FITTING AN ALTERNATOR

[69S4]

I bought a 45 amp alternator from here http://www.lukemotorsport.co.uk/lukemotorsportua.html - it was about ?30 a couple of years ago. It has worked fine and I was advised 45 amps was more than enough.

I used this diagram on the Elan.net site and found it easy to understand http://www.lotuselan.net/uploads/elan_m ... wiring.pdf

Richard

I bought one of Luke's alternators as well last spring and it's been a worthwhile upgrade. The 45 amp output has been more than enough. I just modified the dynamo mounting bracket by welding in a few brackets in such a way that a few minutes with an angle grinder will remove them if I ever want to replace the dynamo.

[elanfan1]

I am thick

Yup

[bob_rich]

Hi

I would fit a fuse in the main feed between the alternator and the battery. fuse needs only to be rated for the chosen wire. If 4 square mm wire used (56/0.3mm) a 75A fuse should be OK. IF the Alternator main rectifier diode(s) fail short circuit very high current could flow in this wire.

Hope this helps best of luck

Bob

[rcraven]

If you fit a fuse and it blows for any reason and you don't stop the engine fairly soon, your alternator will have been running without being connected to the battery, which is generally a bad thing, isn't it?

[bob_rich]

Hi

Fusing the alternator is a vexing question. Most modern production cars do it. Problem is the alternator is directly connected to the battery and unless a safety switch is in use it mean the connection is always live and if a fault develops a car battery, even in poor condition, can easily burn out a 4 square mm or even thicker wire with a very good chance of a fire. If a 75A fuse blew (on a 40A alternator) it means there is a serious fault and that sort of current may damage the alternator anyway.

On balance if I have to choose the "best" bad thing I think I would prefer a blown fuse, dead alternator, and my Lotus otherwise OK!

On my cars I fit a safety switch to totally disconnect the battery when not in use and the 30A alternator output is fused to 60A.

Guess there may be other comments on this!

hope this helps

Bob

[paddy]

I don't see why it's a problem for the alternator if the fuse blows in that situation. The alternator will still generate a voltage but no current will flow and I don't think that can do any harm.

Also, as someone said, if the alternative is a fire then it's hardly an issue anyway.

Paddy

[billwill]

If you fit a fuse and it blows for any reason and you don't stop the engine fairly soon, your alternator will have been running without being connected to the battery, which is generally a bad thing, isn't it?

I think running the alternator with no electrical load will be OK.

[rcraven]

I think the reasoning behind the warning against running with the battery disconnected from the alternator is that the voltage could rise high enough to damage the alternator or other electrical components in the car. Maybe this is more relevant to modern cars with sensitive electronics.
It's not something I'd want to prove by experimentation, but I recently had to change a broken down alternator and when I took it in for exchange the autoelectric store immediately said the problem was the corroded main lead connection at the rear of the alternator which meant it had internittently been running without being connected to the battery.

[billwill]

That's a different situation.

If a fused lead to the alternator goes open circuit, the only component with a higher than usual voltage is the alternator itself.

[Higs]

Hi

I would fit a fuse in the main feed between the alternator and the battery. fuse needs only to be rated for the chosen wire. If 4 square mm wire used (56/0.3mm) a 75A fuse should be OK. IF the Alternator main rectifier diode(s) fail short circuit very high current could flow in this wire.

Hope this helps best of luck

Bob

a thinwall 4 mm2 wire is only rated
at 39 amps so should a fuse of less than this.

[bob_rich]

Hi Higs

U R correct with the wire and fuse rating. I looked up my tables a bit too quick and was wrong!! for 75A fusing 10 square mm with 105C rating insulation. should be comfortable with a 45 A alternator and a 75A fuse should not have any problem with random nuisance failures with a 45A circuit

Link for wire rating I use is
http://www.energy-solutions.co.uk/cable_conductor.html

hope this helps

Bob

[Higs]

Hi Bob

That is an interesting table. I was using the figures normally quoted for thinwall cable which appear to be very similar to the column labelled "85 to 90c" from the table.

So I agree, a 10mm2 cable is the one to go for (I am rewiring my Plus 2, adding relays and fuses etc. and this is what I am doing). As to the fuse, I am putting one midi fuse straight after the power lead comes from the solenoid. The information I have is that these fuses don't quite work as we all imagine. Take a look at this page:

http://www.littelfuse.com/data/en/Data_Sheets/498.pdf

So a 50amp fuse will last for at least 100 hours at a continuous 50 amps (and that is quite a long journey!!). But to blow within a guarranteed 15 secs it needs to be handling 150amps! Even at 75amps it would last a minimum of 90 secs. So I have been conservative and gone for a 50amp fuse rather than a 70amp one on the basis that I want to protect my wiring.

I have worked out that it is theoretically possible to draw 70-80 amps from the battery but you would need everything to be on continuously (lights, ignition, indicators, heated rear screen, reversing lights, stop lights, heater fan, wipers etc.) and not have got roung to starting the engine (and thus get some current from the alternator). In that situation the 50amp fuse would blow in 90 secs - and it would have been my fault for being stupid.

I suppose the other comforting thing is that if there is a catastrophic short circuit through the alternator, or a chafed wire, the current is likely to go very very high and thus blow any fuse pretty quickly.

Finally, all the above is a good reason not to get an 80amp alternator - you will need to put in even thicker wire to cater for the event that the alternator tries to charge a flat battery. Stick to the 45amp variety.

Richard

[rcraven]

Since fuses take so long to blow why not use circuit breakers?

[bob_rich]

Hi All

Talking about fusing and electrical safety--I think this thread will never end!!!
whether fuses or circuit breakers are used ( in the context here of motor vehicles where elaborate and expensive electronics based trips are generally not used)) there is always the vexed issues of choosing the protection level. Generally the breaker device should protect the wiring so the wiring does need to be sure that it can safely clear the fuse AND run continuously at the fuse rating level. Most protection devices if run at close to there trip level will take some time to trip. It is best to be sure that the required current on the circuit is well below the clearing level of the protection device and that the wiring on that circuit is rated to clear the protection device.

The real problem with 12V circuits is that the usual rule ( see link below) for conductor acceptable voltage drop of 3% means that the most fault current you could get is only 33 times load current and as it is most un-likely that the fault at the end of the circuit will be a perfect short so one is into a situation where normal current to major fault may only be perhaps 5 to 10 times normal current.

Fortunately the short term overload capability of electrical wiring as used in cars is quite good and the change in the fuse resistance as it gets near to melt will assist in protecting the wire.

The IEE wiring regulation 16th edition page 95 table 54C gives a useful table to calculate the overload capacity of wiring

hope this helps

Bob

http://www.engineeringtoolbox.com/amps- ... d_730.html

[Galwaylotus]

Since fuses take so long to blow why not use circuit breakers?

Quick-blow fuses?!