Electric motors are one of the most commonly used pieces of equipment in industry; they drive everything, ranging from pumps, and fans, to conveyor belts and compressors. The motors come in various shapes and sizes, but their underlying design is similar. A basic electric motor has a magnetized stator that surrounds a rotor; the magnetic interaction between the two causes the rotor to spin.
Faults in motors can develop due to resonances, imbalance, misalignment and foundation problems or improper mounting. These faults can lead to bearing failures and cause vibration.
Some of the electrical and mechanical faults unique to motors include:
- Eccentric or loose rotors or stators
- Broken rotor bars
- Bowed rotors
- Uneven air gap between the rotor and stator
- Loose laminations
- Electrical discharges between the various electrical components
When analyzing vibration data for motors, it is typical to observe a vibration component at twice the line frequency (at 100/120Hz). The line frequency (50/60Hz) is the frequency at which AC power is supplied to the motor; this causes changes in magnetic attraction between the rotor and stator at twice the line frequency. The varying magnetic forces cause small dimensional changes in the iron material, leading to vibration.
In 2-pole motors, it may be difficult to discern differences between the 2X frequency and twice the line frequency(100/120Hz) and it is important to use a high enough resolution when collecting data. One method to verify the existence of a peak in the vibration at the 2X frequency is to take measurements while the motor is running, and then cutting off power; any peak at twice the line frequency should disappear once the power is turned off, leaving the 2X frequency if it exists.
Frequencies that may show up on the spectral data include:
- Slip frequency - the difference between the motor’s running speed and the synchronous speed; it is typically small in value and will be present as a sub-synchronous frequency.
- Pole-pass frequency - it is equal to the number of poles times the slip frequency.
- Sidebands spaced at intervals equivalent to the to the pole-pass frequency.
- Running speed:
- 3600 or 3000 RPM for a 2-pole motor
- 1800 or 1500 RPM for a 4-pole motor
- 1200 or 1000 RPM for a 6-pole motor
Eccentrically positioned rotors cause a variable air gab between the rotor and stator which leads to a pulsing vibration. Once again, a vibration component at twice the line frequency will be present, however it will have sidebands spaced at intervals equivalent to the pole-pass frequency; the sidebands will also surround the 1X frequency.