In a squirrel-cage induction motor, current flows through a set of conductive bars embedded in the rotor. When one of those bars cracks or breaks, the fault barely shows up in vibration, but it leaves a distinct fingerprint in the motor's electrical current. That fingerprint is a pair of sidebands around the supply frequency, spaced at a small offset tied to the motor's slip. Reading this current signature is how broken rotor bars are detected, and it catches an electrical fault that vibration sensors alone can easily miss.
Broken rotor bar in one line: A broken rotor bar signature is the pattern that a cracked or broken conductor bar in an induction motor's rotor leaves in the stator current spectrum: sidebands on either side of the line frequency, spaced away from it by twice the slip frequency. The amplitude of these sidebands relative to the line-frequency peak indicates the severity of the rotor bar damage.
A squirrel-cage rotor carries current in a cage of bars shorted together at each end. Under normal operation, current distributes evenly among the bars and the rotor's magnetic field is smooth as it turns. When a bar cracks or breaks, current cannot flow through it, so the current redistributes among the remaining bars and the rotor's field develops a periodic asymmetry that rotates with the rotor. That asymmetry reacts back on the stator windings and modulates the stator current, imprinting the fault onto the electrical signal the motor draws from the supply.
The technique that reads this is Motor Current Signature Analysis, or MCSA, which takes a spectrum of the stator current rather than of vibration. In that current spectrum the dominant feature is a large peak at the supply line frequency. A broken rotor bar adds two smaller peaks, sidebands, one just below and one just above the line frequency. These sidebands are the signature; their presence indicates the rotor asymmetry that a broken or cracked bar produces, and their absence is a sign the cage is intact.
A practical strength of MCSA is that it uses the motor's own current, measured at the supply, so it does not require a sensor mounted on the machine itself. The current can be picked up at the motor control center, which means motors in awkward or hazardous locations can be assessed from a safe, accessible point. The measurement must be taken under load, though, because the fault couples into the current through slip, and slip is only meaningful when the motor is doing work.
The spacing of the sidebands is what confirms they come from a broken bar rather than something else. The sidebands sit away from the line frequency by twice the slip frequency. Slip is the difference between the synchronous speed of the rotating magnetic field and the actual rotor speed, the small lag that lets an induction motor produce torque, and it grows with load. The offset of twice slip means the sideband spacing is small at light load and widens as the motor is loaded more heavily. Because that spacing is tied directly to slip, matching the sidebands to twice the slip frequency is how an analyst verifies the signature is genuinely a rotor bar fault.
This slip dependence has two consequences for measurement. First, the motor must be loaded, because at no load the slip approaches zero and the sidebands collapse into the line-frequency peak where they cannot be resolved. Testing under a reasonable load separates the sidebands enough to see them clearly. Second, because the spacing depends on the actual slip at the time of measurement, the analysis needs the operating speed or load to place the expected sideband locations correctly, otherwise the small sidebands can be missed or misidentified.
Severity is read from the amplitude of the lower sideband relative to the line-frequency peak. A healthy cage shows sidebands that are very far down from the line frequency or effectively absent; as bars crack and break, the sidebands rise. The larger the sideband amplitude relative to the fundamental, the more severe the rotor cage damage, and a growing sideband over successive tests indicates the fault is progressing, often from one cracked bar toward multiple broken bars. This makes the signature not just a yes-or-no fault detector but a way to gauge how far the damage has advanced.
The reason the current signature matters is that a broken rotor bar is an electrical fault that produces little mechanical vibration until it is advanced. A vibration program watching bearings and alignment can easily overlook a cracked bar, because the rotor keeps turning and the imbalance it creates is subtle. The current spectrum, by contrast, is directly sensitive to the rotor's electrical asymmetry, so MCSA detects the fault while it is still confined to the cage. This complementarity is the whole argument for adding current analysis alongside vibration: each catches faults the other tends to miss.
Because the measurement is electrical and taken at the motor control center, it fits naturally into a SCADA-based monitoring picture. Merobix can bring motor electrical data and the derived current-signature indicators together with the rest of a site's operating parameters, so a rising rotor-bar sideband on a critical pump or compressor motor becomes visible to operators who may be nowhere near the machine. For the many motors that run remote and unmanned across oil and gas operations, being able to flag a developing rotor fault from the electrical signature, without a technician physically present, extends predictive maintenance to a failure mode that vibration coverage alone leaves exposed.
As with other predictive signatures, the value compounds when the sideband indicator is trended over time. A single reading confirms whether bars are broken now; a series of readings shows whether the fault is stable or worsening, which drives the decision of whether to run the motor to the next outage or intervene sooner. Historized current-signature trends, correlated with the motor's load, let the platform account for the slip dependence and raise a confident alarm when the sidebands genuinely grow. Combined with vibration and temperature history on the same motor, the current signature completes a picture that spans both the mechanical and the electrical health of the machine.
It appears as sidebands, two smaller peaks on either side of the line-frequency peak in the stator current spectrum, measured using Motor Current Signature Analysis. These sidebands are spaced away from the line frequency by twice the slip frequency. Their presence indicates the rotor asymmetry that a cracked or broken cage bar creates, and their amplitude relative to the line frequency indicates the severity.
The fault couples into the current through slip, the difference between the rotating field speed and the actual rotor speed, which grows with load. The sidebands are spaced at twice the slip frequency, so at no load the slip approaches zero and the sidebands collapse into the line-frequency peak where they cannot be seen. Testing under a reasonable load widens the spacing enough to resolve the sidebands clearly.
A broken rotor bar is primarily an electrical fault, and until it is advanced it produces little mechanical vibration, since the rotor keeps turning and the resulting imbalance is subtle. A vibration program focused on bearings and alignment can overlook it. The current spectrum is directly sensitive to the rotor's electrical asymmetry, so motor current signature analysis detects the fault early, complementing vibration monitoring rather than duplicating it.
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