PWM noise and supercapacitors - Page 1
Forum section: electronics / beginners

author: liteonit
author_rank: Newbie
timestamp: December 20, 2014, 11:30:21 am
content: Hey all,I've recently upgraded the cooling setup in my car from a single mechanic fan to 2 heavy duty electric fans.These fans are being controlled by a PWM controller to reduce noise and power consumption as they can pull up to 50A @ 12V under full load.Cooling is awesome now, but the fan motors are "buzzing" when running under <100%, also known as PWM noise. it's a horrible sound, and I need to get rid of it :-)The designer of this PWM controller told me that I need to use a big (1F for example) capacitor on the power feed of these fans, and the noise should go away.Since i'm really interested in all sorts of electronics and like to try stuff, I was thinking about using supercapacitors.Besides these electrics fans, i run a lot of other electronics and car audio.Instead of using a single "weak" 1F capacitor for the electric fans, I was thinking about adding a supercapacitor bank together with the car battery so this has benefits for all electronics in the car.My car a big high compression V8 in it - so starting this engine from a capbank should make a huge difference.The biggest question I have is.... would a supercapacitor solve my PWM noise too? These things have much more capacity then a average crappy 12V 1F car audio cap, so I guess it should have more effect then a small 1F cap would have.Can some electric pro's tell me if this is going to work, before I move over to my next question about wiring/running a capbank in the car

author: max_torque
author_rank: Super Contributor
timestamp: December 20, 2014, 01:00:28 pm
content: Quote from: liteonit on December 20, 2014, 11:30:21 amThe "designer" of the PWM controller clearly doesn't really understand how his creation works then.......(what you want is a very low ESR, but relatively small capacity capacitive filter on the power supply, probably in conjunction with some inductance)However, having said that, it's probably not power supply ripple current that is causing the "Noise" during non 100% pwm duty operation either............

author: liteonit
author_rank: Newbie
timestamp: December 20, 2014, 01:48:44 pm
content: Thanks for your replyJust got a new updated manual regarding my PWM controller.They are now recommending to run a 2500uf, 100V cap across each fan. Which should eliminate any fan noise.They're not talking about 1F caps anymore in this manual.Although, I don't see a reason for the 100V cap since it's as 12V system.However, that would mean that using a supercapacitor as power source won't make any difference @ PWM noise?

author: Simon
author_rank: Unknown
timestamp: December 20, 2014, 01:59:36 pm
content: Quote from: liteonit on December 20, 2014, 01:48:44 pmthe designer is an idiot and is trying to blow his own controller up then, if you put any capacitance in parallel with the fan you will loose much of your speed control as the capacitor will do the same as what a smooting capacitor in an old style power supply does and will charge up very fast stressing your PWM controller and then retain a charge that will keep the fan going for albeit a very short time each time the pulse goes off.The idea is very stupid, a capacitor in parallel with the controllers power in is not a bad idea to help stop the controller upsetting other components in the car but you don't put a capacitor in parallel with the load. The way you get rid of the noise is that you can't, what you do is run at a frequency fast enough so that you can't hear it, like 15+KHz, I suspect the controller is working at a low frequency to help stop the drive transistors overheating and chances are they are not driving these transistors correctly but that is a whole other discussion.

author: Simon
author_rank: Unknown
timestamp: December 20, 2014, 02:01:50 pm
content: Quote from: liteonit on December 20, 2014, 01:48:44 pm100V is a bit much in a 12V system, sounds like they are not dealing with the back emf either. No a super capacitor will make no difference at all.You fan controller will be very noisy in an electrical sense and may upset other things in your car, so a bypass capacitor on the power input is a very good idea.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: December 20, 2014, 02:06:30 pm
content: Freewheeling diode across the fan, if you haven't already got one?What frequency pwm?Supercapacitor is an energy storage device not a power source.QuoteAutomotive and pwm circuits both can be noisy and suffer from voltage spikes, normally you want to double a caps rated voltage above maximum operating voltage anyway as an engineering margin, so 100v sounds ok to me.

author: Richard Crowley
author_rank: Super Contributor
timestamp: December 20, 2014, 02:16:31 pm
content: 1) "Filtering" the "power feed" for the fans will NOT eliminate or even reduce the PWM buzzing. The fact that the motors don't buzz at 100% demonstrates that.2) The PWM circuit should not NEED "filtering". The vehicle battery is already the largest "cap bank" in common use around the planet.3) If the PWM circuit is causing such terrible buzzing, then it may be designed at a frequency close to the resonant frequency of the motor mechanism. Suggest that the PWM design needs to be tuned to a different frequency.4) Do you have a big pile of money to waste on a "capbank" big enough to start a large engine? WHY "should make a huge difference"? The vehicle battery is the largest "cap bank" in common use around the planet. Sorry this is just silly. Whatever symptoms you are experiencing have already been resolved using conventional techniques thousands and thousands of times before you. And without use of a single "supercapacitor".5) "Filtering" the output of the PWM controller will have several other undesirable effects up to and including complete failure of the circuit.This all sounds terribly wrong from end to end. I agree with Simon and seriously question whether the designer knows what he is doing.

author: liteonit
author_rank: Newbie
timestamp: December 20, 2014, 02:19:10 pm
content: I've made some screenies from the manual. Should clear things up, just in case I might have described things wrongI've wired it the same way, except that i've placed the fuse on the + coming from the battery to fan.

author: Simon
author_rank: Unknown
timestamp: December 20, 2014, 02:22:16 pm
content: The designer is an idiot! you don't put a capacitor across the load and it shows that, the controller will be stressed and unless it's a really good capacitor it will overheat. The balancing wire between 2 fans is also odd.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 20, 2014, 02:36:09 pm
content: It's funny because the wiring diagram suggests stray inductance on the order of several uH, ballpark. If PWM were in the 100kHz+ range, the cap would actually help. Of course, the fans wouldn't be buzzing at that frequency, anyway. And all that switching noise flows through the entire power distribution system of the car.Not that cars aren't noisy places to begin with, but intentionally making them worse is stupid.I doubt the switching outputs of that 'controller' are buck style (i.e., flyback clamp diode to +V), so it depends on the low frequency to work (motor and stray inductance discharge fully between cycles, hence the 100V peak rating). Torque is necessarily and intentionally pulsed, and therefore the fan buzzes. It's simply doing exactly what it was made to do.If "what it was made to do" is different from what you desire... dump that POS and get a real one.Also, Comic Sans. Do you need any more reason?Tim

author: liteonit
author_rank: Newbie
timestamp: December 20, 2014, 02:37:53 pm
content: Hmmm - hopefully i didn't purchase any crap ;-)On the eye the controller itself seems to be build pretty well, but I know that doesn't say much ofcourse.I also don't know the switching frequency of the PWM controller.The controller itself works great though, so that's the good side of it for now.Is there any proper way to get these fan motors silent, without burning up any electronics?

author: Simon
author_rank: Unknown
timestamp: December 20, 2014, 02:42:12 pm
content: According to your manual it's running at 200Hz (Cycles), that will make noise and nothing can stop it unless the unit is redesigned. To be fair running 15KHz with 125A will be a righteous paint in the ass. I'm working on a similar unit and planed to use 50Hz

author: IanB
author_rank: Super Contributor
timestamp: December 20, 2014, 03:01:31 pm
content: Quote from: liteonit on December 20, 2014, 02:37:53 pmMy car has an electric cooling fan. The fan turns on when the temperature reaches an upper threshold and turns off when the temperature reaches a lower threshold. I can't hear when the fan is running even though the engine and car are very quiet.This is the proper way to keep the fan motor silent. There is no need for electronic speed control unless the fan makes too much noise at full speed.

author: liteonit
author_rank: Newbie
timestamp: December 20, 2014, 03:10:01 pm
content: Here is a vid of the noise - made the video during the first test before I finished the install.Fans where running at very low RPM, and on this video the sounds is horrible loud.In real life it's not that loud, and with the hood closed it's not really noticeable - actually with the engine running it's hard to hear it. But still I don't like to sound, and I don't think it's healthy for the fan motors too.Is this really a 200Hz frequency what hearing? Sounds like a higher frequency too me.I really hope there is some way to get rid (or at least quieter) of this irritating noise.The last think I want is buying another controller, with no 'guarantee' to have a silent setup.I'm trying to avoid a on/off solution - I only need the fans running @ 100% in the summer when i'm stuck in traffic for example.The reason for a PWM controller is mainly less noise, but also less power consumption since these fans can pull near 50 amps @ full tilt.

author: ConKbot
author_rank: Super Contributor
timestamp: December 20, 2014, 04:36:20 pm
content: Switch the fans so that they are in series for low and parallel for high. 3 FETs (or 2 FETs and a very chunky diode) are all thats needed, or a large DPDT relay. Turn on low at a certain set point, and high at a second slightly higher set point (I.e. on low at 180F and high at 190F, or whatever is applicable to your vehicle.No growl, no pwm harmonics all over everything, no fets that need to switch 125A @ 30 KHz.

author: IanB
author_rank: Super Contributor
timestamp: December 20, 2014, 05:53:09 pm
content: Quote from: liteonit on December 20, 2014, 03:10:01 pm50 amps is not really so much, considering the cold crank amps of a battery is in the many hundreds of amps. Besides the fan would not be running all the time, so the average power consumption is going to be similar to variable speed control. The average power consumption may even be better with the fan running full speed in on/off control since the fan and motor will probably be operating in their most efficient region.

author: Simon
author_rank: Unknown
timestamp: December 20, 2014, 05:53:47 pm
content: Sorry but you won't get what you want. And honestly this fantastic bit of kit has no automatic control and needs you to set a manual pot ?To stop the noise you need:1) a new controller2) no controller3) the setup described above4)Brushless fans and a proper signal speed controller with temperature input. (I recently designed one)

author: Simon
author_rank: Unknown
timestamp: December 20, 2014, 05:58:34 pm
content: Your other option is run them at a low frequency like 5-10Hz so that you don't get the hum but chances are you will still get a clicking sound, you still need to chuck that controller

author: liteonit
author_rank: Newbie
timestamp: December 20, 2014, 06:14:22 pm
content: Quote from: Simon on December 20, 2014, 05:53:47 pmWell that big pot on the controller is just there to set the desired temperature.The controller will change the RPMs based on the coolant temperature the sensor measures.Once the temperatures goes beyond that 'limit', fan speed will increase until the temp. goes below that limit. So during driving, fan speed is 0 or close to zero.Coolant temperatures are rock solid that way.So yes, the controller is automatic, I just need to set the desired temperature one-time :-)

author: Simon
author_rank: Unknown
timestamp: December 20, 2014, 06:19:03 pm
content: err, thats just a pot sitting there loose with all that vibration and it controlls the temperature ? gee good luck, if your lucky it won't move and either overcool and cost you more in petrol or go over temperature. Most engines to run efficiently need to be at about the same temperature, it should have been programmed into the controller with maybe some sort of way of setting "jumpers", realistically your target temperature will mostly be 85-95C and 2 degree steps would have been fine.

author: max_torque
author_rank: Super Contributor
timestamp: December 20, 2014, 10:59:02 pm
content: ALL OEM coolant fans are speed controlled these days. Look for the "shiny" metal box with cooling fins mounted next to the fans in the fan shroud!i.e.:These speed controllers usually take a 100Hz control signal , generally generated by the engine control ecu.The signal parameters are generally as follows to ensure the fan runs under wiring fault conditions (fails safe)PWM Duty 0 - >15% = full fan speedPWM DUTY 15 to 85% = FAN speed linearly controlled from 0 rpm to max rpmPWM Duty 85 to 100% = Full fan speedIn this case, if the control wire shorts to ground, to the vehicle battery or is cut, the fans run continuously at full speed and so will prevent the engine overheating at low vehicle speeds.Best option is to buy one of these OEM controllers and make a small circuit to read a std temp sensor (typically an NTK sensor, with a characteristic resistance of 2k @ 25degC) and output a suitable duty signal to command the fan controller. Any small microcontroller can easily do this, or you could do it entirely in discrete components with 555 timers etc)

author: Zero999
author_rank: Super Contributor
timestamp: December 21, 2014, 12:44:55 am
content: Quote from: IanB on December 20, 2014, 05:53:09 pmNo, variable speed control is much more efficient. It uses far less energy to run a fan at 50% of its maximum speed, rather than turn if off and on, over a period of time.This is because drag is not linearly proportional to the speed but goes up to the square of speed. In order to double the speed if a fan, you need to quadruple the power.http://www.explainthatstuff.com/aerodynamics.htmlThis is why companies invest money in upgrading their air conditioners to use variable speed drives, rather than just switching the fans and compressor on and off. In the long term, the savings mount up and it pays for itself.

author: Simon
author_rank: Unknown
timestamp: December 21, 2014, 12:54:54 am
content: A continuous lower power draw is also easier to manage than a large short ones, batteries last longer on continuous low power than bursts of high power, alternators can feed the required lower continuous power and so the battery can be smaller the alternator smaller. The whole automotive industry is going more efficient and on large electrical systems this has a benefit. Cables won't be expected to carry large sustained currents, voltages drops and losses will be less.

author: IanB
author_rank: Super Contributor
timestamp: December 21, 2014, 01:35:28 am
content: Quote from: Hero999 on December 21, 2014, 12:44:55 amQuote from: Simon on December 21, 2014, 12:54:54 amVery well, these points are all valid.(I think you will find, by the way, that the power of a fan varies with the cube of the speed. From the standard "fan laws" (good engineering rule of thumb), the pressure developed varies with the square of the speed and the flow varies directly with the speed. The required power is proportional to the flow times the pressure rise, so it therefore varies with the cube of the speed.)Another factor, however, is the aerodynamic design of the fan. It will be optimized for a given speed, and if you run it at a much slower speed it will certainly consume less power, but the power consumed per cubic foot of air moved will go up and it will waste energy for a given amount of cooling.If a fan is going to be used with variable speed control, the design of the fan blades should be adjusted to suit the normal range of (lower) operating speeds.

author: Seekonk
author_rank: Super Contributor
timestamp: December 21, 2014, 03:35:39 am
content: I didn't like the noise of my fan. So, I took it out. I haven't had a fan in my vehicle for years.That said, this would be a nice little project for a UNO and a couple of FET's. I have a friend with a custom built car that has asked me to put some LED turn signals on it. I was thinking about using a UNO but couldn't justify it. The other day he had a section of 2 inch pipe with a 12V pump in parallel. The radiator is at the other end of the car and he said if he is at idle for a long time he has to turn on the pump. I didn't think about it till I read your post. He needs to turn on the pump and control the speed of the fan automatically. Now I can justify the UNO. Thanks!

author: SeanB
author_rank: Super Contributor
timestamp: December 21, 2014, 07:07:49 am
content: Quote from: Seekonk on December 21, 2014, 03:35:39 amYou either do not drive more than 2km in a trip, or only travel on a freeway and never are in any traffic jams. The only time my fan runs are if I have done a slow trip, or have been in traffic for more than 3 minutes. As the car has a 2 speed thermostat which does the regulation the simple way by using a resistor block on the one fan I rarely hear it. High speed is only going to occur when the engine is approaching 95C instead of 90C regular running temperature. While the ECU can select the high speed with the AC compressor running, it also has a mechanical thermoswitch to act as back up as well.

author: Simon
author_rank: Unknown
timestamp: December 21, 2014, 10:25:43 am
content: Quote from: IanB on December 21, 2014, 01:35:28 amDoubling the speed of a fan requires the power to go up by a cube. I was looking for a very fast motor one time for work and spoke to a guy at a company and I couldn't make up my mind if the motor (driving a fan) needed to run at 10Krpm or 11Krpm, he said please go back and check because that little 1Krpm your not sure about could make all of the difference to the motor design and the power required and it can make or break the design.Lower speeds are not less efficient for airflow at all. Faster fans create more turbolence and turbolence is lost energy and an obstacle to the air itself, if it was such a bad idea why are so many manufacturers designing expensive fans with 15-100% speed ranges ? the world is going brushless not only for the sealed longer life motors but for the ease of control as they are inherently speed controlled motors. I have designed a tiny box that can do what the OP wants because it drives a brushless motor that has all of the power drive electronics built in, my equivalent for brushed fans is vastly larger because of all of the power switching.

author: IanB
author_rank: Super Contributor
timestamp: December 21, 2014, 05:19:10 pm
content: Quote from: Simon on December 21, 2014, 10:25:43 amYou misunderstand. Variable speed is not a bad idea. But fans are designed to operate optimally at a given speed (this is the aerodynamic design of the shape and size of the fan blades). If a fan is designed to be installed in a variable speed application it will be designed to work optimally at the most common speed encountered, probably somewhere in the mid-range. A variable speed fan will (should) therefore have a slightly different design than a constant speed span. If you retrofit a variable speed drive to a constant speed fan it won't perform quite as well as a purpose designed fan in the same job.(With small fans and low powers like PC case fans this is all a wash and scarcely matters. But with big fans the efficiency and power consumption are important.)

author: Simon
author_rank: Unknown
timestamp: December 21, 2014, 06:03:10 pm
content: Yes you're probably right I have fan datasheets that show some sort of efficiency line on the graphs but it seems to amount to the highest volume you get which is generally midpoint between maximum pressure and maximum flow which looked kind of obvious to me and being an Italian datasheet I took it at some wankery although speaking to another fan manufacturer last Friday he said their datasheets also have a similar concept but they have an area of best operating conditions on theirs so you will have the pressure versus air flow line and then a section of that determined as being the most efficient and what the fan is designed to work in.The other thing is you don't want to go over cooling your engine so often some speed control may be necessary in some applications and yield better overall efficiency.The fan on my car is broken and I generally only gets a problem in the summer if I have to sit in traffic for more than a couple of minutes and then usually the cabin heater is enough to keep the engine stable. I have in fact just placed a cover in front of half my radiator to stop over cooling in the winter and as the fan does not work I have an engine air intake pipe stuffed into the fan's casing so that the engine gets warm air off the radiator which in the winter makes quite a bit of difference to consumption, performance and the time it takes to get the engine to temperature.

author: Simon
author_rank: Unknown
timestamp: December 21, 2014, 06:16:28 pm
content: Quote from: 3roomlab on December 21, 2014, 06:11:36 pmYes but I believe it you're referring to a switch mode power supply whereas here controller is supposed to be pulsing the fan to convert the design into a very powerful switch mode controller would require very large inductor and capacitor as the frequency is a mere 200 Hz so it's a bit of a tall order. I think the best option is to switch at a much lower speed to get rid of the noise or reduce it to a bit of clicking or speed need increasing but then the MOSFETs might overheat if they are not properly driven on their gates. I suggest the design inside the controller may be a bit cheap.

author: Zero999
author_rank: Super Contributor
timestamp: December 21, 2014, 06:18:42 pm
content: Quote from: 3roomlab on December 21, 2014, 06:11:36 pmThere shouldn't be a post PWM filter, unless it's after a huge inductor, otherwise the current through the capacitor would be huge.One approach is to make the PWM frequency high so the motor's internal inductance will smooth the waveform, although even this isn't really necessary, so long as it has enough inertia to smooth out the vibrations and it doesn't excite any resonances.

author: Simon
author_rank: Unknown
timestamp: December 21, 2014, 06:22:30 pm
content: Quote from: 3roomlab on December 21, 2014, 06:14:17 pmWell at the other thing is having read that page on aerodynamics is that to double the speed of an object through air requires four times the power. The blades of a fan are not much different to the wings of an aeroplane so to double the speed of the fan would require more than twice the power, so I think really all things considered it's about the same either way. The blades may be designed for a certain speed but you won't stop the drag factor. Fans I have controlled will produce quite a decent draft with just 5 amps but to get full speed in fresh air required 20 A.There is to be considered that generally for more airflow fans get larger. Where I work we supply fans of up to 1.2 m in diameter which go on hydraulic motors in very large vehicles. These fans generally ran at about engine speed and I would guess they are pretty efficient being much larger.

author: IanB
author_rank: Super Contributor
timestamp: December 21, 2014, 08:02:22 pm
content: Quote from: Simon on December 21, 2014, 06:22:30 pmYou've got to be careful here with your details. Drag represents wasted power and is not a good thing. Fan blades, like aeroplane wings, are designed to minimize drag. In an ideal world you would like all of the power from the fan motor to be spent moving air and none of it to be wasted on drag. Real fans cannot be perfect and so they don't have perfect efficiency. However, the area of best operating conditions you mentioned in your other post is the operating region where the efficiency is maximized.Drag varies with the square of the speed as mentioned in that aerodynamics article, but efficient movement of air through a fan varies with the cube of the speed. Since the primary job of a fan is to move air the cube law wins out.

author: Simon
author_rank: Unknown
timestamp: December 21, 2014, 08:32:54 pm
content: So what your saying is that if the speed doubles the power requirement is the square but the amount of air moved cubes ? But then I'm told that to double the speed of a fan requires an increase in power of a cube so the power to airflow ratio is linear but the drag is the square anyway ?

author: Simon
author_rank: Unknown
timestamp: December 21, 2014, 08:48:07 pm
content: Quote from: 3roomlab on December 21, 2014, 08:36:33 pmYes that is the sort of graph i have seen, but then efficiency compared to what ? 25% of what ?, You would think that the best operating point would be 100% so the graft is a little confusing to me and seems to be simply a representation of where you get the most air moved. For example in an air conditioning system we have at work the customer has put a silly amount of ducting on the outlets. So my quest has not been to look at so-called efficiency graphs but to actually find the fan that will develop the most pressure because if we don't develop pressure nothing will move in the first place. What needs to be remembered when you are working with fans is that most of the time you are trying to push the air someplace, either through ducting for air conditioning or through a radiator for engine cooling so you will need to overcome the pressure drop of what ever there is after the fan or if the obstacle is behind the fan it still has to draw air across that pressure drop.

author: Zero999
author_rank: Super Contributor
timestamp: December 21, 2014, 09:09:37 pm
content: Quote from: 3roomlab on December 21, 2014, 08:36:33 pmIs that with the motor run at full speed?It seems to be a plot of the efficiency vs load, which makes sense. When there's no pressure increase (uncovered), no work is being done, so the efficiency is zero and when there's the fan is covered the pressure is maximum and no work is being done as the air flow is zero, so the efficiency is zero. In the middle there's a sweet spot with some pressure increase and reasonable airflow.

author: IanB
author_rank: Super Contributor
timestamp: December 21, 2014, 09:35:50 pm
content: Quote from: Simon on December 21, 2014, 08:32:54 pmNo, what I am saying is this:The "Fan Laws"1. The volume of air moved (cubic feet per minute, cubic metres per hour) varies directly with the speed2. The pressure rise (psi, bar, kilopascals) varies with the square of the speed3. The motor power (hp, kW) varies with the cube of the speedLaw 3 arises from laws 1 and 2, since the power required depends on the air flow times the pressure rise.

author: Simon
author_rank: Unknown
timestamp: December 21, 2014, 10:24:54 pm
content: Quote from: IanB on December 21, 2014, 09:35:50 pm2. Right so is that 2x speed = 4x pressure or 4x speed = 2x pressureI'm assuming you need 4x the speed to double the pressure.

author: IanB
author_rank: Super Contributor
timestamp: December 21, 2014, 10:37:30 pm
content: Quote from: Simon on December 21, 2014, 10:24:54 pmThe pressure varies with the square of the speed, so 2x speed = 4x pressure (= 2²), 3x speed = 9x pressure (= 3²), and so on.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 22, 2014, 12:20:01 am
content: Quote from: 3roomlab on December 21, 2014, 11:04:41 pmI can't see what it is you're simulating, but it doesn't look representative. But that's okay, you'd need an inductor in the 10-100mH range to handle 200Hz. The obvious answer for any other situation is: by throwing caps at it, you're just going to make things hotter and (electrially) noisier.My point from my first reply still remains... if it does everything the way you want, fine. Don't complain about it. It is exactly what it is, no more and no less. Attempting to address undesirable features (like beating the shit out of the motor at 200Hz) is like trying to train an elephant in ballet. You need a fundamental redesign, a device which isn't made for crap.Tim

author: Leadfootin
author_rank: Contributor
timestamp: December 22, 2014, 01:16:17 am
content: First I would try some fairly high current flyback diodes, say 10 or so amps and check the noise difference. 50V minimum, try paralleling a few smaller ones to test out. Often an old UPS or inverter will have some Schottky's suitable for the job.

author: Simon
author_rank: Unknown
timestamp: December 22, 2014, 06:56:14 am
content: Quote from: IanB on December 21, 2014, 10:37:30 pmThat makes sense as in my work application where we need pressure we are going for a fan that is the same size but it runs a bit faster. However pressure does not always equate to more air flow as if the pressure drop is low putting more pressure in with no ability for airflow will still not get you far hence I suppose those efficiency graphs although it's the characteristics of the whole system that will tell you if a speed change will make much difference.A lot of cars seem to have put large fans in, far more powerful than needed, Bosch developed a brush-less 12V 48A fan for Audi quite a few years ago now, and I've seen audi's that have a mechanical fan in that runs all of the time anyway so the electric fan is either well overpowered or is an extra safeguard so running full speed and drawing all of that power is a really bad idea.

author: SeanB
author_rank: Super Contributor
timestamp: December 22, 2014, 11:38:35 am
content: Audi fan will be a viscous coupled unit, the extra fan is there for when the AC is running and developing high head pressure from being stuck in traffic, and typically will have a 2 speed switch and a resistor. In traffic on a hot day with AC on you will hear it cycling on in a 20 second cycle on 30 second off or so. It will run as well when the radiator reaches 87C.

author: Simon
author_rank: Unknown
timestamp: December 22, 2014, 11:41:08 am
content: The fans I refer to are called QBA fans and they are speed controlled with a 5V signal straight from the ECU, the last thing you want is one of those going full blast, no one has yet matched them in the automotive market that I'm aware of. If they loose a signal they will default full speed.

author: max_torque
author_rank: Super Contributor
timestamp: December 22, 2014, 09:17:28 pm
content: The reason you have speed controlled fans on a passenger car these days is not directly to do with any "efficiency" of the fan, motor, or controller, and EVERYTHING to do with the fact the fan peak power requirement is massively intermittent!During testing and sign off, the cooling system will be loaded under a worse case scenario (usually a 45degC ambient hill climb in a low gear at max vehicle train weight). As modern cars are powerful (at peak output) the cooling system needs to be able to reject a LOT of heat (around 1hp per flywheel hp), so under this condition it takes a powerful fan to produce sufficient Air flow to remove that heat at a sensible DeltaT. However, for the vast majority of it's operation, your car is under a much lower engine load, and ram airflow is sufficient to provide the heat transfer.As a result, older cars just switched a big powerful fan in when needed, but as cars became quieter and NVH expectations grew, OEMs started fitting dual speed fans, and then fully speed controlled ones to limit the impact of suddenly turning on up to 750W of fan all of a sudden!Currently, pretty much all cars use speed controlled brushed motors, with some manufacturers moving to brushless motors for reliability reasons (no brushes to wear or carbon build up etc)