Inverter for my split level heat pump

vince

My Fujitsu heatpump (9RLS3) runs on 220 V and requires a 15 A breaker. The max starting current is 3.3 A, and the running current for cooling and heating are 3.3 and 2.5 A. As I'm only planning to run this heatpump at the low setting as my needs are only 3000 BTU per hour, I was thinking that a 1500 watt inverter might be enough for instance a cotek 1500sk-248. The input power for heating and cooling are rated at .66 and .5 kW. My battery bank is 48 V, 225 AH. This heat pump is not my primary heat/cooling source but is the most energy efficient and would run 10 hours or less per day. Would I be able to run it safely from a generator with modified sinus wave output of 3kW? Thanks

BB.

What kind of genset do you have with an MSW output?

MSW waveforms can be very hard on some electronic power supply front ends.

And asking the mfg may not get you any answers.

-Bill

vince

Generator is a predator from Harbor Freight. Been very reliable so far and able to run all my 15A powertools.

inetdog

vince wrote: »

Generator is a predator from Harbor Freight. Been very reliable so far and able to run all my 15A powertools.

MSW should only come from a generator with inverter based output like some Honda and Yamaha models as well as other brands.
A normal regulated AC genset should have at least close to sine wave but with maybe high harmonic distortion ( particularly clipping at the top of the waveform). The square edges of MSW do not come from any generator winding that I know of.

Dave Angelini

You do know that a heat-pump can only limit it's power consumption by keeping the differential from program to actual room at 2 degrees or less. What this means in practice is someone has to be there to do what you are writing about. If the building is not well insulated and the room warms, your power system will need to supply full rated power to the pump. Use a good inverter for equipment like the Fujitsu.

The Honda inverter generators have a very clean output the last time I looked at them on a scope. Cleaner than an XW inverter BTW

vince

In my situation, the hourly heat loss is calculated to be around 3000 btu, so even the smallest heat pump has a significant overcapacity that I don't plan to use. Given that inverters go up in price with higher capacity, I wonder how much capacity I would need to run the heatpump for my needs.

Stix

Just curious. How did you come up with that number for heat loss? Who did the calculations and by what means?

Dave Angelini

There are two kinds of people, those who have the capacity to do the job, and those who want more.
If this is a home that you live in, you will want more!

The Fujitsu specs a 15 amp circuit. I think it is not cost effective to buy a cheap/small inverter over the long haul. I still have an old working Trace inverter from the mid 90's in my garage that still starts a 1 HP motor on my transfer pump. As long as you are home to start a generator and do not plan to run cooling unattended this might work. If I were doing this I would use 3 times the solar and 4 times the battery for unattended cooling, especially in Kentucky. Good Luck!

vince

Stix wrote: »

Just curious. How did you come up with that number for heat loss? Who did the calculations and by what means?

Using basic software for calculating heat loss, entering square footage, r-values, window sizes etc.. As the space is only 100 sf, and well insulated to energy star standards, the heat loss is in the 2500-3500 Btu range. Using a little propane heater, Mr Buddy, I was able to confirm this with outside temperatures around 0 F. At night, having curtains over the windows lowers the heating needs closer to 2500 Btu. .

Stix

vince wrote: »

Using basic software for calculating heat loss, entering square footage, r-values, window sizes etc.. As the space is only 100 sf, and well insulated to energy star standards, the heat loss is in the 2500-3500 Btu range. Using a little propane heater, Mr Buddy, I was able to confirm this with outside temperatures around 0 F. At night, having curtains over the windows lowers the heating needs closer to 2500 Btu. .

Really need a blower door test. Infiltration is the killer. Such a small area and you have done some testing. All the testing without this bit of info is tough.

Dave Angelini

I misunderstood the 100 square feet and had 1000 in my brain. So this is a little easier but still a challenge as the strategy I tested (many times) involves tracking the sun, or a large battery bank.
You do have other loads beside the heat pump? The main reason for more solar is bad solar weather and tracking gives you more hours off the battery. A nice quiet generator seems to be need here also.

vince

Here is the follow up. My first inverter, Cotek SK1500-245,(1500 cont./1700/3000 surge) died after several months, only running a few times, not longer than an hour each time. Returned the unit under warranty and received a new one. The new unit gets the heat pump going but after about 4-5 minutes, it shuts down, restarts after a few min and shuts down again after a running for a minute or 2. Cotek service department refuses to take the inverter back and blames my setup for the problems. Claiming that the failure rate is so low, that having two inverters fail must be a installation/design problem. Looking at the specs of the split duct AC, the max/rated current rating for heating is 10/3.3 A and for cooling is 9.4/2.5 A.  The max/rated power use for cooling is .85/.5 kW and 1.94/.66 kW for heating. The start up rating is also a reasonable 3.3 A. I'm told to get a bigger inverter like the SK3000.  Did I read these specs wrong in picking my 1500W inverter. The 3000W inverter has a standby power load of < 1A, and will be much more inefficient running my typical load of .5kW but that is what I'm advised to get.

BB.

Does not sound right... Your loads should be pretty small.

Have you measured the DC input voltage on your Inverter (under load/charging)? Low or High DC voltage could cause problems.

Have you measured/monitored the DC current to the inverter to ensure that you are using less than 1kWatt of power on the DC side?

Is this a pure off grid system? Or is this a mixed AC Mains/Off Grid system?

-Bill

vince

This is pure off grid. The DC input voltage stayed around 50V+, on the inverter there is a green input LED that remained green suggesting sufficient input. The current on the Kid CC was going up to 13A or 700W. The load LED confirmed that by flickering orange and returning to green or a load of less than 500W. I can not see any evidence of a possible spike and the load is clearly within range. I used the power conservation setting, the low noise setting and lowest fan setting on the AC to minimize a power surge.

BB.

It sounds like you did everything correctly.

Cotek is supposed to be a decent brand (for low cost units). We have had a person or two here that had issues with them.

-Bill

vince

Thanks Bill, I'll compare the sk3000 to the sk1500. If I need the extra power to run the AC and can handle the extra inefficiency, I just write it off as one of those miscalculations.

Dave Angelini

Something is wrong with your wiring or you really were that unlucky to have 2 bad inverters. 
I assume you are in the US so why torture yourself and buy equipment that is not fully supported here like Outback or Schneider?

vince

Suggest an 1500w, 48v, 220v inverter.

Dave Angelini

If you are talking to me you have to be a client for specific advice. General advice, Outback or Schneider. I have 87 offgrid homes using these. Good Luck!

Vic

Hi vince,

You certainly should NOT need to pay anyone here for advice on a good Inverter for your needs.

Wind-Sun does Consulting,  and are good,  but bet that you should not need to hire them either.

If you are able to bite-the-bullet,  stepping up to an Inverter/Charger,  might serve you well.   MidNite Solar is working very hard on their line on inverters,  but your need seems immediate.

Outback Power makes some good inverter/chargers,  which have a good reputation.   The Idle power to a larger inverter/charger is at least 480 Watt hours per day ...   have not looked at this value for your Cotek 1500 or at the 3000,  for comparison.   EDIT,  and the Radian which does have 240 VAC output seems like a bit of overkill for you,  an does the XW line,  as well.  The Magnum inverters seems to be an OK value,  but they are reputed to have S L  O   W   response in regulating output AC voltage ...  <<

Personally would advise against the Conext SW series of inverter/chargers,  although Schneider may have gotten rid of some of the worst bugs/flaws ...

More later,     Vic

vince

Wondering if my inverter problems may be related to a grounding, bonding problem. I have one earth stake and have a connection to the ground busbar of the combiner box (have all the ground wires of metal enclosures join there, as well as the negative of the battery dc), and have separate ground wires from inverter and heatpump hooked up to the same ground stake (due to the proximity) The heatpump has the neutral bonded to the ground, the inverter does not have the neutral bonded to the ground. Do I need to change anything or any suggestions for trouble shooting why my inverter stops after a few minutes of producing power?

mike95490

Vic ..........

Personally would advise against the Conext SW series of inverter/chargers,  although Schneider may have gotten rid of some of the worst bugs/flaws ...

I would add the XW line as being known good, I use it with my genset and it flawlessly shifts from charging to load support and back, Maybe low quality gensets can cause an issue, as I have recently seen my lightweight 3Kw genset not deal with Power Factor, and if a odd load comes on the inverter may disqualify the genset.

Dave Angelini

The SW is now finally working correctly. Still nothing like an XW with convection cooling.

vince

I'm still confused about grounding of the Dc part, as my system includes a GFP63 and adding a second ground would affect the operation of the GFP or not? Attached is the electric diagram before the grounding was implemented. I connected the ground busbar in the combiner box with 6 awg bare wire to the ground rod, and also made connections to the same rod with the same wire to the heat pump and the inverter.

BB.

I am not quite sure I completely understand the grounding in your drawing--But some comments.

• First, do you have significant lightning risks in your area? Was there any lightning around when the inverter(s) failed?
• I, personally, do not like the DC Ground Fault Breakers. They protect only against a few types of solar array faults and are (in my humble opinion) are unsafe (placing a fuse/breaker between battery ground and "safety ground" is never allowed in "real" power systems. I would just remove the DC GFP. Wire the solar panels as normal to the Midnite MPPT solar charge controller (some Midnite solar charge controllers have "arc fault" detection internally--Configure that if you want the "new" solar safety (I am OK with this for safety use--But Arc Fault can be "false trip", so you have to watch the system does not turn off solar charging without your knowledge).
• With the "new" DC ground bus--Run 6 AWG from battery ground bus to ground rod(s) system. For DC powered devices, run heavy cable (typically the same cable you use for +/- DC power runs to your DC loads/Inverter DC input, etc.) from DC Ground bus to AC inverter cased ground, etc. More or less, 6 AWG is good for ~200 Amp maximum branch circuit safety grounding. If your DC branch circuits have larger fuses/breakers, you need heavier wire to carry fault current safely.
• On the AC side, establish an AC safety ground common ground point (typically in your main AC panel/breaker box). Run your AC grounding from AC devices (your loads, AC outlets, etc.) back to this AC ground. And take a 6 AWG AC ground wire from your AC ground bond back to a common ground point (typically your common AC/DC ground rod/Water Pipe/Gas pipe/etc. ground "window" (ground window is a telecommunications term for the common building ground where all ground wires go to). You want avoid "ground loops" where DC and AC return current can have several paths back (i.e., return current for negative/neutral wires can share a path through your green wire safety ground).
• Neutral Bonding of your AC inverter output... I would need the exact model (or link) for your AC inverter... I the US, we use 120/240 VAC neutral bond grounded AC power. In other 220 VAC countries, they may or may not. If you have lighting in your area, neutral bonding your AC output can help reduce the chance of lighting damage (to your inverter and equipment). Whether you AC neutral bond or not your 220 VAC output, you should probably get the Midnite Surge Protection Devices and install those in your AC main panel or output of your AC inverter--They can help reduce the change for lighting damage.
• Also install SPD(s) on your solar array wiring. A the entrance to the building, and possibly even on the array combiner box (if the combiner box is installed at the array).

Comments?

-Bill
 

vince

. The inverter never performed as expected since hookup. No lightning around when the inverter failed. Where the Tiny house is now located, in a valley, the chance of lightning is average.
. the midnite kid cc does not have a arc fault detection as far as I know.
. My system is designed for 30 A current so the wiring is mostly 10 awg, less than 3-4 ft length.
. my AC side is only the connection between the inverter and the heatpump or occasional another plugin appliance. There is no panel box, yet, and no surge protector yet. I have the ground lugs of the inverter and heatpump connected to the ground rod. Regarding avoiding ground loops, I have the equipment grounds all connected in the ground bar in the connectors box and from there a bare copper wire to the earth stake.
.I have the Cotek sk1500-248 and a cotek sp3000-248, neither of which works as it should. They don't have the neutral bonded to ground as far as I can tell. The heatpump has the neutral and ground bonded internally. I will
. I have a metal roof, metal racking and have not focused on the lightning protection so far. Will tackle that after I get system to work reliably. Thanks for your comments as

BB.

You are correct. The Midnite Kid does not have arc fault protection (as far as I know).

It is highly unusual for equipment (like your A/C) to have neutral+earth bonding internally (other than possible surge suppressors and/or bypass capacitors for EMI reduction). And if you have neutral bonding in both the load and the AC mains panels--That is an example of where operational current will flow in both the white and green wires (parallel current path).

10 AWG wiring for a 3kWatt 48 VDC input inverter--That is ~84 amps at 3kWatts and 42 volt input... And the fuse/wiring should be rated at 105 Amps maximum:

• 3,000 Watts * 1/0.85 inverter eff * 1/42 volts low battery = 84 amps
• 84 amps * 1.25 NEC derating = 105 Amp minimum branch circuit rating.

Of course, if you do not plan on running at 3,000 Watts, you can downsize the wiring and breakers.

The manual recommends a minimum of #4 wire and a 275 amp fuse/breaker with a maximum of 6 feet of wiring.

http://www.cotek.com.tw/downloads/filemag/10000/1000/96/data_20160421144056.pdf

The manual talks about L/N/G connections, but does not specify if the N+G are bonded in the inverter or not.

I am not sure that this L/N/G and Bonding (inverter, A/C load, etc.) is causing you any issues (i.e., damaging the inverter)--At this point, if it was "terribly" wrong, you could have a dead short on the inverter output. But it is working to a degree--So that does not sound like the issue.

Is the Mini-Split a unit designed for the US? Or is it a foreign model? Does it have a L1/L2 input? L1/L2/N input? Or a L/N input? (plus ground).

-Bill

vince

I had it wiring designed for the 1500 W inverter as the heatpump nominal rating is 3.5A.  The heatpump, fujitsu 9rls3, is for US market so it has L1/L2 input. For the cotek 1500-248, awg 10 was recommended with a breaker of 90 A, for the 3000-248 it calls for awg 6 and a 175 A breaker minimum. I have a breaker for 50A as I don't plan to use max power output.
The 1500W inverter starts the heatpump for a few minutes but then shuts down and restarts after a minute or two etc. The cotek 3000 gives me an orange fault stating that the start up process is abnormal and does not proceed from there.
 I'll double check the info on the heatpump having the neutral and ground bonded internally. Thanks

BB.

If marked L1/L2 (plus, I assume, some sort of Ground connection), then there is no N connection? If no N connection, then in the US, both L1 and L2 are floating (not bonded to earth ground). And in the US, even if there was an N connection, there would still not be any N to G bonding in the unit.

My only suggest on looking for faults--Look at the DC input voltage on the inverter terminals and see what it does (low or high voltage?).

-Bill

Vic

vince said:

I had it wiring designed for the 1500 W inverter as the heatpump nominal rating is 3.5A.  The heatpump, fujitsu 9rls3, is for US market so it has L1/L2 input. For the cotek 1500-248, awg 10 was recommended with a breaker of 90 A, for the 3000-248 it calls for awg 6 and a 175 A breaker minimum. I have a breaker for 50A as I don't plan to use max power output.
The 1500W inverter starts the heatpump for a few minutes but then shuts down and restarts after a minute or two etc. The cotek 3000 gives me an orange fault stating that the start up process is abnormal and does not proceed from there.
 I'll double check the info on the heatpump having the neutral and ground bonded internally. Thanks

Hi vince,

You simply cannot be using #10 AWG cables for the DC connections from the inverter to the battery,  can you?   A 50 A breaker is really not  really going to be NEC-compliant protection for #10 AWG cable,  although it is probably not a real fire risk,  IMO.

But #6 AWG inverter to battery cables,  protected by a 175 A breaker is a real risk.

Is it possible that the cable vs breaker size recommendations are for sizing the Grounding conductor,  based on the size of the inverter DC breaker??

AND on the inverter running a few minutes and then shutting down,  this has always seemed to me to be a thermal issue  --  something heats up,  inverter shuts down,  something cools,  and inverter comes back on ...   etc.

Later,   Vic

vince

The input voltage on the inverter was good as indicated by the green led and measuring the voltage directly. As my load on the inverter is known and only involves the heat pump, mostly staying in the 500-800 watt range, the wiring size and breaker is ok. I got the 3000w inverter upon the advice of a cotek technician who didn't belief that the load could be that low for a heat pump and thought that I overloaded the 1500w inverter.