How To Repair Large Squirrel Cage Motors

How to Repair Large Squirrel Cage Motors

If you own a giant squirrel cage motor, you may be wondering how to repair it. While large squirrel cage motors are simple, trouble-free machines, they are still subject to failure. In some cases, the rotor may be defective and need to be replaced, but this is not a difficult process if you know what to look for. Here are a few things to look for. Induction heating, Diagnostic tests, Rotor core, and Offline condition assessment.

Induction heating

Induction heating for large squirrel cage motors is a method of heat-transfer for wind turbine rotors. It is used to braze rotor bars up to 400 kW. This method uses a transistor generator with a frequency range of 40 to 80 kHz. The process has several advantages over other types of heating. For example, the frequency range allows for more precise dosing of energy, resulting in a high efficiency rating.

Induction heating for large squirrel cage motors can be customized to minimize starting current and maximize low-speed torque. These motors are widely used in industry, ranging from milliwatts to megawatts. In addition to their simplicity and low maintenance, they also feature self-starting and stable performance under all loads. Generally, these motors are available in NEMA standard frame sizes, making them interchangeable and streamlined for various applications.

Offline condition assessment

A study by Ahmad and colleagues found that a simple and inexpensive method called instantaneous power analysis (IPA) is capable of detecting rotor defects and eccentricity. The method uses motor current, voltage, flux, and temperature measurements as data sources. This method is not only inexpensive, but it also requires access to the machine for sensor installation. Its reliability is questionable, as well, as it is difficult to perform under no-load conditions.

When conducting an offline condition assessment of squirrel cage motors, it is crucial to observe the rotor windings and their condition. Although they are not insulated, electrical insulation in the rotor and stator windings can degrade rapidly. The most common causes of failure include mechanical failure of the rotor bars, shorting rings, and connections. However, even if the rotor bars and their windings do not show immediate problems, they can result in deterioration of other parts of the motor and costly fixes.

Diagnostic tests

A diagnostic test of large squirrel cage motors can be performed to detect a fault at any point in the motor’s structure, including its armatures, stators, and capacitors. The ideal procedure uses a surge voltage tester (MTC2) to measure the current that passes through the motor and to detect a short circuit between its rotor and stator winding. To perform the test, two test probes are connected to the motor outside the slots, with the first transmitting a signal, and the second receiving a signal. The received signal is measured in the MTC2 and the signal strength is displayed on the screen.

Recent advances in diagnostic technology have made it possible to detect an incipient fault in a squirrel cage motor using its stator current. The traditional symmetrical-component method has been shown to lose its preconditioning effect due to noise and harmonics. Furthermore, the negative sequence component is difficult to obtain and the faulted rotor bars are difficult to discriminate in the current spectrum. To overcome these problems, an improved diagnostic technique based on a fundamental component extraction method is proposed. It entails the transformation of the extracted signal to a DC value and can also detect a fault in the rotor.

Rotor core

A rotor is the revolving part of a motor. It is made of steel laminations or punchings that are compressed between the ends of the stator shaft. These bars are then brazed to the copper end rings. The slots in the rotor core are not perfectly straight and they can be slightly skewed to increase torque. The skewed laminations also minimize the risk of the rotor locking in magnetic fields, which would result in the motor rotor remaining stationary and unable to rotate, even when the current is supplied.

The working principle of a squirrel cage induction motor is similar to that of a typical induction motor. The key difference is the specific interaction between the rotor and the stator. This is achieved through a complex electromagnetic induction effect. This is a powerful form of motor that is used for a variety of applications. For example, squirrel cage induction motors can be used for a wide range of tasks.

Shaft key

For a squirrel cage motor, the shaft key is a crucial component. The key joint consists of two parts that are integral, with the first part having the same width as the shaft. The second part, which is more narrow, cooperates with the keyway in the core discs and secures the rotor 5 to the shaft. This way, the assembly of the motor becomes more compact and easier to maintain.

The core of a squirrel-cage motor is composed of steel laminations that are compressed between two end rings. The laminations are not perfectly straight, but they are slightly skewed to increase the torque and reduce the risk of the rotor locking up in a magnetic field. In the event of such an occurrence, the rotor will remain stationary or stop rotating, even when the motor is provided with current.


Bearings for large squirrel cage motors are necessary for these devices. The rotor consists of steel laminations. A magnetic field generated by the stator acts on the rotor, which rotates. The shaft key is the only connection between the rotor shaft and the load, so it must be able to sustain the full load characteristics of the motor. The end ring compresses the steel laminations, while the shaft key sits within a groove.

The rotor shaft is surrounded by anti-friction bearings, which are installed in recesses in the end bells. The bearing is retained by a c-clip and an opposing bearing is installed using a spring washer. These components ensure smooth rotation and minimal friction. The shaft may rotate at a high speed, which requires high-quality bearings. For best results, choose bearings with anti-friction properties.

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