Any 12 volt techies out there?

[Island_Moose]

I've got a bench top 12 volt power supply, which is putting out 13.7 volts. I am trying to power a 12 volt marine blower fan. When I power it up it won't spin unless I start it with my finger. The same fan on a 12 volt battery spins instantly.

What's going on? Is it an amperage issue? The power supply is supposedly rated to 9 amps, the fan draws 4 amps.

[wokkaboy]

Hey Moose,
Weirdbits will have to answer this one but I'd guess the voltage and amps of the supply don't match the blower fan's requirements

[Island_Moose]

The bloody thing is integral to my spray booth project, which in turn has derailed my Shelby Daytona TL-1 hot rod Telecaster build.

[wokkaboy]

yeah ok no one needs delays with a build. Have you got a spare 12 volt car battery you can use till the power supply issues are solved ?

[Rabbitz]

Hi Moose,

An electric motor can require a significant albeit brief spike in current to start.

So the battery is able to deliver this (as it is very short), on the other hand, the power supply may not be able to generate the current fast enough.

A solution to this is to put a big capacitor across the motor between it and the power supply.

An easier solution may be to float the battery on the power supply. In other word have the power supply connected in parallel to the battery and motor.

The power supply is then supplemented at start up by the battery and also charges the battery.

It is pretty much the same as putting a capacitor across, as batteries and capacitors are similar electrically.

[Rabbitz]

Kinda like this:

6070

[Alm_63]

Is the voltage reading, loaded or unloaded?
A power supply will have have a higher reading than if you just whack a meter across it.
13 volts is ok.
Put a 12 volt bulb across the out puts and then read the voltage.
If it's a lot lower than 12 you might need to re-Capp the supply?

[Swanny]

How old is the fan? It is designed to start spinning when voltage runs through the coils, but a build-up of dust/gunk in the bearings may be enough to prevent it from starting to spin.

Another thought, would it be better to use a brushless motor in the fan, in case there are any sparks between the armature and the brushes... ie spraying flammable substances into an ignition source?

[keloooe]

Brushless fans are also just a better option IMO

[DrNomis_44]

Technically, the power supply should be able to power the blower fan with no problems, car batteries usually measure about 13V when fully-charged, the thing is, a car battery is capable of supplying at least 100 Amps when fully charged.

[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...)