Hacking DC supply into DC inverter mini split.

[hwy17]

I recently learned about this product after having just recently ordered my first mini split system, a 27k Mrcool 3 zone DIY.

YMGI's Solar Assisted DC Inverter Single-Zone Wall-Mounted Mini-split models | YMGI Group

Information, Specifications, and Downloads for YMGI's Solar Assisted DC Inverter Single-Zone Wall-Mounted Mini-split models

ymgigroup.com

The concept makes a lot of sense, all these new mini splits use a DC rectifier and then invert to a variable frequency 3 phase motor drive. YMGI has broken those functions out so you can provide straight DC supply.

What do you think the chances of hacking this into mini splits without the built in DC supply function?

First challenge would be physically getting a good tap onto the DC portion of the electronics in the outdoor unit, but then I don't know if there would be insurmountable hurdles in the electronics, i.e. will the rectifier disagree with batteries there and does the controller rely on any inputs from the rectifiers that confirm it's getting all it's power from the AC side.

 

[Samsonite801]

Have you seen the Hot Spot Energy 48v DC mini-splits yet?

Solar & DC Air Conditioners | 48v DC Solar & Telecom Air Conditioner Heat Pump | Off-Grid Air Conditioning

As far as modifying traditional AC-powered mini-splits, it would be hard to say if it could be done or not (relatively easily), without having all the circuit board diagrams or unless you were an expert electronics techie who had a unit to dissect, reverse engineer and figure out how one of these units in question would be all wired up.

I have a moderate level of electronics understanding and even fixed easy problems like say some blown resistors, caps, or transistors, but to just be able to say whether there would be some simple DC bus you could tap into is doubtful at best. They likely use inverter-type power supply boards (with possible multiple output buses on them), and I'm not sure if those are 3-phase motors/compressors, or if they are actual (standard 2-wire) DC motors, stepper motors or what, it might even depend on the brand and model.

If I could see all the diagrams of the electronics on some example unit, it would be easier to say how possible such conversion effort would be involved.

 

[Quattrohead]

In a Gree service manual I am seeing 450v rated capacitors and the compressor label shows 260-350v DC 900-7200rpm.
It is a 3 phase DC motor...kind of !!! As in not a 50/60hz motor.
This is a 2-3 ton system though.

 

[hwy17]

I believe 3 phase AC compressors are the standard across basically all mini splits now and any reference to them as "DC motors" is a confusion of terminology. They go AC to DC to variable frequency 3 phase AC. And this is how YMGI/Hotspot have easily been able to design DC supply units.

What I am not sure of is whether the DC bus on an AC supplied unit is a typical 12, 24, or 48v or something exotic like higher voltage DC or an atypical low voltage. Or potentially plenty of other complexities like multiple bus as you say.

I will have my unit wired up on AC soon and maybe get around to poking a meter at it.

 

[justgary]

I will have my unit wired up on AC soon and maybe get around to poking a meter at it.

When you poke your meter around at it, be very careful.

The specification sheet for the YMGI Solar mentioned above clearly shows that the input DC has to be between 250V and 400VDC. It also shows that the power supply is 310VDC. At those voltages, I'm leaving the hood on my mini split.

 

[Quattrohead]

The specification sheet for the YMGI Solar mentioned above clearly shows that the input DC has to be between 250V and 400VDC. It also shows that the power supply is 310VDC.

Interesting, that falls into the middle of the range I saw in the Gree manual.
Probably an industry standard.

 

[hwy17]

This paper describes the concept as well, with a 311v tap point illustrated in figure 5.

https://www.sciencedirect.com/science/article/pii/S2405844020325421

I wonder if the YMGI unit even contains any particular electrical modifications, or just provides you the taps onto the 311v bus.

If solar is supplying >311v onto the DC bus will the AC-DC rectifier inherently back off and reduce the AC load proportionately? Maybe.

 

[Warpspeed]

Your biggest problem will probably be motor starting without the dc supply and full available grid power behind it to assist startup.
Motor starting current is usually multiples of the running current.
You cannot soft start a piston compressor against significant back pressure.

It will probably need a high voltage battery to provide the massive required surge in starting power without the internal dc supply.

A 300 to 350 volt battery is very serious stuff....

 

[hwy17]

Your biggest problem will probably be motor starting without the dc supply and full available grid power behind it to assist startup.
Motor starting current is usually multiples of the running current.
You cannot soft start a piston compressor against significant back pressure.

It will probably need a high voltage battery to provide the massive required surge in starting power without the internal dc supply.

A 300 to 350 volt battery is very serious stuff....

Looking to set this up with grid power supply intact, just solar DC input to offset AC consumption, no batteries.

 

[Warpspeed]

O/k, for some reason I just assumed it was for off grid.
Should work fine.
Just need to ensure full open circuit no load solar voltage does not get too high, and that max peak power rated solar voltage is about roughly the same as rectified grid power.

 

[justgary]

A DC-DC boost converter might be your best bet. The solar panels can make whatever voltage they want, and the converter will make a constant 311 volts.

My 18KBTU mini split uses about 6 Amps of 240 VAC running on high, so it probably uses 8 or 9 Amps of 311 VDC.

By the way, the peak voltage of 240 VAC is 339 volts, so it isn't too surprising that they might just rectify and filter the input to get their DC. I suppose that 120 VAC units might run on 160 VDC or so.

 

[hwy17]

A DC-DC boost converter might be your best bet. The solar panels can make whatever voltage they want, and the converter will make a constant 311 volts.

My 18KBTU mini split uses about 6 Amps of 240 VAC running on high, so it probably uses 8 or 9 Amps of 311 VDC.

By the way, the peak voltage of 240 VAC is 339 volts, so it isn't too surprising that they might just rectify and filter the input to get their DC. I suppose that 120 VAC units might run on 160 VDC or so.

I am going to continue with my assumption that the YMGI units are not running special circuitry until proven otherwise.

If that is true, then the DC bus can tolerate and make use of variable voltage in the 250-400v range supplied raw from the array.

Thank you for the peak voltage insight! 331 must be the peak of the 230v design supply. I wonder if that has something to do with YMGI's spec that only 250v+ will offset AC consumption, like you have to cross the AC RMS threshold to start displacing the rectifier. Idk, I am no EE.

 

[Warpspeed]

Nominal voltages vary all over the world, 220v America, 230v Europe, 240v Australia, and there are also going to be variations well above and below those limits. Dc bus will reflect that, as its just rectified mains.

Usually, the 400v upper limit is due to component ratings, typically the big bulk filter capacitors after the rectifier.

Now the motor drive circuit that generates the three phase to run the compressor will compensate for the expected wide input voltage variation, so there is no need to have any voltage regulation on your solar input.
As long as it can never reach the 400v maximum, it will be fine.
If your panels have a rated max power output voltage close to around 330v that should work very well.

Don't sweat on getting the voltages exactly right, 300v to 350v solar should be easily achievable with ten or perhaps eleven panels in series.

 

[hwy17]

Mini splits apparently have a DC "Reactor" which is a separate part from the control board, and the connections to which might provide the easiest tap point.

I wonder if I should connect the solar before or after this reactor? My gut says before.

https://www.mitsubishitechinfo.ca/sites/default/files/005_HiW%20Reactor.pdf

 

[Warpspeed]

It probably does not matter...

 

[hwy17]

https://imgur.com/a/9jMBm9J

Edit: Darnit, I've posted AC wiring. I will not be answering questions about the bare copper ground, thank you. I know what I did.

 

[Warpspeed]

That makes things a lot clearer.
There is a bridge rectifier BR1, then the reactor, then three large electrolytics in parallel E1, E2, and E3.
All pretty standard stuff for variable speed motor drives.

You could feed your solar straight into the three electrolytics, through a series diode.
Or via the rector into the three electrolytics.
It really does not matter, the reactor does nothing when feeding in steady dc.

Just realize that neither side of the electrolytics will be at ground potential.
Both sides of your solar will be "hot".
So be careful !

The electrolytics will contain potentially lethal energy for quite some time after the power is switched off.

 

[hwy17]

The DC points on the rectifier read 330v idle, 215v under load.

The electrolytics read 330v idle, 330v under load.

I wonder if this is a difference in pre-reactor and post-reactor DC, with the pre-reactor DC being dirty. I can see how solar tap before or after wouldn't matter, since the solar wouldn't need to be smoothed.

 

[Warpspeed]

The pre reactor voltage coming out of the rectifier will be pulsing, and have a pretty high high ripple voltage on it.
Votage directly across the electrolytics will be much closer to pure dc.
Probably better then, to feed solar in directly across the electrolytics.

You will be adding a steady dc solar current, even if the actual voltage is a bit of 120Hz ripple on top.
That should not make any practical difference to the outcome.

 

[hwy17]

Am I missing any obvious negative side DC taps? The positive comes out via quick connect fitting to the reactor and back which would be real convenient, but I don't see anywhere the negative does anything similar.

Otherwise the backside of those electrolytics that we're looking at looks like my best connection point.

 

[Warpspeed]

Easiest access will be right across one of the three electrolytics.
Just bring out a pair of suitable wires from there.
It will need a BIG soldering iron, the heat really spreads across the associated very wide copper areas.

 

[hwy17]

Thank you very much for your help. This thread will probably go to bed for a while now while I shop for a set of used panels, hopefully will be testing by Spring.

 

[hwy17]

DC fed into a bridge rectifier passes right through it right?

The 12v, 5v, etc. control circuits maybe use switching power supplies that also begin with a rectifier.

Resistance heating elements don't care.

Every fan indoor and outdoor runs on an inverter.

It possible that mini splits would accept an adequate 250v+ DC supply right on their main AC input and be entirely happy with it. Of course that would probably require batteries and straight solar would probably be a wreck of low voltage or over amperage shutdown errors.

 

[Warpspeed]

It possible that mini splits would accept an adequate 250v+ DC supply right on their main AC input and be entirely happy with it. Of course that would probably require batteries and straight solar would probably be a wreck of low voltage or over amperage shutdown errors.

You would first need to make absolutely sure there are no small transformers or ac powered blower fans inside that work directly off the 50/60Hz incoming ac power. If you fed just dc straight in, it might be o/k or it may smoke some parts.

Also, a single big dc supply would need to have enough surge power grunt to start up the compressor.

Much safer to power the whole thing intact off it own inverter, or the main home inverter in a total off grid situation.

The best way of all is probably to do it your way.
Power the thing direct from the grid as originally intended, and feed supplementary solar dc into the big electrolytics that supply dc power to the compressor variable frequency drive.
The grid then supplies all the motor inrush surges, and the solar contributes whatever it can once its running.

 

[Quattrohead]

Also, a single big dc supply would need to have enough surge power grunt to start up the compressor.

That is not how these things work, they have a very soft start. They slowly ramp up over 30 seconds or so to a level and then sit there for a minute or two and then do some temperature measurements to decide how much more or less they need to run to maintain the set temperature.