Resistance of small DC computer fan motor

[splocious]

To prove that the fan was knackered I firstly applied 24Vdc and then measured the resistance ( ~300Kohms). Satisfied i threw it out.

Out of interest i measured the resistance of the new fan motor before i installed it (~300Kohms)

Can someone please explain why the resistance is so large.

P=6.2W

I=~0.250mA

V=24Vdc

Cheers in advance

 

[kme]

Because it isn`t a standard 24VDC motor mate - I think you`ll find its pulsed.....

As a general rule, if you check the spec. of the bog standard power module for a PC, what you WON`T find is a 24VDC line.

Most of the fans are pulsed by the system, so that they can accurately control the speed - newer BIOS can report the speed a fan is running..........

(I`m sure someone can provide specifics )

KME

 

[splocious]

Fair enough. I can understand its pulsed to control the speed, just thought 0.3Mohm was pretty high.

Cheers

 

[Sidewinder]

Well by Ohm it should be over 96k based on R=V/I from your figures.

It is an inductive load, thus the DC resistance will be lower than the running resistance/impedance.

OK, this is a DC supply, BUT, it is a motor which must have a rotating field to operate, thus, there will be an ammount of effective impedance, and there will be some back emf etc. etc.

As far as motor windings go a straight forward DC resistance measurement is quite ineffective!

PLUS the external control mechanism will have an affect on how the designer designs the motor, thus the values of the motor parameters.