DC to AC conversion and power relation

[Alkemist]

I saw this "AC anywhere" last week and was wondering how it works? How we can get 230 volts out of 12 volt DC supply? Isn't it drawing more power from less? [Below is the description of the Device which I'm discussing about]

Running out of power is a thing of the past with the AC Anywhere. Simply plug the unit into any 12-volt DC vehicle cigarette lighter socket and the AC Anywhere converts your vehicle's battery power to 230 volts AC (common household current.) The AC Anywhere is perfect for charging batteries for your mobile phone, portable computer, electric shaver, TV/VCR, or virtually any device while travelling. The AC Anywhere inverter features a convenient On/Off switch and fuse protection along with an audible alarm that sounds if the vehicle battery voltage drops to 10.6 volts DC.

 

[davenn]

hi Alkemist

yes they are wonderful devices :) I have one in my car for running ni cad battery chargers, laptop and other things, when out on the road stormchasing.

Isn't it drawing more power from less?

No, you can never do that, nothing is 100% efficient, let alone 120% or more :)

now, my unit can supply 150W @ 230VAC so ~ 1.5A. But its drawing ~ 15Amps @ 12V from the car battery to get that. Its rated efficiency is 90%.

very basically the 12V is supplied to an oscillator to generate an AC voltage. That voltage is then stepped up in a transformer to 230V

Dave

 

[Alkemist]

Thanks for your reply Dave.

So can I say that if a battery runs for 6 hours at 12 volt output will be drained within an hour (or may be little less or more) if we draw 230 volts from it? I mean there must be some power limitation?

Secondly can I run any kind of device on it. Like a motor of 5 HP, which requires 230 volt to operate?

hi Alkemist

yes they are wonderful devices :) I have one in my car for running ni cad battery chargers, laptop and other things, when out on the road stormchasing.



No, you can never do that, nothing is 100% efficient, let alone 120% or more :)

now, my unit can supply 150W @ 230VAC so ~ 1.5A. But its drawing ~ 15Amps @ 12V from the car battery to get that. Its rated efficiency is 90%.

very basically the 12V is supplied to an oscillator to generate an AC voltage. That voltage is then stepped up in a transformer to 230V

Dave

 

[davenn]

Thanks for your reply Dave.

So can I say that if a battery runs for 6 hours at 12 volt output will be drained within an hour (or may be little less or more) if we draw 230 volts from it? I mean there must be some power limitation?

Secondly can I run any kind of device on it. Like a motor of 5 HP, which requires 230 volt to operate?

generally I keep the car engine running if I am using the inverter, as its a pretty heavy drain on the battery. So no it wouldn't last long if it wasnt being contineously charged.
The limitations would be
1) the capabilities of the inverter
2) the car battery
3) the charging capabilities of the car system to keep the battery charged above what is being drained from it by the inverter

well yes you can run anything from it .. within the wattage rating of the inverter.
I don't know what current rating your 5 HP motor is. For example sake let's say its 5 Amps @ 230V V x A =W so 5 x 230 = 1150Watts.
So obviously my 150 W inverter aint going to do the job. I have seen inverters up to ~ 5000W. As you could imagine their current drain at 12V would be huge !

http://performanceshop.com.au/power-inverters.html?gclid=CNur1emT7qYCFQbhbgod9wfpGQ

Dave

 

[Alkemist]

generally I keep the car engine running if I am using the inverter, as its a pretty heavy drain on the battery. So no it wouldn't last long if it wasnt being contineously charged.
The limitations would be
1) the capabilities of the inverter
2) the car battery
3) the charging capabilities of the car system to keep the battery charged above what is being drained from it by the inverter

well yes you can run anything from it .. within the wattage rating of the inverter.
I don't know what current rating your 5 HP motor is. For example sake let's say its 5 Amps @ 230V V x A =W so 5 x 230 = 1150Watts.
So obviously my 150 W inverter aint going to do the job. I have seen inverters up to ~ 5000W. As you could imagine their current drain at 12V would be huge !

http://performanceshop.com.au/power-inverters.html?gclid=CNur1emT7qYCFQbhbgod9wfpGQ

Dave

I'll check the rating of Motor I have. I have one more question. Can we get 230 v DC from 12 volt DC battery in similar way? or can we Transform that 230 AC volt output to 230 DC volt using rectifier?

 

[davenn]

I'll check the rating of Motor I have. I have one more question. Can we get 230 v DC from 12 volt DC battery in similar way? or can we Transform that 230 AC volt output to 230 DC volt using rectifier?

for that sort of output 230VDC you would have to invert it to AC then rectify it.
for much smaller voltage changes say 12VDC to 24 or so volts DC there are nice DC to DC regulator circuits called Buck Converters.

look at datasheets for IC's like the Linear Technologies LT1071 its quite a versatile beastie :)

Dave

 

[russ_watters]

For clarity: Alkemist, voltage isn't power and you don't draw 120 V from a 12V battery, you draw 12V DC and convert it to 120V AC with an inverter and transformer.

 

[davenn]

For clarity: Alkemist, voltage isn't power and you don't draw 120 V from a 12V battery, you draw 12V DC and convert it to 120V AC with an inverter and transformer.

Hey Russ

A small but important technical point, thanks for pointing that out :)
I hadnt picked up on that.

Dave

 

[Fish4Fun]

Davenn,

To further clarify what Russ stated:

Power = Volts * Amps

To convert voltage from one level to another:

V1 * A1 = V2 * A2 * CE

Where CE=Conversion Efficiency, and CE < 1

So, if:

V1 = 12V
A1 = 100A

then P1 = 1200W

If V2 = 240V
CE = 0.90
then
A2 = 5A * 0.90 = 4.5A
P2 = 4.5A * 240V = 1080W

Hopefully that will make it clear.

Fish

 

[davenn]

Davenn,
To further clarify what Russ stated:
Power = Volts * Amps
To convert voltage from one level to another:
V1 * A1 = V2 * A2 * CE
Where CE=Conversion Efficiency, and CE < 1
So, if:
V1 = 12V
A1 = 100A
then P1 = 1200W
If V2 = 240V
CE = 0.90
then
A2 = 5A * 0.90 = 4.5A
P2 = 4.5A * 240V = 1080W
Hopefully that will make it clear.
Fish

ahhhhhhhhh... hahaha... you have done something I hadnt figured out b4, that is how to factor in the conversion efficiency.
thanks for that

Dave

 

[Alkemist]

So today I checked few 3 HP and 5 HP Motors in shop. They had ratings on it something like this:

For 3 HP motor :
1] Output = 2.2 /3 kw/HP [this means 2.2 Kw and 3 HP motor], 440 Volt, 6 amp. They have not mentioned the Resistance anywhere.
2] HP = 3, Amp = 11, Volt = 230/250

For 3 HP motor the KW was in the range of 2.2 Kw - 2.80 kw.

For 5 HP motor : I just remember it is somewhat in 250 - 440 volts range and Amp of 6 - 13.
The Kw for this might be somewhere in the range of 4 to 5 Kw.

So now my question is can we run these motors on 12 - 24 volt DC inverter devices? How I can determine which inverter I should choose to run these motors? I mean KW factor? Should I need to bother about resistance factor of motor?

 

[vk6kro]

If the source of power is a 12 volt car battery, you might consider 20 amps as a reasonable upper limit for continuous current.

So, if you accepted this, the maximum power available would be 240 watts. (ie 12 volts times 20 amps). This is about 0.32 HP and it will be less if you include less-than-perfect efficiency.

So, although almost anything is possible, running 3 HP motors off a 12 volt system isn't really a practical proposition.

3 HP is 2238 watts and a 90% efficient inverter would have to be supplied with 2487 watts.
2487 watts at 12 volts is 207 amps.

A car battery may be able to supply this for half a minute while starting a car, but it would run flat very soon and possibly be damaged if it had to produce this current continuously.

 

[russ_watters]

...So the only way around that would be a bank of batteries in parallel.

 

[vk6kro]

That would be one way.

The Toyota Prius uses a large bank of NiMH batteries giving 201 volts at 36 HP or 27 KW.
That would be drawing a current of 134 amps.

http://www.toyota.com/prius-hybrid/specs.html

So, a series / parallel arrangement would be good, however, you would have to find a suitable inverter for whatever voltage you ended up with.

 

[russ_watters]

There are a bunch of 24V inverters out there, which I should have guessed because boats and solar panels are 24V, so yeah, series/parallel car batteries at 24V would be an easy way to drop the amperage a little. Looks like 5hp will run about $2,000:

http://www.google.com/search?source...210&sa=N&biw=1269&bih=828&fp=8d9de31011ecae5a

 

[Alkemist]

Why I'm trying to do is, I was thinking it would be cheaper to use batteries than using a Diesel engine to run 3 or 5 hp water pump motor in Agriculture irrigation purpose. Banking of batteries could be considered if it runs for couple of hours.

The place where I live have unreliable electricity supply and getting diesel is twice expensive about $ 6 / gallon. So considering the usage of around 200 - 300 hours in a year would be cheaper.

 

[russ_watters]

Why I'm trying to do is, I was thinking it would be cheaper to use batteries than using a Diesel engine to run 3 or 5 hp water pump motor in Agriculture irrigation purpose. Banking of batteries could be considered if it runs for couple of hours.

The place where I live have unreliable electricity supply and getting diesel is twice expensive about $ 6 / gallon. So considering the usage of around 200 - 300 hours in a year would be cheaper.

Well, you'll have to do some calculations to see if it is really economically viable: you didn't say what electricity costs and you'll have to know that to know what it costs to charge the batteries. Do the full calculation to get $ per kWh for grid power and battery power, multiply the difference by the number of hours in a year, then compare with the cost of an inverter to see how long it takes to pay back.