Root cause insight into the common BMW blower motor resistorfailures

[Scott Dorsey]

I've seen 20W power resistors in TO-220 packages.

It does indeed look like a TO-220 from the pin spacing (since there are no
actual transistors in the photos, just spots from which they were removed).

But if it had been a PWM device, there would have been some filtering in
there, inductors and capacitors to keep the noise from getting into the
power lines. Designing clean and quiet PWM controllers is not quite as
trivial as some folks have made it out to be.

This is possible, if it is the transistors that are failing. I don't see
any big protection diodes in there either.

If it's a RoHS soldering issue, though, I would not be surprised.
--scott

 

[Jamie]

It does indeed look like a TO-220 from the pin spacing (since there are no
actual transistors in the photos, just spots from which they were removed).

But if it had been a PWM device, there would have been some filtering in
there, inductors and capacitors to keep the noise from getting into the
power lines. Designing clean and quiet PWM controllers is not quite as
trivial as some folks have made it out to be.




This is possible, if it is the transistors that are failing. I don't see
any big protection diodes in there either.

If it's a RoHS soldering issue, though, I would not be surprised.
--scott

A few car manufacturers use resistor wire in the harness leading to
the motor connection at the blower box.

This lowers the Q significantly giving you a voltage drop of course,
but it also reduces electrical noise and helps suppress the wheeling
voltages.

Blower motors in this case are normally designed to operate lets say
8 volts for example, for full RPM.

I learned this years ago when going through the pain of removing the
blower in an air box of a Chrysler product, only to find there was
nothing wrong with the motor. Symptoms led on to the fact there was
since the output of the speed control circuit was alive and happy but
no obvious connection to the motor. If I had unplugged it from the air
box before pulling that all apart, I would of noticed it.

The problem was the resistor wire in the harness which had open at the
crimp point, also in the harness.

Jamie

 

[jim beam]

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.

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://www.flickr.com/photos/38636024@N00/5068043855>
<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.

 

[amdx]

that looks like a linear semiconductor controller - an incredibly

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

 

[tm]

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

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

 

[[email protected]]

Is that cheaper than roll your own?

Definitely not, if you already have a multimeter - but it is easier
for the guys who can't figure out how to do it without butchering the
wiring harness.

 

[Jamie]

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

Sure, if you don't mind heating the motor up...

Jamie

 

[Scott Dorsey]

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.

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.

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.

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

 

[jim beam]

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.

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.

It's a cheap, reliable way of doing the job, if it's done right.

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.

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.

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.

 

[jim beam]

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

interesting.

 

[jim beam]

Sure, if you don't mind heating the motor up...

Jamie

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?

 

[jim beam]

All those photos only show one side. I full reverse engineering
should be done to draw a full schematic but I've never had my hands on
that module. It would probably take a couple of them, because ot the
potting.

if it's just a two-layer board, maybe. assuming you get the specs on
the chips of course. but you'll need more than two units and a whole
lot of patience trying to reverse the schematic if it's 4 or more
layers. and you still don't achieve anything more than having a broken
light bulb in your hand.

what you need to do is get the operational capacities of the /working/
unit, and work with those. that the unit is a black box is completely
irrelevant.

Have you looked at the National Semiconductor (Now part of T.I)
'Simple Switcher' series of controllers? Generally only one inductor
and a couple small electrolytics. A lot simpler than older designs, and
little noise because of the small footprint.

/and/ a smaller footprint. the "because" is entirely due to the soft
switching they've achieved. soft switching is still "big science" -
there are still phd's being written on how to implement and design, both
with the types of semiconductor, and the circuits in which they're used.

 

[Scott Dorsey]

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.

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

 

[jim beam]

The more you rub off, the quieter the motor is!

true, but you don't want to "integrate" the full power individual pulses
down to a lower average otherwise it won't run or torque.

You start throwing stuff
in the KHz range into the motor, and it whines from magnetostriction.

they whine pretty effectively at centihertz too.

The more of that you remove, the quieter it goes. Of course, if you make
it too quiet, people complain....

they will surely complain if it doesn't run.

 

[Bimmer Owner]

Why would you have to cut the wires????
Simply remove the fuse and connect the ammeter. You guys make your
lives so difficult

The 40 amp fuse is barely accessible (as can be seen from the photos).
It's not even easy to pull the fuse & even harder to replace it.
So, all I was saying was that it's actually rather difficult to insert
test leads into the empty fuse #F76 fuse holder.

I'd wager it 'can' be done - it's just going to take an hour or so
to get the leads in place.

 

[Bimmer Owner]

Take a blown fuse and use it for a test connector

That's an idea.

 

[Bimmer Owner]

It does indeed look like a TO-220 from the pin spacing (since there are no
actual transistors in the photos, just spots from which they were removed).

I doubt anything was removed since all the authors of those pictures
are expressly NOT trying to remove anything.

Those round metal "dots" in the pictures are the five pin connectors
of the harness connector.

 

[tm]

The 40 amp fuse is barely accessible (as can be seen from the photos).
It's not even easy to pull the fuse & even harder to replace it.
So, all I was saying was that it's actually rather difficult to insert
test leads into the empty fuse #F76 fuse holder.

I'd wager it 'can' be done - it's just going to take an hour or so
to get the leads in place.

So go back to measuring at the battery terminals. Do you have or can you
borrow a clamp on DC ammeter? They are great for this sort of problem. You
don't even lose any skin that way.

 

[tm]

I doubt anything was removed since all the authors of those pictures
are expressly NOT trying to remove anything.

Those round metal "dots" in the pictures are the five pin connectors
of the harness connector.

Are there any other pictures of the module showing if or where the
transistors are attached to the heat sink?

Has anyone ever fully dissected the unit so you can see what is under the PC
board? Maybe an X-ray from several angles done at the dentist?

 

[[email protected]]

The 40 amp fuse is barely accessible (as can be seen from the photos).
It's not even easy to pull the fuse & even harder to replace it.
So, all I was saying was that it's actually rather difficult to insert
test leads into the empty fuse #F76 fuse holder.

I'd wager it 'can' be done - it's just going to take an hour or so
to get the leads in place.

Bet I can do it in less than half an hour - without the MaxiTester
and in about 5 or 10 minutes with it.