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#### ipbr21054a

Jun 10, 2012
44
Hi,

I wish to use a small motor away from my house being powered from a 12v Ring Inverter "not yet purchased"

The small motor has a label on it which reads the following.
Motor Power 120w
Voltage 220v
Frequency 20Hz

Currently its being used in my garage on mains no problem & the motor has a 10amp plug fitted.

This is the inverter i am looking to purchase.
http://www.halfords.com/workshop-to...automotive-powersource-inverter-300w-rinvu300

1, Will the inverter be suitable.
2, Is the 10amp fuse installed ok or to high.

#### kellys_eye

Jun 25, 2010
6,514
What is the motor powering/driving? Sure it's not 50Hz? ('course it is!).

If the inverter is rated at 300W continuous output it should be ok - it doesn't actually specify (makes me suspicious).

For a full rated 300W output you would need a fuse of 30A and cables to match - none of this 'let's use mains cable for the input' malarky.

#### ipbr21054a

Jun 10, 2012
44
Hi,
You are correct it IS 50Hz,slip of the old finger.
All i can say is that its a Ring power source 300w
I am looking to use it on a small key cutting machine.
I would only have it switched on for the duration of the cut which would be say 3 minutes maximum etc.
Since my post i found some equation.
It was as follows.
10 x 60 "car battery amps" = 600
600 / 120 "motor wattage" = 5
5/2 = 2.5 hours of use ????

Not sure how good this equation is hence waiting for an expert reply.

Many thanks

#### kellys_eye

Jun 25, 2010
6,514
Nope.

The 120W motor will draw 0.5A at 240V (0.5 x 240 = 120) and twenty times that at 12V - so 10A.

This assumes 100% efficiency which you NEVER get so add 25% for leeway - call it 12.5A from the battery, minimum.

If your battery is rated at 65Ahr (typical small car battery) then, in theory, you can run the motor for (65/10) = 6.5hrs.

The 30A fuse is specified to protect the CABLE, not the equipment, and the cable should always be rated for the maximum current draw that may be expected - not what you might use (someone else might use it for something else )

Please ensure you have the correctly sized cables for the 12V connection bearing in mind that the longer the leads from the battery to the inverter the thicker they have to be.

#### ipbr21054a

Jun 10, 2012
44
Hi,
I would only use the cable that are supplied with the item.
I think they are about 2 foot long.
Crocodile clips one end for the battery & the other end has like ring connection which go to the inverter and then a round nut is used to tighten it up,covered in plastic etc so no metal is seen.

My car battery is rated at 60Ahr

So the result is that i can run this key machine for 5 minutes no problem at all.

Jun 25, 2010
6,514
Yup.

Jun 10, 2012
44

#### ipbr21054a

Jun 10, 2012
44
Hi,
Just an update to say i have now tried this motor from the car battery and it works fine.
I think if not mistaken that it doesn't run as fast as on the mains,is that correct.

Also "but i forgot to try" would running the engine then increase the speed ?

Many thanks.

#### kellys_eye

Jun 25, 2010
6,514
Most inverters have a regulated output that shouldn't change much over a range of input voltages. The motor may also be 'frequency dependent' i.e. tied to the 50Hz AC signal. But the inverter, like a lot of cheap ones, probably has a 'stepped' output (called a 'modified sine wave') rather than a pure sine wave that can affect AC motor operation.

#### ipbr21054a

Jun 10, 2012
44
Thanks for clearing that up for me.

#### ipbr21054a

Jun 10, 2012
44
Nope.

The 120W motor will draw 0.5A at 240V (0.5 x 240 = 120) and twenty times that at 12V - so 10A.

This assumes 100% efficiency which you NEVER get so add 25% for leeway - call it 12.5A from the battery, minimum.

If your battery is rated at 65Ahr (typical small car battery) then, in theory, you can run the motor for (65/10) = 6.5hrs.

The 30A fuse is specified to protect the CABLE, not the equipment, and the cable should always be rated for the maximum current draw that may be expected - not what you might use (someone else might use it for something else )

Please ensure you have the correctly sized cables for the 12V connection bearing in mind that the longer the leads from the battery to the inverter the thicker they have to be.

Hi,
I think I would like to hardware 2 cable from the battery under the bonnet to the rear of my jeep, for running this motor. The run from front to rear is say 7-8 feet.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Use the thickest cable you can reasonably use. Something rated for 4 to 5 times the current you expect should be sufficient. At low voltage and high current losses add up very quickly.

Use the same rating of fuse (30A)

#### ipbr21054a

Jun 10, 2012
44
So on this 12v system you're saying to buy 40amp cable correct.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
40A would be a minimum.

Let's look at 10awg wire. It can carry up to 50A, and has a resistance of about 3.4 ohms per 1000ft.

You have a run of 16 feet. That's a resistance of 0.0544 ohms, which at 12.5A will drop 0.7V. (5% loss)

Going up to 8awg wire. It has a resistance of 2.2 ohms per 1000ft. at 12.5A, the voltage drop will be about 0.4V (3% loss).

Those figures are acceptable, but remember that you will also have losses in your cable connections.

If I were doing it, I would probably use 4awg.

#### ipbr21054a

Jun 10, 2012
44
The run is 7 to 8 feet not 16 or are you multiplying it by 2 as there is 2 cables ?
This is all new to me and the way you explain it.
Looking on eBay for wire it's sold like
2x28/0.30 = 25 amp

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Yeah, 16 feet because you either need to run 2 wires, or rely on the uncertain resistance through the chassis of the vehicle.

Here is where I got the wire size/resistance stuff.

2x28/0.30 means 2 wires, each made up of 28 strands of 0.3mm diameter wire. That works out to be close to 14awg. (0.3/2)² x π x 28 = 2. 14 awg is 2.1 (mm²)

You want stranded wire with a total cross section of at least 5mm²

#### kellys_eye

Jun 25, 2010
6,514
The 5% volt drop is a target to meet as a minimum requirement for cable size however, more importantly, the mechanical and physical security of the cabling is paramount.

Such high current draw will easily result in fire if not properly done so the correct cable - as in cable intended for automotive use - is recommended and secure fixing, proper fusing (at the battery), routing it clear of any sharp edges, away from sources of heat and moving parts, no tight bends, grommets used on any holes you pass through and properly crimped (or soldered) and bolted termination should ALL be ensured.

You only need to get it wrong once......

#### ipbr21054a

Jun 10, 2012
44
Thanks for all the info.
Measuring the o/d of the 2ft cable with the inverter its 6mm so 5mm you advised is spot on.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Not the outer diameter, the diameter of the copper inside the insulation.

#### kellys_eye

Jun 25, 2010
6,514
the cable should always be rated for the maximum current draw that may be expected - not what you might use (someone else might use it for something else )

Please ensure you have the correctly sized cables for the 12V connection bearing in mind that the longer the leads from the battery to the inverter the thicker they have to be.

The inverter is rated at 300W which equates to a minimum of 25A (and with losses this would actually be around 30A but with potential overload conditions even 50A!) so don't scrimp on the cable used.

Personally I'd fit 6mm c.s.a. as a minimum and still fuse it at 30A.

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