Any 12 volt techies out there?

[Island_Moose]

The fan is brand new, and its a marine bilge fan, sealed motor for venting flammable fumes. The 13.7 volts was read under no load. Another guy at work also suggested the capacitor route, as though the motor needs a 'kick-start' to get spinning. This confirms my experience, in that the fan runs very well as long as I give it a flick with my finger to get it going...it's as though there's just not enough current to overcome the first revolution.

Rabbitz, any idea of what size/rating capacitor would be required? The fan draws 4 amps, the power supply is rated to 9amps max, 7 amps duty.

[Swanny]

John,
I checked out a pool pump that I remember had the same problem a decade ago... it has a 20uf 400V capacitor. That is for a 240V AC motor, though, but it's worth trying.

[stan]

just flick start it, a pain, but free...

[Muzza]

I don't know if this has been resolved, but here's why it won't work. If a motor is rated at 4 amps, that is its rated RUNNING current. Electric motors can draw 5 to 8 times its running current when starting, so your motor would be drawing anything from 20 to 32 amps. Your power supply is most likely an electronic transformer, the regulator won't allow more than its rated current to flow.

The unloaded transformer will show over 13 volts unloaded. That's normal. As the load (current draw) increases, the terminal voltage drops. If you're exceeding your transformers output current rating by 2 to 3 times, the terminal voltage at that moment would be significantly lower than 12 volts.

A car battery can handle a MUCH higher current draw before dropping its bundle. Much, much, much, much, much higher.

Better late than never...

[Muzza]

John,
I checked out a pool pump that I remember had the same problem a decade ago... it has a 20uf 400V capacitor. That is for a 240V AC motor, though, but it's worth trying.

No. That's a completely different issue.

Single phase pool pumps are 'capacitor start' induction motors. They are a completely different animal to a DC motor. The capacitor is needed to simulate a rotating magnetic field in the stator, which induces a voltage into the rotor, which has no electrical connections to the outside world

Once it's running, the capacitor is automatically removed from the circuit. And the reason you use a 400V capacitor on a 240V device is because 240V is the RMS voltage. (Kind of like the 'average' of the sine wave) A 240V rms sine wave has a peak voltage of 340V. So that's why we use 400V capacitors.

(Looks like I'm putting my hand up as the resident electrician...)