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Industrial frequency inverters (or "variable speed drives") can be thought in three parts: A mains rectifier, that converts two- or three phase input current to DC, a DC energy storage (large capacitor) and a three phase driver, that converts DC current to three phase AC currents of the wanted frequency.

The mains rectifier is often just a passive rectifier circuit.

The output driver usually is made of microcontroller driven FET bridges.

Question is, if the rectifier stage of one inverter can be removed, and replaced by the same circuit as the output driver stage taken from another inverter. Like we would treat mains power just as another motor.

With right control, the inverter then would become fully symmetric and would be able to feed current either way, from the mains to the motor but also from the motor to mains (recupation).

As the motor is also meant for running of the mains directly, voltage and current of both (motor and mains) somehow fit well, and I guess the driver circuit could serve both of them. Would this be feasable?

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  • \$\begingroup\$ I'm not sure what you are trying to accomplish, here. Sounds almost like you want to power a motor from mains, but feed power back to the mains when you rotate the motor (using it as a generator) without having to switch anything connections. \$\endgroup\$ – JRE Jan 14 '16 at 11:31
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    \$\begingroup\$ And there are many applications where mechanical work is stored but not used up. For example, presses for forming sheet metal use a flywheel to get a strong punch force. If only small parts are formed ocassionally, the energy consumed to spin up the flywheel is many times larger then the energy used to punch. If the machine is then switched off, it could return the power softly until the wheel is stopped. \$\endgroup\$ – dronus Jan 14 '16 at 11:36
  • \$\begingroup\$ How does this question differ from electronics.stackexchange.com/questions/211277/… ? If you are attempting to run the motor and generate power simultaneously, please clarify that, so we can close both questions as "perpetual motion" nonsense. If you are talking about regenerative braking when something else is driving the motor (like running downhill) yes of course that's possible. \$\endgroup\$ – Brian Drummond Jan 14 '16 at 11:59
  • \$\begingroup\$ DC Motors can regenerate, because they typically max out at approximately the line voltage. On 480VAC, that is 500V. For an AC drive, on 480VAC, the buss that you need to dump is up to 750V. If you try to PWM 750V onto the supply, you are asking to blow things up. It is done, and possible with (typically) a transformer to lower the regenerated voltage presented to the line. Active front ends can also do this, but at the cost of extreme filtering. This wheel has already been invented and built. \$\endgroup\$ – R Drast Jan 14 '16 at 13:52
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Yes. Fully regenerative variable frequency drives (VFDs) have been available practically throughout the history of VFDs. The characteristics of the AC source are not exactly the same as the characteristics of motors, but the motoring and regenerating functions can still be incorporated into the same control system. The cost is too high to justify using regenerative VFDs where the only regenerative function is occasional braking. They are most attractive for controlling dynamometers used to absorb power when testing engines. They have also been used in processes in which one motor pulls and stretches a material while another motor holds the material back. An electric vehicle like the Tesla that uses an AC motor could also benefit, but I don't know if they are doing so at present.

Question is, if the rectifier stage of one inverter can be removed, and replaced by the same circuit as the output driver stage taken from another inverter. Like we would treat mains power just as another motor.

Yes. Something like that can be done. The same switching devices connected in the same way are used in the input and output power sections. The control section needs to have appropriate software programming and the capability to provide the necessary signals to control the switching devices and the feedback sensing and signal system needs to be configured appropriately. Mains power can not exactly be treated as just another motor. In addition to the feedback signal and software programming mentioned, some additional filtering is required.

Emerson / Control Techniques apparently has Unidrive models are configured so that two identical units can be connected as described. The system is described in some detail in "Control Techniques Drives and Controls Handbook" (see diagram below). Other manufacturers may use the same or similar input and output bridges and add other components as required in a product that is specifically constructed as a four-quadrant, regenerative drive.

enter image description here

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  • \$\begingroup\$ I like to hear such devices are readily available. However, would it be possible to build one by the way I propose? \$\endgroup\$ – dronus Jan 15 '16 at 13:34

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