Amperage via inverter
Submitted: Saturday, Dec 02, 2006 at 08:43
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Member - Gary W (NSW)
If a 240 volt fan is rated at 40 watts, then it is using 6 amps.
If the same fan is run off 12 volts via an inverter, is it still drawing 6 amps?
Gary.
Reply By: joc45 - Saturday, Dec 02, 2006 at 08:50
Saturday, Dec 02, 2006 at 08:50
Hi Gary,
No, Watts = volts x amps, so a 240v fan rated at 40 watts is:
40/240 = 0.16A
At 12v thru an inverter, the 40w translates to:
40/12 = 3.33A
But there will be less than 100% efficiency (about 80-90%), so increase the amps by that much.
Cheers,
Gerry
AnswerID:
208110
Reply By: Notso - Saturday, Dec 02, 2006 at 08:52
Saturday, Dec 02, 2006 at 08:52
A 40 watt fan using 240 volts would use about .16 amps, at a nominal 12 volts the inverter would use 6 amps plus any losses in the system due to the fact that an inverter is less than 100% efficient.
AnswerID:
208111
Follow Up By: Notso - Saturday, Dec 02, 2006 at 08:54
Saturday, Dec 02, 2006 at 08:54
Actually Joc is correct about the amps from your battery. My mistake there.
FollowupID:
468058
Reply By: Member - Gary W (NSW) - Saturday, Dec 02, 2006 at 09:25
Saturday, Dec 02, 2006 at 09:25
Thanks Gerry,
Fast response and
well explained.
Gary.
AnswerID:
208121
Reply By: Derek from Affordable Batteries & Radiators - Saturday, Dec 02, 2006 at 10:18
Saturday, Dec 02, 2006 at 10:18
Hi Gary,
I have found it is better to buy a 12V fan unless you will be running other items off the 240V inverter at the same time.
We don't stock them but Camec have a good range.
Regards
Derek.
AnswerID:
208129
Follow Up By: joc45 - Saturday, Dec 02, 2006 at 11:09
Saturday, Dec 02, 2006 at 11:09
Hi Derek,
I'm inclined to agree.
Most small 240v fan motors are of the shaded-pole induction type motor, which have lower efficiencies due to the losses in the copper short circuit turn in the shaded pole. A capacitor-start induction motor will be more efficient, as will those with a centrifugal start switch, but these are rare in cheap small fan motors.
As I understand it, most 12v computer fan motors are of a brushless, permanent-magnet design, which are very efficient.
A standard DC brush motor is a different kettle of fish, tho. In any DC motor, tho, a permanent magnet design will be more efficient than those with energised field (compound) windings.
Gerry
FollowupID:
468074
Reply By: Grungle - Sunday, Dec 03, 2006 at 09:12
Sunday, Dec 03, 2006 at 09:12
Hi Gary,
A general rule of thumb is to divide the 240V appliances wattage by 10 to find out what Amps it _Affordable_Storage_Drawers.aspx off a 12V system through an inverter.
For example a 40W fan if powered through an inverter would draw 4A @ 12V; If you run a 600W drill then it would be 60A @ 12V; And a 900W Microwave would draw 90A @ 12V. This is one of the reasons why high wattage
inverters have high amperage dc cables and fuse / cct breaker recomendations.
Below is an exert from a SEA inverter manual (page 10)
http://www.solaraustralia.com.au/downloads/TempoManual.pdf :
Step 1 Take the power rating of your appliance and divide this by 10 (divide by 20 for 24volt systems.) this is the approximate current the inverter will draw from your battery.
Step 2 Take the Ah (Ampere Hour) rating of your battery and divide it by two, this is the conservative amount of useable power in your battery.
Step 3 Now divide the current draw into the useable battery capacity , this is the maximum number of hours you should use this appliance. This equation presumes that no other charging has taken place.
Example: A 67 watt TV running on a 100Ah battery.
Current draw = 67/10 = 6.7Amps DC
Useable battery capacity = 100/2 = 50Ah
Time = 50/6.7 = 7.4 hr
Regards
David
AnswerID:
208283