Using a motor+flywheel to supply short peaks of power beyond what the inverter can provide

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davideandrea asked Oct 4, '22

Using a motor+flywheel to supply short peaks of power beyond what the inverter can provide

In our application, the load may pull short peaks of power that are 5 times the continuous rating of the Victron inverters. The inverters shut down momentarily, then recover. That cause flicker, which is undesirable.

Of course, we could stack up more inverters to power these short peaks.

But, we are exploring the alternative of adding a 3-phase motor and a flywheel to the 3-phase line to power these peaks and keep the inverters from flickering off and on.

Is that reasonable? Do you have any experience to share in this regard? Can you recommend an off-the-shelf product to do that? A "ride-through generator"? A "synchronous condenser"?

I looked at a flywheel UPS, but that includes high power inverters, and that defeats the goal of not having to add mode Victron Inverters to our system.

Thank you,

Davide

peak

blockdiagram.jpg (29.9 KiB)

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Kevin Windrem answered · Oct 04, 2022 at 06:51 PM

If the Victron inverter shuts down, it expects no voltage on it's output. The motor/flywheel would be feeding power back into the inverter and could cause problems and possibly damage.

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davideandrea commented · Oct 04, 2022 at 09:35 PM

Well! That's good to know! Thank you!

However, I have some doubts. In a normal application there will always be some motor in the load that keeps on spinning. Therefore, it must always be the case that the Victron inverter is exposed to voltages on its output when it is off. And that hasn't been a problem.

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sharpener davideandrea commented · Oct 04, 2022 at 10:11 PM

Only with a synchronous motor, which are not very common. As noted above, the usual (induction) motor does not generate in the absence of an external voltage on the terminals. The power it produces is the product of the external voltage and the current in the rotor which is proportional to both the voltage and the slip (i.e. difference between actual speed and synchronous speed). No voltage => no power produced.

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davideandrea sharpener commented · Oct 04, 2022 at 10:55 PM

OK, I think I get it. Thank you very much!

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Answer 1 · 2022-10-04T16:50:05Z

ludo answered · Oct 04, 2022 at 04:50 PM

Maybe if you can give me an idea of your complete installation I might come up with some suggestions. For this I normally take inverters with 600..800v dc , the motors driven in 4q mode, recuperating power to the grid.

With 12 to 48v dc circuits the currents are much to high to be effective in any way, but they are created for ships and RV 's, using these in industrial applications doesn't really make sense.

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davideandrea commented · Oct 04, 2022 at 05:05 PM

I added a block diagram.

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Answer 2 · 2022-10-04T17:02:38Z

sharpener answered · Oct 04, 2022 at 05:02 PM

For an induction motor to work as a generator and provide power into your system it would have to be turning faster than synchronous speed. But in providing power it will give up kinetic energy so its speed will drop. So I think you would need a variable-speed drive, either electronic (very much like an inverter) or mechanical, which would be complicated.

Wind turbines do something similar, they have a 3-phase generator, with a slip-ring wound rotor which is energised from a bi-directional inverter and so achieve a 2:1 speed range. But the slip rings are a significant source of unreliability. There are also designs which avoid this with complicated stator windings but they still need an inverter drive.

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davideandrea commented · Oct 04, 2022 at 05:08 PM

> For an induction motor

During the short interval that the Victron inverter is off, there is no frequency because it is not generating any voltage. My assumption is that the motor would be generating voltage, albeit at a lower frequency. That would reduce the flicker. Wrong?

What about a synchronous motor instead?

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Phil Gavin davideandrea commented · Oct 04, 2022 at 08:49 PM

I think this will work with an induction motor. Its mechanical rotational inertia will momentarily and for a short time make it act like a generator and enable your system to override "dips".

I know this because large motors contribute to fault current in power system faults on the grid.

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ludo Phil Gavin commented · Oct 04, 2022 at 08:54 PM

Possibly, but how much energy will it be using all day ?

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Phil Gavin ludo commented · Oct 04, 2022 at 09:07 PM

No indication was given as to whether these dips are random or at predictable times. I try not to assume.

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davideandrea Phil Gavin commented · Oct 04, 2022 at 09:39 PM

Our study shows that they occur at random times. We assume it is when someone or something turns on some equipment.

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Phil Gavin davideandrea commented · Oct 04, 2022 at 09:46 PM

Then as @Ludo pointed out you may get various technical solutions that are not economically viable.

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Phil Gavin ludo commented · Oct 04, 2022 at 09:13 PM

This is another technique:

https://www.youtube.com/watch?v=Tg66lLMRBEE

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davideandrea Phil Gavin commented · Oct 04, 2022 at 09:49 PM

Interesting video. Thank you.

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Answer 3 · 2022-10-04T17:37:45Z

sharpener answered · Oct 04, 2022 at 05:37 PM

Ah I see from yr diagram it is off grid, which helps a bit.

An induction generator need external excitation from an AC source (a bit like an AC-coupled PV system in that respect), so it would need the Victron inverter to keep going but allow the frequency to fall over time, don't know how you would do that.

Synchronous motor e.g. with permanent magnet or DC-excited rotor would not need the inverter to stay online but would lose speed while riding through the load peak so the inverter would have to re-synchronise with the lower frequency when it comes back.

I think as @ludo hints a mfr of 4-quadrant variable speed drives e.g. Schneider or Control Techniques might be able to help. To start with you would need to have some idea of what energy you need in Joules to get through the peak and then calculate the change in speed/frequency this would result in for a motor of known inertia.

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davideandrea commented · Oct 04, 2022 at 05:43 PM

> you would need to have some idea of what energy you need

We are still taking data. We do know the value of the current peak (200 A) but we don't know the duration because the data we already took didn't have enough time resolution, so we don't know the energy yet.

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Answer 4 · 2022-10-04T21:21:23Z

Kevin Windrem answered · Oct 04, 2022 at 09:21 PM

I'll say this again. DO NOT backfeed power into the Victron inverter charger output!!! It is not designed to do this when it's shut down or faulted due to overload.

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Answer 5 · 2022-10-04T23:13:33Z

Kevin Windrem answered · Oct 04, 2022 at 11:13 PM

What about a soft start for the motor? These significantly reduce the startup current of the motor by spreading the initial peak across a longer period of time. I use one on my RV air conditioner (which is a single phase motor) and I hardly notice when the compressor kicks in. Your milage may vary with a 3-phase motor as the locked rotor current is much less. But there still may be solutions. Question is if the cost is less than additional inverters.

Note that Victron inverters typically handle 2x the running current for a short period of time. This of course assumes the battery bank can output the energy.

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davideandrea commented · Oct 05, 2022 at 04:28 PM

> What about a soft start for the motor?

It's not our motor. It's our customer's motor. We can't tell the customer how to drive their motor. Even worse, we don't even know who the customer is beforehand.

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sharpener davideandrea commented · Oct 05, 2022 at 05:33 PM

No but you can suggest to your customer that he would get better performance out of your product and/or be able to buy a smaller size if he fitted a soft start to his motor if that is what is causing the problem. Probably more cheaply than you fitting a motor/flywheel inside your product to solve it.

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Answer 6 · 2022-10-05T16:44:36Z

ludo answered · Oct 05, 2022 at 04:44 PM

Ok, so you're selling the wrong product ?

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