Motor inverter as a battery to mains power inverter - Page 1
Forum section: electronics / projects

author: haxby
author_rank: Contributor
timestamp: July 11, 2020, 02:24:36 am
content: Hi all,Short story:I have a high current 200V DC source that I want to convert to 240V AC at 5KW.I also have a motor inverter Variable Frequency Drive (VFD) rated at 11KW that can accommodate a DC input voltage.I need to know if I can connect the VFD to the 200V DC source, then run the output to a common transformer (instead of a motor for which it was designed) and use the transformer output as my power.Would this work???More detailed story:I have a Toyota Prius that I would like to use as a portable power generator. The prius traction battery is around 200 to 230V DC, and I know that it is good for at least 5KW output. The output has to pass through a transformer as any earth leakage on either positive or negative battery terminal will shut off the Prius and generate an error code. I propose to use the VFD on the DC side of the battery, with the output to drive a transformer. Most probably this transformer will be one of those 110V to 220V iron core transformers used when using US appliances in Australia or vice versa...... will connecting the VFD to the transformer work?

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 11, 2020, 02:50:38 am
content: More economical than trying to get such a large transformer is to get multiple 48V "telecom rectifiers" and connect the outputs in series. They're really cheap on the surplus market as in $20 or so for a 2kW unit.The VFD will work fine if you add an output filter and can run it in V/Hz mode with phase imbalance detection disabled. In fact, if the load is a (single phase) motor, you can do some limited V/Hz control.

author: haxby
author_rank: Contributor
timestamp: July 11, 2020, 03:06:46 am
content: Thanks for that. Sounds promising. What will the output filter do?

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 11, 2020, 03:19:11 am
content: The VFD outputs high frequency PWM, so you need the filter to prevent it from causing a lot of EMI.

author: haxby
author_rank: Contributor
timestamp: July 11, 2020, 04:01:51 am
content: Thanks for that. Any leads for the 48V telecom rectifier transformers? I tried ebay with no luck... $20 sounds amazing for the copper alone!

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 11, 2020, 04:25:26 am
content: Quote from: haxby on July 11, 2020, 04:01:51 amTry searching alternative terms like "48V power supply". They're switching supplies that will run on AC or DC. Beware that some of them have the output internally tied to ground and will need modification to be used in series. Here's what's inside one:https://www.eevblog.com/forum/reviews/teardown-fuji-48v-40a-psu/If the VFD needs a bus voltage of 200V (sounds low for a 11kW unit, it's usually in the 300V range for 208V output), use 4 of the power supplies in series to power it.

author: haxby
author_rank: Contributor
timestamp: July 11, 2020, 06:03:29 am
content: Oh i dont think you understand what I am doing..... I'll be getting the 200V DC input from the Prius battery. No need for a SMPSU.The 200V DC connects to the VFD input, then I want to connect a 110V to 220V step up transformer to the VFD output. The appliances are then connected to the secondary of the step-up transformer, and are isolated from the car's electronics.

author: Bloch
author_rank: Unknown
timestamp: July 11, 2020, 06:56:49 am
content: That are you questions ?Are you asking if the VFDQuotewill explode with 200VDC on the input ?To low input to give you wanted output voltage ?To high input voltage for the VFD data spech.QuoteI have no experience to connect an transformer to the VFD output - only to motors.

author: richard.cs
author_rank: Super Contributor
timestamp: July 11, 2020, 08:03:58 am
content: Quote from: haxby on July 11, 2020, 06:03:29 amHe's suggesting putting an SMPS between the vfd and battery pack as an equivalent way of providing the isolation you need, same effect as having a transformer at the output. It would be a physically smaller and lighter approach. It does mean you need enough voltage into the VFD to get 220V out which would actually need a different vfd and a 350V dc bus. I don't think you mentioned it was a 110V vfd with a planned 1:2 output transformer so we were all puzzling what kind of 220V output vfd would accept 200V in.

author: haxby
author_rank: Contributor
timestamp: July 11, 2020, 08:35:06 am
content: My main question is if there is anything terribly wrong with connecting a transformer to the output of a VFD instead of a motor??I would set the VFD to 50hz and connect the transformer to 2 of the 3 phase wires...

author: Siwastaja
author_rank: Super Contributor
timestamp: July 11, 2020, 08:38:04 am
content: The VFD expects significant inductance motor windings provide. Good transformers have little leakage inductance.I wouldn't except it to work. You could use a VFD as a building block with an added output filter, and if you are very lucky, the V/f modecouldwork, although I wouldn't count on it. It's designed to drive motors.

author: richard.cs
author_rank: Super Contributor
timestamp: July 11, 2020, 04:01:48 pm
content: Essentially what you propose will work, but you will need a filter before the transformer to turn the VFD PWM into a sensible sine wave. If you rely on transformer leakage inductance to do that for you everything is a bit uncontrolled and the core losses in the transformer will also be high. This relies on the VFD being able to be set into a "dumb mode" where it ignores the fact that only two outputs are connected.As an alternative ebay etc. is flooded with low-cost inverter boards that take a high voltage DC input produce sine wave outputs, often including the filter on the board. Many are built for 380 V in to 220 V out but would require minimal changing (just feedback components basically) to do 200 V to 120 V. e.g.https://www.ebay.co.uk/itm/2000W-Pure-Sine-Wave-Inverter-Power-Board-Post-Sine-Wave-Amplifier-Board-DIY-Kit/133197461127- note that if you operate at 120 V the available current is not increased so the power rating is halved. All the cheap ones seem to use a EG8010 controller, unfortunately so far as I can tell it does not support syncing multiple devices.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 11, 2020, 04:34:30 pm
content: A better approach would be to get a second Prius inverter (~$100) and connect the battery input to 3 or 4 telecom rectifiers in series. Then use the built in boost converter to step the voltage up to about 400V and then use 2 of the output phases for 240V out. To allow that kind of control, replace the controller with a STM32.https://openinverter.org/wiki/Toyota_Prius_Gen3_BoardIt would also be helpful to know exactly what kind of load you need to power, for example electronics work great on a filtered square wave while motors run best with a PWM sine wave but only need minimal filtering to avoid standing wave issues, or no filtering if the motor is close to the inverter. It might be easier to have separate outputs for electronics and motors.

author: haxby
author_rank: Contributor
timestamp: July 12, 2020, 02:49:28 am
content: Thanks Richard, the ES8010 is looking to be the most promising way to do this.... The pre-assembled ES8010 modules with MOSFET drivers on eBay look convenient. I'm surprised there is enough of a demand for a low level board like this to be available from so many suppliers. Any tips or links on what the filter between the MOSFET outputs and the transformer input should look like? I assume it will be dependent on the characteristics of the transformer used? If I design the power electronics with quality components I hope I'll end up with a reliable solution.Regarding the EG8010 data sheet, on page 10 it shows a recommended diagram for a low frequency transformer inverter. There is no filter between MOSFETs and transformer. But there is a 2.2uF cap on the secondary windings. The note says "T1 needs to use low power frequency transformer. Transformer filters PWM high frequency signal by connecting its secondary turns to a 2.2uF/400V capacitor of CBB. After filtering, it outputs 50Hz/60Hz sinusoid".Is this enough or should there also be a filter on the primary side?Thanks for your help!

author: richard.cs
author_rank: Super Contributor
timestamp: July 12, 2020, 08:48:54 am
content: Those modules with the gate drivers turn up in all kinds of inverter products, both as a final-stage for an inverter with a high-frequency DC-DC, and as the first stage of an inverter with a low-frequency transformer. Be aware that by default those boards don't connect up all the protection features (like overcurrent) that the ES8010 offers. Boards like the one I linked that contains one of those modules and some output FETs are much less common, but probably won't do the power output you need anyway.It can work without an explicit filter, but to do so well the transformer has to be carefully chosen (or more likely in a commercial product as this chip is intended for, designed) for the right amount of leakage inductance low core loss at the switching frequency, etc. If you add a filter it's everything (including the filter) is much less dependent on the transformer characteristics, the 3.3 mH / 2u2 filter on the preceding page is probably fine as a functional waveform-reconstruction filter.EMC is a rather separate issue and may require additional filtering at input an output, but this would be at higher frequencies and could use more modest component values. Be aware that (with any inverter) unless you add a lot of bulk capacitance to the input, and maybe a low-frequency inductor, your DC input waveform will mostly consist of 100 Hz ripple. Prius batteries probably won't complain too much about that, but the rms will be higher than you expect so be careful when sizing cables.

author: haxby
author_rank: Contributor
timestamp: July 12, 2020, 08:27:43 pm
content: Ok after contemplating many options I'll probably put this together:https://www.fieldlines.com/index.php?topic=149427.0It uses the same control chip and looks like the most robust low frequency inverter I've seen.