Fuse rating for radiator cooling fan.

[fireblade]

I would like to know the correct fuse rating for the radiator cooling fan. My Lotus is a 1969 Elan SE DHC fitted with the standard cooling fan. Currently the circuit is protected with a 8 Amp fuse which has recently blown several times. I think the fan motor draws quite a high amperage and my feelings are this should have a higher rated fuse fitted. Your comment would be appreciated.

[ericbushby]

Hello,
I just checked my spare fan on the bench. It drew 4.5 Amps at 14 volts.
An 8 Amp fuse should be OK for that.
I suggest you check the current on yours with a view to repairing the motor if it is much higher.
Hope this helps
Eric in Burnley
1967 S3SE DHC

[Andy8421]

Motors suffer from 'inrush current' where the motor draws many times its steady state current at switch on until the motor gets up to speed and reaches equilibrium.

It is quite possible that a motor could draw 3 or 4 times its steady state current during startup - at which time it becomes a race between the motor speeding up and the fuse melting.

[ericbushby]

It may not be quite as you say.
On the fan which I tested the current when stalled was 6.7 Amps at 12 volts. I used a battery for this as my bench power supply is limited at 5 amps.
At 14 volts the current would not be sufficient to melt the fuse during the time it takes to get up to speed.
Some motors do as you say, especially series wound types or AC induction motors. They can be destroyed by the high starting current if not allowed to rotate and not correctly protected.
Eric in Burnley.

[Craven]

Remember a fuse protects the wiring not the component connected by it.

[fireblade]

Thank you all for a quick response. I did overhaul the fan motor a couple of years ago and it appears to be in good condition and spins freely. The front harness was replaced some years ago along with the fuse box. All the wiring is in good shape. So do I replace the fuse with one of a higher rating ie 16 Amp?

[ericbushby]

No, it should not be needed. measure the current and if it is too high find out why.
The motor could have a fault which although still running is drawing too much current. For instance, shorted turns on a winding, which will get worse as it continues to overheat.
If the motor has a commutator some segments may be shorting. (I have not had one in pieces yet)
Maybe a broken carbon brush bridging two segments.
If you do want to try a bigger fuse then do it on the bench, not in the car, just to see what happens.
If there is a fault that will probably find it, although you might not like the result.
Eric

[derek uk]

Andy's comment about "inrush current" seems to be normally accepted. It happens on all circuits when you switch them on to some degree. In this case the fan needs a bit of push to get it revolving so draws some extra current. I'd be happy to use a 16 amp fuse. Revotec with their aftermarket fans advise a 20 amp fuse. Fuses are often not quite what their ratings are, so you might have some that are a bit "weak". That can work the other way round of course. Is this circuit switched via a relay? If so this might be a bit tired.

[TomR]

I second Eric's comments. Measure the current and see what happens. There'll be an inrush current but it shouldn't exceed the stall amperage as Eric measured which was still less than 8 amps. If the current is over 8 amps even if just briefly at the start I'd be suspicious of the fan motor regardless of how freely it spins. The internal windings could be shorted. TomR

[ericbushby]

Good morning fireblade,
I see from another thread that you have now resolved this issue. Please will you give us details of your solution.
My main reason for asking is so that anyone in the future searching for the same problem can learn from your experience.
For instance if you replaced the motor what did you use and where from.
We are all here to help each other. Thanks.
Eric in Burnley

[fireblade]

Morning Eric

My solution for the fuse rating problem (which I had intended to post for everyones benefit) was quite simply replacing the aged fan motor with a new replacement 'Smiths' motor that was more or less identical to the original. Sourced from Ebay (charlieccu). Expensive but my problem solved.
I dismantled the original motor to find the brushes had virtually disintegrated, the mica board on which they were mounted was burnt and cracked so there was my problem.
The replacement' Smiths' motor fitted the mounting bracket but I had to shorten the motor spindle to avoid hitting the radiator cooling fins.
The new fan seems very efficient and there have been no more fuse blowing incidents.
Many thanks for all your input on this matter.

Roger.

[ericbushby]

Roger, well done and thanks
Eric