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Inductance of 24VDC fan?

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FvM

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I would expect a filter capacitor parallel to the supply pins as most BLDC fans have.

If my assumption is right, you need an external inductor for effective PWM operation.
 
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treez

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Thanks,
How did you know it was brushless.?.....do you think the inverter transistors are somewhere inside this fan?
Do you mean there is such a capacitor inside this capacitor?

When you say an external inductor is needed, you mean that we will need to basically make this fan the load of an external buck converter? (as you know, we have a 48v rail to drive this 24v fan with.)
 

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All DC fans on the market have BLDC motors. Perhaps you'll want to disassemble a defective exemplar in your reach.

I'm not sure if all have input capacitors, but I found it with many types, e.g. a several µF ceramic capacitor. The PWM circuit with series inductor and possibly additional filter capacitor forms in fact an external buck converter.

It has been reported in previous edaboard threads that PC boards are utilizing low frequency PWM for fan control. This should work too if you don't mind the switch-on current peaks
 
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godfreyl

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The PDF mentions a third wire. That could be worth investigating.
 
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FvM
It has been reported in previous edaboard threads that PC boards are utilizing low frequency PWM for fan control. This should work too if you don't mind the switch-on current peaks

you read my mind......that is my central interest.

I have such a circuit, its just a micro pwm'ing a low side fet of a buck converter with the fan between the external buck inductor and the 48v supply rail. -But for some reason the engineer has put an electrolytic 100uf cap across the fan.....I want to change it for a 10uF ceramic.....I don't see any reason why I shouldn't be able to?
I am worried though that when using a lower value capacitor across the fan there will be more AC in the fan (switching current ripple), and thus more hysteresis loss in the fan's iron core?

I suppose its a single phase BLDC motor, ...I wonder what happens in the dead time of the bldc inverter?...I suppose it can't draw current from its parallel cap during the deaf time?
 

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