Wiring for a second fan

[Geoffers71]

Advice please! I currently have a single 7" cooling fan fitted to the Marsten radiator of my S2, which is very marginal in traffic especially on a hot day. I want to fit a second identical fan next to the first, but need advice as to the best option for the new wiring. Currently I have a single relay wired conventially operated by a manual switch. I've see two options for the wiring of a second fan operated by the same switch. The first shows the second fan connected in parallel to the first AFTER the single relay. The second shows another relay connected to the second fan in the conventional manner, but the feed to the two relays is from a common fused supply, it being split BEFORE the connection, with each branch joining spade 86 of each relay. Have I explained it clearly enough?
So, which is preferable? The first is easier and doesn't involve me buying another relay (tight ol' bu66er that I am ) But the second may be the safer option?? I just don't know. Any advice please?

[john.p.clegg]

Stop being a tight bugger and get a kenlowe ( or other ) fan fitted with a thermostat , you will only need the manual switch as a back-up in dire circumstances...

John

[billwill]

It all depends on the electric rating of the current single relay.

If the contacts have sufficient amp rating for the start-up current of two fans, you do not need to add a second relay.

[rgh0]

It all depends on the electric rating of the current single relay.

If the contacts have sufficient amp rating for the start-up current of two fans, you do not need to add a second relay.

Most 12 V automotive relays are rated at 30 amps which is plenty for 2 fans so I would just wire your second fan off the same relay as the first.

cheers
Rohan

[john.p.clegg]

As Bill said , the start-up current could be a lot higher...

John

[Geoffers71]

Thanks guys, that's very helpful and just what I want to hear.

[billwill]

As Bill said , the start-up current could be a lot higher...

John

Unless you stall (jam) the fan motor I suspect the startup current would only be about 3 times the running current, but that is a pure guess

[ericbushby]

OK, you made me curious.
I am just fitting a relay so I am into the fan circuit just now.
The 11" fan on mine draws 6.8 Amps running and 8.5 amps stalled.
The original Lotus type in the cupboard takes approximately 5 amps running and 7 amps stalled.
Yes, I know, it surprised me as well.
It also surprised me that the modern one takes more current than the old inefficient 50 year old one, although it is a larger diameter.
Eric in Burnley
1967 S3SE DHC

[mbell]

It also surprised me that the modern one takes more current than the old inefficient 50 year old one, although it is a larger diameter.

Being new doesn't necessarily mean it's more efficient and pulling a higher current doesn't mean it's less efficient. The new fan could being pushing a lot more air (higher CFM) so if you look at CFM per amp it could be a lot more efficient than the Lotus unit.

They may also be some effect from the mounting of the fan directly to the radiator rather than a few inches away as the Lotus fan is.

[ericbushby]

Yes, I understand that, but I didn`t expect it.
Eric

[john.p.clegg]

Is the stall current the same as the start-up current?

John

[ericbushby]

John,
Yes it must be, as it is the current drawn when it is not moving. As soon as the torque developed overcomes the inertia of the rotor and blades and they start to move, the current falls.
Incidentally, if you reduce the blade speed physically, the current increases, so a larger fan or stiff bearings will cause the motor to overheat.
Eric

[Craven]

Hi,
Your result is surprising, all of the modern 12v DC motors I?ve seen are PERMANENT magnet motors,
As there is no field current needed there are considered more efficient.
Ron.

[William2]

I am not surprised the single fan is struggling, 7" is far too small. You should be able to fit a 10" unit without any problems. I agree that wiring a second fan in parallel using a single relay should be fine current wise.