Driving a big alternator.

[Graham_Wright]

I have a standard 40A alternator for the starter battery and a 130A alternator for the domestic bank.

There is only one groove left on the crankshaft for the 130A alternator and it may be impossible to remove it and replace with a twin or multi-V with the engine in situ.

We have a local belt shop, A C Belting, with very helpful staff. They have suggested a different belt form deeper than the standard "A" and "B". They have also mentioned an on-line design facility (by Fenner?) which will produce recommendations given certain parameters. This they are feeding with the data for my problem and will advise.

They made the observation that alternators increase the load on the drive belt as the speed drops leading to earlier failure. I don't understand this and they could not explain the theory just reported the empirical facts.

Any observations please?

 

[rogerthebodger]

I am using a single fenner SPA belt driving my domestic 120 Amp alternator and I find it OK but as I have solar panels charging my domestic bank if find my alternator has never needed to deliver that amount of power

With regard to belt/bearing loading it is a simple function of belt tension, the torque and pulley size. What they may be referring to is that for the same power transmitted as the speed reduces the torque would increase thus driving belt tension would increase thus the bearing load. This bearing load would be the same no matter the number of drive belts just the drive tension in each belt would be lower for a given power /speed conditions

 

[pvb]

What's the capacity/type of your domestic battery bank? Unless it's huge (or possibly AGM), you'll rarely see anywhere near 100A charge rate, so a standard belt should suffice.

I had a 130A large-case alternator with Adverc booster as a second alternator on my Volvo 2003T, and rarely saw much more than 90A charge rate into my 660Ah domestic bank. I took it off, because it was too heavy and vibrated enough to cause bracket problems. Now have a 90A small-case which is fine.

 

[Graham_Wright]

What's the capacity/type of your domestic battery bank? Unless it's huge (or possibly AGM), you'll rarely see anywhere near 100A charge rate, so a standard belt should suffice.

I had a 130A large-case alternator with Adverc booster as a second alternator on my Volvo 2003T, and rarely saw much more than 90A charge rate into my 660Ah domestic bank. I took it off, because it was too heavy and vibrated enough to cause bracket problems. Now have a 90A small-case which is fine.

I bought it brand new for a song at Beaulieu! Apart from the 800 Ah bank, I would run the engine whilst weighing anchor. That, and the fridge and inverter should keep it loaded.

If you have to run the engine, then I believe it should be loaded as far as possible. It is good for the engine and reduces the frequency of necessity.

 

[lw395]

A modern alternator is capable of delivering a lot of current at low RPM.
A 110A Bosch alternator might limit at 110A at 6000rpm (engine), but still deliver 80A at little more than tickover.
They are designed that way to cope with lights, heaters, heated seats, windows etc, electric power steering etc etc.

So the power out at low rpm is quite high.
The power in will be nearly as high as at high rpm,

Power =work /time
work = force x distance

So Power =force x distance/time = force x speed

So the slower the belt is going, the more force is on it.
So if the belt squeals, increase the RPM and the force will get less.
Because the alternator regulates to a constant power, or whatever the load demands, whichever is less.

 

[wotayottie]

They made the observation that alternators increase the load on the drive belt as the speed drops leading to earlier failure. I don't understand this and they could not explain the theory just reported the empirical facts.

Any observations please?

Basic physics. The alternator will reach full output well below the max revs of the engine so the energy that has to be transmitted through the drive belt will be almost constant once full load is reached - say at 1500 rpm. The energy being transmitted through the belt to the alternator is a direct function of the load on the belt and the speed it is running at, so as the speed gets lower the load gets higher until you reach the point where the alternator output starts to drop off.

Do you go to the gym? If so have you ever noticed how the load on your legs as you pedal the exercise bike drops off when the speed goes up? Same issue.

 

[CreakyDecks]

The important equation is
Power = torque x speed of rotation (x some constant relating to units of measurement)
So for a constant power, as you reduce speed you increase torque, like Roger said.

 

[rob2]

Just an extra thought... The wrap on the pulley is important in how much torque can be transmitted without slipping. Many lower power alternators have suprisingly little wrap, so the potential slipping problem is almost built into the design of the mounting. It may help if you can add anidler to deflect the belt and so increase the wrap on the pulley.

Rob.

 

[nigelmercier]

We have a local belt shop, A C Belting...

Thanks for that info, they may be able to help with uprated pulleys.

Web site is http://acbelting.co.uk/default.asp BTW.

 

[SHUG]

It's very seldom that your alternator will deliver its full output so you can be somewhat conservative on the belt rating.

 

[Bobobolinsky]

You have to be able to change the bottom pulley, if you can't remove the pulley in situ, to add a second groove, it is unlikely that you will be able to go to a larger belt profile, as this also needs the pulley removing. Personally I like the flat belts, less wear, less black dust, less vibration, higher torque, greater reliability.