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Have installed and configured three Victron Multiplus II - 5000/48 as a 3ph System in an Off-Grid constellation. For power saving reasons I would now like to switch off the inverters for L2 and L3 during night times and connect all loads, that need to be alive, on L1.
Is there any strategy to configure the system to achieve this?
Thank you for input!
Michael.
No, the system needs to see all 3 inverters "ON" in order not to go into error mode. The only way to get some power saving is to enable AES. This can save up to 50% energy during low power periods. It helps also to keep the loads evenly balanced so the load on each inverter is less than the AES threshold.
You can add fourth smaller just inverter and use it for the night loads when main system is switched off.
Thank you very much for your input!
Michael
@Mike Dorsett
Thank you very much for your clear answer.
Anyway, are you sure, that AES works in 3ph-systems?
Due to the handbook, it's said, that this only works in single mode.
Could you confirm this?
Thank you very much!
Michael
For that freezer you can install "slow start" unit to take care of the inrush current.
No, unfortunately, from all what I know, you can't.
Every heat pump or compressor (and a freezer is nothing else) has to overcome the pressure against the dead center point on start.
A slow start unit does nothing else than limit the power to the load on start. However, limiting the power in the moment of utmost power energy demand of the motor against the gas pressure on dead center point means, that either the motor will get stuck and burn down, or, in case the slow start unit is a good one, the motor will NOT start to rotate at all, or, if the freezer motor has protection, its current limiter will respond. Some heat pumps have a mechanical decoupling in the moment of start, combined with a mechanical flywheel mass, however, freezers have to stay affordable units and therefore the expenses for such gimmicks are too high.
Test it yourself! Stop your freezer when running by interrupting the power supply and then plug it in some seconds later. You will have a good chance, that the thing refuses to start for some minutes, until the coolant has released its pressure via the expansion valve. This is usually the case during normal operation, but not, when the plug is briefly pulled. It may well be, that the motor protection will respond for quite a while, until even the background power of a public grid is strong enough to overcome the rest pressure inside the coolant system for just this one particular millisecond.
In case of electric motors built in bigger electric angle grinders, for example, that do not have to overcome any dead points on start, a slow start unit works perfect. And in case of most inverters, they have that inbuilt anyway.
Thank you for your input and if I missed something in my statement or put something wrong, please let me know.
Michael
There are 2 types of a slow-start units.
1. based on power limiting that use thyristors. Motor gets less power at the beginning and full power at the end. This method is the one you are describing.
2. based on frequency adjustment. Motor gets full power from the beginning but at lower frequency, therefore motor is rotating slower. When frequency is slowly raised up to the nominal value motor is slowly increasing its RMPs up to nominal.
I have a Heat Pump at home that uses that second method at startup.
I am not saying that that second method will work for your freezer. You should ask someone who knows your freezer for opinion on this mater. There may be a problem if the motor is a single phase type.
Thank you for your correction! Very much appreciated.
But the point is: In a compressor-like heat pump you have to have the torque to overcome the counter force at the moment of start. In addition, you have the mass inertia of the accelerating motor.
No matter in which way you try to "fade in" your energy fade up, either frequency or pulse width manipulation, both has the same result: In the moment of utmost power demand the mechanic doesn't get the energy to overcome the dead center point.
Because that's the way my freezer is constructed. After that first moment, the rotation motor has built up enough flywheel mass to overcome the next dead center point, that the freezer runs using 70W only.
But in the moment of start, it needs way more than 2000W (an inverter of that size, having inbuilt startup capabilities, can't move the thing, I have tested that before.)
Let's take an example: If you pull a nail into wood, you can either press the nail in using a leverage press (which would be according to a soft start unit) or you can use a hammer (which compares to a electrical grid with endless power reserve). Let's assume that we need a force of 1000kg (which is physically not correct, but just for easier imagination) to drive the nail in.
What will happen with the nail, if your leverage press only has 800kg max. available? It can't provide more, because the lever of that press would brake otherwise (According to our electrical motor, that would burn down, when receiving that much energy without being able to turn). Would it help to soft-start the leverage press up to that maximum point? No, for one millisecond, your load (the nail) will need a force greater than 1000kg and if you can't get this from your energy background, the nail will not move.
What do you think?
Michael
I agree with you, and this is why I suggested to consult this with local supplier of such slow-start unit.
It could be possible to use a slow-start after selecting the startup speed correct for your freezer. Even half a second startup time could lower the inrush current significantly.
It depends on the type of the compressor.
In my heat pump I have scroll type and it works but slow-start was selected and programmed by the maker of the pump. But it is a 3 phase unit so it is much easier to do.
I never used it but Victron Phoenics 1600VA inverter should be able to work with your freezer without slow start if the inrush power is less than 3kW.
https://www.victronenergy.com/upload/documents/Datasheet-Phoenix-Inverter-Smart-1600VA-5000VA-EN.pdf
If not that one there is a stronger one 3000VA with 6000VA peak power.
https://www.victronenergy.com/upload/documents/Datasheet-Phoenix-Inverter-1200VA-5000VA-EN.pdf
Extra start-up power
A unique feature of the SinusMax technology is very high start-up power. Conventional high frequency technology does not offer such extreme performance. Phoenix Inverters, however, are well suited to power up difficult loads such as refrigeration compressors, electric motors and similar appliances.
Hm, I have provided, that the inrush current is not the problem.
I have provided, that this is a mechanical problem of the construction of this particular type of heat-pump. And I have provided, that I have used a 1200VA inverter (Victron Phoenix 48-1200 with 2200Wp), that couldn't start the freezer, and I have provided, that even the grid doesn't start that particular freezer after plugging out and in during run, because the motor isn't strong enough to overcome counter-pressure as long as the expansion valve hasn't degraded the pressure inside the compressor. I can't find any argument, that could bring that further.
Your heat pump must have accompanying mechanical constructions that minimize the coolant pressure. It has probably not a fixed expansion valve and it has a much stronger electric drive to manage the much stronger overall energy turnover. Such drives must have a soft start or kind of frequency modulation, otherwise your lights in the house would flicker, when the thing turns on.
I assume, that your heat pump first of all opens the expansion valve, as soon as the motor stops. This has the effect, that there is no counter force or upper dead point at all anymore. The coolant pressure in that moment is the same all over the gas system. In scroll type pumps, the upper center point effect is mechanically minimized anyway. Just because to get soft start capabilities from the mechanical side.
My freezer obviously is an old fashioned construction with a fixed and therefore uncontrolled expansion valve as almost any cheap freezers are constructed. However -besides the start problem- my old guy almost doesn't need any energy and runs on very low overall consumption rates.
No matter how: Thank you very much for your input and time, because it forced me to think myself deeper into that manner and any input and experience is helpful to do so.
Michael
Thank you for the technical explanation of how the compressor works. Really, I did not know such details.
As for my Heat Pump, they sell them with and without slow-start unit. It depends on the size of the compressor. Mine has a fixed expansion valve.
I suspect your freezer has piston type compressor that has low startup momentum and single phase motor.
Mine is rotary and 3 phase.
How your freezer deals with short power outages?
"I suspect your freezer has piston type compressor that has low startup momentum and single phase motor."
Exactly. On short power outages, the coolant pressure BEFORE the expansion valve stays quite high until the valve has released quite an amount of gas into the expansion zone of the system. About one minute or so later. If you try to restart the freezer within that minute, even the public grid can't manage to start it. The internal motor current guard shuts the motor immediately out.
Are you sure, that your 3-phase heat pump has no variably controllable expansion valve? This is one of the ways to control the performance of a heat pump.
Michael
If you keep the door shut you could probably use a timeswitch to turn off the freezer for the whole night. It will probably last eight hours or more. Maybe you would need to lower the thermostat a bit but worth doing the experiment.
