Fan speed control

[leorest]

Apologies if this is deemed to be in the wrong forum, Mods please move it if you see fit, but there doesn’t seem to be a technical automotive electronic forum.

The problem
How to provide variable speed control for a 12Volt 16Amp fan.

Constantly variable would be good but two speed would be fine.

Currently I am using a dc-dc converter which outputs 5Volts. This is perfect for slow speed.

For high speed 12Volts is too much, not so much to do with the speed but more because of the high current and its effect on the wiring.

The three options available are
1 Voltage regulation – On the face of it this seems the simplest if not the most efficient but it is not easy to find a circuit design which can control 16Amps

2 Current limiting – either a complex circuit or a massive Watty resistor.

3 PWM control - I have tried two types of PWM drive and depending on the drive frequency they exhibit one or more of the following problems.
Undesirable audible noise linked to the drive frequency (especially when driven with a frequency in the audible range).
Undesirable electronic noise (interference heard on the radio).
With excessively high or low PWM frequency the motor does not run.

Having purchased two types of quite expensive PWM drive and found each to be unsuitable for some of the reasons above I have decided that PWM is not the way forward.

So this brings me back to option 1
Slow speed is already taken care of so I need a circuit which can provide a fixed voltage somewhere in the range of 7.5-9V and capable of delivering 10-12Amps

Having done plenty of Googling I have found several circuits suitable for controlling the voltage but no specific circuit capable of working at the high current. In the following link the circuit can manage 4.3 Amps but would need additional or different pass transistor (2N2955) to get to 10-12 Amps
http://www.eleccircuit.com/lm317t-voltage-regulato...
Are there any electronics gurus who can help me with this seemingly simple circuit design.

It’s been too many years since college.

[stevieturbo]

I think Autospeed did an article on building such a device a while back. get googling.

[anonymous-user]

Sounds like you just havent done the pwm bit properly. All current DC motors in a car are speed controlled with PWM! (including the big current rad cooling fans etc) Easy to drive 20 or more amps with a single MOSFET H bridge etc!

Basically, if you're not that fussed about a) efficiency, or 2) torque ripple (which you won't be for a fan drive) then use a slow pwm frequency, i would suggest something like 200hz, (or if you care about the previous a) and b) use a pwm frequency above the audible spectrum (say 20kHz)

As for the "electrical noise" a simple RC snubber circuit accross the motor should fix any major issues (assuming your H bridge design is Ok (i.e. no excessive parasitic inductance and clean gate drive signals etc!)

i have a fan drive board (that also drives things like fuel pumps in my rally car) thats 4x7cm small





However, if you don't care about REAL inefficency then just get a cabin heater "hedgehog" resistor and put that in series with the fans (some old rad cooling fans also had these resistors)

[anonymous-user]

Should also have added that at 16A, ANY kind of linear circuit is going to be a no-no due to the ohmic heating! you deffinately need some kind of switcher.

Also, what is the load? have you details of it's series resistance and inductance (a quick sim in LTSPice or similar would give you a good heads up as to the best pwm frequency and snubber requirements etc)

[stevieturbo]

Of course on the other hand....why do you need variable speed ?

[Pigeon]

leorest said:

For high speed 12Volts is too much, not so much to do with the speed but more because of the high current and its effect on the wiring.

Seems to me you're going to a heck of a lot of trouble and expense just to avoid the simple cheap option of putting thicker wiring in.

[leorest]

Thanks for the replies,

In the light of the information provided I have decided to purchase another PWM driver this one works outside the audible range (32kHz).

Will let you know how it works.

[anonymous-user]

leorest said:

Thanks for the replies,

In the light of the information provided I have decided to purchase another PWM driver this one works outside the audible range (32kHz).

Will let you know how it works.

If you still get "noise" issues (although i'm not sure what you mean by this exactly (radio interference etc?) then adding a few capacitors (with a nice low ESR) and a series inductor infront of the pwm stage power input will kill a lot of any noise that might get radiated back down the power supply wiring into the rest of the car.

[Steve_D]

Max_Torque said:

.....i have a fan drive board (that also drives things like fuel pumps in my rally car) thats 4x7cm small.........

Hi I'm interested in what you are using for pump control. Is it something you manufacture or a commercial product you bought?

I PMed you but got no response.

Thanks
Steve

[anonymous-user]

It's a pcb i have designed/made myself, uses a AVR to read in a signal, and outputs a PWM amplified signal to drive a brushed dc motor (half bridge configuration) Input can be either digital (on-off) analogue (0-5v) or pwm / frequency, and there's enough processing power onboard to do PID control of the output. Also has onboard measurement of output current & MOSFET temperature for control / diagnostics etc.

[leorest]

Max_Torque said:

leorest said:

Thanks for the replies,

In the light of the information provided I have decided to purchase another PWM driver this one works outside the audible range (32kHz).

Will let you know how it works.

If you still get "noise" issues (although i'm not sure what you mean by this exactly (radio interference etc?) then adding a few capacitors (with a nice low ESR) and a series inductor infront of the pwm stage power input will kill a lot of any noise that might get radiated back down the power supply wiring into the rest of the car.

Yes it was interference on the radio but who knows what else it could be affecting! My order is showing as dispatched so shortly I'll try out the new 32kHz drive and let you know how it goes. Unfortunately I don't have any manufacturers specifications on the motor I only know that when powered by 12Volts it draws 16Amps. You sound like the right person to be talking to so thanks for your input.

[anonymous-user]

Also, to minimise radiated noise, mount the pwm unit as close as possible to the motor, use extra noise blocking on the input power (Caps + inductors) add a snubber across the motor (cap + series R, google RC snubber for more info) and maybe even shield the phase wires (add grounded braided shield over wires to motor) The fact that you were getting radio interference with the last pwm device suggests that it had a lot of switching noise at multiples of the base pwm frequency.

[Huff]

Cheap 'snubber' - Maplin part RG22Y is close enough for almost any purposes (100nF +100R in series, in one package).

[leorest]

Max_Torque said:

...The fact that you were getting radio interference with the last pwm device suggests that it had a lot of switching noise at multiples of the base pwm frequency.

The first PWM circuit was cheap and nasty. It didn’t work at all well so I wouldn't be surprised if the output had harmonics and switching noise.

Huff said:

Cheap 'snubber' - Maplin part RG22Y is close enough for almost any purposes (100nF +100R in series, in one package).

Cheers for that.

When I get the new driver I will look to see if it incorporates any snubber circuitry across the outputs and if not then include one of the Maplin snubbers. As already said the driver to motor wires need to be beefy and short. That said is it best practice to fit the snubber circuit nearest the motor or the driver? Thanks for all the quality advice

Edited by leorest on Friday 15th October 11:30

[leorest]

I have the PWM driver now and it works perfectly on the test bench. Just need to install it to see if I get any RFI.
[pun mode]Currently[/pun mode] working out wire size for a 1.5m run. The 32/0.2 I have in stock seems to run quite warm.

[stevieturbo]

leorest said:

I have the PWM driver now and it works perfectly on the test bench. Just need to install it to see if I get any RFI.
[pun mode]Currently[/pun mode] working out wire size for a 1.5m run. The 32/0.2 I have in stock seems to run quite warm.

Surely any controller you now use, will mean a higher current at the fan...and hence even larger cable and fuses ?

You could maybe give that guy looking a variable controller for his fuel pump a shout too lol

[leorest]

stevieturbo said:

Surely any controller you now use, will mean a higher current at the fan...and hence even larger cable and fuses ?...

No, any additional current would be in the circuit between the battery and the driver rather than “at the fan”. You could argue that with the controller set to 100% then there will be a minute increase but not enough to move up cable sizes.

stevieturbo said:

...You could maybe give that guy looking a variable controller for his fuel pump a shout too lol

Thanks, but what I now have seems to work.