Reuby, pretty sure I have something suitable to donate to the cause

Thanks DD
Didn't think of the extra power that the 120 amp unit would need. besides, it would be a waste. Its worth $300.

It looks like weekend1000 has an alternator for me. Ill be dropping round this arvo to raid his stash.

Just need the new electric motor. Im thinking about a 1hp should be fine.

Motor Shaft Power = Volts x Amps x 2 in Watts
(the 2 is because the setup will be about 50% efficient)

Hope that helps.

Thanks for that lamb_d

so by that reckoning
12 volts x 50 amp alternator x 2 (efficiency loss) = 1200 motor shaft power.

So then i need to find a motor that has 1200 watts or 1.6hp at the shaft (according to the link below).

http://www.rapidtables.com/convert/power/watt-to-hp.htm

Most electrical motors are rated by their input i think(eg 2hp) rather than their rated output.

Ill call an electrical motor shop tommrrow and ask them.

cheers

Figure 15V instead of 12V. Charging voltage is 14-15 volts. Even more at freezing temps.

If you're using an AC induction motor (like what's on most air compressors) you can over size it and it wont matter. Most small induction motors are around the 75-85% efficient mark. In fact, an air compressor motor would probably be about right size.

A few other things things to consider,

-an AC motor in Aus will do just shy of 3000rpm (single pole) 1500rpm (two poles) or 750rpm if you get a 4 pole motor. It'll say on the nameplate what rpm it is at 50Hz.
-Most alternators have a cut in speed of about 1500rpm (i.e. 2x idle speed).

An AC induction motor is what i am chasing.

Looking at new prices for a 2hp from a local shop
http://www.machineryhouse.com.au/Sear ... ter=Category&FilterID=378

a 2hp unit at 2800rpm is about $250 new.

Thats far too pricey for this project. Ill keep scrounging about for a 2hp motor from somewhere. Im sure a working second hand unit will turn up. The 1/4hp 1440rpm unit I have was free from my local machinist. he fished it out from an old dusty box of spares.
I actually have an old 3 phase motor from my hoist that is collecting dust. Its a pity I cant use it.

Soooo..... any perthites have a dead compressor with a 2hp motor on it?

You can actually run a 3 phase motor off single phase, it's a little rough and you only get 1/3 to 1/2 of the power but it works.

Less stuffing round if you can get a single phase motor though.

mabee....

Quote:

So to get about 2hp, i would need about 5-6hp in three phase. Ill check the name plate on the motor, but i doubt its that big. It was only to run my car hoist and all it did was turn the hydraulic pump, which is low volume, high pressure I imagine = low motor power needed.

Ill keep fishing for s single phase unit. im not enough of an electrician to be rewiring a three phase motor to single phase.