• Have you tried out dark mode?! Scroll to the bottom of any page to find a sun or moon icon to turn dark mode on or off!

diy solar

diy solar

low frequency inverter better for fully electric home (heat pumps)?

jmilanski

jmilanski

New Member
Joined
Apr 13, 2023
Messages
13
Location
Boulder, CO, USA
hey folks!
i recently electrified my house - heat pump for heating and cooling, heat pump for water heater. grid tied 10kW roof solar panels with battery backup, want to prepare for extended grid outages.

low frequency inverters (victron) handle surge loads better than high frequency (eg4, ruixu).

should i be considering low frequency inverters over high frequency? Thanks!
 
The #1 thing you can do for a standard HVAC heat pump is install a soft start box to reduce the surge (inrush) current.
Then it won't matter as much which kind of inverter you get.

Yes, the LF inverters have a reputation for handling surge loads better, but newer HF designs are pretty darn good.

You typically don't need soft starts for modern mini-split heat pumps because they are inverters.
Don't know about water heater heat pumps.

I've got two EasyStart 368s, search the forum for "Soft Start" or "EasyStart" for lots of commentary.
 
What matters is starting surge current of the motor, plus other loads present at the same time.
The heat pump may quote Locked Rotor Amps LRA on the label, use that. If not, multiply Full Load Amps FLA x 5 to get estimate of surge.
Or get a clamp ammeter that reads "Inrush" and measure it.

If HF inverter's rated power exceeds peak starting current and other loads combined, should be no problem.
If HF inverter has a rated surge power lasting for at least 3 seconds, that also should be sufficient.

Many LF inverters like my Sunny Island are rated for 3 second surge around 2x operating power.
Some HF like Midnight Rosie are even rated for 60 seconds surge.

Cheaper HF traditionally had poor surge, but not all of the newer ones do.

SMA has a hybrid Sunny Boy Smart Energy, it's surge is only about 1.25x operating power.

The world is definitely going high frequency. It will also be going high voltage battery.
There are several really good low frequency inverters, but likely they will be discontinued and support & related parts will become harder to get.
There are deals on Schneider LF around $1000 (about 75% off), also Sunny Island. You can't go wrong with those except regarding future support including features related to grid interaction.
HF inverters especially AIO are more likely to keep getting updates to whatever utility companies require to keep grid stable with higher levels of PV on homes.
 
The only reason now days to prefer a low frequency inverter is to start up large power tools which have a traditional AC motor on them with a large induction start up load. For instance, my table saw has a 5 HP motor and the induction is like 34,000 watts to start up. On the other hand, my 5 ton Trane XV20i start up is only 16 amps at 240 volts or 3,840 watts, which is easy for any size inverter of 6k or more. Heat pumps use the same startup technology and draw very little current to operate.

Also, a low frequency inverter is less efficient than a high frequency inverter because the large transformer in the low frequency model creates a lot of mechanical heat that has to be vented and dissipated using mostly fans.

Now, all of that aside, I did have an Outback Radian inverter, 8K capacity which is transformer low frequency and it was an absolute tank and a great operating inverter. I ran this for 8 years without a hiccup. So if you want low frequency, that is a great inverter and they can be installed in parallel to up the power outputs.
 
Last edited:
In my opinion, LF is about the only way to go if one wishes to power motors. One problem that has been carefully described by the electronics experts elsewhere here on the forum is that the startup of an inductance load (motor) will alter the sine wave of the electricity, essentially lowering the power factor (PF)...sometimes to as low as 0.4. Many inverters will choke with a power factor of less than 0.7, or even higher. This is the reason that an HF inverter rated for 12kw cannot startup a motor rated for 3kw. Even though the HF inverter could theoretically handle the power requirements of the motor, it cannot handle the offset to the PF as the motor is starting up.

I'm not the expert, and may have presented the situation inaccurately, but this is as best I remember from a post I read somewhere here not long ago.

LF inverters are more expensive to build, which is one reason manufacturers prefer making the HF ones. It's much easier to turn a profit from cheaper equipment that still purports to support the same wattage load. For myself, I would prefer an LF inverter hands down. It's the only kind I'm interested in buying. I got a 3kw LF inverter that starts and runs a metal chopsaw with ease, claiming to reach only 60% of its capacity as the motor spins up. Well pumps are another load that is a cinch for LF but might present difficulty for HF inverters.

I think it may be true that the modern refrigeration/air conditioner/heat pumps/etc. that are of the "inverter" type would likely not trip an HF inverter. But I want the peace of mind of knowing the inverter will start any motor within its range. And I've heard the HF inverters may reduce the lifespan of motors. I do not understand fully the electrical rationale for this, but I suspect it amounts to brownout conditions upon startup--not providing the full power factor and/or wattage the motor needs. Motors don't do well with brownouts.
 
Norwasian said:
This is the reason that an HF inverter rated for 12kw cannot startup a motor rated for 3kw.
Click to expand...
Maybe using technology from ten years ago.
I have a 3.5kW SRNE inverter that runs all kinds of power tools flawlessly. Log splitter, saws, air compressor, literally anything I've ever tried to run with it. It just smiles and keeps on trucking.
 
My inverter heat pump only creates a surge load when you flip the heat pump breaker on. It's a very strong inrush current to fill the capacitors.

In normal operation it's totally soft start.

It also has a really poor power factor in standby mode, like 1 amp at 240v just for 20W of real draw. I don't know what the effect of that is on a high frequency inverter. On my Schneider XW I assume this means it takes .2kW out of our total capacity all the time when it's off. Plenty of people run heat pumps on HF inverters though and I don't hear complains about this being a problem.
 
LakeHouse said:
Maybe using technology from ten years ago.
I have a 3.5kW SRNE inverter that runs all kinds of power tools flawlessly. Log splitter, saws, air compressor, literally anything I've ever tried to run with it. It just smiles and keeps on trucking.
Click to expand...
It does fine to start the 12kw heat pump compressor here ( without any soft or hardstarts
 
A key factor I think for either the HF or LF inverters is that you must have a battery bank of sufficient capacity to deliver the amps needed to start up any motor, When I had my Radian and was starting my Unisaw 5HP motor, I had also a 44,400 watt Trojan lead acid battery bank with a large inrush current on 4/0 wire to the Radian so the Radian could pull the necessary current to support start up. So plan for that in your battery bank.

And this raises the question of the person who installs a HF inverter along with a high voltage low amperage battery bank (ex. SMA hybrids). Could that type of setup start up a load as heavy as an HF inverter with a low voltage high amperage battery bank?

I don't know that answer.
 
The SMA HF inverters have anemic surge power rating, maybe 1.25x full load power.
That was addressed in the introduction presentation, but I don't spot it in the data sheet.
And you can't parallel them.


I don't think SMA has introduced equivalent of ABU, Automatic Backup Unit, for SBSE yet. There is one for SBS (which also has anemic surge rating.
Not available as a complete system yet.

We may see HV HF inverters from other brands with good surge, those designed primarily as backup and off-grid inverters.
These SMA products are a follow-on to their grid-tie Sunny Boys, with battery added for peak shaving.

Tesla of course is also HV HF inverter, initially an AC coupled battery but now also a hybrid model.

Lead-acid we can take any surge current we want. Lithium could deliver it, but BMS sets limits. As you say, plan for it. Those of us with LV batteries end up paralleling several. Only someone who makes DIY 4p16s might be limited. 16s4p would have four BMS in parallel. The HV batteries often have fairly low current rating; they could design or this application but may ignore it.
 
Schneider is selling a 7.7 kw hi frequency hi voltage (420v) inverter which they claim can surge to better than 15kw.
 
I'd be interested in hearing if anyone who has used lithium batteries with an LF inverter has had trouble with pulling a surge from the battery. It seems to me that is a problem for the HF inverters. I haven't noticed people on LF inverters talking about soft-start devices--the inverter seems well able to handle fine on its own.

My own battery setup is a 48-volt (16s1p) setup and it has handled everything thrown at it so far with no indication of overload. Granted, I have not intentionally pushed its limits to see what they might be--but as I said, it handles a chopsaw just fine, and the LF inverter is only 3kw.
 
jmilanski said:
hey folks!
i recently electrified my house - heat pump for heating and cooling, heat pump for water heater. grid tied 10kW roof solar panels with battery backup, want to prepare for extended grid outages.

low frequency inverters (victron) handle surge loads better than high frequency (eg4, ruixu).

should i be considering low frequency inverters over high frequency? Thanks!
Click to expand...
A few of thoughts;

People think LF inverter surge capacity is due to stored energy in the transformer. People will say that is for only one cycle or 1/60 of a second but they don't understand the mechanism. Its more like a pendulum then a pulse. There is also the slow switching frequency on the DC side and the less switching in the mosfets is easier on them = more load. Most high start loads are also high reactive power and LF handles it much better.

That is not the only reason to go LF, the value of transformer isolation is a BIG plus. On an HF, your mains are connected DIRECTLY to the mosfets/switching gear and are exposed to high voltage DC. If you have a MOSFET short on an LF, the transformer would likely peak saturate before you blow up your nice TV and other expensive things in your house.

Then there is the extended life with and LF since transformer saturation/inertial helps with things like reactive power and that is much better for switching gear life.

The lower switching frequency extends the life of the mosfets and other components.

The design is simpler. Less parts to fail.

Thats the quick version but there are other benefits like having the ability to double ground + and - DC leads from your panel array to earth to prevent PID.

There is no case where it wouldn't be better for and LF over and HF.
 
sunsurfer said:
A few of thoughts;

People think LF inverter surge capacity is due to stored energy in the transformer. People will say that is for only one cycle or 1/60 of a second but they don't understand the mechanism. Its more like a pendulum then a pulse. There is also the slow switching frequency on the DC side and the less switching in the mosfets is easier on them = more load. Most high start loads are also high reactive power and LF handles it much better.

That is not the only reason to go LF, the value of transformer isolation is a BIG plus. On an HF, your mains are connected DIRECTLY to the mosfets/switching gear and are exposed to high voltage DC. If you have a MOSFET short on an LF, the transformer would likely peak saturate before you blow up your nice TV and other expensive things in your house.

Then there is the extended life with and LF since transformer saturation/inertial helps with things like reactive power and that is much better for switching gear life.

The lower switching frequency extends the life of the mosfets and other components.

The design is simpler. Less parts to fail.

Thats the quick version but there are other benefits like having the ability to double ground + and - DC leads from your panel array to earth to prevent PID.

There is no case where it wouldn't be better for and LF over and HF.
Click to expand...
I like it. Now can some one please tell me why Outback Power fails to package a Radian inverter into an All In One Hybrid box like Midnite and others have done?
 
glandpuck said:
I like it. Now can some one please tell me why Outback Power fails to package a Radian inverter into an All In One Hybrid box like Midnite and others have done?
Click to expand...
Have you ever tried to lift a Radian inverter? Adding in 8kw of charge controller would break my back.:)
 
glandpuck said:
I like it. Now can some one please tell me why Outback Power fails to package a Radian inverter into an All In One Hybrid box like Midnite and others have done?
Click to expand...
If you look at most HF AIO inverters, there is a narrower operation window for input voltages on the MPPT. Most have a minimum voltage where they work. I suspect there is issues with getting a good boost circuit to blend with a buck circuit on the high voltage DC side of things. Imagine trying to blend a boosted MPPT say around 100v while a cloud passes by then blend over to bucking voltage as you get full sun again. This is while the MOSFETS are switching like mad trying to maintain the sinewave. Add in at the same time blend mode from grid power.

Most AIO are built around being cheap. To do it really well with and LF and really good MPPT, its likely more expensive with a shared bus then to just keep them separate. I think mostly what makes it work with HF is the high switching frequency allows enough "time" to make it all mostly work with good software algorithms. EDIT - I wanted to add that with a transformer in the mix with all its time vs saturation nuances and the slower DC switching side, it might be harder to play "NICE" with software switching algorithms. There is not enough "time" due to slower switching to just compensate for the extra pieces (MPPT) as easily, likely there has to be prediction in the mix even with HF speeds. Its all to complicated if you ask me. I think just keep it simple and separate. Why chase down a few percent and over managing features? Just give me an on/off switch.

Personally, I avoid the AIO. I think its better to have the MPPT hooked just to the batteries. Why complicate things?

How about a car where the transmission and engine are the same unit? Sounds nice right? Might as well put the brakes in there too.
 
Last edited:
You must log in or register to reply here.

Similar threads

E
Low or high frequency inverter
4 5 6
Replies
130
Views
3K
ksmithaz1
ksmithaz1
C
Low frequency or high frequency inverter
Replies
8
Views
888
sumpower
S
ddxv
DC Solar vs Solar to Inverter AC for Heat Pumps (off-grid)
Replies
3
Views
206
timselectric
T
J
Reliability on Sungold Low Frequency Inverter
Replies
1
Views
289
SparkyJJO
SparkyJJO
C
Using a mini split for heat in the winter?
7 8 9
Replies
200
Views
7K
Brucey
Brucey

diy solar

diy solar
Back
Top