Introduction
Rotor failures in induction motors may occur in the rotor bars, the end rings, or in the connections between them. Rapid detection of rotor faults after their occurrence is extremely important. Otherwise, the fault may propagate, eventually leading to motor shutdown and significant economic losses.
Among the most important causes of rotor faults are mechanical stresses, thermal stresses, and manufacturing defects in the rotor structure. These factors are discussed in the following sections.
Mechanical Stresses
Mechanical stresses in squirrel-cage induction motor rotors mainly result from centrifugal forces acting on the rotor during operation. Since centrifugal force is proportional to the square of the rotor speed, its magnitude reaches the highest level at rated operating speed.
If the motor operating conditions involve frequent start-stop cycles, the stresses caused by centrifugal forces will be repeatedly applied. Over time, this cyclic loading may lead to fatigue failure of rotor components, particularly the rotor bars and their connections.
Thermal Stresses
Each direct-on-line (DOL) start of an induction motor produces a significant amount of heat in the rotor. During startup, the rotor current may exceed five times the full-load current.
The longer the rotor takes to reach rated speed, the higher the temperature rise experienced by the rotor.
In large motors driving high-inertia loads, repeated startups within a short time interval can be particularly problematic. Heat generated during the first startup may not be fully dissipated before the next startup occurs. As a result, the rotor bars may already be at an elevated temperature when the second startup begins, leading to excessive thermal stress and overheating of the rotor.
Manufacturing Defects
Manufacturing defects are another common cause of rotor faults.
Depending on the application and rated power of squirrel-cage induction motors, rotor construction may be based on:
- Cast aluminum rotors
- Fabricated copper rotors
Cast rotors are generally less expensive to manufacture. Due to the casting process, the rotor bars take the shape of the rotor core slots, allowing various bar geometries to be achieved.
For high-power motors, however, fabricated copper rotors are often preferred. Copper has lower electrical resistance and higher mechanical strength, allowing it to withstand both mechanical and thermal stresses during motor startup more effectively while also reducing operational losses.
The most common defect in cast rotors is the formation of voids or porosity at the junction between the rotor bars and the end rings after the casting process.
In fabricated rotors, the most frequent issues include:
- Poor welding between rotor bars and end rings
- Insufficient mechanical fixation of the bars within the rotor core slots
Development of Rotor Bar Faults
Rotor faults typically begin as a small crack or a localized high-resistance point in a rotor bar. This location then experiences localized overheating, which may eventually lead to complete fracture of the rotor bar.
Once a rotor bar breaks, the rotor structure becomes electromagnetically unbalanced. Because current can no longer flow through the broken bar, the current and thermal distribution within the rotor becomes uneven.
As a result, higher currents flow through the neighboring bars, subjecting them to increased thermal and mechanical stresses. This may lead to further cracking and eventually cause severe damage to the motor.
Broken rotor bars can lead to several operational symptoms, including:
- Speed oscillations
- Torque pulsations
- Changes in the frequency components of the motor supply current
- Increased temperature
- Electrical arcing within the rotor
- Increased motor vibration
Detection Using Stator Current Signal Analysis
An asymmetric rotor structure in a squirrel-cage motor induces a voltage and current in the stator windings at a frequency of f(1 − 2s) Hz.
This phenomenon can be demonstrated using equivalent circuit analysis of the motor under steady-state operation at constant speed.
Early analytical models were purely electrical in nature and therefore only predicted a current component at frequency f(1 − 2s). This component is known as the lower sideband component located at twice the slip frequency below the supply frequency.
However, these equivalent circuit models cannot accurately predict the exact amplitude of this component (in amperes) as a function of motor design parameters and the presence of one or more broken rotor bars.
In the early twentieth century, Digital Signal Processors (DSPs) had not yet been developed. As a result, it was not possible to measure the frequency and amplitude of the f(1 − 2s) current component with sufficient accuracy.
Effect of Motor Load on Broken Rotor Bar Fault Detection
Motors often operate at loads below their rated capacity, resulting in reduced slip. In practice, operators usually do not increase the load to full load solely for performing an MCSA test.
When motor load decreases:
- Slip decreases
- Rotor bar current decreases
Under no-load conditions, the rotor bar current becomes much smaller than at full load. Even if several rotor bars are broken, the 2sf sideband components may not be detectable, because the slip at no load is very close to zero and the sideband amplitudes become extremely small.
Extraction of the Current Component at f(1 − 2s)
The rotor current frequency can be calculated as:
where f₂ represents the slip frequency (Hz).
As illustrated in the figure below, asymmetry in the rotor cage caused by a broken rotor bar produces a reverse rotating magnetic field (Nsb) with a speed of sNs, rotating in the opposite direction to the forward rotating field produced by the rotor currents.
Fig.1: Forward and reverse rotating magnetic fields in the presence of a broken rotor bar
The speed of this reverse rotating magnetic field with respect to the stationary stator reference frame is:
The corresponding frequency of the magnetic field generated by broken rotor bars is:
This means that the rotor bar fault generates a rotating magnetic field at frequency f(1 − 2s). This field induces an Electro Motive Force (EMF) in the stator windings, producing a measurable current component. Consequently, the fault can be detected through spectral analysis of the motor supply current.
This frequency component (fsb) is referred to as the lower sideband located at 2sf below the fundamental supply frequency f.
Origin of the Current Component at f(1 + 2s)
The periodic variation of rotor current caused by broken rotor bars results in torque oscillations at twice the slip frequency (2sf). These torque oscillations cause small speed fluctuations in the motor.
Speed variation reduces the amplitude of the f(1 − 2s) component and generates an additional current component at f(1 + 2s).
If the mechanical system has high inertia, it resists torque and speed oscillations at 2sf, and therefore the amplitude of the upper sideband (2sf⁺) is usually smaller than that of the lower sideband (2sf⁻).
Thus, as shown in the following figure of a real motor tested by MCM1 device, broken rotor bars produce two sideband components around the supply frequency f:
Fig.2: ±2sf sidebands.
Fig.3: MCM1 in action.
Several factors influence the magnitude of the ±2sf sideband components, including:
- Variations in motor load and slip, assuming constant rotor asymmetry
- Defective bar-to-end-ring connections, which create rotor asymmetry (common in large motors)
- Porous cast-aluminum rotors and the resulting internal arcing
- Partially broken rotor bars that still maintain high-resistance contact with the end rings
- Rotor bars that are partially cracked along a portion of their depth
