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davideandrea avatar image
davideandrea asked

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.


blockdiagram.jpg


Thank you,

Davide

peak
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7 Answers
Kevin Windrem avatar image
Kevin Windrem answered ·

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 avatar image davideandrea commented ·

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 avatar image sharpener davideandrea commented ·
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 avatar image davideandrea sharpener commented ·
OK, I think I get it. Thank you very much!
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ludo avatar image
ludo answered ·

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 avatar image davideandrea commented ·
I added a block diagram.
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sharpener avatar image
sharpener answered ·

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 avatar image davideandrea commented ·


> 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 avatar image Phil Gavin davideandrea commented ·

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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sharpener avatar image
sharpener answered ·

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 avatar image davideandrea commented ·

> 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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Kevin Windrem avatar image
Kevin Windrem answered ·

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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Kevin Windrem avatar image
Kevin Windrem answered ·

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 avatar image davideandrea commented ·

> 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 avatar image sharpener davideandrea commented ·
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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ludo avatar image
ludo answered ·

Ok, so you're selling the wrong product ?

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