Abstract
Rotor bar breakage in squirrel cage induction motors is one of the most common electromechanical faults, leading to reduced efficiency, increased losses, and in the worst-case scenario, direct contact between the rotor and stator. This paper examines the structure of the squirrel cage rotor, the mechanism of rotor bar breakage, the causes of this fault, and diagnostic methods based on vibration analysis and stator current analysis. A comparison between these two diagnostic methods is also presented, including their advantages and limitations. Finally, using a real-world scenario, the study addresses a key question: how do current and vibration signals behave under rotor bar breakage stress?
Introduction
Squirrel cage induction motors are widely used in industry due to their simple construction, high reliability, and low maintenance cost. However, rotor bar breakage is one of the main causes of failure in these motors. Timely detection of this fault can prevent costly downtime. The rotor in squirrel cage motors consists of conductive bars (usually aluminum or copper) placed in the slots of the iron core of the rotor. These bars are short-circuited at both ends by end rings, forming a structure similar to a squirrel cage.
The bars are typically manufactured using either die-casting or fabricated methods, each with its own pros and cons that affect motor performance, cost, and lifespan. Die-cast rotors have bars and end rings formed as a single unit by injecting molten metal (usually aluminum) under pressure into a mold. This method is generally used for small to medium-power motors. Key advantages include lower cost, uniform connection between bars and end rings (which reduces electrical resistance), and light weight (suitable for high-speed motors). However, lower mechanical strength makes them more prone to cracking under thermal and mechanical stress. There’s also a risk of porosity (internal cavities) forming during manufacturing or startup.
In fabricated technology, the bars (usually copper) are prefabricated and individually inserted into the rotor slots. The end rings are then connected to the bars using brazing or mechanical fastening. This method is used for large motors and heavy-duty industrial applications. Its key advantage is higher mechanical strength, making it more resistant to thermal stress and vibrations. However, it has drawbacks such as higher manufacturing cost and less uniform connections (especially if brazing is poorly done), which can increase electrical resistance.
The choice between these two technologies depends on cost, application, and operating conditions. For critical applications requiring high reliability, fabricated rotors are a better option, while die-cast rotors are more economical for general, low-cost uses.
Rotor bar breakage typically occurs in the following locations:
- a) At the ends of the bars near the end rings due to concentrated thermal and mechanical stress.
- b) In the middle of the bar due to fatigue caused by alternating electromagnetic stresses.
Thermal stresses arise from the heat generated by starting currents, which can increase rotor losses up to 200 times the nominal conditions. Mechanical stresses result from centrifugal forces and vibrations caused by imbalance. Structural defects such as casting porosity, material impurities, or poor bar design can exacerbate fatigue, especially with frequent start-stop cycles, worsening the condition of rotor bars.
Methods for Detecting Rotor Bar Breakage
Rotor bar breakage leads to the appearance of frequency components corresponding to twice the slip frequency in the motor’s rotational speed. Therefore, by measuring the vibration signal from the motor housing near the bearings and performing a frequency domain analysis, this fault can be detected. An increase in the amplitude of these peaks indicates the progression of the breakage.
Another method of detecting this fault is by analyzing the stator current. Rotor bar breakage introduces sideband frequencies around the fundamental frequency in the stator current, corresponding to twice the slip frequency. However, a shared limitation of both methods is the need for motor loading to achieve accurate detection. Nevertheless, both are significantly simpler than removing the rotor from the stator for visual inspection.
Based on test experience, current analysis is more sensitive than vibration analysis in detecting rotor bar faults, although it requires substantial loading (more than 70%) for accurate results. According to the conducted tests, a combination of both methods provides the most reliable outcome for confirming rotor bar breakage. Particularly for motors powered by drives, vibration analysis serves as a complementary method to current analysis in proving rotor bar failure.
Real Test Case
In this section, a 400V, 160 kW, 4-pole motor was tested. The motor load was 102 amps, which is ideal for diagnosing rotor bar breakage. The motor was equipped with a drive, and its electrical frequency was 46.361 Hz. The effective vibration velocity was measured at 6.2 mm/s, which, according to ISO 20816, places the motor in a warning condition and indicates it should be taken out of service.
To identify the fault type, the MCM1 device was used, which can simultaneously measure current, voltage, and vibration signals. Figure 2 illustrates the signal sampling process using the MCM1 device on a sample motor. Figures 3 and 4 show the time-domain current and vibration signals, respectively. Figure 5 presents the current signal in the frequency domain, where rotor bar breakage frequencies are clearly noticeable. Figure 6 shows the vibration signal; although the bar breakage frequency growth is visible, its magnitude is significantly less than that of the current signal, emphasizing the importance of current analysis.
Interestingly, the signal associated with rotor mass unbalance grew in service conditions—while it wasn’t detectable when the motor was cold, it became apparent in vibration analysis during operation. The reason is that rotor bar breakage, due to its asymmetrical effect on rotor current profile and uneven surface heating, caused asymmetric thermal expansion and ultimately led to dynamic thermal unbalance.
Another noteworthy point is that the motor bearings suffered mechanical stress damage in short cycles and had to be replaced. Furthermore, based on the analyses, due to uneven magnetic pull in the air gap, symptoms of eccentricity fault were observed in both vibration and current spectra.
After identifying the fault and repairing the rotor, the motor was re-energized. First, the rotor bar breakage frequencies disappeared. Second, the frequency indicating mass unbalance significantly reduced. Third, the effective vibration velocity dropped to 0.9 mm/s, which, according to ISO 20816, falls within the acceptable range.
