450watt fan

[Borg Warner]

Scratched me 'ead a bit for this then got the old books out. DC motors operate on the interaction of two magnetic fields. When the armature of the motor rotates through the magnetic fields there is an E.M.F induced in them which is in opposition to the current taken by the motor. This is known as back E.M.F. and is what controls the current through the armature when the motor is running.

When the motor starts it needs a resistance to limit the inrush current to the armature, otherwise it would burn out and is what the Op measures initially. The resistance is switched out when the motor increases in speed and the back E.M.F. then has the limiting effect of the current; this is what is being measured when the motor is running by the OP.

Lenz’s Law and Faraday’s Law apply.

Think I've got that right - been a long time

[Phil-T4]

And as the start up current was measured at 36A, then that is the 450W as stated, its a peak value not a constant one.

[SCP440]

As much as I agree this seems to be the logical answer I have tested a fan fitted to another car rated at 450 watt and that was pulling mid 30's while running and its starting draw was another 10 amps. If the above conclusion is correct that fan should be rated at about 600 watts but it clearly states 450 on the label.

Maybe some manufactures rate them on the starting current and others rate them on the running current?

[Borg Warner]

Unfortunately forum restrictions wouldn't allow me to illustrate the maths supporting the fundamental principles, so a quick Google and:

http://www.ni.com/white-paper/14921/en/

Hope this does

[Phil-T4]

Originally Posted by SCP440

As much as I agree this seems to be the logical answer I have tested a fan fitted to another car rated at 450 watt and that was pulling mid 30's while running and its starting draw was another 10 amps. If the above conclusion is correct that fan should be rated at about 600 watts but it clearly states 450 on the label.

Maybe some manufactures rate them on the starting current and others rate them on the running current?

When designing electrical circuits you always go by the peak current for wiring and fuse protection, the other fan you tested was not a new one and therefore probably near its end of life.

For example, the VIS motors on a V6 should draw about 80mA when running, i have a pile of them here that draw 3 to 5 Amps but still run! - Some of them draw 8+Amps and then burn out, this is why you shouldn't repair the boards by soldering a piece of wire across the burnt out track, it burnt out for a reason!