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I need a transformerless inverter which generates a 3 phase 230 VAC modified sine wave output from 325 VDC input. Unfortunately, it seems these are rare, at least high power models. Can a VFD (with DC bus) be used instead? As I understand it, most modern VFD use Sinusoidal PWM. Are there any models which can be configured to output "six step" instead of PWM?

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    \$\begingroup\$ what currents? what is "high power"? There used to be six-step VFDs, but as far as I know (not a domain expert), they are largely obsolete. Also, most VFDs that actually modulate output voltage are in fact not transformerless at all – what's your reason for not wanting transformers? \$\endgroup\$ – Marcus Müller Jan 13 at 11:42
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    \$\begingroup\$ 6 Step VFD is older technology prone to higher harmonic currents with no advantages other than it used stable fixed voltages with switches instead of PWM or DAC controlled voltage controlled SMPS to produce sine waves \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 13 at 12:15
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    \$\begingroup\$ Most VFDs do not use transformers except as part of the voltage converters to convert DC bus voltage to control system voltages. There are six-step VFDs for down-hole oil well pumps and other applications with motors that are particularly susceptible to repetitive "ringing voltage" transients. Some of those may be made by replacing or modifying the controls systems of PWM VFDs. \$\endgroup\$ – Charles Cowie Jan 13 at 12:18
  • \$\begingroup\$ @Marcus Müller: About 100 A = 70 kW for a second, about 16 A = 11 kW continuously. One reason for not wanting SPWM is that the inverter is not directly connected to the motor: a soft starter is in between. If anyone has links to actual products which can do six-step, I'd be happy. \$\endgroup\$ – Peter Åstrand Jan 14 at 22:30
  • \$\begingroup\$ um, your inverter should also take the role of controlling the start, problem solved? \$\endgroup\$ – Marcus Müller Jan 15 at 13:03

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