Using 3-Level Inverters to Reduce Bearing Damage

Pumps & Systems , June 2008

A look at how 3-level inverters can help reduce electrical damage to bearings.

PWM inverters introduce motor shaft voltages and bearing currents. The bearing damage in inverter-driven motors is mainly caused by the shaft voltage and bearing currents created by the common-mode voltage and its sharp edges [1]. All inverters generate common-mode voltages relative to the power source ground that cause coupling currents through the parasitic capacitances inside the motor. The main source of bearing currents is the capacitance-coupling currents that return via the motor bearings back to the ground. 

This paper describes the common-mode voltage in inverter-driven AC machines and compares them in 2-level and 3-level inverters. The relationship among common-mode voltage, motor shaft voltage and bearing currents are discussed using parasitic capacitances and its mathematical representation inside the motor. Test results of shaft voltage and bearing currents are presented to prove that 3-level technology has advantages over the 2-level inverter with regard to shaft voltage and bearing currents, which result in reduced bearing damage.

Common-Mode Voltage in Inverter-Driven Machines

Common-Mode Voltage

In a three-phase AC system, the common-mode voltage can be defined as the voltage difference between the power source ground and the neutral point of a three-phase load. If the load is an AC motor, the neutral point of the load means the stator neutral of the motor. It is important to define the common-mode voltage in mathematical terms in order to compare its characteristics among different types of source and load combinations.

In a three-phase AC system, the phase to ground voltage can be written as the sum of the voltage from phase to the neutral point of the load and the neutral point of the load to system ground. As per the definition, the common mode voltage is the voltage across the neutral point of the load and the system ground. Since in a balanced system, the sum of all three phase-to-neutral voltages is zero, the voltage from neutral to ground (common-mode voltage) can be defined in terms of phase to ground voltage as shown in Equation 1.

In Equation 1, it is assumed that the load is balanced so that the sum of all three phase-to-neutral voltages is zero (åV_a,b,c-N = 0). If the source is also assumed to be balanced and ideal, then the sum of all three phase-to-ground voltages is zero (åV_a,b,c-G = 0). Under such an ideal case, for a balanced AC motor driven by a balanced three-phase AC source, from Equation 1, the common mode voltage  V_N-G  will be zero. However, in the case of an inverter-driven AC machine, a common-mode voltage exists because the voltage source inverter does not constitute an ideal balanced source. Figure 1 shows a typical 2-level voltage source inverter-fed AC machine.

In an inverter-driven system, the common mode voltage  (V_com or V _N-G) can also be defined as the voltage across the stator neutral (N) and the DC bus midpoint (M) because from a high-frequency viewpoint, the DC bus midpoint (M) is same as the electrical ground (G) of the system. Using this definition, the common-mode voltage can be redefined as in Equation 2. This definition would then be valid for 3-level inverters as well.

In Equation 2, it should be noted that the source voltage nomenclature has been changed from  V_a,b,c-G  to  V_u,v,w-M  to reflect the source as the voltage source inverter.

The common mode current  (i_com)  is defined as the instantaneous sum-total of all the currents flowing through the output conductors. Stray capacitances of the motor cable and inside the motor are the paths of this current, and a source of EMI noise problems.

2-level Inverter

2-level voltage source inverters have eight different switching states for the six inverter-switches, and the voltages across the output terminals and the DC bus mid-point  (V_U-M, V _V-M and V _W-M)  can be either +E/2 or -E/2 according to the inverter switching states. The three output legs could 1) all be connected to the positive or negative rail of the DC bus; 2) two legs can be connected to the positive rail and one leg to the negative rail or vice versa. Given these constraints and Equation 2, the inverter output neutral with respect to the DC bus midpoint will have a voltage of ±E/2 for condition 1) and a voltage of ±E/6 for condition 2). Figure 2 shows an example of the switching states and the common-mode voltage waveform.

During a PWM cycle, the change in voltage from -E/2 to -E/6 constitutes a change of E/3. When the level changes from -E/6 to +E/6, the change in voltage is again E/3. Since this change in voltage is proportional to the DC bus voltage and has a frequency equal to the inverter carrier frequency, the change in the common-mode voltage level is steep and typically occurs in hundreds of nanoseconds.

Figure 2. 2-level inverter switching states and the corresponding common-mode voltage

Since the motor windings are fed from PWM pulses having fast rising and falling common mode voltage edges, there exists a leakage current from each phase to ground due to the existence of various parasitic capacitances that include cable capacitance formed between the power leads and ground and other capacitances between the stator winding to the grounded frame. This leakage current that flows only during the step change in the common mode voltage is called common mode current.

Motor Shaft Voltage and Common-Mode Voltage

Parasitic Capacitances Inside the Motor

Figure 3 shows the various parasitic capacitances in an AC motor that become relevant when the motor is driven by a PWM voltage source inverter. The high dv/dt of the common mode voltage applied across the stator and grounded frame of the motor causes pulsed currents to flow through the parasitic capacitances shown in Figure 3. The parasitic capacitances shown are:

• Stator to Frame Capacitance ( C_SF): The primary