# Force necessary to spin an alternator at the rated RPM?

1. Oct 24, 2009

### eli_lied

Can anyone tell me how to determine the force necessary to spin an alternator at the rated RPM?

2. Oct 24, 2009

### HallsofIvy

Re: alternator

No, because that depends upon properties of the specific alternator. In particular, the friction in the alternator itself and the time in which it is brought up to that RPM.

3. Oct 24, 2009

### Staff: Mentor

Re: alternator

....and the electrical load on the alternator.

4. Oct 24, 2009

### eli_lied

Re: alternator

ok hypotheically then, let's say it's a 6000 watt generator head taken from a portable home backup generator. if i have 6400lbs-ft of constant force and the generator head operates at either 3600 rpm or 1800 rpm...without knowing the specific details such as mass(approx. 60-80lbs of rotational parts) or friction, is it possible to determine the force necessary? also does anyone know of a website that offers detailed information on alternators and/or suppliers that I could purchase one from?

5. Oct 24, 2009

### Ranger Mike

Re: alternator

about 5 horsepower for 100 amp alternator

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6. Oct 24, 2009

### Bob S

Re: alternator

Torque (Newton-meters) in angular rotation is the equivalent to force (Newtons) in linear motion. For an alternator, the power output (watts) is the product of the angular force times the angular speed. Specifically

T x 2 pi RPM/60 = W watts, where the torque T is in Newton-meters.

So for a 6000 watt alternator output (at 100% efficiency) at 3600 RPM, the torque is

T = 60 W/ 2 pi RPM = 15.9 Newton-meters.

6000 watts (at 100% efficiency) is equal to 8.04 HP

Bob S

7. Oct 25, 2009

### Ranger Mike

Re: alternator

be advised that in the real world alternators are 40% efficient which is why there is a difference between calculated HP and actual parasitic HP required to turn the Alt.