The vast majority of electrical car (EV) traction motors require sinusoidal 3-phase alternating present, with frequency immediately proportional to RPM. A well-liked resolution for attaining that is the triple half-bridge voltage supply inverter (VSI).
On this inverter configuration, every swap of the bridge can join its output terminal (motor section winding) both to the optimistic or adverse aspect of a voltage supply (the EV battery, labeled as +Vbatt and -Vbatt), disregarding the ineffective choice of each switches being off and the damaging choice of each being on. This design is characterised as “2-level.”
The output waveform of a 2-level inverter on the basic frequency is basically a sq. wave. Whereas it is possible to drive AC motors utilizing sq. waves, this technique is just not ideally suited due to the excessive harmonic content material. The ensuing harmonics generate extreme warmth, lower most achievable torque at numerous RPMs, and enhance vibration. A standard technique to reduce harmonic distortion includes chopping every output pulse into a number of segments and modulating their on-times, or responsibility cycles, in a sinusoidal method. The inductance of the motor windings helps combine this pulsed voltage right into a smoother sinusoidal present, resulting in improved torque, decreased vibration, and decrease power losses. As the heart beat width modulation (PWM) frequency will increase—leading to extra segments for every output pulse—the full harmonic distortion (THD) within the present waveform decreases, which is usually useful till it reaches diminishing returns.
One important problem with rising the PWM frequency indefinitely is that the time spent within the switching transitions (from off to on and vice versa) will increase except the switches themselves develop into proportionately quicker. The switches act like resistors throughout these transitions, resulting in elevated switching losses because the PWM frequency rises. Utilizing quicker switches, like the most recent SiC MOSFETs and GaN HEMTs (Excessive Electron Mobility Transistors), presents its personal challenges. Regardless of enabling switches able to dealing with 400 to 800 V in 10-30 ns, such fast switching generates important radio frequency (RF) noise. It additionally induces dangerous common-mode currents that may harm winding insulation and shaft bearings. A rule of thumb means that the efficient RF bandwidth in MHz of a switching transition, measured in ns, is 350 / dV / dt. For instance, a ten ns switching time can generate appreciable RF power as much as 35 MHz.
A possible resolution to mitigate sharp switching transitions with out slowing down the switches is to include an LC low-pass filter immediately after every inverter output. This method is especially efficient if the motor is positioned quite a lot of meters away, because the connecting cables can act as efficient radio antennas. By setting the filter’s cutoff frequency to one-tenth the efficient bandwidth frequency (e.g., 3.5 MHz for a ten ns switching time), sharp transitions could be softened, considerably decreasing RF noise emissions with out introducing sufficient section shift to disrupt vector motor management schemes. Nonetheless, these dV/dt filters are much less efficient in decreasing common-mode currents because the motor’s voltage waveform consists of pulses reaching the total battery voltage.
To adequately clean the chopped voltage waveform into a correct sine wave (matching the present waveform), the LC filter’s cutoff frequency needs to be restricted to one-tenth the precise PWM frequency. This is able to require a bigger and dearer filter, whereas possible introducing sufficient section shift to have an effect on vector management schemes except addressed by {hardware} or software program options.
Another technique to decrease THD and reduce the magnitude of common-mode currents is to extend the variety of ranges within the voltage waveform generated by the inverter, thereby making a multilevel inverter (MLI). A multilevel inverter can produce waveforms that extra carefully resemble sine waves in comparison with a standard 2-level inverter. In a 3-level MLI, for example, every PWM pulse makes use of solely half the battery voltage, whereas the 5-level MLI makes use of one-quarter of the battery voltage, resulting in decrease THD and decreased common-mode currents. Nonetheless, multilevel inverters usually introduce extra complexity in comparison with their 2-level counterparts, doubtlessly offsetting the advantages when evaluated towards less complicated options like further LC filtering or enhancing the motor’s resilience to common-mode currents.
MLIs require entry to separate voltage sources for every output voltage stage. In some circumstances, further methods like capacitive voltage dividers could be employed to generate these ranges not directly. As an illustration, Cascaded H-Bridges are a sort of MLI that makes use of a number of DC voltages however is impractical for EV functions as a result of complexity of getting remoted battery packs and chargers.
A extra easy method to generate numerous voltage ranges in an MLI is thru capacitive voltage dividers. For instance, connecting two equal capacitors in sequence will halve the bus voltage, establishing a brand new 0 V reference for the MLI. This system could be prolonged additional by introducing further pairs of capacitors, yielding new voltage ranges, thereby enhancing the MLI’s efficiency.
Whereas decreasing the voltage swing of the pulses is helpful, the complexity and dimension of the capacitors required can escalate rapidly, as extra ranges necessitate bigger capacitance values to handle ripple present successfully. Moreover, balancing the cost on the divider capacitors incurs challenges.
Two unique multilevel inverter topologies appropriate for EVs are the Impartial Level Clamped (NPC) and Flying Capacitor (FC) sorts. Each make the most of the identical primary energy stage construction, however they differ in producing the zero-voltage output stage. In NPC designs, the 0 V stage is created by activating particular switches, whereas in FC designs, the extent is achieved via a flying capacitor that should be pre-charged, including complexity to manage and operation.
Different superior MLI topologies, together with Energetic Impartial Level Clamped (ANPC) and T-type MLIs, present mechanisms to higher keep cost steadiness on divider capacitors whereas accommodating various energy issue masses, resembling induction motors. The ANPC replaces passive clamping diodes with lively switches, permitting for flexibility in controlling cost steadiness. The T-type MLI simplifies the design by using a single bidirectional swap, which hyperlinks a section winding to a divider midpoint and allows bi-directional present movement.
The rising complexity of assorted MLI topologies presents a serious problem to their adoption, particularly when in comparison with less complicated 2-level VSI options enhanced via further LC filters. Furthermore, the event of those superior inverter schemes typically leads to larger prices and longer timelines. Contemplating that the capacitor necessities for MLIs can devour important quantity, the case for his or her use in electrical autos stays advanced and troublesome to justify.
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