Vibration analysis for electric motor

The fault detection and diagnosis is very important for the safety of the system. Like any other equipment group, electric motors can have a limited number of issues. Some issues arise, however, only in electric motors.

It is necessary to further divide electro-mechanical issues in electric motors into induction and synchronous engines. In induction motors, the following electro-mechanical problems occur mainly:

1. Motor out-of-magnetic center

2. Broken bars in the rotor

3. Turn-to-turn shorts in the stator windings

4. Vibration at line frequency, 50 or 60 cycles

5. Siren effect

6. Overloading

Vibration causes:

Motor base vibration: The standard vibration near the base should be 30% of the rated vibration at motor bearing.

Bearing friction: Bearing vibration presents in all type of rotating systems. Antifriction bearing are important for motor operation. When friction in bearing increases the vibration at bearing increase.

Broken rotor bar vibration: If the rotor bar broken. This will create magnetic unbalance between the two opposite side of the motor, side with broken bar and side with unbroken bar. This will causes a twice line frequency vibration.

Motor unbalance: Induction motor must be balanced to have easy, quiet operation, this unbalance influence the whole operation of the motor.

Vibration Analysis:

Induction motors are intended for fixed speed operation. For example, the most common speeds are 900,1200,1800 and 3600 RPM. These engines, however, rarely work at synchronous speed.

Generally, they operate below the synchronous speed. The difference between the synchronous speed and operating speed is called the slip frequency.

The stator windings are called poles. The number of poles determines the speed of the motor. Frequency is equal to events times speed.

1.Motors Out-of-Magnetic Center:

This situation usually happens when the rotor in the axial or lateral direction is not placed in the stator’s magnetic core. If there is an irregular air gap between the rotor and the stator, an out - of-magnetic center situation may happen in the radial direction.

2.Broken rotor bar

A broken rotor bar, sometimes referred to as open iron, produces a rotor dead place. The resulting electrical imbalance at one and two times RPM can produce important vibration rates. Because this issue is not interactive with line frequency, there may not be a spectral line at 60 Hz. Data must be processed with sufficient resolution to be diagnosed accurately

3. Turn-To-Turn Shorts In Windings:

Windings of the motor stator are often referred to as poles. When some cables are shortened together in the winding / pole, the engine often slows down. Slip frequency improves when the engine speed is lowered.

The figure above contains the data from an 1800 RPM motor that has turn-toturn shorts in the windings. Motor speed is 29 Hz x 60 = 1740 RPM, and the sidebands are 4 Hz apart (delta F = 4 Hz). These sidebands are still equal to slip frequency x number of poles, i.e. 1 Hz X 4 = 4 Hz.