Diagnosing Electrical Faults in AC Motors

1. Introduction: Electrical Faults as a Vibration Source

While vibration analysis is typically associated with mechanical faults like unbalance and bearing defects, it is also a very powerful tool for detecting problems within AC induction motors. Electrical faults generate pulsating magnetic forces that cause the motor’s stator and rotor to vibrate. These vibrations are transmitted through the motor frame and can be detected by an accelerometer.

The key to diagnosing electrical faults is to look for specific patterns at frequencies related to the electrical line frequency (50 or 60 Hz) and the number of poles in the motor.

2. Stator Faults

Stator problems, such as loose iron, coil looseness, or shorted laminations, can cause the stator to become eccentric or distorted. This results in an uneven magnetic field.

• Vibration Signature: The primary indicator of a stator fault is a high-amplitude vibration peak at 2X the line frequency (2xFL). For a 60 Hz motor, this is 120 Hz (7200 CPM). For a 50 Hz motor, this is 100 Hz (6000 CPM).
• Characteristics: This 2xFL peak is typically very steady in amplitude and is not sensitive to the motor’s load. The vibration is often highest in the direction of the stator mounting feet.

3. Rotor Faults (Broken Rotor Bars)

Cracked or broken rotor bars are a common failure mode in AC induction motors. When a bar breaks, it disrupts the flow of current in the rotor, causing a localized heating and a pulsating torque.

• Vibration Signature: The classic sign of rotor bar problems is pole pass frequency (FP) sidebands around the running speed (1X) peak and its harmonics.
• Pole Pass Frequency (FP): This is the rate at which the rotor “slips” past the rotating magnetic field of the stator. It is calculated as: FP = Number of Poles × Slip Frequency. The slip frequency is the difference between the synchronous speed of the magnetic field and the actual running speed of the rotor.
• Characteristics: Look for a 1X peak with two clear sidebands, one at (1X + FP) and another at (1X – FP). As the rotor damage becomes more severe, you may see sidebands around the 2X and 3X harmonics as well. Unlike stator problems, this signature is highly sensitive to load. The sidebands will increase in amplitude as the motor’s load is increased and may disappear entirely under no-load conditions.

4. Eccentric Air Gap

The air gap is the small clearance between the rotor and the stator. If this gap is not uniform all the way around, it creates an unbalanced magnetic pull, forcing the rotor to vibrate.

• Static Eccentricity: The rotor is centered in the bearings, but the stator core is out of round. The narrowest point of the air gap is fixed in space.
• Dynamic Eccentricity: The rotor itself is out of round, so the narrowest point of the air gap rotates with the rotor.
• Vibration Signature: Both types of eccentricity produce pole pass frequency (FP) sidebands around the 2X line frequency (2xFL) peak. In severe cases, you may see a complex pattern of sidebands at 2xFL ± FP and also sidebands around the running speed harmonics.

5. Confirmation and Best Practices

• High-Resolution Spectrum: Diagnosing electrical faults requires a high-resolution FFT spectrum to clearly separate the running speed harmonics from the line frequency harmonics and their sidebands.
• Load is Critical: For rotor bar issues, the motor *must* be under significant load (typically >75%) for the defect to be visible.
• Confirm with Other Technologies: Electrical faults can be confirmed using other technologies like motor current analysis (MCA) or infrared thermography, which can detect the localized heating caused by broken rotor bars or shorted laminations.

← Back to Main Index