Bearing Currents, Consequences and Precautions to be Taken

Significant changes in energy savings have occurred with the use of frequency converters for the control of induction motors. In the simplest term, both process and energy consumption were optimized by providing control of torque and speed. Energy savings up to 50% have been provided because they provide the opportunity to use as per demand. Thanks to these savings rates, redemption period of frequency converters, which are relatively high in investment costs, has generally been below 1 year. However, with the use of frequency converters, problems that were not previously encountered in induction motors started to emerge. After extensive research and tests, the problem could be identified. Bearing currents… Bearing currents caused serious problems even for a new motor in just a few months and this situation forced the motor manufacturers to take some precautions.

The reasons of bearing currents should be investigated in order to be able to select the most efficient way to prevent bearing currents. There are 120° difference between the phases of 3 phase induction motors designed according to 50 or 60 Hz sinusoidal waveform. In this way, the vector sum of the 3 phases is zero and no voltage or current occurs at the zero point (can be called as star or common mode point as well). However, the voltage at zero point in induction motors fed by frequency converter is different than zero. Phase voltages are composed of thousands of square waves for each phase. The voltage is either zero, positive or negative, and this change occurs at the same rate as the switching frequency of the frequency converter. The figure below represents the formation of the common mode voltage.

Voltage pulses supplying the motor cause capacitively coupled common mode voltage on the motor shaft. This voltage induced on the shaft tries to find a way to flow into the ground. The shortest way for motors is bearings. When shaft voltage exceeds the dielectric withstand voltage of the grease in the bearing, it is discharged via this path. This causes damage on the bearing as shown in the figure below during discharge.

In addition to the capacitive (EDM-electrical discharge machining) currents discharged through the bearing, the circulation currents caused by the imbalance of the magnetic flux also cause serious damage to the motors. The circulating currents of 75 kW and above follow a path from the motor shaft to the motor frame over the bearing and then to the motor shaft again via the other bearing. The paths for both type of currents and the power values that they appear at are given at the figure below .

An extremely low resistance path alternative to the ground through the shaft can prevent capacitive EDM currents. Usually a grounding ring with a microfiber brushes or coal type grounding brush work for these types of applications. However, different precuations should be taken to prevent circulating currents. In most cases, only the rear (non drive end-NDE) bearing is isolated to break current path and prevent circulating currents. However, the system should be examined well and it should be ensured that the circulating currents do not find a way over the load connected to the motor. The figure below represents an example of these kind of systems.

If we examine the system given in Figure 4 (a); isolating bearing A to break current path will suffice to prevent circulating currents. If bearing B is isolated alone, the circulating currents can complete the circuit through bearings A and C. The coupling between the motor and the load must be isolated to prevent this.

If we examine the system given in Figure 4 (b); in double-shaft systems, isolating bearing(s) A and/or B is sufficient in most cases, but at least one of the couplings must be isolated as well.

Let’s consider a system such as that shown in Figure 4 (a) with a motor of 75kW or above and evaluate the optimum solution. An isolated bearing A and a grounding brush on the load side (drive end-DE) bearing will prevent both capacitive EDM and circulating currents. However, in such systems, the bellhousing used to protect the coupling side may cause difficulties in the coal exchange of the grounding brush on the B side. To avoid this, this problem can be solved by using the earthing brush on the A side and the isolated bearing on the B side. Capacitive EDM currents are also prevented in this way, but the circulating currents continue to flow through the bearings A and C. The coupling must be isolated to prevent this. And thus, the whole system is completely protected from the bearing currents.

REFERENCES

[1]. ABB- Technical guide No. 5- Bearing currents in modern AC drive systems

[2]. AEGIS Presentation- Preventing Bearing Failure in VFD Driven Motors

[3]. Gül?en, G., Frekans Konvert?rlü ?ndüksiyon Motorlar?n Harmonik Analizi ve Optimum Filtre Tasar?m?n?n Belirlenmesi, Yüksek Lisans Tezi, Mersin üniversitesi Fen Bilimleri Enstitüsü, Mersin, 2018.