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

[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

[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]

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?

[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.

[russ]

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

An Oregon country mechanic - home trained.

[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?

[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.

[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.

[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]

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.

[PhillyTitan]

2) Cycling. A typical A/C, running under a moderate setting, might cycle on for 1 minute every 3 minutes to keep the room cool. So if your average power draw is 500 watts, it is taking 1500 watts while it is on and just a few watts when it is off to run its fan.

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.

[PhillyTitan]

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. Could there possibly be anything that would do the same in a 120VAC arrangement? I would think someone would have mentioned that by now but it doesn't hurt to ask.

To the OP, when is these 5 to 7 hours operation time? during the day when sun is shining or in the dark night when you go sleep?

Daylight hours would be the bulk of when it's running. Maybe turn on for a couple minutes at a time during the other hours. Does that make a difference in getting the batteries charged?

[Sunking]

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.

Stop it, you are embarrassing yourself. You have no clue what you are talking about and making ham radio operators look like fools and morons. Your credibility is completely gone here, you have proven many times you do not know a thing about electric power.

[russ]

This poor fellow is off to research stuff in the woods of Oregon.

[K7ABE]

Are you sure you are a ham ? calculate what useable power is in the cap from 13V to 12.5V



oh boy - Capacitors can not store AC. They can phase shift AC signals. You are not a ham for sure, or else you had someone else take the test.


So at some point, this poppycock is going to stop, or we are going to either clean the thread, put a disclaimer on it of "this contains factual errors", or lock it down. Choice is yours.

The capacitors I am talking about are 5F and 10F at 35V designed for use in 12 volt systems to handle the hi serge in stereo amps (I use one 'AT' my mobile HF amplifier to help with the peaks on side band). The 12 Volt compressors designed to run straight from the solar panel without any charge controller or inverter use panels rated at 12V with 22V peak.

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.

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

When the reader is not able to understanding what the writer is writing about, it does not mean that the writer does not know what they're talking about.
Abe