Bram Corne
Technical Services Committee Member
Orbits
Kortrijk, Belgium 

Introduction
In modern industry, electric motors driven by variable frequency drives (VFDs) have become the standard for energy efficiency and process control. These drives rely on pulse width modulation (PWM) to regulate motor speed and torque, aiming on current signals that approximate a sine wave. While highly effective for control, PWM introduces high-frequency transients that can compromise motor reliability if left unchecked.

As inverter technology has evolved, the switching speeds of insulated gate bipolar transistors (IGBTs) have increased significantly. This results in voltage pulses with fast rising slope, often several kilovolts per microsecond, commonly referred to as dV/dt (the rate of change of voltage over time, as illustrated in Figure 1). Combined with the capacitive and inductive characteristics of cables and motors, these pulses generate stray or leaking currents that may cause insulation degradation, bearing failures and electromagnetic interference.

The Risks of High-Frequency PWM Effects
One of the primary impacts of high-frequency switching is winding insulation degradation. The steep voltage transitions produce intense electric fields concentrated in the first few turns of the motor winding, where insulation stress is highest. Over time, this leads to partial discharges and ultimately insulation failure, especially in motors not rated for inverter-fed operation. In contrast, inverter-duty motors are equipped with enhanced insulation systems, often including reinforced slot liners and improved winding materials, designed to withstand steep voltage gradients and the repetitive transients typical of modern VFDs. These construction features significantly increase resistance to insulation breakdown under high dV/dt conditions.

Another critical issue is bearing damage. Common mode voltages between motor phases and ground create parasitic currents through the motor’s internal capacitance. These currents often discharge through the bearings. Though each event is small, the cumulative effect leads to fluting, pitting, grease degradation and eventually early bearing failure.

Additionally, reflected wave phenomena add to overvoltage issues. Fast-rising voltage fronts reflect at the impedance mismatch between cables and motor windings, resulting in voltage peaks that can exceed twice the DC bus voltage. With long cable lengths, these peaks can easily surpass insulation ratings.

Filtering as a Solution
To address these challenges, engineers apply filters, passive electrical networks composed of inductors (L), capacitors (C) and resistors (R), to reduce harmful high-frequency components in voltage and current waveforms. These filters, carefully dimensioned and placed, allow the desired low-frequency signals to pass while attenuating unwanted high-frequency content.

The most basic filters are low-pass networks. Inductors resist rapid changes in current, and capacitors resist rapid changes in voltage. When combined, they form RL, RC or RLC circuits that block or absorb transients. A first-order LR or RC filter offers moderate attenuation, while second-order LRC filters provide a steeper roll-off and better transient response. The ratio between L, R and C determines both the filter’s cut-off frequency and damping behavior.

Figure 2 illustrates the frequency and step responses of RL and RC (first order) and RLC (second order) filters. These highlight how properly selected filter parameters directly influence the rise time of the voltage waveform. 

Dimensioning these filters requires balancing attenuation performance against practical constraints. Higher inductance improves filtering but increases voltage drop and physical size. Capacitance offers additional smoothing but must be carefully selected to avoid resonance. Resistance can dampen oscillations but introduces thermal losses. The goal is to reduce dV/dt below critical thresholds—often under 1 kV/µs (IEC 60034-17).

Filter selection
Understanding the distinction between differential mode and common mode interference is critical for selecting and designing effective filtering. Differential mode disturbances arise from voltage differences between any two phases and travel along line-to-line paths, directly stressing motor windings. To suppress these, filters are installed in series with each motor phase. The function is primarily to reduce the slope of the voltage waveform, extending the rise time and minimizing peak voltages.

 Load reactors are among the most commonly used differential mode filters. These three-phase inductors are installed at the output of the drive. They smooth out switching transitions and absorb part of the energy carried by steep voltage edges. Their simplicity, low cost and ease of installation make them a practical choice for applications with short to moderate cable lengths.

Another differential mode solution involves the use of ferrite cores placed around individual motor leads. These high-frequency suppressors are particularly effective at absorbing very fast switching spikes. By increasing the impedance for high-frequency components, the ferrite cores reduce local voltage overshoot at motor terminals. However, they are ineffective against common mode currents.

Ferrite cores wrapped around all three phases of the motor cable, including the shielding, function as common mode chokes. These components present a high impedance to any current that is the same in all conductors, effectively blocking common mode disturbances while leaving differential mode currents largely unaffected. 

Among the more advanced filtering solutions are dV/dt filters and sine wave filters. The dV/dt filters are designed to limit the voltage rise time and overshoot at motor terminals. These second-order low-pass filters are more selective than load reactors and provide improved attenuation of steep voltage transients. They are effective in applications where cable lengths are moderate and where inverter-rated motors are used. Although they do not eliminate high-frequency components, they significantly reduce insulation stress and improve electromagnetic compatibility.

Sine wave filters go a step further by reshaping the PWM waveform into a nearly pure sinusoidal output. These filters consist of multiple stages of inductors and capacitors that eliminate most of the high-frequency content (Figure 3). The result is a smooth voltage waveform with minimal harmonic distortion. Sine wave filters are indispensable in systems where motors are not rated for inverter use, where cables exceed 50 to 100 meters (165 to 330 feet) or where sensitive electronics are present in the environment. However, their added impedance means they are best suited to applications with stable load dynamics. Some of the filter selection characteristics are summarized in Table 1.

System-Level Considerations
Filter selection depends on a careful assessment of the entire drive system. The VFD, motor cables and motor together form a distributed impedance network that shapes the electrical behavior of the system. Any filter introduced must be compatible with this network. An oversized filter may introduce resonance or unnecessary losses, while an undersized one may offer insufficient protection.

Rather than relying solely on generic design rules or worst-case assumptions, a more robust approach involves direct measurement of electrical parameters in the application environment. Capturing voltage rise times (Figure 4), peak amplitudes and spectral content at the motor terminals allows for a data-driven assessment of the actual stress levels encountered by motor insulation and bearings. These measurements, when analyzed over multiple switching cycles, provide statistical insight into the system’s behavior.

Such measurement-based filter selection enables engineers to apply targeted solutions. If the primary concern is bearing damage, common mode mitigation techniques such as ferrite rings or common mode chokes can be applied. If insulation stress is the main issue, dV/dt filters or sine wave filters may be warranted. In some cases, improvements in grounding or cable layout can provide substantial benefits without the need for additional components. Ultimately, this approach helps avoid overengineering, reduces unnecessary costs and ensures that the chosen filter is both technically appropriate and economically justified.

Conclusion
Filtering is no longer an optional add-on in modern motor systems: it is a necessity. As switching frequencies increase and system integration becomes more complex, the risks associated with high-frequency PWM effects continue to grow. Fortunately, with a solid understanding of filter types, impedance behavior and real-world system dynamics, engineers can deploy effective mitigation strategies.

Whether the issue is premature bearing failure, insulation breakdown, EMC compliance or sensor malfunction, the right filtering solution, carefully selected and properly dimensioned, can resolve the problem at its source. By thinking in terms of disturbance modes and impedance placement and by using dedicated measurements, it becomes possible to design robust, reliable and cost-effective motor systems for today’s demanding industrial environments.