Broken rotor bar detection with motor current signature analysis

Why rotor bars show up in the current

The rotor of a squirrel-cage induction motor carries current in bars joined by end rings. When a bar cracks or breaks, or a bar-to-ring joint degrades, the rotor's magnetic field becomes asymmetric. That asymmetry modulates the stator current at a rate set by the motor's slip, which puts small sidebands on each side of the supply frequency.

Motor current signature analysis (MCSA) measures one phase of stator current, usually with a clamp-on current transformer at the motor control centre, and looks for those sidebands in a high-resolution spectrum. The motor doesn't have to be stopped or reached physically.

The sideband frequencies

s      = (ns − n) / ns
f_sb   = f · (1 ± 2 k s),   k = 1, 2, 3 …

where f is the supply frequency, n_s the synchronous speed (120 · f / number of poles, in rpm), n the actual running speed and s the slip. The pair at k = 1, f · (1 − 2s) and f · (1 + 2s), is the one used for grading. The spacing between each sideband and the supply frequency, 2 · s · f, is the pole pass frequency.

Worked example

A 4-pole motor on a 50 Hz supply has a synchronous speed of 1500 rpm. Running at 1470 rpm under load, its slip is (1500 − 1470) / 1500 = 0.02. The rotor bar sidebands sit at 50 · (1 − 0.04) = 48 Hz and 50 · (1 + 0.04) = 52 Hz, 2 Hz either side of the supply peak.

Because the sidebands are close to a peak that is far larger, the spectrum needs fine resolution, typically 0.05 Hz or better, which means long acquisitions of several tens of seconds at steady load.

Grading severity

Severity is read from the difference in dB between the supply frequency peak and the larger of the two k = 1 sidebands. A widely used rule of thumb:

Thresholds vary with motor design, size and load. Trending the same motor at the same load is more reliable than any single reading against a table.

What makes MCSA readings misleading

• Low load. At light load the slip is small, the sidebands move close to the supply peak and lose amplitude. Readings near full load are the most trustworthy.
• Load oscillation. Reciprocating compressors, crushers or gearboxes with a slow-turning stage can modulate the current at frequencies that sit near the rotor bar sidebands.
• Supply and drive effects. Variable frequency drives and supply disturbances add their own components; the supply frequency must be measured, not assumed.
• Rotor design. Some rotors, for example with skewed or double-cage bars, show the effect differently, so a baseline from the same motor helps.

Combining current and vibration

Rotor bar problems also appear in vibration as sidebands at the pole pass frequency around 1 × running speed and its harmonics, often with a beat in the time waveform. Seeing the same fault in both current and vibration raises confidence and helps separate rotor faults from mechanical causes such as misalignment or a load oscillation.

How Misoki uses this

Misoki Prism AI reads motor current alongside vibration, ultrasound and process data, and electrical faults such as broken rotor bars are among the faults it detects. Findings that matter are checked by a vibration analyst through specialist review before they reach your team.