Alternator 70, 90 or 120 amp

[TREVR6 0]

My alternator is dead :(

 

Can anyone please tell me if it's ok to put a 120amp alt on to my 93 k reg VR6. It's only got a 70 amper on at the moment. Will I send my car in to melt down :?:

Or will it just charge better?

 

Thanks people,

 

Trevr6

[Roger Blassberg 0]

If it fits, it will have the capacity to charge the battery better whilst running all your electrical equipment at the same time. It will still be rated at the same nominal 12 Volts, so it shouldn't do any harm to anything.

 

Best wishes

 

RB

[dr_mat 0]

The only downside to having a bigger alt is that it will have a slightly higher inertia and a higher resistance to turning, so you might lose a couple of bhp, but it's pretty minor.

[Chris VR6nos 0]

TREVR6,

I seem to have the same problem with my alternator and wondered what the difference in electromagnetic drag would be between the different alternators so here's a little insight to the answer, this one's for Kev Haywire too!

So how can you measure the drag?

 

Let's assume your alternator has uniform density, and a moment of inertia of I = 1/2 m r2

Then, the rotating coil has a total rotational kinetic energy of E = Iω2

Now, if your alternator rotates at 1000 rpm, this is 1000*2π radians / 60 seconds/minute, or 104 rad/sec. Thus, for a 10cm radius coil with 1000grams of mass, the rotational energy is .55 Joules. A change in the rotational kinetic energy is caused by a torque τ. The change in energy, or the work, due to the drag force would be

W = ΔE = 1/2 Iω12 - 1/2 Iω22

Now work has the units of force * distance. In the rotational case, this is force * angle swept, or

W = τ Δ φ = τ (ω1 - ω2) t

where t is the time the torque is applied. Then, torque is defined as

τ = r x F = rF sinθ = rF for our case.

So, given the change in the rotational speed and the time it took to change the speed, you can measure the drag force F exerted on the alternator.

 

Good luck,

 

Chris

[Henny 0]

The alternator will only produce 120A if the car is drawing 120A... Most of the time a bigger alternator is producing the same amount of drag as a smaller one as it's only being asked to provide the same amount of ampage... it's only when you start asking more of the alternator (electrical heaters, stereo equipment, bright lights etc) that the load would increase causing more power loss from the engine...

[dr_mat 0]

Or, much more simplified, it's 120A, which is 1400W, which is about 2bhp. This alternator is some 50A higher capacity than your current one, which is about 0.8 bhp by that reckoning.

But of course, there's more to it than that, as it's not 100% efficient, so:

I'd say at 80% efficiency, the extra 50A will cost you around 1.0 bhp.

So not a lot then...

 

(But as Henny pointed out, most of this electrical drag is load dependant anyway. If your're just running the engine, and no other electricals the only difference is the slight increase in rotational inertia of the stator and coils, which is even less significant.)

[dinkus 10]

...but only when you're actually drawing the extra 50A

[Kevin Bacon 5]

Tractor with NOS, LOL, cheers for that!

 

The 120 amp alterantor is physically larger and needs something changing on the engine to make it fit apparently. PhatVR6 is the man for that.

 

70 or 90 amp is fine unless you regularly plug in a garden waste compactors, industrial PA stystems and fridge freezers etc....

 

And a 70amp one is less than £100 exchange from the dealer. 90 and 120 amps significantly more....

[Chris VR6nos 0]

Cheers Kev, thought of you when typing the techy bit, thought you'd appreciate it m8! ;-)

 

THe 70amp ones are only that much from the stealer! less than £100 exch! but how much less, £1?

 

 

Chris

[Supercharged 2]

42...?

[Chris VR6nos 0]

Only in the movies Supercharged

 

 

sirhC.Bsc

[bigpants baby 0]

i have had 5 engines in with 120a alts,there the same physical size

[PhatVR6 0]

I have a 150a one, it's defintiely bigger than the 90a one, but I never got round to fitting it so I don't know what needs to be done.