MCSA analyses an induction motor's supply current to diagnose faults β most famously broken rotor bars β without stopping or touching it. The current is clamped at the MCC, so it's safe, cheap and non-intrusive.
A broken rotor bar disturbs the rotor field once per slip cycle, putting sidebands around the supply frequency at f(1 Β± 2s), where s is the slip. The amplitude of those sidebands, in dB below the supply peak, grades the damage.
It needs load to work (no load β almost no slip β the sidebands collapse onto the main peak and hide). Survey loaded.
1 · The motor as a sensor
An induction motor turns because its rotor "slips" slightly behind the rotating stator field β that slip is what induces rotor currents and makes torque. Anything that disturbs the rotor's magnetic symmetry (a cracked bar, an off-centre rotor, a fluctuating load) modulates the stator current the motor draws from the supply. So the current itself carries a record of the rotor's health. MCSA reads that record from a current transformer at the motor control centre β nothing on the machine, nothing shut down.
It complements the others: vibration sees the mechanical structure, thermography sees heat, but rotor-bar and electrical-rotor faults are often clearest β sometimes only clear β in the current.
2 · Slip and the rotor-bar sidebands
The synchronous field turns at the supply frequency; the rotor turns a little slower. The fractional difference is the slip:
A broken bar interrupts the rotor current path, creating an asymmetry that pulses at twice the slip frequency. In the stator current spectrum that appears as a pair of sidebands straddling the supply line frequency:
The supply peak is enormous, so we read the sidebands in decibels below it. The further down they are, the healthier the rotor:
| Lower-sideband level (dB below line) | Rotor condition |
|---|---|
| > 54 dB down | Good β healthy rotor cage |
| 48–54 dB | Rotor deterioration β high-resistance joints / a cracking bar; monitor |
| 42–48 dB | Likely one broken bar β plan repair |
| < 42 dB | Multiple broken bars β serious; risk of cascade and rotor damage |
The analyzer below builds the current spectrum around the line frequency. Add load to spread the sidebands, then add broken bars to watch them rise toward the danger zone.
Interactive — Current spectrum analyzer
Live modelStator current spectrum (around the line frequency)
s β 0.005 + 0.025Β·load, sidebands at f(1Β±2s), level β β58 dB rising with rotor damage. Real MCSA resolves these with high-resolution FFT, accounts for pole pairs and load, and confirms with a stop test β but the sideband location and severity logic are exactly this.3 · Beyond rotor bars
- Air-gap eccentricity (static or dynamic) β the rotor not centred in the bore; shows as sidebands around the line frequency spaced at the rotational and slip frequencies.
- Bearing faults β bearing defect frequencies modulate the current too, though vibration usually sees these earlier.
- Load & coupling problems β anything that cyclically varies the load (a bad gear, a reciprocating load, misalignment) imprints on the current.
- Stator faults β winding asymmetries and turn-to-turn issues (often with current/voltage analysis, ESA).
Why it earns its place: a broken rotor bar can be nearly invisible to vibration yet obvious in the current β and the measurement is taken at the MCC with the cabinet closed. For large or inaccessible motors, MCSA reaches a fault class the other techniques can miss, which is why critical-motor programmes combine it with vibration and thermography.
Key takeaways
- The current is the sensor β non-intrusive analysis from the MCC, nothing on the machine.
- Broken bars create sidebands at f(1 Β± 2s); their dB level below the line peak grades the damage.
- Load is mandatory β no load, no slip, no sidebands.
- It catches rotor and eccentricity faults that vibration can miss β best used alongside the other techniques.