Other Motors Topics
Induced Bearing Currents
Problems produced by electric currents passing through the bearings in an ac motor
(expecially motors rated in the hundreds of horsepower) has been recognized since the 1920s.
Today, however, adjustable-speed (A-S) drives and plants generating their own power
are putting a modern twist to this problem that significantly shortens bearing life.
Causes of induced shaft voltages
Magnetic imbalances and harmonics in the power line are the primary causes of induced
shaft voltages that, in turn, produce damaging bearing currents. Other causes include
improperly grounded electric arc welding and static electricity from manufacturing processes,
such as pumping or compressing applications.
Magnetic imbalances, which are produced by the motor design or its application,
are considered the primary causes of bearing currents, especially in motors in the
hundreds of horsepower. These imbalances are typically caused by some form of
nonuniform magnetic flux paths.
Harmonics in the voltage supplied to the motor are becoming more common because more
facilities are generating their own power, more motors are powered by adjustable-speed
drives, and more drives now use IGBT rather than SCR and GTO technologies in the drive
inverter section. As harmonic content in an adjustablespeed motor-drive system increases,
motors that previously had no problem with shaft currents may begin to develop rapid
bearing failures.
This shaft voltage seeks a complete circuit through its two bearings to ground,
or through its outboard bearing and the connected machinery. Unless prevented from
reaching high levels, over 0.5 V, it can cause chemical changes in the insulating grease,
breaking it down and thereby making the grease act like an electrolytic in a capacitor.
When a motor operates on sinusoidal power, a safe low level for voltage along the
length of the shaft is less than 0.1 V. If a motor operates off an adjustable-speed power,
highfrequency transient-voltage spikes can cause this voltage to measure appreciably higher.
|
