Diagnostics of rotor and stator problems in industrial. The process executed, for this simulation, was by choosing manual mesh and. Squirrelcage rotor almost 90 per cent of induction motors are squirrelcage type, because this type of rotor has the. Figure 1 fft showing advanced broken cracked rotor bar symptoms. The squirrel cage is also considered the secondary windings of the motor figure 17. Loose rotor bars extremely unusual and never found in cast rotors.
This final thesis will mostly be dedicated to induction motors but also to synchronous permanent magnet and reluctance motors. The rotor bars are magnetized and by the use of iron filingshacksaw blade you determine a discontinuity of current flow through a rotor bar or a cracked bar. The rotor is a moving component of an electromagnetic system in the electric motor, electric generator, or alternator. This is accomplished by end rings which may be welded to the ends of the rotor bars, or the end rings and rotor bars may be injection molded around the core, effectively forming a single, homogenous structure. Lamination is necessary since a voltage is induced along the axial length of the steel as well as in the stator conductors. The electrical effect of the broken or cracked rotor bar is an increase in the rotor circuit impedance and, therefore, the stator impedance as well. Numerical and experimental studies have confirmed that the proposed approach is effective in diagnosing broken rotor bar faults for improved induction motor. Also i would like to thank dr michael robinson and pope electric motors. The basic idea of an electric motor is to generate two magnetic fields. Pdf detection of broken rotor bars in induction motor using. Chris says that a standard motor repair practice should include a rotor bar and stator slot count when the equipment is serviced. The rotor will constantly be turning to align its magnetic field with the stator field. The rotor has salient poles and a cage so that it starts like an induction motor, and runs like a synchronous motor.
The frequency, then, is the number of windings slots x rpm winding slot pass frequency or wspf. It consists of a cylinder of steel laminations, with aluminum or copper conductors embedded in its surface. Nov 01, 2004 in this case study, the motor was operating at less than full load with a current of 250 a and therefore at lower slip higher speed. In some induction motors, what is called the deep bar effect, or distribution of rotor currents due to eddy currents, is taken advantage of. Broken rotor bar is one of the commonly encountered induction motor faults that. Pdf modeling of broken rotor bars in a squirrelcage. An electric motor is a device which converts an electrical energy into a mechanical energy. Pdf effect of different type of rotor bars on performance using.
Squirrel cage rotor wound type rotor or slip ring type rotor squirrel cage rotor in this type rotor as the name suggests it is a rotating part of an electrical machine, in which current is induced by transformer action from rotating magnetic field. The motor designs differ mainly in the design of the rotor bars. Broken, cracked rotor bars, bad joints between end rings and rotor bars and end ring problems have unique and easily recognizable symptoms. A large swing in measured current as you move the motor is an indication of problems. Getting the rotor back up and running machine design.
Diagnosing rotor bar issues with torque and current. Broken rotor bar detection by power and current based fft. The rotor cage is made up of parallel conductors shorted together on each end by a shorting ring. Electric motor electric motor construction of induction motors. As a magnetic field is passed over the bars, they induce a current flow that generates a second magnetic field. Regional sales manager at industrial motor services in richmond, virginia. The inner most section of the rotor bar clos electric motor tip on rotor bars. Broken rotor bar diagnosis for induction motors jinjiang wang 1, robert x.
Unsubscribe from dreisilker electric motors, inc cancel unsubscribe. Past efforts on broken rotor bar diagnosis have been focused on current signature analysis using spectral analysis and wavelet transform. During the startup process, the outer most section of many rotor bar designs can heat up as much as 300 degrees celsius in less than 10 seconds. Evaluation of electric motor condition using motor circuit. For example, on an 1800 rpm motor with 36 rotor bars, the frequency would be 64,800 cpm. Symptom of high vibration cause test or check correction. When an electric motor is started, the fields within the rotor are at line frequency ie. Typical induction motor rotors the magnetic field generated in the stator induces an emf in the rotor bars. For most shore facility applications, this is the case. The flux density frequency spectrum at position b at rated load. Intermittent behavior is common in rotor bar problems. As can be seen in the figure, the copper rotor has substantially lower resistance over the whole frequency range, but the difference is largest at running conditions.
The number of winding slots will be between about 25 and 100. Rotor interaction both the rotor and stator magnetic fields interact in the air gap between the rotor and stator. Jun 06, 2015 induction motor rotor bar current density yahya zakaria. Select current density in rotor bars and end rings depends on ambient temp, cooling conditions, and duty cycle, and from rotor bar and end ring currents get their cross sectional areas. Broken rotor bar is one of the commonly encountered induction motor faults that may cause serious motor damage to the motor if not detected timely. Brokenrotorbar diagnosis for induction motors iopscience.
Heating in the stator winding, proportional to i 2t, is 10 to 50 times rated conditions and the winding is without benefit of the ventilation normally produced by rotation of the rotor. Methods of detecting cracked rotor bars in electric motors. The impact of broken rotor bars to the stator current can be determined by analyzing in the frequency domain this approach to detect rotor bar failures is typically called a motor current signature analysis mcsa. During routine emax testing in june 2003, motor engineer enrique rodriguez observed that the peak level of the pole pass sideband in a 4,160volt, 3,500horsepower, 3,590 rpm ac induction motor was 0. I have seen a number of motors operating at their rated load with an open bar. The rotor magnetic field will attempt to line up with the stator magnetic lines of force. In some cases, a motor may also be used as a generator. Uses soft magnet rotor that is magnetized by the application of current in the stator coils 3. Software applications like mcegold can help identify the number of bars, but.
Even at this load, the harmonics from shaft rotation may lead a user to raise a false alarm of broken rotor bars if not correctly identified as such. The motor is figure 1 is a squirrelcage induction motor. Electric motor construction of induction motors britannica. At higher frequencies the smaller skin depth of copper causes the impedance of those bars to increase. An ac induction motor converts electrical energy into mechanical motion. It is true that single phase motors are less efficient substitute for 3phase motors but 3phase power is normally. The basic diagram view a shows a circle with two leads labeled t1 and t2. In turn, a current is produced in the rotor bars and shorting ring and another magnetic field is. Second, the number of induction motor rotor bars and thus slots must be slightly different from the number of stator slots to minimize both electromagnetic and acoustic noise.
Sequentially, the bars create their own magnetic field. Some singlephase induction motors are also called squirrel cage motors because of the rotor s similarity to a circular animal exercise wheel. The most frequent used detection methods are 2 motor current signature analysis. Besides a simple visual inspection, below is a summary of the most common methods of detecting cracked rotor bars as well as shorted laminations in an induction electric motor s rotor. Diagnostics of rotor and stator problems in industrial induction motors by fang duan b. How can an instrument 100 to 1,000 feet or more away from the motor provide information about the condition of the rotor.
The rotor will constantly be turning to align its magnetic field with that of the stator field. The motor rotor shape is a cylinder mounted on a shaft. An epa study completed in 1996 found that the 1 billion electric motors in the united states consume 1700 billion kwh per year. The stator frame consists of laminations of silicon steel, usually with a thickness of about 0. The 3phase set of currents, each of equal magnitude and with a phase. Conversely, a generator converts mechanical motion into electrical energy. An introduction to electric motors stmicroelectronics. Recall that leakage reactance is the reactance due to the rotor flux lines that do not also couple with the stator windings. Its rotation is due to the interaction between the windings and magnetic fields which produces a torque around the rotors axis. Aug 17, 2017 preferably the rotor bars are extruded from oxygen free copper.
Additionally, chris adds that a nameplate should be attached on the motor with the data as well as in the standard repair report. When higher loads are present, an alloyed steel such as chromiummolybenum crmo is frequently used, and for applications with extreme corrosion or hostile. There are two major components of an ac induction motor. Damage to the rotor laminations is another defect that can be catastrophic to the motor. As discussed in chapter 16, the squirrel cage comprises the bars and shortingrings that make up the rotor windings. Design and stress analysis of a high speed rotor for an advanced induction motor matthew t. How to know the number of rotor bars in induction motor.
Designing squirrel cage rotor slots with high conductivity. Telecommunication engineering, southwest jiaotong university, china, 2005 thesis submitted for the degree of masters of engineering science in school of electrical and electronic engineering the university of adelaide, australia august 2010. Control of motorcharacteristics by squirrelcage rotordesign. This explains how a motor could run for years with broken rotor bars, possibly performing worse or better as more bars fail. By applying space vector v s, the stator voltage equation written in stator axis can be written as in 3. Typically k r is in the range of 1 to 5, and k x is in the range of 0. The rotating magnetic field cuts through the rotor, inducing a voltage in the rotor bars. The electric motor operation is based on the following points. Investigation of broken rotor bar faults in threephase squirrelcage induction motors 483 a b c fig.
The squirrel cage induction motor rotor is shorted and so there is no rotor excitation, the rotor voltage equals zero. Rotor construction highfrequency induction brazing of the rotor bars to the end rings ensures mechanical integrity and results in uniform strength and electrical conductivity at the brazed joints. Rotor design for an electric motor tesla motors, inc. The rotor consists of conducting bars along the motor axis. To detector is evaluated experimentally using one stator and three rotors from identical. In really simple terms, there are a couple of factors. When rotor bars fail, and the motor is accelerating and running at full speed, you may hear a distinctive beat frequency noise, the volume or amplitude modulating.
In this part 1 video, we discuss the forces within rotor bars through experimentation. They are generally performed by mechanical andor electrical monitoring. Its rotation is due to the interaction between the windings and magnetic fields which produces a torque around the rotor s axis. A defect in the rotor bar of an induction motor causes the modulation of the stator current. The rotor also consists of a set of slotted silicon steel laminations pressed together to form of a cylindrical magnetic circuit and the electrical circuit. The axial currents in the rotor bars will interact with the radial flux wave to produce the driving torque of the motor, which will act in the same direction as the rotating field, the rotor being dragged along by the field. The rotor and stator are separated by an air gap which allows free rotation of the rotor. When the rotor is stationary or turnin g slowly the frequency of rotor currents is relatively high, the currents crowd to the top of the rotor bar and the resistive part of impedance is relatively high. Rotor testing with mce pdma a leader in electric motor. The power transmission system is integrated with riserpipes. The magnet is spun by a 3 phase ac motor with no rotor voltage 4.
The amount of space available for the rotor slot is dependent on the stresses in the rotor teeth the more area used for the slot, the higher the tooth stresses. During the operation of an induction motor, many types of faults can be avoided, involving electrical and mechanical faults. The brokenrotorbar singlephase electrical model of an induction motor. Single phase rotor test this test is performed while the motor is still together. Detecting broken rotor bars prevents catastrophic damage. The electrical circuit of the rotor is of the following nature squirrel cage rotor consists of a set of copper or aluminium bars installed.
Many of these tests need to be done in an electric motor repair. Investigation of broken rotor bar faults in threephase squirrelcage induction motors 479 im s ii b n dn e 2blv2 l 2 60 1 where b im is the amplitude of the fundamental component and theith harmonic. The unusual aspect of this fault is that it will show up at a frequency equal to the rpm times the number of rotor bars and is referred to as the slot or rotor bar pass frequency. Control of motorcharacteristics by squirrelcage rotordesign the reactance x2 in an induction motor equivalent circuit represents the referred form of the rotors leakage reactance. Here are tips on how to find the problem, fix the rotor, and get the motor running again. The thin piece of metal, commonly a hacksaw blade, is then placed on top of the nearby rotor bars. The output voltage is taken from the rotor by the sli p rings and brushes. Pdf a new pattern for detecting broken rotor bars in induction. Detection and analysis of rotor faults in induction motors.
During data analysis or a troubleshooting effort on a threephas e squirrel cage induction motor, we are often left wondering ho w many rotor bars are manufactured into the rotor. Mar 21, 20 in one aspect of the invention, an electric motor rotor assembly is provided, the assembly including i a rotor shaft. Rotor bars, for example, can be conservatively sized to allow compact motor dimensions. Investigation of broken rotor bar faults in threephase. The insulation system must be sufficient to protect the winding from ground faults and turntoturn shorts throughout the operating envelope. At least one of the two magnetic field is generated by a solenoid carrying a current. Motor shaft materials for most motor applications, hot rolled carbon steel is a good choice.
If a rotor bar is broken, the alternating voltage at the location of the break will cause the thin piece of metal to vibrate. The stator field induces a voltage into the rotor cage. Nearly 80% electrical motors used in industries are induction motors and hence industries depend on the performance of them to. Diagnosing rotor bar issues with torque and current signature analysis professionals who maintain motors need to be able to answer the following questions. You are right, a singlephase test can be done to attempt to detect rotor problems. Construction of squirrel cage induction motor explained.
Austin hughes, bill drury, in electric motors and drives fifth edition, 2019. In operation, the nonrotating stator winding is connected to an alternating current power source. Cracked rotor bars or cracked rotor bar shorting ring connections are a major source of problems for many electric motors, and detecting them can require the use of specialized rotor testing equipment. Characteristics of an open rotor bar electric motors.
Gao, and ruqiang yan2 1 department of mechanical engineering, university of connecticut, storrs, ct, usa 2 school of instrument science and engineering, southeast university, nanjing, china email. Various techniques have been developed for broken rotor bar fault diagnosis, including vibration 5, rotor speed 6, motor axial flux 7 and radial flux analysis 8, parks vector. Root cause ac motor failure analysis with a focus on shaft. Symptom of high vibration cause test or check correction l. A squirrelcage rotor is the rotating part of the common squirrelcage induction motor. One slip ring is attached to each end of the rotating loop. We invite you to send us your engineering drawings or bar. The rotor, acting as the armature, rotates in the field, cutting the lines of force and producing the desired output voltage. In its simplest form a threephase induction motor is comprised of a stator and a rotor. The alternating current ac induction motor is one of the most rugged and most widely used machines in industry.
First, the total mass of the induction motor rotor bars must be sufficient to handle the heat generated by the high currents during a transient start condition. Design flexibility is enhanced by the use of copper. Ger4212 ge generator rotor design, operational issues, and. Induction motor broken rotor bar detection based on rotor. If you have an open bar, the torque capacity of the motor will drop, and iff you had sufficiently high load on the motor, it will begin to stall and then the current would certainly rise. Rotor bar problems the electrical problem that is most effectively diagnosed and monitored through vibration analysis.
From 1974 onwards, rotor nl started manufacturing special motors to meet the demand for electric motors tailormade to specific working conditions and applications. Laminations from typical squirrelcage induction motor rotors, showing the cross section of the rotor bars. Herbst the university of texas at austin center for electromechanics austin, tx 78712 j. Motors employing this type of rotor are variously known as phasewound motors or wound motors or as slipring motors. Just as in the threephase motor diagram, the motor shows the power supply lines as being identified with the t. Winding design, modeling, and control for polephase modulation induction motors. Depending on the design, a motor may be stator limited thermally or rotor. Try searching this forum for single phase test, possible with motor andor rotor for more detailw. Pdf winding design, modeling, and control for polephase. The phase relation between the rotor and stator magnetic field i.
How real electric motors work educating global leaders. Thanks to our large stock of components, we can adapt very. Internally it contains longitudinal conductive bars usually made of aluminium or copper set into grooves and connected at both ends by shorting rings forming a cagelike shape. As a loose rotor bar passes a winding slot, the magnetic force causes it to momentarily lift and then drop. The rotor conducting bars are usually not parallel to the shaft, but are purposely given slight skew. The purpose of the stator is to develop the rotating magnetic field. Pdf different rotor bars type, lower in iron loss, total loss and thicknesses of steel. This mechanical energy then can be supplied to various types of loads. D, in electrical submersible pumps manual second edition, 2018. In small motors, the rotor is fabricated in a different way. We specialize in the supply of copper components for the repair of ac rotors in the field. X depend on the shape and size of the rotor slot, the conductor material and the rotor current frequency. Modeling of broken rotor bars in a squirrelcage induction motor. The stator is composed of laminations of highgrade sheet steel.