Cooling fan motors

[Bitsyncmaster]

Ever wonder why the OEM fans seem to draw more current when they age? This has always bothered me for as long as I've owned my D. My thoughts about this were just rekindled when an owner with my fan fail unit said it is flashing an overheating warning.

Years earlier I did take an old OEM fan motor apart and cleaned the commutator and brushes thinking that would reduce the current draw. Well the current went from 13 amps prior to cleaning to 15 amps after cleaning. This just amazed me since I thought that was the cause of current change of the motor. Now bearing wear could also cause some increase current draw if the bearings started to add drag. But more thought I started to think maybe the permanent magnets in the motors may be loosing magnetism. I would think loss of magnetism would reduce motor current but I could not find info to prove that thought.

It looks like older magnets did loose magnetism more so than the newer alloys used today. Also you can demagnetize a magnet using an AC field held close to the magnet. The old TVs you would degauss the metal around the CRT. So I wonder if turning the fan power on and off many times would produce a field that could degauss the permanent magnets. My fan relay does turn the fan current on and off slower than a mechanical relay so not sure if that is better or worse for permanent magnet motors.

So if anyone has nothing to do one day, try finding some information on permanent magnet DC motors to see what effect of magnet loss would have on fan current.

[dmcnc]

I'm definitely following this thread. Here's an article I found on motor energy losses: https://www.fmanet.org/blog/2013/12/04/motor-energy-losses-occur

[Bitsyncmaster]

Heat is another thing that can cause loss of magnetism but not sure if our fan motors can get hot enough.

I always thought supper efficacy could be helped by using silver wire but nobody has ever tried that. Copper resistance goes up when it heats so I would guess your current would drop along with the power of the motor.

[David T]

My guess for higher current draw is bad connections. As the electrical system ages and each connection gets less conductive, more current gets "lost" or converted to heat. Also there could be breakdown of the insulation on the windings inside the motor sending more current to ground and between the windings.

[Bitsyncmaster]

Bad connections do not increase current unless your unit has an electronic power unit which maintains power. The fan motor current drops as voltage to them drops. Bad connections cause a voltage drop on the bad connection so current to the unit drops because the voltage is lower.

[Rhye]

IMHO, weakening the magnetic field increases the current draw by the rotor windings. This is because the voltage generated by the field's back EMF (electromotoric force) counteracts the external voltage applied. It's easy to understand if you imagine powering up the motor with no magnets at all (or with a rotor standing still) - current draw will obviously reach it's maximum value (voltage applied divided by windings' resistance). You can find more details here:

https://www.edn.com/Pdf/ViewPdf?contentItemId=4440813

I wonder if it's possible to find a modern replacement for the OEM magnets - that would quickly confirm the theory.

I think another source of increased current draw could be a deterioration (oxidation) of the rotor windings' electrical insulation - but that's just my guess.

[Bitsyncmaster]

IMHO, weakening the magnetic field increases the current draw by the rotor windings. This is because the voltage generated by the field's back EMF (electromotoric force) counteracts the external voltage applied. It's easy to understand if you imagine powering up the motor with no magnets at all (or with a rotor standing still) - current draw will obviously reach it's maximum value (voltage applied divided by windings' resistance). You can find more details here:

https://www.edn.com/Pdf/ViewPdf?contentItemId=4440813

I wonder if it's possible to find a modern replacement for the OEM magnets - that would quickly confirm the theory.

I think another source of increased current draw could be a deterioration (oxidation) of the rotor windings' electrical insulation - but that's just my guess.

Thanks. That is a good read and most people can understand since it does not use complex math. By the way I did test the OEM startup current with the stock fan mounted in the stock shroud mounted on the stock radiator (no AC condenser). The start current ran about 55 amps for each fan.

[DMC1890]

Well the current went from 13 amps prior to cleaning to 15 amps after cleaning. This makes sense if you just think of the dirt as being additional series resistance, which, along with bad connections, small wire, etc., would reduce the current.

I thought you were a big proponent of newer fans?

[Bitsyncmaster]

This makes sense if you just think of the dirt as being additional series resistance, which, along with bad connections, small wire, etc., would reduce the current.

I thought you were a big proponent of newer fans?

Yes, I always recommend all owners get some new lower current fans. That test I did was on an old fan I use when testing the fan fail design. I have Toby's fans in my car.