Using 3-Level Inverters to Reduce Bearing Damage - Pump Systems Problems, Pump Repair, Pump Installation, Pump Help & Resources

Using 3-Level Inverters to Reduce Bearing Damage

Written by Ron Koehler and Rafi Wilkinson, Yaskawa Electric America

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3) Common Mode Current Flow Through Shaft Due to Poor Grounding (i₃): If the motor frame is poorly grounded and the motor shaft is connected to a mechanical load with much lower ground impedance, the common mode current that flows at every edge of the common mode voltage through the capacitor C_SRand charges up the rotor structure now finds a way to flow through the shaft into an external ground that has a lower impedance. This is exactly what happens when an external grounding brush kit is used to ground the rotating shaft. The current bypasses the bearing and makes its way safely into a lower impedance ground through the shaft or the load structure connected to the shaft.

4) Circulating Bearing Currents (i₄): The shaft voltage, due to asymmetry in the magnetic field from one end of the rotor to the other end of the rotor, is prevalent in long axial machines. This asymmetry induces a shaft voltage across the length of the rotor and is basically an electromagnetic induction phenomenon opposed to the capacitive coupled phenomenon discussed above. This phenomenon is observed only in long axial machines that are used for large horsepower applications typically greater than 110-kW [2]. Yet another distinction is that the induced voltage is of very low frequency and depends on the fundamental excitation of the motor. The circulating current flows along the axis of the rotor, through the bearings, circulates through the stator frame and returns back from the other bearing end. This current is generally not significant in small power AC machines less than 110kW [3].

Bearing Current Reduction

Here are some approaches that prevent bearing current damage of AC machines:

External Passive/Active Common-Mode Filters: Common-mode noise filters are a good solution to cancel common-mode noise of the system, but typical common-mode noise filters consist of magnetically coupled three-phase inductor and capacitor components. These filters are bulky and expensive; in addition, filters reduce efficiency, and can cause voltage oscillation if parameters of the passive components are not tuned properly.
Motor Shaft Ground Brushes or Insulated Bearings: The grounding of the motor shaft by connecting a brush between the motor shaft and the motor frame is an effective way to shunt the current path that normally would flow through the motor bearing. However, regular maintenance is required due to limited lifetime of the brush. Insulated bearings such as ceramic bearings can also prevent bearing current problems, but require the replacement of the existing bearings in the motor.
Multi-Level Inverter Technologies: Reducing the amplitude and voltage transition step of the common-mode voltage can reduce bearing currents. One way of achieving this is to use a multi-level inverter topology. Progress in inverter technology has made it possible to introduce a 3-level inverter to the general purpose-inverter market [1]. The advantages of a 3-level inverter are discussed in the next section.

Features and Advantages of the 3-Level Inverter

General Features

Figure 6 shows a typical neutral-point clamped 3-level inverter. In order to determine the common-mode voltage in a three-level inverter, it is important to understand the various switching combinations in a 3-level inverter. In contrast to a 2-level inverter, a 3-level inverter has four switches (IGBTs) per phase, totaling twelve switches (IGBTs) for all three phases. According to the switching signals, each output phase voltage with respect to the DC bus midpoint can have three distinct levels, i.e., E/2, 0, and -E/2. Consequently, this arrangement is called a 3-level inverter.

Figure 7 shows various switching states and common-mode voltage waveforms among 27 different switching states of the 3-level inverter. By comparing the common-mode voltage of a 3-level inverter to that of a 2-level inverter as shown in Figure 2, it is clear that in a 2-level inverter the difference in voltage level from one state to the other is always ±E/3.

In the case of a 3-level inverter, the voltage level is generally ±E/6; this means that the transition level of the common-mode voltage in a 3-level inverter is typically one-half that of the 2-level inverter. In a 3-level inverter, the amplitude of the common-mode voltage can be lower than a 2-level inverter in the high voltage region. In fact, the maximum and minimum values of the common-mode voltage in a 3-level inverter at higher voltage (i.e. at higher speed) reaches only ±E/3 as shown in Figure 7(b), while the common-mode voltage reaches ±E/2 in the case of a traditional 2-level inverter as shown in Figure 2. The lower transition level of the 3-level inverter also results in a lower common-mode current compared to the 2-level inverter, an important advantage of the 3-level inverter over the traditional 2-level inverter.

Reduced Bearing Current and Increased Bearing Life with the G7 3-Level Inverter

The steep voltage transient in the shaft voltage causes current to flow through the bearing insulation, which leads to the breakdown of the bearing grease insulation and discharge of the shaft voltage. Since the change of the common-mode voltage is smaller in the 3-level inverter, this provides a significant advantage over the 2-level inverter with regard to shaft voltage and bearing currents. Figure 8 shows the comparative test results of the shaft voltage and bearing current for the 2-level and 3-level inverters. In these tests, insulation material was inserted in between the bearing and the housing so that the current through the bearing could be observed. Figure 8 shows that the bearing current of the 3-level inverter in Figure 8(b) is significantly smaller than a 2-level inverter in Figure 8(a).

Actual longevity tests were conducted to verify the superiority of the 3-level inverter. The tests simulated extreme conditions including temperature, types of grease and motor speed. The results of the bearing life test are shown in Figure 9. Note that during normal operation the normal bearing life would be longer than that shown here. Figure 9 clearly proves that the use of a drive with a 3-level inverter topology can yield a significantly longer bearing life.

References

H.P. Krug, T. Kume, M. Swamy, "Neutral-point clamped three-level general purpose inverter - features, benefits and applications," IEEE Power Electronics Specialists Conference, pp. 323 - 328, 2004.
J. Erdman, R. Kerkman, D. Schlegel, and G. Skibinski, "Effect of PWM inverters in AC Motor Bearing Currents and Shaft Voltages," IEEE APEC Conference, Dallas, TX, 1995, CD-ROM.
A. Muetze, A. Binder, "Experimental evaluation of mitigation techniques for bearing currents in inverter-supplied drive-systems - investigations on induction motors up to 500 kW," IEEE International Electric Machines and Drives Conference, pp.1859 - 1865, vol.3, 2003.

USE OF TECHNICAL INFORMATION

Technical content and illustrations are provided as technical advice to augment the information in manual, not supercede it. The information described in this document is subject to change without notice. Yaskawa assumes no responsibility for errors or omissions or damages resulting from the use of the information contained in any technical document. All warnings, cautions and product instruction for product use must be followed. Qualified personnel should carry out installation, operation and maintenance.

Ron Koehler is the director, next generation product and Rafi Wilkinson is the senior product marketing manager for Yaskawa Electric America, Phone: 262-754-2220, Fax: 847-785-2730, http://www.yaskawa.com/.

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