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Will this addition be adequate to start the heat pump?

pvdude

Solar Enthusiast
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Florida
Just completed installation of a Schneider Conext 6848 XW+ Pro inverter, and associated accessories.
It was initially to run the well pump and some other smaller loads.
Works great for that. Lots of extra capacity available.

We did an experiment to see how it would do running the entire house.
Turned off:
- WELL PUMP
- POOL PUMPS
- DRYER
- WASHER
- DISHWASHER
- HEAT PUMP
- WATER HEATER

Typical house load w/ the heavy loads disconnected is about 1500 watts.
Tried to start the heat pump, no luck.

The 6848 specs claim it will sustain a 12kw load for 30 seconds. (50A of 240vac)
That is more than enough to start the well pump, LRA = 41A.
The unmodified heat pump compressor LRA is 121A, far over the load rating tor the inverter.

However, I put a “Hard Start” kit on the heat pump compressor some years ago, as the 21kw Diesel generator was not happy starting the heat pump either.

Kit worked great, the generator can start & run the heat pump easily, after the modification to the heat pump.
I have no idea how many amps it takes to start the heat pump now, after the starting circuit modification.
But it requires less than the 87A from the genset, and more than the 50A capacity of the Schneider 6848 inverter.

So if I add a second Schneider 6848 to #1, (I think Schneider calls it cascading the inverters, of some such), there would be 100A of 240vac available, which would probably start the heat pump compressor easily.

Note that after the initial start spike, the heat pump compressor and air handler blower run @ about a total of 3kw.

What do you think of the idea to add a second inverter?
 
That should work but you may want to check to see if you could use a variable frequency drive for your heat pump as that may be less expensive and more efficient.
 
I’ve seen deep well pumps staggered from too light of cables so I’ve got to ask — Can you temporarily move the inverter & battery close, as in right on top of the compressor, with shortest and heavier DC/AC leads and see if it starts, that the existing AC power line might be too light & long, or too many junctions etc.?

Also the old Trace 5548 etc. inverters called out keeping (+) & (-) cables zip tied together as much as possible to reduce capacitance effects on surge & high current loads.

And somewhere along here it is likely smarter to get a new heat pump with variable speed inverter drive with the soft start built in… just double up on line noise & lightning protection as the controls circuits are their weak links
 
The heat pump is fairly new (2017), and has a variable speed compressor and air handler.
The problem is the “one second” of initial startup current draw.
The “soft start” circuit I installed works great to reduce that instant of starting load, but it is a 5 ton compressor, and needs a big “jolt” to get going, even w/ the soft start.

After the first second, the current to keep the heat pump running is minimal, very energy efficient.
Not sure how the heat pump electronics would react to a VFD.
(I have a TECO-Westinghouse VFD on the mill, but it is a simple motor, no electronics)

The solar equipment is installed in a separate building about 200ft behind the house, can’t think of a way to move it to the house for experiments.

solar_installation_2021.jpg

So I’m going to go ahead and start saving $$$$ for a second inverter, and the related equipment required to combine inverter#1 and inverter#2 together.
Good Winter project to look forward to!
 
Wondering if your batteries are able to supply the surge current required to support the heatpump startup draw? Doesn't the XW manual stipulate the minimum battery capacity required? That bank of L16 type batteries probably doesn't have sufficient capacity to support such a load. Lithium batteries have a much lower internal resistance. Also, if the batteries weren't in 100% condition, internal resistance may have built up that limits surge current. You might get a meter that can read fast enough transients to check what the battery bank gets drawn down to while trying to start the heat pump. That will answer the question if the problem is the batteries. The XW has the reputation of being able to supply massive surges, as long as provided with sufficient battery capacity. I think that 8 L16 batteries is towards the minimum battery capacity recommended for the XW.
 
I’ll know soon if stacking works so as to start the heat pump compressor.
The batteries are all I could afford for phase-1 of this experiment.
I followed this recommendation from the manual in selecting 445Ah capacity:

xw_pro_battery_capacities.png

The XW Pro is configured to source up to 150A. (That must be the default.)

invertermaxdischargecurrent.png

Not sure if 7500 watts is adequate.
I will ask Schneider what I can set the “Max discharge current” value to.
The Rolls S6-L16-HC have a CCA rating of 1040A, MCA of 1299A.
Thus they can source much more than 150A.

Have received most of the additional equipment to add the second XW Pro!

more_schneider_parts_jan2022.jpg
 
You initially said “hard start” then later, “soft start”, then “variable speed”. Which is it??? I hope your 200’ a/c circuit is properly sized. Non-variable speed heat pumps dislike undersized wires. I run into many that are.

Variable speed heat pumps and air handlers, like my American Standard 20 SEER have no inrush amps. No hard or soft start kit needed. It slowly ramps up from 0-8A or even 12A TOTAL, depending on the load.
 
I learned that “hard start” kits for the heat pump compressor did not work like I assumed.
Anyway, the “5-2-1 compressor saver” hard start kit I installed a couple of years ago was a complete
waste of time and $$, the crappy relay contacts in it melted quickly.

Reading this forum educated me about what a “soft start” device is.
(Seems to operate similar to the VFD on my mill)

I measured 132LRA on my compressor start.

So if the inverter upgrade won’t start the compressor, I’ll install either a “Hyper engineering” or a
“Micro-air easy start”.
 
HVAC is my profession. I’d install the soft start kit either way. Make sure your run capacitor and contactor is good. Your starting amps will be 30-40% with a soft start and even with the second inverter, it’s much better on them, your contactor, disconnect, breakers, windings and other appliances

Yes, hard start kits are made for locked-up compressors. They usually just buy one some time until the compressor or unit gets replaced. I’ve had some burn/blow up a week later and a few compressors lasted 2-10 years longer.

Try two smaller hp systems when you get that one replaced. Redundancy, more efficient and even cooling throughout house. Then a breakdown becomes a nuisance instead of an emergency.

I also installed a SPST toggle switch on my electric heat kits and defrost thermostat circuits. I don’t want them determining when to energize. I’ll light the wood burner below 40F. Many defrost cycles are unnecessary, even when set on 90 minute intervals during drier winter conditions.
 
Regarding battery capacity as related to surge capacity:

1) CCA is an automotive standard related to current supplied to starting motors - as a measure of current supplied allowing 60% voltage sag at that current. 1040 A CCA doesn't necessarily mean that the batteries under load can supply sufficient voltage at 1040 A to keep the inverter supplied. The inverter probably clips the waveform more and more as the battery voltage supplied collapses under higher and higher starting surge current loads.
2) At a nominal 48 volts (which the battery bank surely won't supply at peak draw due to internal resistance) the 150 A DC software limit on inverter draw would equate to 7200 Watt surge (neglecting battery sag). At 240 VAC output, assuming say 80% overall efficiency (90% inverter, less say 10% cabling losses) an inverter input limited to 150 Amps at 48 volts equates to 24 Amps AC output, less than the 87 A that the generator has to supply to start the heat pump compressor.

I'd check with Rolls to find out what the internal resistance of your batteries is so that you can calculate voltage drop under extreme loads. You'd then have to compare with the inverters input voltage specifications.

I don't mean to rain on your parade but ff the battery resistance (surge capacity) is the limiting factor, having two inverters won't necessarily be the solution. However, even with insufficient battery current available, there may be some benefit in terms of surge capacity from the energy stored in saturating the coil of the second inverter.
 
I adjusted the “Max discharge current” value to 200A.
(Will probably go higher after I have Schneider review it.)
Schneider rates the peak output of two stacked 6848 XW Pro @ 17kva (about 70A)
The starting LRA of the heat pump compressor is an instantaneous peak, lasting less than one second.
The actual current load drops to about 12A, with the compressor and air handler running.
Looking forward to completing the next phase to find out how well it works!
 
I adjusted the “Max discharge current” value to 200A.
(Will probably go higher after I have Schneider review it.)
Schneider rates the peak output of two stacked 6848 XW Pro @ 17kva (about 70A)
The starting LRA of the heat pump compressor is an instantaneous peak, lasting less than one second.
The actual current load drops to about 12A, with the compressor and air handler running.
Looking forward to completing the next phase to find out how well it works!
@pvdude any updates to this thread. Very interesting topic and I’d like to undetsnd the final results.
 
Update!
I installed the Micro-Air Easystart on the heat pump compressor.
LRA dropped from 132A to 38A.
Schneider inverters start the heat pump with ease now.
During the day, w/ full sun available, the timers have the pool pumps running from 10AM to 4PM.
2500 watts continuous.

When the heat pump turns on, that adds about 2100-3000 watts.
So there are typically about 4500 - 5300 watts from the PV / charge controller running the loads via the inverters during those hours.

The well pump (41 LRA) cycles a couple of times during the day, adds 1800 watts to the load for about 60-90 seconds needed to fill the pre-charge tank.

Clouds attenuate the PV output.
I have “Grid support” enabled in the inverters, so they draw from the grid to make up for temporary PV energy shortages.
Looking forward to much lower energy bills this Summer!
after copy.jpg
 
Glad to hear Micro-Air Soft Start worked. I have them on two out four of our split hp's. Other two have variable speed compressors, outdoor fans and indoor blowers. Sol-Ark 12k's love them too.
 
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A lot depends on particular unit. Heat pump does not mean they are inverter variable speed compressor. Inverter-compressor does not mean they have power factor corrected power supply to run inverter. Inverter-compressor does not draw less power for given BTU size unit until inside air temperature and humidity is brought down. LRA is not the maximum current that a single-phase induction motor can draw.

A hard start kit is intended for the case where you have too much AC voltage drop to unit during startup surge. It can actually increase the startup surge current. It is just putting a large start capacitor temporarily in parallel with run capacitor to increase start winding current.

To improve start current on a regular compressor with run capacitor you need a coordinated combo hard start capacitor boost to start/aux winding and a soft start voltage ramp, like EasyStart, SureStart, SecureStart, etc. unit. These units will not provide much benefit for motors with just large starter caps with centrifugal switches to disengage start capacitor, like well pumps and air compressors.

Cap Start Induction Motor_2.png
 
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