IGBT inverters raise risk of motor bearing failure

Marek Lukaszcyk outlines how the traditional problem of motor bearing failures due to electrical discharge has taken on new impetus with the widening use of PWM IGBT inverters.

Bearing failures in electric motors due to shaft-induced voltage (SIV) were first studied and understood in the early years of the 20th century.

Extremely damaging in its consequences, SIV is an effect that generates an electrical current, which then flows (discharges) through motor bearings, deteriorating the lubricant and electro-eroding the bearing raceways, leading to eventual bearing failure.

In the 50 years following the identification of SIV, technological advances in machinery and materials were made, and vast improvements in the tolerances and quality of electric motors achieved, all of which combined to reduce its amplitude to a safe level.

Then came the 1960s and the first electronic devices started to appear.

Cycloconvertors and frequency convertors built with BJTs (bipolar junction transistor), SCRs (silicon controlled rectifier) or GTOs (gate turn-off thyristors) all began to be marketed.

These devices could only be switched at very low frequencies, usually up to 600Hz, and so did not greatly affect the improved situation with regard to SIV.

However, in the 1990s, the IGBT (insulated gate bipolar transistor) came onto the scene.

These represented a huge improvement in drive technology, increasing the switching frequency up to 20kHz, reducing harmonics and audible noise.

Recently though, it has become apparent that these improvements have been bought at a price: IBGT technology has resurrected bearing problems due to electrical discharge, creating a new challenge to manufacturers of electric motors.

The new problems arose because PWM inverters equipped with IGBT inverters distort the sinusoidal supply generating high frequency harmonics and high dv/dts.

The inverter switching mechanism also creates what is called common-mode voltage.

Due to the high switching frequencies of IGBT inverters, parasitic capacitances between stator winding and stator, and between rotor and stator winding become relevant.

These capacitances result from the common mode voltage and lead to a common mode current flowing through the motor bearings.

They are called on to handle two types of bearing currents that have been identified.

The first these, conductive-mode bearing current, is discharged continuously during a period of time when bearings exhibit good conductivity.

In contrast, the second type, discharge-mode bearing current, is discharged in discrete time intervals.

The former prevails at lower speeds, because the good electrical contact between the rolling elements and bearing raceways connects the rotor to ground through the outer bearing race, whereas the latter is more significant for higher inverter output frequencies, as the electrical conductivity of the bearing decreases, enabling the capacitive voltage to build up till it is able to break down the dielectric resistance of the grease.

Although both types of currents are present at the same time, it can be said that the discharge bearing current is the more critical.

The conductive bearing current is usually less harmful to bearings, as it is a low-amplitude current that flows continuously without arcing.

However, it increases bearing temperature, accelerating grease deterioration and reducing bearing life.

On the other hand, the high energy level of the discharge bearing current works like an electro-erosion machine, resulting in bearing pits or flutes.

The amplitude of bearing currents depends on operating conditions such as speed, temperature, lubrication type, motor size etc.

From all these factors, motor size is probably the most significant, as the larger the motor the larger its parasitic capacitances.

Motor design can also have reasonable influence over bearing current amplitudes.

Manufacturers offer a number of options as a means of overcoming the damage to bearings caused by electrical discharge.

The most obvious of these is insulated bearings, which are used where it is desirable to achieve perfect insulation of the bearing from its application environment.

However, the method increasingly used to achieve insulation is ceramic coating, which is very expensive, typically adding anything from GBP 600 to 700 to the price of motors with frame sizes in the range from 315 to 355.

Another option for applications where some passage of current can be accommodated is a shaft grounding brush.

A much less costly option than insulated bearings, the shaft brush reduces stray current through the motor bearings by half, as a result of short-circuiting the path between rotor and stator.

By employing a shaft brush it is possible to keep voltages below the so-called "fritting voltage" which is responsible for the development of bearing defects due to electric current discharge.

Although damage cannot be completely prevented by employing this measure, the extent of damage can be kept within such limits that the life expectancy of the bearing is not affected.

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