Solar powered AC air conditioning, a bummer or a challenge?

[PhillyTitan]

Many engineers seem unable to understand how things really come together to make things work.

That's the most ignorant thing I've ever read and you just made me angry. Engineers are the only ones who know what's happening down to the lowest, most obscure levels. That's why they know how things work. Do you actually know why a letter appears on your screen when you type it on your keyboard? It's not a magical demon that lives in your computer. Only an EE or an ET with significant electronics schooling can understand it or explain it. You can't learn this stuff perusing the internet.

[J.P.M.]

That's the most ignorant thing I've ever read and you just made me angry. Engineers are the only ones who know what's happening down to the lowest, most obscure levels. That's why they know how things work. Do you actually know why a letter appears on your screen when you type it on your keyboard? It's not a magical demon that lives in your computer. Only an EE or an ET with significant electronics schooling can understand it or explain it. You can't learn this stuff perusing the internet.

It's been my experience that some folks who would call themselves engineers wouldn't make a wart on an engineer's butt, but feel qualified for the title and superior to the great unwashed masses due to having been fortunate enough to sit in a classroom but take up little more than space and come out with an attitude and a tuition receipt they call a diploma. I suspect those are some of the types K7ABE is referring to. They're pretty ubiquitous. Others quietly go about their business and practice real engineering.

Just my $.02 having interviewed several hundred engineers over the years, hiring some and firing a few.

[PhillyTitan]

It's been my experience that some folks who would call themselves engineers wouldn't make a wart on an engineer's butt, but feel qualified for the title and superior to the great unwashed masses due to having been fortunate enough to sit in a classroom but take up little more than space and come out with an attitude and a tuition receipt they call a diploma. I suspect those are some of the types K7ABE is referring to. They're pretty ubiquitous. Others quietly go about their business and practice real engineering.

Just my $.02 having interviewed several hundred engineers over the years, hiring some and firing a few.

Notice I didn't say "all" engineers can understand it. I said "only" engineers.
That clown certainly has no room to make a statement like that.

[billvon]

Please chime in folks because this is important. I was under the impression that an air conditioner rated at "500 watts" was the power it's running while at full blast, not the average power running at different cycles.

If you look at the nameplate rating and you see "1000 watts" then it will draw a max of about 8 amps during operation (~960 watts.) If it has a thermostat that's cycling on and off you'll see close to zero watts then something close to 960 watts for a while. That doesn't include the surge, which is generally much larger. Since a short starting surge won't blow most residential AC breakers it's ignored.

However nameplate ratings are notoriously inaccurate, both because they may not be measured at the point you think they are (i.e. they may be worst-cased at 132 volts instead of 120 volts) and because they are sometimes just plain wrong. A quick test with a Kill-a-watt (pretty good) or a scope + current probe (best) will give you much more accurate numbers.

[billvon]

A high Farad capacitor from the automotive stereo industry Would handle the inrush for a 12 volt DC system. I am not aware of any capacitors for a 120 volt AC system that will handle that load.

Capacitors don't work to reduce surge on AC systems. Adding one will generally just make things worse by creating a poor power factor.

The fan motor on all of the larger AC units I am aware of will not start without a capacitor. The capacitors give a DC Jolt to the AC motor to start it spinning, then it runs on AC. Capacitor start AC motors have been around for a long time it helps keep the size and expense down. And as I said I do not know of any capacitor that will start a 120 AC unit.

This is really jumbled. There is no "DC jolt" on starting.

Run capacitors on motors provide a current lag on a secondary winding of the motor. A single phase AC induction motor will not start reliably because a single phase cannot create a rotating field. By adding a capacitor and a secondary winding, you create the spinning field that will always start the motor. These capacitors are connected all the time.

You can also use a start capacitor that creates this spinning field only during startup. Once the motor is running reliably you can disconnect it. These help with startup but are more complicated since you need a switch. (Once it's running you can either use a regular run capacitor or allow it to free run with the single phase only.)

For larger motors, where efficiency is important, three phase power is used. This generates a spinning field without any capacitors required.

Many engineers seem unable to understand how things really come together to make things work.

You seem to not understand induction motors. Are you an engineer?

[russ]

You seem to not understand induction motors. Are you an engineer?

An Oregon country mechanic - home trained.

[PhillyTitan]

If you look at the nameplate rating and you see "1000 watts" then it will draw a max of about 8 amps during operation (~960 watts.) If it has a thermostat that's cycling on and off you'll see close to zero watts then something close to 960 watts for a while. That doesn't include the surge, which is generally much larger. Since a short starting surge won't blow most residential AC breakers it's ignored.

However nameplate ratings are notoriously inaccurate, both because they may not be measured at the point you think they are (i.e. they may be worst-cased at 132 volts instead of 120 volts) and because they are sometimes just plain wrong. A quick test with a Kill-a-watt (pretty good) or a scope + current probe (best) will give you much more accurate numbers.

Thank you for that. I will certainly test any A/C current and wattage now before going through with hooking it up to an inverter.

[PhillyTitan]

You can also use a start capacitor that creates this spinning field only during startup. Once the motor is running reliably you can disconnect it. These help with startup but are more complicated since you need a switch. (Once it's running you can either use a regular run capacitor or allow it to free run with the single phase only.)

I can probably build a simple automatic switch circuit for this if that is the only solution to get the current draw down on startup (not that I'd enjoy doing this). But how do I know the size and how to connect the capacitor?

[billvon]

I can probably build a simple automatic switch circuit for this if that is the only solution to get the current draw down on startup (not that I'd enjoy doing this). But how do I know the size and how to connect the capacitor?

It will be unlikely that you will be able to do anything with a standard window A/C compressor to reduce its starting surge. Induction motors want to match the speed of their field (i.e. 60Hz or 3600rpm for a one pole motor) instantly, and playing with the cap value won't help. (You also can't even _get_ to the compressor since it and the motor are sealed in a hermetic container.)

One thing you can do is to drive just the compressor with a variable frequency inverter motor drive (VFD). In other words, start at a lower frequency (=lower speed) and ramp up from there, which is what a lot of mini-splits do. This, however, will tend to be difficult and expensive and probably not worth the effort.

[PhillyTitan]

It will be unlikely that you will be able to do anything with a standard window A/C compressor to reduce its starting surge. Induction motors want to match the speed of their field (i.e. 60Hz or 3600rpm for a one pole motor) instantly, and playing with the cap value won't help. (You also can't even _get_ to the compressor since it and the motor are sealed in a hermetic container.)

One thing you can do is to drive just the compressor with a variable frequency inverter motor drive (VFD). In other words, start at a lower frequency (=lower speed) and ramp up from there, which is what a lot of mini-splits do. This, however, will tend to be difficult and expensive and probably not worth the effort.

Bummer.
Since that is the case I've been working on a prototype off-grid air conditioner.
The fan battery was charged and ice was made in the grid-tied side of the house. LOL

[Sunking]

Hey gentlemen, I've been absent for a couple of days. One thing (probably the only) that Abe mentioned did make sense: I believe a high farad capacitor would be helpful for a startup surge in a DC situation.

Will not do anything. A battery is a extremely large capacitor that does not leak and has magnitude of several hundred times more capacitance than a super capacitor. What you are wanting to believe is you can change the level of a lake by peeing in it. Good ole ABE is a crack pot who knows nothing of electronics or power.

The issue is not with the battery, it is the inverter itself that cannot supply high starting currents and low Power Factors. Inverters are rated in KVA at some power factor. The best of inverters can only handle a power factor of .7 That would mean a 1 Kw inverter can only supply up to 1.4 Kva starting power. If you had say a 600 watt motor with a 7 x FLA it would take an inverter of 4500 KVA @ .7PF or a 3200 watt inverter to run a 600 watt motor. No way around it.

This is exactly why off-gridder use DC powered refrigeration equipment, and do without Air Conditioning unless money is no object. You are going to have to accept that fact. YOu wanna stay cool, use AC power, or dump a lot of bucks needlessly. If you just have to take your Air Conditioner off-gir you are going to have to use a DC split Air Conditioner. They are very expensive but less expensive overall trying to power conventional Air Conditioning with AC power from solar.

If you are smart and wise with your money, forget trying to cool or take anything off-grid. Take that money and invest it. In 10 years you should catch a double or quadruple. Go solar and it is all gone.

[paulcheung]

Will not do anything. A battery is a extremely large capacitor that does not leak and has magnitude of several hundred times more capacitance than a super capacitor. What you are wanting to believe is you can change the level of a lake by peeing in it. Good ole ABE is a crack pot who knows nothing of electronics or power.

The issue is not with the battery, it is the inverter itself that cannot supply high starting currents and low Power Factors. Inverters are rated in KVA at some power factor. The best of inverters can only handle a power factor of .7 That would mean a 1 Kw inverter can only supply up to 1.4 Kva starting power. If you had say a 600 watt motor with a 7 x FLA it would take an inverter of 4500 KVA @ .7PF or a 3200 watt inverter to run a 600 watt motor. No way around it.

This is exactly why off-gridder use DC powered refrigeration equipment, and do without Air Conditioning unless money is no object. You are going to have to accept that fact. YOu wanna stay cool, use AC power, or dump a lot of bucks needlessly. If you just have to take your Air Conditioner off-gir you are going to have to use a DC split Air Conditioner. They are very expensive but less expensive overall trying to power conventional Air Conditioning with AC power from solar.

If you are smart and wise with your money, forget trying to cool or take anything off-grid. Take that money and invest it. In 10 years you should catch a double or quadruple. Go solar and it is all gone.

Can a Capacitor bank increase the power factor on the inverter or the motors? In Jamaica there is a new trend that some engineers is sell some capacitor bank to increase the efficiency on motors on the house hold circuit. They come to the house and measure the power factors on the refrigerators, water pumps and wash machines etc. If they find any item that has bad power factors they put a capacitor bank to increase the power factor close to 1. they charge from $150 to $500 US per capacitor bank. Some people say they do save a good amount electricity consumption.

In the OP situation I wonder if he use most of the time on sunny time. Can he get 1200 watt PV panels and a 1500watts pure sine inverter with 3000watts surge capacity, and a 24 volts bank of AGM batteries with 200 amps hours. only use on sun hours and may be one hour after hour? would that work?

[Sunking]

Can a Capacitor bank increase the power factor on the inverter or the motors? In Jamaica there is a new trend that some engineers is sell some capacitor bank to increase the efficiency on motors on the house hold circuit. They come to the house and measure the power factors on the refrigerators, water pumps and wash machines etc. If they find any item that has bad power factors they put a capacitor bank to increase the power factor close to 1. they charge from $150 to $500 US per capacitor bank. Some people say they do save a good amount electricity consumption

It is complete FRAUD. For starters there are not enough inductive loads to worry about in a residence. Industrail is different where the can be running multiple large horse power motors.

But the nail in the coffin, is utilities do not monitor, record. or bill for Vars. So those folks who claim it lowered their bills are brainwashed. Utilities only charge for True Power, not VARs

[PhillyTitan]

In conclusion

Well, I guess the answer to this thread's title is overwhelmingly BUMMER.

I found a YouTube channel called DIYWORLD where this guy did successfully run his 120VAC air conditioner overnight one time time in his off-grid camper trailer. But I believe he had several batteries, and at least 2kW worth of solar panels and his batteries saw significant drain (which I have learned here will lower your battery cycles significantly. So this is not a good idea and I shouldn't even try.

I will setup a solar panel system still, but I will have to revisit the drawing board to see how I'm going to do it.
I will now retire to my dark corner and silently weep.

[billvon]

Can a Capacitor bank increase the power factor on the inverter or the motors? In Jamaica there is a new trend that some engineers is sell some capacitor bank to increase the efficiency on motors on the house hold circuit. They come to the house and measure the power factors on the refrigerators, water pumps and wash machines etc. If they find any item that has bad power factors they put a capacitor bank to increase the power factor close to 1. they charge from $150 to $500 US per capacitor bank. Some people say they do save a good amount electricity consumption.

That won't save any money. Electric meters measure real power, not reactive power.

If your power factor is very bad then correcting it will help with an inverter trying to drive that load. However the load has to be pretty bad (i.e. you are running large three phase ventilation fans or pumps) for it to make a big difference. For facilities like municipal water pumps utilities often use large capacitor banks to correct power factor because large pumps can be very reactive.

In the OP situation I wonder if he use most of the time on sunny time. Can he get 1200 watt PV panels and a 1500watts pure sine inverter with 3000watts surge capacity, and a 24 volts bank of AGM batteries with 200 amps hours. only use on sun hours and may be one hour after hour? would that work?

That would work for a while. But spend enough summer evenings with the A/C on and you'll kill the bank pretty quickly.