Summary
- EV traction motors require sinusoidal 3-phase AC with frequency proportional to RPM
- Triple half-bridge VSI is commonly used for this purpose
- Higher PWM frequency leads to increased switching losses and RF noise
- Multilevel inverters can reduce harmonic content and common-mode currents
- Different MLI topologies have varying complexities and advantages, with challenges in implementation for EVs.
Article
The most common way to supply sinusoidal alternating current to electric vehicle traction motors is through a triple half-bridge voltage source inverter (VSI). This type of inverter connects the output terminal of each bridge switch to either the positive or negative rail of the voltage source, creating a square wave output waveform. To reduce harmonic content and improve torque, vibration, and losses, the output pulses are chopped into slices and modulated sinusoidally. However, increasing the pulse width modulation (PWM) frequency can lead to higher switching losses if the switches are not fast enough, which can result in RF noise and common-mode currents.
To address the challenges of increasing PWM frequency, an LC low-pass filter can be added after each inverter output to soften the switching transitions and reduce RF noise emissions. However, this does not help with reducing common-mode currents in the motor voltage waveform. Another approach to lowering total harmonic distortion and common-mode currents is to use multilevel inverters (MLIs). These inverters generate voltage waveforms with additional steps, reducing the THD and common-mode currents at a given PWM frequency.
MLIs require access to separate voltage sources for each output voltage level or use capacitive voltage dividers to create the voltage levels indirectly. The Neutral Point Clamped (NPC) and Flying Capacitor (FC) MLI topologies are examples of 3-level MLIs, each with different methods of generating the 0 V output level. While these topologies offer advantages in reducing voltage ripple and THD, they come with complexities in control and pre-charging components during startup. Other MLI topologies, such as the Active Neutral Point Clamped (ANPC) and T-type, use active switches to maintain charge balance on divider capacitors and handle loads with a wide power factor range.
Despite the benefits of MLIs in reducing THD and common-mode currents, the complexity and cost associated with these inverters pose obstacles to their adoption in electric vehicles. Additionally, changes to power stage switch control schemes are required for MLIs, making their development more costly and time-consuming. While MLIs offer advantages in reducing RF noise emissions and common-mode currents, the argument for their implementation in EVs remains challenging due to the simpler alternative of adding LC filters to a 2-level VSI output.
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