Math question?? figuring out watts, power...

[Southern3.0]

Never thought I would say it but I shoulda payed more attention in school, I can remeber sitting in class thinking "this if F'ing stupid, when am I gonna need to know this?" and now that I havent been in school for 2 yrs im always trying to figure somthing out and I can think back to them trying to tech it to me, oh well...

Anyways Im building a small generator with a 95 amp alt off my old Taurus, I got the wiring figured out but now Im trying to figure out what size engine I'll need to run it.

I used to know how to figure it out but completly forgot, I think I need to find out how many watts the alt will put out going by 95 amps at 14 volts, then covert watts into Horsepower, Then round it up to factor in losses through the belt drive and heat/ friction loss or whatever.

I found this chart that shows alt output and it looks like the alt I have can put out 115 amps but idk.

Anyone have any ideas?

 

[silveranger93]

Well...

Power=Current*Voltage

P= 14*95
P= 1.3 kW

1 kilowatt = 1.7835593797 horsepower

P= 1.7835593797 hp

I think this is what you wanted. But this assumes 100% efficiency, so take that into account.

 

[srteach]

Alright. 14V at 95 Amps gives you 1330 Watts (Volts x Amps = Watts). Horsepower needs will be a factor of the driver to driven ratio on the pulleys. Each 1330 Watts will require 1.784 horsepower. In other words, a 1:1 ratio on the pulleys will require 1.784 horsepower per 1330 Watts of generation, not counting any losses (belts, heat, friction, etc.).
You will want to bump that up to 1.8Hp to 2Hp (minimum) per 1330 Watts to account for losses. Different ratios will change the requirements. Use this for the base conversions to find pulley ratios.

Conversions are not mine. They come from OnlineConversion.com and more specifically the power page onlineconversion.com/power on that site.

I could be wrong, so I invite anyone to correct me.

 

[Southern3.0]

Yup thats exactly what i was looking for, Im geussing the 3.5 hp briggs I have should do the job.

But how did you figure 1.74 hp from 1.3 kw? I tyed it my self and did

14*95=1330/1000=1.33*1.34102209=1.7835593797 hp.

Its close to yours but Idk if mines right or not?

Edit: after reading Streach's post I think I got it right.

 

[silveranger93]

You're right, it is 1.7835593797 hp. That was a typo on my part. I try to help and I can't even type right...doh.

 

[srteach]

Your numbers are correct. You have 1.7835593797 per 1 Watt. I have the numbers per the output of your alternator. Southern has it based per watt. They are all the same, and all within 5% tolerance.

If you search online, you can find anything. That is where I got the conversions. I am in school for electrical engineering and knew the formulas except for Watts to Horsepower.

You got less than tolerance difference in the answers from both Southern and I. Both of us rounded from the same numbers. Yours is not rounded, but is accurate to the ten millionths of a horse accuracy. You don't need that much accuracy. the components of the alternator are probably only 5% components.

So, you have 3 different people saying the same thing independently (You, Me and Southern). Southerns' numbers are 3% off of mine. You have what you need.

 

[Southern3.0]

Haha it happens.


Thanks for all the help!

 

[srteach]

Hell! I can't even figure out who I'm talkin to.

 

[almostclueless]

746W=1Hp, but honestly I'd err to the side of caution and use a 3Hp engine.

 

[Will]

I see a certain portable generator that uses a 3hp engine for a 1400 continous watt generator. So that's a good estimate 'clueless. It's Northern Tools #164065-2801.

One thing I would be cautious about is the duty rating of the alternator. I wouldn't load it very close to maximum output. It's meant to quickly top of a starting battery, not power a 1400 watt load continously. I might aim for a 75% continous load--1,000watts.

 

[almostclueless]

I hear ya will.....Im sure there are plenty of 140-plus amp alternators in junkyards....