Root cause insight into the common BMW blower motor resistor failures

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Does anyone have insight into what is the root cause (and repair) of the FSU failure that plagues almost every 1997 to 2003 BMW? http://www.bimmerfest.com/forums/attachment.php?attachmentid 6060&d94115994
Also, does anyone have an idea HOW TO TEST a "repaired" FSU?
The "blower motor resistor", which also goes by FSR (Final Stage Resistor) or by FSU (Final Stage Unit), is known to fry itself in almost every single E46 (3-series), E39 (5-series), and E38 (7-series) BMW. http://www.bimmerfest.com/forums/showthread.php?t 3393
The problem with replacing this ~$100 part is that the new replacement FSU fries itself just as often as the old one did, so you end up repeatedly replacing your fried FSU every few years or so. http://www.bimmerfest.com/forums/showthread.php?tR8566
That's fine for most people (although the DIY is a PITA) - but I ask this newsgroup whether anyone has any insight into WHAT is actually breaking - and - why? http://www.bimmerfest.com/forums/showthread.php?t09399
Here is the best (admittedly sketchy) wiring diagram we have so far:

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On 03/20/2013 08:08 PM, Bimmer Owner wrote:

that looks like a linear semiconductor controller - an incredibly antiquated concept for a modern car.
old resistor packs for fans were open wire that sat in the fan's air stream for cooling. they were generally very reliable if their alloy wasn't too susceptible to salt.
that unit looks like it still sits in the air stream with that honking great heat sink and i estimate it's trying to dissipate >100W. that can only mean it's a linear controller because a modern pwm device can control high motor currents with very little heat dissipation <10W.
bottom line, a linear controller is always going to get hot and end up frying itself over time. the only thing you can do is either replace it with another unit that will ultimately meet the same fate, or undertake a significant modification.
for the latter, you can try putting an even bigger heat sink on it - but i doubt there's a lot extra room available. you can also "pwm" it. i built a similar unit to deal with a linear controller over-heat issue on my 89 civic.
<http://arduino.cc/en/Tutorial/PWM
depending on how much time you want to spend on a project like that, pwm can control superbly and offers benefits like motor speed not being so susceptible to supply voltage [engine idle voltage drop] etc.
the down side of pwm is that it can generate electrical noise. [poor stereo installations can be particularly susceptible.] the ideal solution is to implement pwm with "soft switching", but that's getting quite advanced.
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On 03/20/2013 09:13 PM, jim beam wrote:

well, you live and learn. apparently the reason they use a linear controller is because it allows the fan to run near silently at low speed. with pwm control the fluctuating magnetic fields in the motor coils cause it to vibrate and make a humming noise at the pwm control frequency.
that doesn't of course get around the fact that the unit in question here is apparently badly under rated, but the above does at least explain why it's used.

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>> that looks like a linear semiconductor controller - an incredibly >> antiquated concept for a modern car.
>well, you live and learn. apparently the reason they use a linear >controller is because it allows the fan to run near silently at low >speed. with pwm control the fluctuating magnetic fields in the motor >coils cause it to vibrate and make a humming noise at the pwm control >frequency.
>that doesn't of course get around the fact that the unit in question >here is apparently badly under rated, but the above does at least >explain why it's used.
I see many Motor Speed Control Manufacturers upped their PWM frequency to be between 16 K to 22 K to eliminate much of the noise. Mikek
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That would also make the LPF very much smaller.
Even going to 40 to 60 kHz makes the magnetics smaller. We still do not know what is under the PCB for that unit.
tm
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amdx wrote:

Sure, if you don't mind heating the motor up...
Jamie
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On 03/24/2013 02:30 PM, Jamie wrote:

good point - how much? sure, big motor coils, big inductors -so how to balance against pulse frequency for a bigger motor like a blower fan?
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jim beam wrote:

THe problem is eddy currents at high pulse rates. Between the wire used and the core you can get heating in the motor..
Most inverters or pulsers for industrial drives tend to operate in the 8kHz range, that seems to be a good compromise.
Another way to do this, is to have an inductor on board with the speed control circuit. You would PWM that inductor in series to a filter cap on the output which will then give you a clean variable DC. THe inductor will be doing all variable voltages. This is a form of a PFC type of supply and the only heat you get is from the DC coil R in the inductor and the heat from the switching MOSFET, which should both be rather low. BY doing it this way, you could operate a switcher at lets say up in the 100kHz or more range and this would keep the size of the caps and inductor small.
It is cheaper to use the motor as the inductor for PWM control but, it also can radiate noise on the lead wires. I guess one could actually mount the speed control directly on the motor using PWM and the noise problem should be minimum.
Calculating the cost between the two, BMW most likely decided to go with the basic linear type, Because why should they care? It'll most likely make it past it's warranty!
Jamie
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On 03/25/2013 05:57 PM, Jamie wrote:

ok, as i understand it, and as i said to scott earlier, this is a problem because it mungs low speed motor start and low speed torque.

this seems to be the problem. you can get good efficient control at a few hundred hz, but it's super noisy. if you take the frequency up above audio, bigger motors seem to be more of an issue.

you got that right - and it would go a lot further past its warranty if it was better rated.
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Not really, it's feeding the motor with variable DC from that integrator stage. It's not much different than a big rheostat in terms of starting torque. It's just more expensive and less reliable and quieter. --scott
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On 03/26/2013 05:58 AM, Scott Dorsey wrote:

i understand that - and variable voltage is the problem. the secondary [bordering on primary in some applications] advantage of pwm is low speed start and torque. if a motor starts at low dc voltage, not only is the start speed inconsistent, it has little torque. pwm can start a motor slower and at much higher torque. it's a big deal.

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jim beam wrote:

A properly working blower motor does not need extra torque to start at low RPMs.
Torque is only needed when RPMs increase and mass air flow is increased, thereby, putting a strain on the motor. Basic resistor systems will vary in speed if air pressure isn't constant, and in most cases it isn't..
When there is no air flow or the flow has been restricted somehow, there is little to no torque demand, other than mechanical of the blower blades and those should turn easy, sine bearings and balance permits this.
PWM is just a cheap way of speed control, it does not mean it's better, in fact in some ways it's not, due to over head in noise..
A linear control with feed back will provide the needed torque but they do tend to run hot when throttled back, because of the resistance being present between the 12V and the motor terminals. THis is where PWM comes in a winner but then you need the added cost of noise reduction engineering.
I can only assume the linear module at least uses a feed back to maintain output voltage, if it is so cheap that it does not even do that, then maybe they are trying to emulate a real resistor or they are just shitty engineers or tightwads.
Jamie
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On 03/26/2013 05:16 PM, Jamie wrote:

you're right, except that there are more variables. very cold days, very windy days, blown snow powder, leaves, all kinds of things can mess with the motor starting at a low speed.

from what i can see, the /only/ drawback is noise. power efficiency, controllability, speed consistency, and yes, sometimes price, all are wins for pwm.

well, they're clearly failing at something if they're trying to provide an engineering solution. if however they're providing a financial solution with a per-determined failure rate, then they're right on target.
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jim beam wrote:

If that being the case, I guess we now know why the module keeps burning out :)
But there is factor that maybe you have forgotten or didn't know, and that is, the resistance of the DC motor. stall torque can be limited to what the DC R value is, in otherwords, this value forms a voltage divider and thus low voltage at stall current could seem like no voltage and not turn.
DC PM/SHUNT motors attempt to compensate when load is dragging it down in speed, that also includes a slow start. So as long as the speed control can maintain a low voltage set point even when the motor is calling for high amps - the motor R, it'll still start. However, there is another factor, the speed control may not be performing armature feed back and simply supplying current only.. If this is the case, then the motor will stall at low speed demands in conditions that make it hard for the motor to start.
I've also seen them allow the motor to run in torque mode to adjust for air density. It'll simply self adjust naturally, and in those cases you do not want armature feed back but torque feed back. Of course, this will cause heating of the module when torque demand is low..
Jamie
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On 03/26/2013 07:16 PM, Jamie wrote:

didn't know.

which would be another factor in favor of pwm...

that would be my guess.

i guess that's another part of what we're looking at here.
thanks for the feedback!
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On Tue, 26 Mar 2013 19:16:06 -0500, Jamie wrote:

Most of the people on the BMW forums think it's one of these.
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On 03/24/2013 08:52 AM, amdx wrote:

interesting.
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Yes, this is why you put an integrator stage after the pwm stage, so that the motor sees nice filtered DC with very little of the PWM leftover.
Problem is that the integrator stage costs money and big electrolytics tend to have limited life, so auto folks don't like doing that.

It's a cheap, reliable way of doing the job, if it's done right. It's clear that it wasn't done right, but I'm still waiting to hear what was done wrong. Given all the RoHS-related failures and the report that touching up solder joints on the transistors fixes the problem, I am suspicious that it's a soldering issue made worse by the extreme temperature cycling. --scott
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On 03/25/2013 06:59 AM, Scott Dorsey wrote:

you don't want to integrate the output, merely rub the shoulders off the square waves to get the harmonics down. the whole point and benefit of pwm is that you have full voltage full power available in each pulse. that's how you can start and control a motor with high torque at low rpm. if you integrate or smooth out the motor's supply, you effectively lose that and the motor won't start or torque the same way or even at all.

in this day and age, that's no longer true. motor control is one of the hot ticket items on the silicon fab agenda, and has been for some time. there are some great pwm options out there, and for not a lot of money.

you definitely have a point there, but given the size and shape of that heat sink, i don't think there's any way that silicon is getting sufficient cooling, and is clearly way up against its ceiling. whether that's accident or design is another matter, but the bottom line is that it's an issue that spans multiple different module manufacturers across multiple continents with different internal designs - that reduces the probability of it being rohs and slaps it firmly into the vehicle manufacturer's lap.
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The more you rub off, the quieter the motor is! You start throwing stuff in the KHz range into the motor, and it whines from magnetostriction. The more of that you remove, the quieter it goes. Of course, if you make it too quiet, people complain.... --scott
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