Unit 3 Condition Monitoring of Induction Motors PDF

Unit 3 Condition Monitoring of Induction Motors Unit 3 Condition Monitoring of Induction Motors Parameters of Induction motors: Induction motors are one of the most common types of electric motors used in various applications. They work on the principle of electromagnetic inducti...

Unit 3 Condition Monitoring of Induction Motors Unit 3 Condition Monitoring of Induction Motors Parameters of Induction motors: Induction motors are one of the most common types of electric motors used in various applications. They work on the principle of electromagnetic induction to generate a rotating magnetic field which, in turn, induces a current in the rotor, causing it to turn. Here are some of the key parameters of induction motors: 1. Rated Power (P): This is the amount of power the motor is designed to deliver continuously under normal operating conditions, usually expressed in kilowatts (kW) or horsepower (HP). 2. Rated Voltage (V): The voltage at which the motor is designed to operate. Common voltages include 230V, 460V, and 690V for three-phase motors. 3. Rated Current (I): The current the motor draws at full load and rated voltage. 4. Frequency (f): The frequency of the power supply, typically 50 or 60 Hertz (Hz) for most applications. 5. Number of Phases: Induction motors can be single-phase or three-phase. Three-phase motors are more common in industrial applications due to their higher efficiency and power capacity. 6. Synchronous Speed (Ns): The speed of the rotating magnetic field in revolutions per minute (RPM) when the motor is connected to its rated power supply frequency. It's determined by the formula: Where f is the frequency of the power supply and P is the number of poles. 7. Number of Poles (P): This is a design parameter and determines the synchronous speed. It's an important factor in determining the motor's speed-torque characteristics. 8. Rated Speed (N): The actual speed of the motor's rotor when operating at full load and rated voltage. It's always less than synchronous speed due to slip. Prof. Patil J. B. EE Dept. Page 1  Unit 3 Condition Monitoring of Induction Motors 9. Slip (s): Slip is the difference between synchronous speed and rotor speed, expressed as a percentage. It's given by the formula: 10. Efficiency (η): The ratio of mechanical output power to electrical input power, usually expressed as a percentage. High-efficiency motors are preferred for energy-saving applications. 11. Power Factor (PF): This is a measure of how effectively the motor converts electrical power into useful mechanical power. It is the cosine of the phase angle between the current and voltage waveforms. 12. Starting Torque (Ts): The torque developed by the motor at startup. Some motors are designed for high starting torque applications. 13. Rated Torque (T): The maximum continuous torque that a motor can deliver at rated conditions. 14. Locked Rotor Torque (Tlr): The maximum torque a motor can produce when the rotor is stationary. 15. Temperature Rise: This parameter indicates how much the motor's temperature will rise above the ambient temperature during operation at full load. 16. Insulation Class: This refers to the class of insulation used in the motor windings, which determines the maximum allowable operating temperature. 17. Duty Cycle: It specifies how a motor is expected to perform in terms of load and time. Common designations include S1 (continuous duty), S2 (short-time duty), etc. These parameters collectively define the performance and capabilities of an induction motor and are crucial considerations when selecting or designing a motor for a specific application. Keep in mind that the actual performance of a motor can be influenced by factors such as ambient temperature, altitude, and installation conditions. Prof. Patil J. B. EE Dept. Page 2  Unit 3 Condition Monitoring of Induction Motors Induction Motor Fault Diagnostic Methods: Condition Monitoring of Induction Motor The causes of faults can be categorized into two groups: i) Mechanical Causes of Failure: -Misalignment, -Mechanical unbalance, -Bearing fatigue, -Fractured rotor bars or end rings, -Overheating. -Loss of cooling, -Improper lubrication ii) Electrical Causes of Failure: -Poor power quality, -Resistance and Impedance unbalance, -Insulation failure, -Excessive loading and current The distribution of induction motor major faults can be listed as: bearing (69%), stator winding (21%), rotor bar (7%), and shaft/coupling (3%) faults. Monitoring of a rotating machine has been a challenging task for engineers and researchers especially in industries. Vibration analysis is a powerful process of performing all of the following: a) Current machine Status. b) Diagnosing defects related with operational machines. c) Monitoring the overall condition of machines. A complete vibration analysis consists of the following three components: a) Exact vibration determination b) Bearing condition determination c) FFT analysis Prof. Patil J. B. EE Dept. Page 3  Unit 3 Condition Monitoring of Induction Motors What is Vibration? Definition: “Vibration is the motion of a body about a reference point caused by an undesirable mechanical force.” Shaft vibration caused by the shaft moving about the centerline of a journal bearing. 1) VIBRATION SIGNATURE ANALYSIS  The vibration signature of a machine is the characteristic pattern of vibration it generates while it is in operation.  The actual signal from a vibration transducer can be considered the signature, but the spectrum of the vibration signal is usually referred to as the signature.  Effective vibration analysis starts with obtaining an accurate signal from standard vibration transducer with the help of an accelerometer.  The analog signal is then converted into digital signal using analog to digital converter (ADC).  The digital signals can be processed directly or can be processed using some formula depending upon the need of the user. 2) MOTOR CURRENT SIGNATURE ANALYSIS (MCSA)  Motor Current Signature Analysis (MCSA) is a technique used to determine the operating condition of AC induction motors without interrupting production by analyzing current and voltage supplied to an electric motor with the help of specific patterns and protocols.  MCSA along with vibration and thermal analysis is used to make machinery diagnostic decisions. MCSA operates on the principle that induction motor circuit is effectively a transducer.  By clamping a Hall Effect Current sensor on either the primary or secondary circuit, fluctuations in motor current can be observed.  Current harmonics in the stator current are analyzed by MCSA. These harmonics are caused by new rotating flux components on account of a fault. It needs only one current sensor and is based on signal processing techniques like FFT.  The equipment set up for measuring motor current is shown in Fig. (1). Data acquisition is achieved by performing FFT on the stator current. Prof. Patil J. B. EE Dept. Page 4  Unit 3 Condition Monitoring of Induction Motors  The data obtained after FFT is normalized as a function of the first harmonic amplitude is then analyzed. Fig. (a) and Fig. (b) illustrates FFT with initial condition of rotor bars and with 3 broken rotor bars respectively. Fig.(1) Set-Up for Measuring Motor Current Fig. (a) FFT with initial condition for broken bars Fig. (b) FFT with 3 broken bars Prof. Patil J. B. EE Dept. Page 5  Unit 3 Condition Monitoring of Induction Motors Table 1. Results of Broken Bar Faults The Induction Motor Fault Monitoring Method and Remedies: The three-phase induction motor is one of the most popular and most widely used electric motors. They are used in many applications due to their simple and rugged construction with almost maintenance-free operation. It is an asynchronous motor that works on the principle of induction. But similar to other motors these motors are also vulnerable to faults that may result in damage to the motor. Thus monitoring of the induction motor has to be done for continuous operation without any interruption. There are many factors like heavy operating duty, poor working conditions, improper installation of motor and manufacturing factors of the motor, etc, that can cause failure to the motor operation. If the faults are not rectified beforehand, can result in large revenue loss to the industry as well as can pose threat to reliability and safety of operation. Prof. Patil J. B. EE Dept. Page 6  Unit 3 Condition Monitoring of Induction Motors Faults in Three Phase Induction Motor: 1) Motor Fails to Start Possible Causes Remedies to overcome Supply may be cut off. Check the main supply with the test lamp The main switch may be in an off position. Check the main switch and switch ON. Fuses may be blown off. Check them physically and replace them. Overload control may be tripped. Let the overload control be cooled and start again. Connections of the control circuit may be Check physically and correct them. wrong. Connections may be loose at the terminals of Check and tighten them. the motor or starter Stator or rotor windings may be open-circuited. Check the windings with a series test lamp or megger and connect the broken ends, if possible or get them rewound. Misalignment of the motor. Check and correct the alignment. 2) Motor is Overheated Possible Causes Remedies to overcome The motor may be overloaded. Check and reduce load. Control may be defective. Check the starter, fuses, main switch, etc, and correct them. Ventilation may be poor. Maintain the proper ventilation. Supply voltage maybe too high. Check with the voltmeter and stop the motor till the supply voltage falls to normal Drive troubles. Check and correct. Stator winding may be short-circuited or Check for short circuit and earth fault with a earthed series test lamp or megger and remove the defects. Prof. Patil J. B. EE Dept. Page 7  Unit 3 Condition Monitoring of Induction Motors 3) Bearings Overheated Possible Causes Remedies to overcome Bearings may be squeezed. Check and re-grease the bearings with proper grading lubricant. Belt may be too tight. Check and reduce the tension of the belt. Excessive gear end thrust. Reduce the gear pressure. The bearing maybe not properly fitted. Set the bearing correctly. Using the wrong grade of lubricants. Use the lubricant of proper grade. Foreign material in the lubricant. Replace the old lubricant with a new one according to I.S.I rule. 4) Motor Starts and Stops Possible Causes Remedies to overcome Starting gear may be defective. Check the contacts and correct them. Connections of stator and rotor windings may Check and tighten them. be loose. Loose connections at the motor's terminals. Check and tighten them. 5) Motor Works With Reduced Speed Possible Causes Remedies to overcome The motor may be over-loaded. Check and reduce the load. Stator winding may be open-circuited. Check with the series test lamp and join disconnected ends. 6) Motor Works With High Speed Possible Causes Remedies to overcome Starting gear may be defective. Check and correct it. Supply voltage increased suddenly. Check the voltage with a voltmeter and stop the motor till it falls to normal. Prof. Patil J. B. EE Dept. Page 8