Running DC motor from low voltage battery

[ady_mech]

Hello all

As a part of a larger application/product, I have a few brushless DC fans that I need to run from four 1.5V duracell C-batteries (connected in series, so total of 6V). All the fans are 12VDC rated. They differ in their flow output and the current they draw (ranging from 0.2A to 0.6A). However, some of those fans (larger ones) don't run when I connect them to the battery pack. I believe the reason for this is that a DC motor draws a much larger current during startup and the battery pack is not capable of providing that large startup current. Now I have the option of having a PCB (that does other things in the product) that I can use to operate the fan. But I don't know what I should advise the PCB designer to incorporate. What would I need to do in the PCB so that I can run these fans from this 4-pack? I just need to be able to startup the fan (by providing whatever voltage/current it requires) from these batteries. Once the fan is running normally, I would supply the power to these fans from a different source (thermoeletric chip).

Your suggestions would be appreciated.

Thanks

[AnalogKid2DigitalMan]

C cells can supply quite a bit of current for short periods >10 Amps. I presume the larger fans are not starting due to lack of voltage. You say they are rated for 12V, is there a constraint from preventing you from supplying them with 12V.>Since they are brushless, there may be internal electronics within them to create the phase signals that need near 12V to start functioning. Starve them of proper voltage, the fan may not run.

Do they run with 12V? Is the polarity hooked up correctly?

[ady_mech]

Thanks for such a quick reply.

I only have these 4 batteries that I must use. I have the option to do whatever with PCB so make it work. But the batteries must remain the same.

One of fans (I will call it 1225M to signify it's 12V and 25mm thick) runs from the 4-pack (even though it runs slower because it's rated for 12V, but that's ok for now). However, a larger fan (1225H) doesn't run from the 4-pack. It does run if I hook it upto a 9V battery (which I tried just to see if it works, I can't use it in my final application).

Is my understanding the start-up current being several times higher than the normal running current, is that true? If it is, how can I startup these fans from the 4-pack? Can I incorporate some sort of electronics in the PCB?

Thanks

[AnalogKid2DigitalMan]

So it is lack of voltage causing them not to run. Actually (I think) a 9V battery has LESS current capacity than a C cell.

Yes, motor devices typically take more curretn to start up than when running steady state.

If you must stick with 4 C cells in series, you probably need a DC-DC converter that takes a lower voltage in (6V) and steps it up to 12V output. I never have used one, but such devices do exist, worthy of a Google search. Such convertes are typically fairly efficient in the 70-90% range.

You will need to look at your application and determine what the best solution is. Note that as your batteries discharge, their output voltage will tend to drop below 6 volts. You thus need some headroom for your converter to operate at less than 6 volts.

If you need to match the form factor of the 4 C cells, I'm sure there are 12V batteries that fit in the same volume as the C cells.

Message Edited by AnalogKid2DigitalMan on 02-12-2007 01:31 PM