Cooling fan start current

[Bitsyncmaster]

You must know by now I love to develop testing for complex electrical situations.

Well my cooling fan testing is continuing. I have a set of used OEM fans that were removed from a friends car because his fused fan fail jumper caught fire. I mounted those fans (still in the stock shroud) onto my removed OEM radiator. Wish I had a stock condenser but don't for the best simulation. Anyway, here is the bench current test results:

12.0 volts = 13.4 amps
12.5 volts = 14.1 amps
13.0 volts = 14.9 amps
13.5 volts = 15.6 amps

Both fans tested at the same current. You probably get about 12.5 volts at the fans with your engine running accounting for the voltage drop of the wiring in the car. Lower power fans would have less voltage drop.

I recorded the fan starting current as shown in the two attached files. Used a clamp on current to voltage probe attached to my oscilloscope.

I also bench tested the stalled OEM fan resistance. The resistance varies a bit as you turn the fan and the brushes contact another coil in the armature. The result was .23 ohms to .30 ohms which would result in 52 amps and 40 amps if your fan was jammed. That relates to my results also on the measured starting current. The current probe was set to one volt equals 10 amps and you see it takes about one second to settle.

[David T]

Your results don't seem to make sense. As the voltage is increased the amperage should go down. Power should be a constant and since I X E = P, with P constant you can't have I and E both go up. There is no scale for your charts so it is difficult to interpret the results. LRA (Locked Rotor Amps) is generally 2.5 to 3 times running amps. As a rule-of-thumb that is why starting current is basically LRA and is 2.5-3 X running ( constant) amps. It is a little different for DC motors but you use the same numbers for an AC motor for fusing and wire sizing. Using these #'s you can see starting current for both motors at the same time can exceed 67 amps if you use the lower amperage figures you derived. Using worst case it can be as high as 93 amps. That's a LOT to hit the electrical system all at once. And on an old fan motor it can probably get even higher. When my fans kick on I can feel it in the gas pedal, the car seems like it is pulling back. When you have the A/C cycling and cycling the fans it is also a big load on the motor when that clutch slaps in too! Anything that can be done to reduce that sudden load will be helpful. That is one of the big advantages to a Fanzilla or anything else that will sequence the fans so they both don't come on at the same time. It cuts the instantaneous starting load in half. That is a BIG difference! BTW to test it under more realistic conditions you do need the condenser coil for the added resistance and a fan blowing air through the stack to simulate the car moving. Won't affect the starting current test much but should make a difference to the constant current reading. Don't forget with it all installed in the car there is some blockage from all the stuff behind the radiator too. Another factor is cold air is harder to move than hot air. With all things considered, to get more accurate results, you really do need to test it as it is installed in the car.

[Bitsyncmaster]

David T, where do you get the idea that motor power is constant with voltage change? I've seen you say this for the fuel pump also which again is not true. When you have bad connections, you get voltage drops and current goes down for everything except switching regulators which are constant power.

My PDFs do show the vertical scale is 500 mv per division vertical and 200 ms per division horizontal.

I do want to get an AC condenser mounted on my testing fans which would also increase the currents a little more and pretty much equal what the fans do in the car when not moving.

[David T]

David T, where do you get the idea that motor power is constant with voltage change? I've seen you say this for the fuel pump also which again is not true. When you have bad connections, you get voltage drops and current goes down for everything except switching regulators which are constant power.

My PDFs do show the vertical scale is 500 mv per division vertical and 200 ms per division horizontal.

I do want to get an AC condenser mounted on my testing fans which would also increase the currents a little more and pretty much equal what the fans do in the car when not moving.

As I mentioned in my previous post, you still won't be simulating the blockage behind the rad and you won't be moving hot air so your bench simulation will not be accurate. I don't understand the need to simulate to measure loads. It shouldn't be that difficult to do real world measurements right on a car. Didn't see the scales in the .pdf. The electrical system voltage can run as high as 14.5 volts and as low as 12 volts.

[tommyrich]

Dave M.,
I want to let you know that there are may of us who appreciate and encourage your experimenting (and taking the time to share the results). When I log on and see one of your posts, it is often the one I will click on first. I find that you often satisfy my curiosity for electrical measurements/experiments. I know you as a very talented engineer.
I don't often post on this (or any) forum, mostly for the reason that I see here. But this thread got me very irritated. You performed a very logical experiment and presented your detailed data and assumptions, only to be jumped all over by other(s) who think they know better. If they want to dispute your results, let them do their own measurements and compare. Then there can be a logical discussion, rather than speculation.
It is very easy to get discouraged (as I did), but I would like to offer encouragement by letting you know that your information is informative and appreciated and, in my opinion, quite valid. Keep up the good work!
Tom Neiland

[Mario]

Your results don't seem to make sense. As the voltage is increased the amperage should go down. Power should be a constant and since I X E = P, with P constant you can't have I and E both go up.

Fans are not constant-power devices, they're constant resistance (though they don't quite act like it, but it's close enough for this situation). Ohm's law says I = V / R, so if your resistance stays the same, your current goes up when your voltage goes up.

[DMCMW Dave]

Dave M.,
I want to let you know that there are may of us who appreciate and encourage your experimenting (and taking the time to share the results). When I log on and see one of your posts, it is often the one I will click on first. I find that you often satisfy my curiosity for electrical measurements/experiments. I know you as a very talented engineer.
I don't often post on this (or any) forum, mostly for the reason that I see here. But this thread got me very irritated. You performed a very logical experiment and presented your detailed data and assumptions, only to be jumped all over by other(s) who think they know better. If they want to dispute your results, let them do their own measurements and compare. Then there can be a logical discussion, rather than speculation.
It is very easy to get discouraged (as I did), but I would like to offer encouragement by letting you know that your information is informative and appreciated and, in my opinion, quite valid. Keep up the good work!
Tom Neiland

Tom - Thank you for posting this. 100% agreed.

[David T]

My posting about Dave's experiment is not to be "jumping all over him". I mentioned some things I didn't think were correct in order to improve his experiment. I don't want to discourage his experimentation but if he is going to do it, and post about it, he is going to get comments about it. I suggested ways he could get better measurements and questioned the ones he has already obtained only to get the most the most accurate ones possible. If we are going to use the scientific method then his experiment must be able to stand up to objective critique and be repeatable. To further this along if he is going to collect voltage and current data he should also try to measure CFM. If, in fact, voltage and current are increasing (and that infers power is increasing) we should see CFM increasing. This would confirm the voltage and current measurements as we see them. Another variable that might be of interest would be RPM of the fans, again, to confirm other measurements. Even after making all of these measurements we can only consider them indicative since they would only be done on a very small sample size. As for others doing their own measurements, the more people who do it the more representative it will be BUT!. We would all have to agree on how it is to be done so every measurement is done the same. I apologize in advance if I offended anyone by commenting on his post, that was not my intent.

[Elvis]

yes DavidT, more voltage, more speed, more current, more power - more airflow (FCM)


I could have it measured here in our lab, but I would need to have the radiator, fans and condenser.


Then we could also compare the "low power fans" and we could see the difference in airflow (wich makes
the power huge reduction).

Thanks Dave M, great work.

[Bitsyncmaster]

After looking at some of the fan shrouds offered by other vendors, I decided to run a test on the OEM shroud to see if both fans running would change the measured current of one fan. My testing was done with just one fan running.

The OEM shroud showed no measurable change in current on one fan with the second fan running or not.

The OEM shroud is shaped so air flow is separated from the other fan since the shroud almost compleatly blocks the second half of the radiator. There would probably be a current change with those metal shrouds of other vendors since I don't see them with a center blocking shape. Now how much current would change, I don't know.

I'm guessing the OEM shroud reduces the air turbulence because of it's shaped air flow to the radiator. What the cooling effect is I don't know

This is just an FYI for anyone else running any current testing.

[Bitsyncmaster]

I've already got what I was looking for (the start current and timing) so I'm doing some other testing just for curiosity.

I've got one of my original fans and another OEM shroud. So I tested that current both without it mounted to the OEM radiator and with.

12.0 volt = 10.5 amps without and 12.4 amps with
12.5 volt = 11.3 amps without and 13.2 amps with
13.0 volt = 12.0 amps without and 14.0 amps with
13.5 volt = 12.7 amps without and 14.8 amps with

I suspect the high currents of OEM used fans is due to brush dust conducting current to adjacent armature coils. So I'm going to try to open this fan motor up and clean it and then retest the currents. I think I need to pull the fan blade off to open the motor and not sure if that will happen without damage. That is why I'm using this OEM fan since I want to keep the other two fans untouched.

[novadmc]

I've already got what I was looking for (the start current and timing) so I'm doing some other testing just for curiosity.

I've got one of my original fans and another OEM shroud. So I tested that current both without it mounted to the OEM radiator and with.

12.0 volt = 10.5 amps without and 12.4 amps with
12.5 volt = 11.3 amps without and 13.2 amps with
13.0 volt = 12.0 amps without and 14.0 amps with
13.5 volt = 12.7 amps without and 14.8 amps with

I suspect the high currents of OEM used fans is due to brush dust conducting current to adjacent armature coils. So I'm going to try to open this fan motor up and clean it and then retest the currents. I think I need to pull the fan blade off to open the motor and not sure if that will happen without damage. That is why I'm using this OEM fan since I want to keep the other two fans untouched.

if you ever need another pair of OEM fans for testing, for extra data-point collection or anything, let me know. Mine are just sitting in a box on a shelf in my garage. their exterior housing is all beat up and paint flaking, but they ran just fine when i pulled them to replace them with Toby's units.

[Bitsyncmaster]

if you ever need another pair of OEM fans for testing, for extra data-point collection or anything, let me know. Mine are just sitting in a box on a shelf in my garage. their exterior housing is all beat up and paint flaking, but they ran just fine when i pulled them to replace them with Toby's units.

Thanks but I think we all know the OEM fans seem to increase there current draw as they age. I did some testing reading CFM and RPM but that was back on the old .com forum. CFM testing is pretty hard to get a standard results since the sensor (i have a hot wire anemometer) measurement position is critical. CFM testing would need to be done with side by side fan testing to just get a comparison of air flows.

What it all boils down to is does your engine stay cool and the AC pressures don't run to high. Now to do it with less current draw helps the electrical system.

[David T]

Thanks but I think we all know the OEM fans seem to increase there current draw as they age. I did some testing reading CFM and RPM but that was back on the old .com forum. CFM testing is pretty hard to get a standard results since the sensor (i have a hot wire anemometer) measurement position is critical. CFM testing would need to be done with side by side fan testing to just get a comparison of air flows.

What it all boils down to is does your engine stay cool and the AC pressures don't run to high. Now to do it with less current draw helps the electrical system.

It isn't that simple. The thermostat controls the temperature of the motor. Once the car is moving, air is rammed into the intake so you don't need the fans over a certain speed. They will be running anyway if the A/C is on but their current demand will be less because of the ram air. I agree measuring CFM can be tricky which is why you must set parameters for the testing. If current demand is dropping and you achieve the same or better cooling effect then implicitly you must be moving more air. Maybe a more reliable indicator would be to read head pressure in the A/C system for comparison purposes. If you are moving more air (and transferring more heat) the head pressures should be correspondingly lower. As for the OEM motors drawing more current as they age, I suspect that is a very small change, the only things that will change are the bearings may lack enough lubrication and the brushes will be worn. Unless windings are shorting the current draw should not change that much over the lifetime of the motor.

[Bitsyncmaster]

Well I lucked out that the two bolts holding motor together are rusted so I could remove the two nuts on the end of the motor without pull the fan blade off.

There was about 5 of the commutator spaces that had brush dust on one end. I blew it out but had to use a pick to get those out.

Now the test still mounted on the radiator.

12 volt = 13.2 amps
12.5 volt = 14.0 amps
13.0 volts = 14.8 amps
13.5 volts = 15.6 amps

What the hell......the currents increased 0.8 amps higher. Maybe the RPM is higher now but I really expected currents to drop.

I wonder if the permanent magnets lost some magnetism if the current would increase or decrease. I would guess you would loose RPM and the currents would fall.