were analyzed with Power Spectral Density (PSD). The results shows that the system can be used to simulate the effects of the unbalanced loading conditions of the induction motors. 1. Introduction One of the most-widely used electrical motors in modern industrial plants is the induction motor. Induction motors play an important role in the safe and efficient operation of industrial plants. Compared to other electric motors, induction motors have many advantages such as simplicity of their structure, high reliability, and relatively low cost. However, there is a possibility of faults when converting electrical energy to mechanical energy. Maintenance and fault diagnosis of induction motors are becoming increasingly important in the industry. Periodic inspection of induction motors and/or preventive maintenance are time-consuming and expensive. The detection of faults in induction motors is important in maintenance work. The detection of faults at an early stage is very important to prevent the total breakdown of the motors and/or processes. If the necessary measures are not taken at an early stage the failed induction motor may be replaced which leads to financial losses. Fault identification and diagnosis in a timely and regular manner can increase the reliability of the system and provide repair/replacement. Also the condition monitoring is important to avoid unexpected and catastrophic failures. The faults of induction motors include stator winding failures, broken rotor bars, misalignment, static and/or dynamic air gap irregularities and bearing failures. The distribution of faults occurring in induction motors is given in Fig. 1. The major faults of induction motors can broadly be classified is as follows: a. Stator winding faults; b. Broken rotor bars or end rings; c. Static and/or dynamic air gap irregularities; d. Shaft related faults; e. Rotor winding faults; f. Bearing faults. Fig. 1. Induction motor failures [1] Mechanical faults of induction motors, such as rotor imbalance and misalignment of the shaft, are most common problems. In most applications, it is important to notice that these mechanical faults affect the safety and/or efficiency of working environment [2, 3]. Mechanical faults may cause torque oscillations and/or eccentricity faults. Torque oscillations may be caused by unbalanced loading, shaft misalignment, gearbox faults and bearing related faults. An eccentricity fault is a nonuniform air gap which may be caused by bearing wear or bearing failure, and bad motor assembly with unbalanced or noncentral rotor [4-7]. Eccentricity faults as shown in Fig. 2, are classified into three groups: static eccentricity (SE), dynamic eccentricity (DE) and mixed eccentricity (ME) [4, 7, 8] . Static Ecc. Dynamic Ecc. Mixed Ecc. Fig. 2. Different types of eccentricity fault [5] The eccentricity faults causes the sideband frequencies in the stator current and vibration signals of induction motors [5][9, 10]. These side-band frequencies can be calculated as  ௘௖௖ = ௦ േ  ௥ = ௦ ቀ1 േ  (ଵ௦) ௣ ቁ (1)