PWM fan control?

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Jon K
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Re: PWM fan control?

Post by Jon K »

Good discussion guys. Stevie I get an error on the site with that link.
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stevieturbo
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Re: PWM fan control?

Post by stevieturbo »

Strange, works for me ok ?

If you do a google search for "max_torque fuel pump speed control" the thread should be some of the results that pop up.

Max_Torque being one of the main contributors to the thread
Alex DTA
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Re: PWM fan control?

Post by Alex DTA »

Works fine for me too. Really interesting discussion as well. Bookmarked to come back to when I have some down time.
stevieturbo
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Re: PWM fan control?

Post by stevieturbo »

You can buy cheap H-Bridge PWM controllers on egay for high current DC motors...I even have one beside me.

Never did try or figure out how the feck it works though lol No doubt it is easy...when you know how.
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ignitionautosport
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Re: PWM fan control?

Post by ignitionautosport »

The good bit from page 2:
Ok, i think this maybe a bit of a red hering really. With Pulse Width Modulated (PWM) signals, the "mean" voltage is set by the ratio of the ON to OFF time. With the switching frequency chosen so that the inductance of the motor reduces the current (and hence torque) ripple to low enough values to not cause either audible noise or motor damage (from vibration).

For the case of a fuel pump, (brushed DC motor, approx 100mH inductance) a PWM frequency above approx 500Hz will be sufficient to limit excessive ripple currents.

The primary advantage of using a high frequency "square wave switched" supply is that the semiconductor switches (the MOSFETS) spend the majority of their time either fully off or fully on. This is the state at which they have the least resistance, and hence the least losses.

If you try to control the output voltage with a "Linear" device, the losses are enormous because you effectively have to continuously "throttle" the applied voltage within some resistance (as opposed to reacting it against a virtually lossless inductance).

Take a pump motor with a 3 ohm resistance for example, if you supply it with the full 14.7 battery voltage (for full speed running) it will consume 4.9Amps (V=IR). Now if you wanted to run it at half speed, you need to only supply it with 7.35V (half the battery Volts). This means you need to "drop" half the supply voltage across a resistor.

In this case we will need a 3ohm resistor in series with the pump (so that the supply voltage is evenly split between the 3ohm pump and the 3ohm resistor).

The problem is now that we are still supplying 14.7V from the battery, but effectively wasting half of it over some resistance that does no "useful" work. (in fact both pump and resistor each use 18watts)

This produces a very inefficient system (and a lot of heat to deal with)

No damage caused to a DC motor if it is stalled with a limited current running through its windings.
(Stalled motor's are worst case because the multiple windings are not sharing the current between them (the rotor is not turing, so the brushes just keep feeding a single set of windings). if you stall a motor and supply the full voltage to it, it's low resistance will quickly cause a huge current flow and overheat the pump to failure.

In actual fact, with a fuel pump in proper working order (i.e. not jammed up by a foreign object etc) the pump never completely stalls, even at a current that is insufficent to generate enough torque to overcome the pressure head, because the roller or vane pump has enough "leakage" paths to allow the pump rotor to slowly turn anyway.
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PhillipM
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Re: PWM fan control?

Post by PhillipM »

Rob Stevens wrote:You could do it with an aux output but depends on the current you intend to present to the ecu.

I have seen a way of running 2 fans in series (6v) then using a relay network to switch them to parallel (12v) for full power. But I can't find the wiring diagram, and it uses 2 ecu outputs
Might have been my car, with two huge 16" cooling fans I have similar issues to the OP, so I have a small relay/diode box which flicks them from parallel to serial (6v to 12v), so it soft starts the fans, it uses a fan switch on the dash for high speed mode and then the 6v mode is controlled by the ECU.
If you used SSR's instead you could run PWM control the higher fan speed so it ramped up from the low but this works well enough for me.
Rob Stevens
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Re: PWM fan control?

Post by Rob Stevens »

Could you post a diagram?
PhillipM
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Re: PWM fan control?

Post by PhillipM »

It's just a couple of DPDT relays and some flyback diodes.
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