ignorant electrical question regarding wiring fans in series.

[zubzub]

This is only bus related because it's for my toad, but I googled it and no leads.
Original cooling system uses 2 fans on rad.  Fans are controlled by relays (3) to either be powered in parallel (high speed) or in series low speed. 

Problem:  changed out fans with aftermarket fans now they work fine in parallel but blow the fuse immediately in series.  I did some trouble shooting, but before I go crazy I would like to know if the construction of some fans is such that they can't be run in series.

Thanks as always.  Patrick.

[Melbo]

I would guess from you description that the resistance is too great when run in series for the circuit that you are on.

Do you have a meter to test the amp draw???

What size wire and fuse are you working with???

Melbo

[Len Silva]

That doesn't make sense.  The current would be less in series than in parallel.  I would recheck your wiring.  Are you talking about the 4104 defroster fans with the funky series/parallel relay?

[zubzub]

Len it's not on my bus  it's on the toad (92 vw eurovan).  I did  double check the wiring...first with the original relay set up then I designed my own figuring the original was malfunctioning.  same condition.  It's a 30 amp fuse so we are talking full short to blow it.  Like I said instant fry of fuse.

[Len Silva]

Sorry, I should have read your post more carefully (I seem to do that a lot lately)

It still doesn't make sense to me. The only thing I can think of is that radiator fans are permanent magnet motors, and polarity may be a factor.

You could try using a large 12 volt lamp (like a headlight) in series while you experiment.  That would just save fuses while you play with it. The lamp will go to full brightness if there is a short.

Have you tried manually wiring them up without the relay, just to verify the wiring?  Might be easier to do on the bench rather than on the car.

[Hartley]

I would suspect that the replacement Fans motors are polarity sensitive and switching to series causes one to try to run backwards or it has a diode in place to make it run one way only. Maybe both of the new ones have diodes.
( polarity checkvalves ) You may need to either remove the diodes or get "original" fans for the VW as they may be designed to run series/para modes.

Dave...

[zubzub]

dr dave your post reminded me...the aftermarket fans I got aren't polarity sensitive in that they can be puller or pushers depending on where you put the + and -.  Come to think of it I guess that does make them polarity sensitive?  Don't know why I didn't try wiring them directly in seires without the relays...was getting late and cold and dinner was waiting I guess.  Finally re getting fans from VW....those people are crazy!  Priced the fan set up a few years ago...around $500.  It is a pretty fancy set up though, with thermo shutters that cut off air to the rads and everything....too bad the fans die after 300,000 kms.

[RTS/Daytona]

are you sure these replacement fans don't have the negative side grounded to the frame of the fan / bus  - switching to series will work ONLY if the fans wiring is ISOLATED from ground - some fans are JUST NOT DESIGNED with ISOLATION

[Lin]

Pete,

I do not know if your answer is right, but I sure like it.

[zubzub]

I like dave's answer as well except these fans work wire either way.  switch the leads(invert polarity?) and they run in the opposite direction....that means they are isolated otherwise they would just blow a fuse when you switched the leads.  My thought on all of this was maybe there is some kind of fan motor set up that can't be run in series.....I have no idea why this would be so as my limited understanding of any electric motor suggests that putting it in series with another motor should just make them both run slower.
FWIW I need to take another stab at it, double check the wiring etc....I was really just checking to see if there was something obvious I was missing re: electrical motor design/wiring.

[Melbo]

That was my original question -- If the fan is designed to run at a certain speed and voltage and you reduce the voltage by running the circuit through the first fan I think it could increase the resistance at the second fan -- break out the meter and see

Melbo

[JohnEd]

You are not missing anything as far as your logic goes.  Temporary wire them in series and see if they work at half speed, as they should.  How many amps is each fan sup[posed to draw?  I'll bet the wires in the fan won't tolerate 30 amps so the ground is somewhere before  the fans.  They don't even grunt....do they?  I would guess that in parallel they might draw close to thirty.  

You cannot apply this logic to AC motors.  In series they would burn up.  You can't slow them down by using a dropping resistor either.  Solid state sped control might work depending on the type of ac motor.

You are on the trail Zub, hang in there,

John

[JohnEd]

Melbo,

The resistance of the fans is not changed by how you connect them in the circuit.  The thing that changes is the resistance of the CIRCUIT in which they are connected.  In parallel, the circuit resistance is 1/2 of the resistance of a single fan.  In series the resistance of the circuit is double that of a single fan.

If this is talking down to you I humbly apologize.  The kids are listening and you know how easily they pick up bad words.


HTH,

John

[Melbo]

Not talking down to me at all

I many times don't understand what is happening in a circuit when stuff get rearranged

I have to use a meter to tell me and then noodle it out from there

The idea that the resistance in the circuit would increase when run in series is where I was at

If you use a meter and check the resistance with each configuration it will give you a chance to work it out from there

I guess my only point is what size wire and fuse are being used and the process of wasting perfectly good fuses over and over again did not make sense to me --- I usually do things wrong at least three or four times before I go GOOOLLLLLYYYY that really doesn't work.

Back to my original point --- break out the meter and do some testing and then go from there.

YMMV

Melbo